Chapter 2: Technical Chloe Fong 12837126
DP363 Product Development
Module Leader: Dr. Derek Covill
TS EN NT CO OF LE TA B
01 Introduction 02 PDS
Performance 03-04 Force Required 04-05 Magnet Specification 05-06 Physical Testing 06 Magnet Calculations 07 Prototype Testing 07 NFC Performance Testing
Material + Part Selection 08-10 Core Timber Species 10 Plug Species Selection 10 NFC Tag Selection
Design For Manufacture 11-12 Manufacturing Method Selection 12 Timber Specifications 13-14 Block Core DFM 15 NFC DFM 15 Magnet DFM
Design For Assembly 16 Assembly Process
Product Testing 17 Drop Test 18 Water Submersion Test
Sustainability 19 Product Lifespan 20 Recycling + Sustainability
INTRODUCTION Encuro Blocks are designed to help users visualise and share problems. It is compromised of a series of magnetic blocks with NFC chips, each representing a problem or a task that needs to be tackled. The blocks are not only designed for people with depression, but also their friends and families. On a subject that is heavily stigmatised, Encuro blocks aims to open up communication channels to help users reach out sooner and not suffer in silence. This report is the sequel to Chapter 1: Human Factors. It documents the technical considerations needed to meet the parameters set by user needs and feedback in chapter 1.
Product Design Specification Performance Material + Part Selection Design for Manufacture Design for Assembly Product Testing Sustainability
3.1 Professional approval 3.1.1 At least 70% of mental health professionals surveyed should agree the product has the potential to help symptoms of depression. 3.1.2 Professional approval must be sought before investment in app development.
3.2 Beta testing 3.2.1 A batch of 200 to be tested with early adopters to prove clinical effectiveness. 3.2.2. Two focus groups to be held after beta unit sales to improve and iterate design. 4.1 Magnets 4.1.1 Children must not be able to access magnets. 4.1.2 Magnets must be securely embedded in the block. 4.1.3 The pull of the magnets must not be strong enough to cause pain if fingers are trapped between blocks.
4.2 Surface Finish 4.2.1 Finish must be food safe, as children may think it is a toy and place it in their mouths. 5.1 Material Selection 5.1.1 The block core must be a British hardwood*. 5.1.2 Magnets must be permanent. 5.2 Material Sustainability 5.2.1 Timber from an FSC certified source 5.2.2 Raw material wastage to be sold to paper pulp manufacturers. * Derived from human factors. 02
6.2 Medium block assembled size* 6.2.1 54x54x54mm 6.2.2. 76-126g 6.3 Large block assembled size 6.3.1. 65x65x65mm 6.3.2. 112-189g 6.4 Unit should compromise of 2 large, 4 medium and 6 small blocks.*
2.1 Product is for year round indoor use in centrally heated UK households. 2.1.1 Temperature range: 12-30 Celsius 2.1.2 Relative humidity: 8-12 %
1.3 App 1.3.1 90% of users must be able to add data to blocks without assistance after viewing tutorial. 1.3.2 NFC tags to be read in under 2s. 1.3.3 Available for free on the Google Play store.
6.1 Small block assembled size* 6.1.1. 43x43x43mm (±2mm) 6.1.2. 43-70g
7.1 Product must have a service life of at least 3 years. 7.1.1 Without the need for service or maintenance. 7.1.2 Product should have a longer life cycle of up to 10 years if Danish oil is reapplied.
1.2 Magnets 1.2.1 Magnets must be of sufficient strength to statically hold 2 medium blocks, end on end, off the side of a large block.* 1.2.2 Magnets must not interfere with NFC read/ write.*
The following PDS compromises of the most important sections from Pugh’s method for Encuro Blocks. For further information on shipping and logistics, please refer to the DP365 Product Launch report.
8.1 Magnets must be retrievable and reused. 8.1.1 Retrieved by water submersion of blocks at the end of life in service.
1.1 Blocks 1.1.1 Must have a universal fitting system.* 1.1.2 Blocks will be arrayed on the X Y and Z axes.* 1.1.3 Each magnetic face of the block must fit all magnetic faces of other blocks.
9.1 There are currently no direct competitors. 9.1.1 Indirect competition from high end diaries (e.g. Moleskines). 9.2. Product should avoid first to market failure rates by adhering to testing principals outlined in PDS point 3, before a full product launch.
Product Design Specification
8.2 The company must provide means for customers to send products back at the end use. 8.2.1 Mail bags will can be requested by users on product website. 8.2.2 Blocks can be sent back to the company to be re-oiled for a cost.
10.1 App bugs should be fixed and updated as soon as they are identified. 10.1.1 There must be a “report bug” option in the settings menu. 10.1.2 Google Play store reviews should be monitored weekly and users are likely to voice app errors in this section. 10.2 There should be no maintenance required to the blocks in the first 3 years. 10.2.1. Users must be able to register for the warranty online using their unit serial number. 10.3. Danish oil should be reapplied when the wood looks dull. 10.3.1 Preloaded re-oiling swabs can be bought on the online shop, so users can re-oil their products.
Product Design Specification
Performance Material + Part Selection Design for Manufacture Design for Assembly Product Testing Sustainability
PART 1: MAGNETS Section A: Theoretical Calculations
Section A1– Force Required Aim: To find the force required to hold 2 medium blocks end on end to a fixed block (PDS 1.2.1) The 2 medium blocks were approximated into 1 rectangular testing block (Block A). This was held against a vertical surfacing using magnets. The weight of block A was increased until it fell. The falling motion was observed. Figures 2a to 2c show the failure of the block as a line drawing. The results show the force in play which causes the magnets to fail first is static friction force, as illustrated in Fig 3.
Figure 1 Illustration of magnet positioning
To achieve static equilibrium: Block A
Time + weight increase
Where Fs is the total static frictional force, in N m is the mass of the block, in kg g is the gravitational constant, which is approximately 9.82 µ is the friction coefficient, and N is the total magnetic force, in N Therefore:
Placing the numbers in context:
Optimum weight ranges for all blocks were determined from human factors, and users indicated that solid blocks of wood increases its desirability. (See PDS point 6 for details)
To find out what material would satisfy these parameters, the density of 5 of the most common British hardwoods were analysed. (See Appendix B1 for data table) Taking a medium block of solid oak (heaviest material) m = 0.11kg µ = 0.25 (Engineering toolbox, n.d.) Figure 2c Fs
Conclusion: Although static frictional force affected the failure of the magnets first, there is still a risk of tumbling (figure 2c) if the moment forces are not in equilibrium.
Figure 3 03
The minimum force required to moment forces to be in equilibrium Assuming that N is a fixed force in one direction: Therefore
Where d is the width of a medium block, in m
Taking a medium block of solid oak (heaviest material) m = 0.11kg
Conclusion: The magnetic force required counteract static friction is greater than that needed to keep the moment forces in equilibrium, therefore:
Section A2 - Magnet Specification Aim: To calculate the optimum magnet size for the force required Method 1: Mathematical Calculations A physics expert was consulted and the following equation was to approximate the size of magnet needed.
In this instance: B = 1.3T for an N42 grade magnet A = πr2 Assuming that the cross sectional area (CSA) of the magnet = pole area h = 4e-3m which is the depth of the slot for the magnet if wall thickness = 6mm r = unknown δ =4e-3m, 2mm from the surface of each side. µ=4πe-7 derived from the permeability of space, which is close to that of wood (Clarke, 2008)
Relationship between magnetic force and radius 1500
Magnetic Force (N)
Where B is flux density in Teslas A is the area of magnetic pole in m2 h is the height of the cylinder, in m r is the radius of the cross section of the magnet, in m δ is the separation of the magnets, in m µ is the permeability of the intervening medium, in Tm/A
Radius of magnetic face (mm)
Using the function above, radius values between 1-7mm were plotted against their respective magnetic force. Chart 1 shows that a magnet with a 7mm radius will produce 1580N. This is clearly incorrect. This was due to the assumption that the pole area was equal to the CSA of the cylindrical magnet. It is obvious that the assumption cannot be made. Further research came to the conclusion that there is no way to measure the pole size without specialist equipment. Other sources should be explored to size of the magnet. 04
Method 2: Online Calculator
An online calculator which could estimate magnetic force produced by neodymium magnets based on radius and height inputs was found (K&J, 2016). The company was then contacted to enquire about the equations they used to calculate pull force. Response According to the manufacturers, the calculator is based on a proprietary algorithm they developed using data from thousands of experiments. They added that most equations cannot accurately predict the pull force of magnets due to the non-uniformity of magnetic force (Maxwell, 2016 ) Therefore the calculator was used to determine magnetic force required. See appendix B2 for correspondence
Other factors Apart from the initial aim to hold 2 blocks end on end, another parameter was set during chapter 1 with regards to magnet strength. Participants found that Ă˜15 x 2mm N35 magnets at 4mm to be most satisfying, followed by the same magnets at 1mm separation. Using the online calculator, data gathered was translated into SI units. Magnet grade
2.31N < Satisfactory Force < 7.92N
As the performance specification exceed those dictated by user desirability, performance was prioritised.
Section B- Physical Testing Aim: To calculate the optimum magnet size for the force required Before ordering 120 magnets, the results above had to be validated. A quick and cheap proof of principal test rig was created with the materials available: Sand, sheet MDF, weighing scales, and Ă˜15 x 2mm N35 magnets. 2x Magnets end on end
Ă˜15 x 2mm N35 magnets
Hypothesis: In this case, the magnets can only provide 7.339N (K&J Magnets, 2016) and therefore will fail to hold 220g. Results: The magnets were strong enough to hold 220g, the failure weight was 308g, disproving the hypothesis. The calculations over predicted the force required. Discussion: If 7.339N can hold up 308g of weight in the dimensions specified, it is safe to assume that lower strength magnets can be used in the product. As magnets are the costliest part of the product, weaker magnets were tested.
Magnet Selection: In order for all blocks to attach to every magnetic face of all other blocks (PDS point1.1.3), each face needs to have two magnets; because opposite poles attract, a north and south magnet has to be on each face (see page 6 for magnet array considerations).
Therefore, the magnetic 7.34N needs to be halved for each magnet = 3.67N 05
Magnetic force, like electricity, acts across the path of least resistance (Busbridge, 2016). Fig 6 shows the layout of the magnets, where d1 is shorter than d2, making the magnetic strength higher between the blocks than across the blocks.
An upper limit of radius was set to be 12mm, to create a safe d2 distance if d1 was set at 4mm.
Using results from the online calculator, two Ø10 x 4mm (4mm separation) may create the sufficient pull force with 6.23N. They magnets were ordered and the test was repeated.
The hypothesis of 6.23N was proven when the test was repeated with the correct magnets. 10mm
Two N35 Ø10 x 4mm neodymium magnets arrayed diagonally (45° across corners) on each magnetic face.
Fig. 7 Final Design
Section C – Magnet Array The following PDS points determined the arrangement of magnets 1.1 Blocks 1.1.1 Must have a universal fitting system. 1.1.2 Blocks will be arrayed on the X Y and Z axes. 1.1.3 Each magnetic face of the block must fit all magnetic faces of other blocks.
Methodology Rhino was used to quickly simulate possible arrangement of magnets as it allows part editing in assemblies. Pegs and holes were made to represent the north and south poles, as opposite poles attract. The red faces seen in figure 8 represent the non magnetic NFC side.
Fig. 9 Final Array
N S N
S N S S N Fig. 8 Peg and hole simulation 06
This net will be sent to manufacturer alongside technical drawings
Section D - Prototype Testing Test 1: On MDF working prototype Background: An MDF working prototype was made using mitre joints. The inter-block magnet distance was set at 4mm. N35 Ă˜10 x 4mm neodymium magnets were used. Aim: To test the if the magnets will work holding 2 cubic blocks instead of 1 rectangular block, in the array specified above Hypothesis: The on a stationary block will provide sufficient force to hold 2 medium blocks end on end. Result: Hypothesis proven. 2 medium blocks end on end can be held on a single large block. Conclusion: Use same magnets for production.
Test 2: On final prototype
Background: A final working prototype was made using the same methods and materials as the product in beta model manufacture.
Aim/hypothesis: To confirm that the chosen magnets will not be affected by the manufacturing methods. Result: The final prototype passed the test, magnets will be used in manufacture.
Part 2: NFC Tags
Although the maximum NFC embedding distance was determined in the ideation process to be 15mm for wood, the test was repeated in both the working prototype and the final prototype to confirm validity.
Test 1: Working Prototype Aim: To confirm the NFC tags can be read and written when embedded 2mm into a block. Method: SmartTec MIDAS NFC tags were placed in a pre cut pocket on the internal face of the block. The blocks were then assembled (p12 for detailed prototyping process). Information was tagged onto the blocks and scanned by 5 different phone models to confirm readability. Fig 11a: Pre-cut pockets
Results: The NFC information was successfully read on all tested devices (table 2).
Test 2: Final Prototype Aim: To find out if the NFC tags will be affected by an oil based wood finish. Background: The final prototypes were finished with 3 coats of penetrating Danish oil, a blend of natural oils and solvents. It was feared that this may interfere or corrode the NFC chip. Method: Data was first inputted into each block, and scanned by 5 different NFC enabled mobile phones as per working prototype test. Results: NFC tags were not affected by a penetrating oil finish. (see table 2) Make
Moto X Play
Samsung Galaxy S6
Table 2: NFC chip test results
Product Design Specification Performance
Material + Part Selection Design for Manufacture Design for Assembly Product Testing Sustainability
Section 1: Core Timber Species Two parameters where set in chapter 1; first, the material should be a hardwood to increase its perceived value, and secondly a weight range for each block. Weight Requirements: Small blocks – 43-70g Medium blocks - 76-126g Large blocks – 112-189g
Using data sheets:
A list of the most common European hardwoods (Smith, 2013) was compiled and their masses calculated using data form Engineering toolbox (n.d): Species
Size of Block
Oak / Ash / Beech
Results: All woods specified above fall within the parameters +/- 5g (tolerance was set by what can be felt by hand).
Expert Review: A furniture designer was consulted with regards to material selection of timber. The conclusion of the meeting was that appearance, sustainability and cost were the most important factors in material selection. The above species are “tried and tested” woods, which will have little variance in performance. He also stated that the incorrect specifications for the cut of the wood and its moisture content would be more likely to affect performance.
Material selection by appearance As directed by the expert, material appearance was used to narrow down the timber species. User preference was sought as aesthetics are not easily quantifiable.
Results: Users were not able to tell the difference between species of timber. Comments such as “they all look like floorboard” and “are they from the same tree” were major contributors to this conclusion.
Method: Images of the species above were shown side by side to 7 participants aged between 18-35, male and female, from the general population (desirability is not influenced by mental state). A zoom in of the wood grain was also placed on the image, but the names of the species were not mentioned. Participants were instructed to point to the most appealing to them. (see appendix c1)
URE CT RU
Aim: To find out which of: birch, oak, ash, douglas fir, and beech had the most appealing appearance.
Fig 12: Example user test card 08
Selection by cost: Method 1: CES Edupack CES Edupack was initially used to estimate cost of wood, the following costs were generated: Species
0.42 – 0.841
European Oak European Ash
Conclusion: These figures seem quite unrealistic as there were no parameters to define the wood’s moisture content, cut of wood, or method of treatment (i.e. kiln dried, air dried, years of drying etc). It is also strange that the cost of oak is up to 10 times the price of Birch and Birch. Further investigation is needed to validate the prices above.
Method 2: Quotes Before the application of design for manufacture, the final dimensions for materials needed could not be accurately estimated. A price list from a single supplier (SL Hardwoods, 2016) who had all the species was therefore used to limit the variability of the prices.
Air dried (£/ft3)
Kiln dried (£/ft3)
Results: Oak has a higher cost, however, it also has a higher perceived value. The variation of price in raw material is not vast, considering a unit of 12 blocks will require only 0.17ft3 of material (See appendix C2 for breakdown). A clear direction in species selection was still required.
Final species selection: In order to manufacture an initial prototype, the most cost effective hardwood was used – firewood. When the logs were dried and sawn open, it was discovered that they had very distinctive black streaks along the grain, as well as striking dual coloured swirling wood grain. An overwhelming amount of lay people consulted (100% of participants preferred the figured wood over the other options previously presented.) Several carpenters could not identity the species. Hence a photo of the wood grain was taken and shown to various people. A guitarist eventually identified the wood as spalted beech, commonly used for figurative guitar tops.
Fig 13: Cross section of firewood from Brighton Woodstore
Final chosen timber species – Spalted Beech
Acknowledged Limitations: Structural – As spalting is a result of fungal decay, weak spots will be present within the timber, this should not cause significant problems unless the are located near the drill holes. However, 80% wastage is still estimated in all cost calculations. FSC certification – The FSC does not certify spalted beech. As spalting is due to rot, which leads to structural instability, it 09
is often found in the reject pile. The market for spalted woods is limited, so there is no incentive for it to be taken from an unsustainable source Uniformity and production rates – As there is no steady supply of this timber, there is a limit to production rates. This will be combated when the product is redesigned for mass manufacture, where coconut timber will be used.
Wood cut selection:
Traditionally timber is either plain, quarter or rift sawn (Dudgeon, 1980) (fig. 14a-c). Plainsawn is the cheapest as it has the least wastage, followed by quartersawn, and then rift sawn. As wood is made of lignin cells, wrapping occurs in relation to moisture content, which is affected by relative humidity (fig 15). Movement occurs mostly when the timber is green (freshly cut). This phenomenon cannot be stopped, but can be limited by seasoning methods (Johnson, 1983). As the blocks are to be arrays along the x y and z axis, the seasonal changes and relative humidity will cause the blocks to swell and shrink, this will be very obvious when they are stacked on top of each other. Quartersawn wood was chosen as it does not move as much as plainsawn timber, but does not carry the high cost and wastage of riftsawn wood.
Section 2 – Other Considerations Plug material:
Fig. 14a Plainsawn wood
Fig. 14a Quartersawn wood
Industrial timber is either air or kiln dried. Kiln drying provides a controllable environment for the wood to dry slowly, reducing the number of defects such as warping and shakes (Johnston, 1983) (see fig 15&16).
Fig. 14a Riftsawn wood
However, kiln dried spalted wood is rare; wood has to be left to rot prior to placing in the kiln, as the kiln kills the fungus which creates to patterns (Robinson, 2009), it is also more expensive.
The plugs are used to create a design feature, rather than disguising them using spalted beech, because • High skilled labour will be needed to line up grain, increasing the cost of manufacture • There will never be a perfect match Furthermore, the species birch was chosen due to: • Readily available as off the shelf sheet material • Lack of grain, to juxtapose the highly figured spalting • A light hue, to accentuate the darker beech • Low cost British hardwood
NFC (Part selection)
At this stage air dried wood was chosen, but subsequent testing indicated the need for kiln dried spalted beech. The equilibrium moisture content in a centrally heated house the UK, where is product will primarily be used, is 10% in winter and 12% in summer (Johnston, 1983). Therefore, the wood will have to be dried to no more than 13% prior to the manufacturing process.
Birch laser ply was chosen as the material for both the NFC and magnet plugs: • Laser plywood is made of glue that does leave burn marks, a high definition etched logo can be placed on the NFC face • Structural stability of plywood • Reduces splintering when hammered into block core • Can easily be toleranced
Fig. 15 Dimensional shrinkage in cuts of wood
Although a SMARTRAC MIDAS tag was used for performance testing, the updated model, CIRCUS will be used in the production of the blocks. • Same cost as old model • Designed for inlay use, increased readability range • Rounded design, easier to fit in pre drilled hole • ISO 14443A and NFC Forum Type 2 Tag applications compliance (See appendix C3 for part techsheets)
Chosen timber: Quartersawn, air dried*, spalted beech with <13% moisture content
Fig. 16 Star shakes due to incorrect drying, found in rejected wood.
*It was subsequently found that air dried timber does not have enough structural stability. 10
Product Design Specification Performance Material + Part Selection
Design for Manufacture Design for Assembly Product Testing Sustainability Please See Appendix X for technical drawings.
Method Selection: A solid cube of wood was preferred from human factors. 3 methods of manufacture were identified and explored by modelling. (ranked from most visually close to a solid cube to least like a solid block)
1. Hollow cubes
2. Solid cube with plugs
3.Casted/Moulded frame with wood panel inserts
Sheet material cut as faces of the cubes, with magnets fixed to internal walls. Filler is then added internally to achieve weight of a solid block of the same volume.
A solid cube with counterbored hole for magnet insert + a wood plug. Flush with the surface of the face.
This methods requires the least skilled labour and has the shortest assembly time.
Another important consideration of manufacturing method was the minimum order quantity (MOQ) for production. The benefit of an all wood product (options 1&2) is the MOQ is essentially 1. As Encuro Blocks are a first to market item, manufacturing in high quantities poses significant risk. 200 units will initially be batch produced to gauge market reception.
Using quantifiable metrics: 5 methods of manufacture were identified to produce the blocks out of timber. They were ranked using quantifiable metrics, see appendix D1 for full breakdown and rationale of scores given (i.e process, machining time, labour cost etc). Table 6 is a weighted matrix to compare manufacturing methods. Mitre + Drilled + Key Metric Weighting Rabbet Mitre Rabbet Finger plugged Machining cost
Labour skill + time
4 of the 5 methods above were prototyped (no access to a rabbet joint router bit), to estimate manual production and assembly times.
Fig 19: Mitre joint profile
Fig 18: Mitre Rabet Joint
Fig 20: Mitre Joint with magnet holes 11
A full set blocks were made from MDF using mitre joints. Through the process of making (see images on right), it was found that the method required an extremely high tolerance, there was a large amount of wastage, and the assembly time was very long. This method was subsequently eliminated for large scale production.
Change of Method of Manufacture Drilled and plugged blocks were next on the ranking of manufacturing methods. Although this method produces blocks that have distinct plug hole, the plugs let the users know the location of each magnet, increasing affordance. It was decided that the plugs would be made into a design feature.
Final Specified Timber Dimensions: Width: 70mm
Specifications for Timber: Sizing: BS EN 1313-2-1999 specifies the dimensional tolerance for wood with a 20% moisture content to be -2mm +6mm, they also specify the preferred nominal dimensions for the timber. There is a preferred thicknesses is 65mm, but due to the accepted -2mm tolerance, the next size, 70mm was chosen. Width of nominal sizes go up in intervals of 10 from 50-90mm, therefore 70 was also chosen. The length, measured in meters, goes up in intervals of 0.05 (-0, +30% tolerance). 200 units will be produced, each unit with total length of 604mm, Therefore, 0.65m (650mm) will be specified. Wastage: 80% wastage will be added to the order to accommodate tolerances, defects and human error and manufacturing in process. This is a conservative assumption based on one provided by a spalted beech timber merchant (Simmons, 2016). See appendix D2 for correspondence emails . Finish: PAR (Planned all round) timber will be specified to limit wastage and ease of manufacture.
User testing on working prototype
Length: 0.65m Thickness: 70mm
01. Cut wood to size (10 min) Equipment: Table Saw
Tolerancing: Perpendicular +1º, -0. Dimensional +2.6, -0. (See figs 12a +b)
Post process: Sand to specified dims. ±0.5mm Fig 22a 1.3mm
QC: Reject if shakes of >2mm in width is present
65 + 2.6mm
02. Fillet Edges (5 min)
Fillets to be routed prior to drilling to prevent drill holes interfering with router bearing.
Equipment needed: Table router. Custom router bits. ( r4, r5, r6) Bit specification: Rounding-over bits with bearings, the bearing would eliminate the need for a jig.
06. Quality Check (<1 min)
An open steel box with the internal dimensions of the blocks will be used for quality check. The fillets will be checked using radius gauges due to the cost and precision needed to add the radii. This option will reduce the costs of manufacturing QC steel boxes with perfect fillets.
Tolerancing: ±1mm as it doesn’t affect performance. More than 1mm on the smallest block will interfere with the magnet plugs. (taking into account variability in hole locations due to tolerances)
PART 1. BLOCK CORE
05. Sand and Deburr Edges
04. NFC Plug Drilling (1 min) Equipment: Pillar drill/ drill press. Bit specification: 24mm Fostner bit (standard size) Hole depth tolerance: Due to the adhesive nature of the NFC chip, which is stuck onto the NFC plug and not the block core, the depth tolerance can be positive (+0.2) without compromising NFC performance. However, as the blocks are sanded to flush post assembly, a negative tolerance can result in the logo being sanded off completely. Accepted limitations: The cross sectional area depth ratio for the NFC holes will result in an uneven hole base. Another catalyst for this irregularity is due the difference in hardness of spalted beech, particularly obvious in the end grain. Sticking the NFC chip to the plug first eliminates this limitation. 13
03. Magnet Hole Drilling (7 min) Equipment: Pillar drill/ drill press. Custom bit, (11mm is not standard size) Bit specification: 11mm HSS drill bit (to reduce burring, better for wearing as each unit has 120 magnet holes) Jig design (see fig. 22e): To locate the hole, jigs would be used to ensure the positioning of the magnets are all uniform across S, M and L blocks. Holes were datumed from lower right corner of each face. This could cause variation in hole positioning. However, the variation is not significant enough to cause performance problem. Holes were placed on the base plate to reduce errors from burrs causing an uneven surface.
3 unique L shaped datum plates, for S, M and L blocks, were designed to fit on the same base plate (only one is used at a time), so the worker can change L plates for the different sized blocks without repositioning the drill bit or unclamping the base plate. Jig alignment: A datum CNCed (laser cut or similar) block must be used to align the drill bit.
Total estimated time for part: 24mins. Assuming wage is ÂŁ10/hr (skilled labour) the total labour cost for part will be ÂŁ4.
Drill depth: Drill depth set to a of 6mm (+0, -0.5) to ease of assembly, errors can be easily spotted as hole depth = Magnet depth + plug depth (see fig 22d). A positive tolerance will result in a reduction of magnetic strength, whereas a negative tolerance will result in a protruding plug, but this can be sanded off post assembly.
The equipment needed can be found in most good wood workshops. If product goes into mass manufacture, the cost of the equipment per unit would be negligible. Please refer to costing part of DP364 product launch for full breakdown.
Top Level DFM Best Practice: Each unit consists of 3 assemblies, one for each sized block, the manufacturing and assembly process of all blocks are the same. Limiting part count: Apart from the size of the block core, the inlay magnets, plugs (both NFC and magnet plugs), as well as NFC tags are all the same. Limiting processes: The NFC and magnet plugs are all made of the same material and thickness, so that both parts can be laser cut on the same bed to reduce labour and machining time.
The peg holes on each L plate are calibrated in relation to the base so that worker do not have to reposition the base plate.
Holes on base plate
Fig 22e Jig design 14
PART 2. PLUGS Method:
Laser cutting will be used to produce the plugs that go over magnets. Although best practice would be to include draft angles in plugs, this cannot be done using a laser cutter. The alternative, using a CNC router, would cost significantly more due to an increase of machining time and wastage. Furthermore, modifications to the vacuum bed will also be needed due to the small part size. The engraving on the NFC plug also cannot be achieved with this method.
Fig 23: laser cutter jig for prototype
The plug diameter tolerances have to be very high to reduce the risk of plugs falling out. Specific tolerances vary by laser cutter, which will be calculated by the manufacturer. The following are actual sizes post cutting (all tolerances specified are from testing data): Part Name Thickness (mm) Hole Ø (mm) Plug Ø (mm) Magnet Plug
Machining time: 5 mins approx. This process and be done concurrently with the manufacture of core blocks.
Engraving Depth Tolerancing: The engraving depth is vital to its final appearance. If the logo is engraved too deep the inner parts of the graphic to come loose (see fig. 24), if the engraving is too shallow, it may be sanded off during the post assembly sanding process. A tolerance of 1mm (-0, +0.2) was specified taking into account the final assembly; plug thickness tolerance and block height tolerance.
11.1 (+0, -0.03)
24.37 (+0.03, -0)
PART 3. MAGNETS The magnets will be bought in on a OEM basis from a Chinese manufacturer, where 97% of the world’s neodymium is produced (Milmo, 2010). This will decrease the price Specification: Ø10 x 4mm N35 magnets, marked on the north pole DFA: The marked north pole as the polarity specified in the technical drawings is essential to product function. Diameter tolerance: The diameter tolerance is +1, -0mm as the holes drilled have a 11mm diameter. A positive tolerance will only cause the magnets to be more powerful, however, a smaller diameter will decrease the performance of the blocks and may fail PDS point 1.2.1. Thickness tolerance: The disk cannot have a negative tolerance as it will not only affect performance, but also cause the magnet to rattle in the hole. A tolerance up to +0.3 can be accepted as the plugs will be sanded to flush post assembly.
Fig 24: logo engraving too deep
A vector graphic of the logo will be provided to the manufacture. They will be instructed to centre the graphic to the plug. The design of the logo was designed with a rotational symmetry of order 4. So they will not appear upside down no matter where the user places the block in the structure. 15
PART 5: NFC TAGS NFC chips are bought in components. The chip will be stuck onto the plug prior to assembly. The diameter of the hole, 24mm is 2mm wider than the diameter of the chip, this is minimise assembly errors.
Product Design Specification Performance Material + Part Selection Design for Manufacture
Design for Assembly Product Testing Sustainability Please See Appendix X for technical drawings.
01. Marking Out Magnet Polarity on Blocks.
02. Marking Out Magnet Polarity on Blocks
Equipment: Vice press, wood mallet
Equipment: Pencil, master cube DFA: The polarity of the magnets is vital to the performance of the blocks. A schematic, in the form of it’s 2D net will be supplied alongside technical drawings. As the array of magnets is the same across S, M and L blocks, the manufacturer will be encouraged to make master blocks to prevent human errors.
Process: • The array of the magnetic poles will cause some magnets to repel when placed in position. Therefore, a plug has to be immediately placed on top to prevent the magnet from flipping. • A wooden mallet will be used to force the slightly oversized plugs in, a standard hammer will only attract the magnet. Alignment will be judged by eye, which did not prove to be a problem in the prototyping stage • A wood vice will be used after all the plugs are placed to further press the plugs in, this will ensure the plugs are placed to a sufficient depth. Additionally, using a vice will ensure uniform pressure across the block face, reducing damage, and preventing the plugs to be pushed shy from the surface. For hole depth DFA see DFM of core blocks on page 13-14.
03. Sand to Flush
04. NFC Assembly
05. Sand and Finish
Sanding must be done before the NFC inlay. This is to keep a flush face to be used as a datum against a disk or belt sander.
The NFC sticker is stuck onto its plug prior to assembly. This tackles the of the NFC hole due to the use of the Fostner bit.
A final round of sanding will ensure the NFC plug face is flush using 300, 800 and 1000 grit sand paper. The prototypes will also be sent to the manufacturer as a sample for smoothness, as there is no standard to measuring surface smoothness of wood. 3 coats of Danish oil will be used to provide a robust, water resistant finish. It will also stain the wood and accentuate the distinct grain.
Assembly QC: Complex parts have their own QC process, bought in components will be QCed upon arrival to the assembly location. Magnets: A compass will be used to test if the magnets poles are correctly arrayed. NFC: A testing rig, with an NFC antenna on its top face, will test if the NFC tags are working. Final checks: A worker will do the last round of QC manually to ensure there are not faults in each magnet.
Estimated assembly and finish time: 1 hour. This was estimated using time required to produce prototype. Although there is 6 hour of drying between each coat of Danish oil, this can be minimised when a larger quantity of units are produced , as the drying times can be staggered for efficiency.
Product Design Specification Performance Material + Part Selection Design for Manufacture Design for Assembly
Product Testing Sustainability
Although the product is a robust, solid block of wood, there is one major potential failure risk. In the inital design, the plugs are not glued in as they are toleranced to be larger than the hole and is held in with friction. The consequences of the plugs failing are severe. There have been cases of young children swallowing the magnets and becoming severely injured due to the magnetsâ€™ attracting each other and puncturing the GI tract. (Rettner, 2013) 2 tests were done on the blocks to find the failure point of the plugs.
Test 1: The Drop Test Set up: A medium sized block was dropped from 2 heights onto 3 types of surfaces, to simulate the user accidentially dropping the blocks off the desk or from a standing position. Direction of drop: BS-EN 60068-2-31:2008 outlines 3 directions of dropping, on the face, on the edge/corner and a topple (Fig.27). The block was dropped using all three methods of dropping.
Fig. 27a Face down drop Fig. 27b Corner down drop
Fig. 27c Topple
Height of drop: The British standard above outlines a fall height for general equipment testing, but the recommendations were not used as the drop height was too low compared to what would happen when the product is in use. Instead, the block was dropped face down and corner down from 1108mm, the elbow to floor height of the 50% percentile standing man (Dreyfuss, 2002). The topple test was pushed from a 72mm height, the UK average desk height (from product catalogues of John Lewis, Argos and IKEA). Floor surface: 3 types of indoor surface was chosen for the drop test. Carpet, wood flooring and vinyl tiles. The tests were repeated on these surfaces. All 3 tests were done on each surface. Results: Carpet
Conclusion: The plugs did not fall out of the block. There was no cosmetic damage.
Fig. 28: Test set up
Test 2: The Water Test Theory: As wood expands and shrinks in relation to its moisture content, a constant fluctuation will wrap the geometry of the holes and plugs, causing the plugs to fail. Preparation: To catalyse the movement of the wood, a spare large block was made up and oiled. Only 2 coats of Danish oil was used (as opposed to 3) to simulate the effects of surface wear.
Procedure: The block was measure on the X, Y and Z Axis and weighed. It was then submerged in water for 24 hours. A second reading of measurements and weights were taken. The block was then placed in a 70 degree fan oven to accelerate the drying process.
This test was used to place the material under a maximum amount of stress at a cellular level to test their point of failure.
After 24h Submersion
After 70Â°C Oven Drying
Dimensions: X: 65.68mm, Y:64.44mm, Z:65.98mm Weight: 145g
Dimensions: X: 66.17mm, Y:66.73mm, Z:66.17mm Weight: 232g Notes: NFC plug falls out Plywood starting to delaminate Warped plugs NFC tag works
Dimensions: X: 65.22mm, Y:65.44mm, Z:66.06mm Weight: 232g Notes: 50% of plugs fell out Severe discolouration Star shakes of 2mm width forming
Under extreme and rapid humidity changes, the plugs fail and fall out. Delamination of the plywood and splitting is the main cause. As expected, the dimensional movement is more prominent along the grain. (X+Y axes) It was interesting to note that the NFC tag was readable when wet after the 24hour submersion. Design Changes: Although the product is not likely to be submerged underwater for long periods of time (note that the block also floats), neither are they likely to be placed in an industrial fan oven. For liability purposes PVA glue will be used alongside the friction tolerance to further secure the plugs in place. The NFC hole depth can also be increased to create a larger surface area for the friction fit. A do not submerge and 6+age limit labels will be added to the packaging and instructions to decrease liability risk. Further Insights: Prolonged submersion in water will cause the plugs to disintegrate and deform, although PVA glue can strengthen the bond between plywood and beech, it will still eventually fail if submerged for long periods of time. This means that at the end of the lifecycle, the blocks will be placed in a large tank of water for a few days until the plugs fail and the magnets fall out. Providing a low energy means to extract the magnets (which sink) from the wooden core (which floats). Please see design for sustainability on pages 19 and 20 for more information on the extration process.
Further Testing: Under normal use, the plugs are most likely to fail from overuse. A stress test would be in done in industry to find out the number of cycles the blocks can connect before magnet fails. A hall effect sensor will be placed on top of the rig to detect the change when the magnet fails. The rig itself will count the amount of cycles before failure. Hall Effect Sensor
Fig 30: Testing rig 18
Product Design Specification Performance Material + Part Selection Design for Manufacture Design for Assembly Product Testing
Product Lifespan NFC tags The scan limit for NFC tags specified by the manufacturer is 10,000 cycles. It is also noted that this is a conservative figure, and may be more in practice (RapidNFC, 2016). The following equation shows the estimated life of the tag assuming that each block will be reprogrammed each day:
10000 cycles / 365 days = 27 years
The tags are themselves water resistant, as they are incased under a PET layer. Results from the water test show that they can be used even after 24hours of submersion. The NFC tags are therefore a robust part in the block assembly.
With reapplication of Danish oil = 100 years With the original coat of oil = 3 years
The lifespan of a wood kitchen work surface was used as a benchmark for the blocks, as they are commonly made of hardwood, frequently finished with Danish oil due to the food safe properties, and undergoes daily wear and tear. The International Association of Home Inspectors (InterNACHTI) estimates that the wood counter tops have an average life expectancy of 100+ years (n.d). Assuming that the timber is kept in intended conditions (centrally heated indoors), the first to wear off would be the Danish oil finish. The lifespan of the surface finish cannot be predicted as it will be largely dependent on the frequency of use and handling. For the purposes of a figure, and expert was consulted and the surface finish lifespan of 3+ years before reapplication was estimated. (Smith, 2016) The re-oiling process for Danish oil is very straightforward and can be done at home. Swaps preloaded with Danish oil will be sold as an addition accessory to the blocks on the online shop.
Magnets: Sintered neodymium (NdFeB) magnets are permanent and have a negligible loss of <1% every 100 years (Magcraft, n.d.). This also means that they can be reused indefinetly using a â€œclosed loopâ€? cycle. (See page 20)
Limitations of Estimates Above
With repeated use, there may be a risk of the plugs falling out as the magnets are constantly pushing against the plugs, (see testing section for details) The risk of NFC technology becoming obsolete will also play a large part in the life cycle of the product. The hardware will possibly to outlast the technology.
Recycling + Sustainability Timber of core block: Spalted beech is a sustainable timber choice as it is classed by furniture makers and structural engineers as “defected” due to a lack of structural stability caused by fungal decay. Spalted beech will only be used in the first 200 units as there is no steady supply of spalted beech due to low demand. (Simmons, 2016) In mass manufacture, the core of the blocks will be made of coconut timber, due to its hardwood-like density, and a dense, close grain similar to that of beech. With the rising popularity of coconut water and beauty products (Riley,2014), the supply of the coconut seed has been increasing in many tropical countries. However, coconut trees can only bear fruit in the first 70 years of their lifespan (Ohler, 1999). Subsequently, farmers fell the trees in order to create space for more. Another advantage of coconut timber is the rate of growth; the tree will reach full maturity in within a decade, as opposed to 80 years for European beech (Hoyt, n.d). of Therefore, there is an abundance of cheap (15USD for a trunk estimated by the UNFAO, 1997), sustainable wood. By using coconut timber for Encu ro blocks, the carbon footprint would be minimised in mass production. It essential to source wood from sustainable coconut plantations in order not to encourage illegal tropical deforestation.
Figure 31 A coconut wood soup ladle.
Neodynium Magnets Neodymium (NdFeB) magnets are made of rare earth materials that need to beHall Effect Sensor mined from the earth’s crust. There is only a small amount of Nd in the ore. During the extraction process, large quantities of toxic solid and liquid waste is generated and leeched into the environment (Milmo, 2010). Neodymium magnets are most commonly used in hard disk drives and wind turbines, and cannot be recycled. It is not economically feasible to retrieve them from their larger assemblies such as laptops and wind turbines. (Lee, 2014) The drill and plug design of the blocks allows the magnets to be easilyFixed retrieved and reused. The plugs will have removed by water submersion, a cost, energy and labour efficient way. Using the insights gained from the water submersion test (on page 18). The following set up was designed to retrieve 100% of the magnets from blocks. The retrieved magnets will be reused to make new blocks. Creating a closed loop recycling process, reducing environmental impact that using virgin materials would otherwise cause.
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