F1 in Schools Car Manufacturing
Design Guide
Contents Introduction
3
How will the Manufacturing Centre machine my design?
5
What size tool will the Manufacturing Centre Use?
6
Will the Manufacturing Centre drill my axle holes?
9
What machining plans will the Manufacturing Centre use to produce my car?
11
What is a stepover?
12
What is a stepdown?
13
What is the maximum length I can make my car body?
14
What is a tailstock?
15
What size should my tailstock be?
16
Why is 29mm the maximum height of my cartridge hole?
16
What tolerance will my car be manufactured to?
16
Manufacturing Job Order sheet
17
Introduction The aim of this guide is to explain the manufacturing process for producing the body of the F1 in Schools car. A number of the teams taking part in the F1 in Schools Technology Challenge do not have access to their own CNC router and rely on the services of a Manufacturing Centre. Manufacturing a design is a complicated process which takes time and can be very difficult if the design has not been produced with the manufacturing process in mind. In order to assist the teams using the Manufacturing Centres this guide will explain the manufacturing process and layout a few design rules which must be followed in order to use the manufacturing service.
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Car Manufacture - Design Guide
How will the Manufacturing Centre machine my design? The Manufacturing Centre will machine your car from the left and right sides of the billet. This means that the tool will only remove material it can "see" from the left and right sides as demonstrated in the image below. LEFT
RIGHT
The tool can remove all the material (shown in green) to produce the design (shown in blue) as it can reach it from the left and right hand sides.
LEFT
The material shaded in red cannot be removed as the tool cannot reach this area from the left or right.
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Car Manufacture - Design Guide
What size tool will the Manufacturing Centre use? The Manufacturing Centre will use a 1/4" (6.35mm) Ball Nose cutter. This cutter is 6.35mm in diameter and has a rounded end. The diagrams below show some points to bear in mind when designing your car. The image on the right shows a design with a perpendicular corner, the ballnose tool cannot clear out the material in the corner because it has a radius of 1.75mm.
The image to the right shows the material which cannot be removed (shown in red) if your design has sharp lines meeting at the corner of features. You should add a fillet to these corners to give a radiused edge.
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Car Manufacture - Design Guide
The image to the right shows a design that has had a fillet applied to the corner of the features. Unfortunately the radius is less than 3.175mm.
The image to the right shows the material (highlighted in red) that will not be removed if the fillet has too small a radius.
The design on the right has a fillet with a radius of more than 3.175mm so the tool has removed all the material required to create this feature.
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The image below shows a billet from above, the design on it has a groove that has to be cut out. Unfortunately the width of the groove is less than 6.35mm.
The image below shows the material (highlighted in red) that will not be removed if the gap is narrower than the diameter of the tool.
Hopefully the you now have a better idea of the shapes that can be machined using the manufacturing centre based on the tool they are using.
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Will the Manufacturing Centre drill my axle holes? The manufacturing centre will drill your axle hole with a 6.35mm ballnose cutter. In order to have your axle holes drilled, it is not enough for the stl to have the holes in it, you must specify the location of your axle holes in relation to the cartridge hole. The image below illustrates how the axle hole locations should be measured.
If you wish for your axle holes to be smaller than 6.35mm they will not be drilled by the Manufacturing Centre, the machining process may not even leave marks showing where the holes should be if you are using a diameter of less than 6.35mm. If the centres of your axle holes are less than 10mm from the base of your car then the material surrounding the axles hole may break away during the drilling process as shown by the red highlighted area in the image below.
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Car Manufacture - Design Guide
Another area of your design which you may want to consider is if having hubs for your axles extruded from the side of the car body. If they are less than 20mm in diameter they are also likely to break away as shown in the image below.
The image above illustrates the part which may break away from the right side of the car, the image below illustrates the same example from above (the area which will break is shown in red)
Another area of your design concerning the hubs around the axle holes is if they are extruded in such a way that when the car is laid on its side they are vertical. This will cause the full length of the tool to be in contact with the extrusion ad has been known to cause the feature to break off. Adding a radius draft angle to the extrusion should prevent breakages.
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Car Manufacture - Design Guide
What machining plan will the Manufacturing Centre use to produce my car? The manufacturing centre will use 2 machining plans on the right hand side of your car and 2 machining plans on the left hand side of your car. The first machining plan will be a raster roughing at 270 degrees raster angle, 80% stepover, and 100% stepdown. The second machining plan will be a raster finishing at 270 degrees, 15% stepover.
What is a raster machining plan? A raster machining plan removes material by moving the tool back and forth over the billet whilst working its way over the billet as shown in the image below. The red line shows the path that the tool will follow for a 270 degree raster angle.
What is the raster angle? The image above shows a raster angle of 270 degrees, this means that it starts on the left and works across to the right whilst rastering in the direction of the Y axis. A 90 degree raster angle is very similar to the 270 degree raster angle, but it starts on the right and works its way over to the left. A 0 degree raster angle will raster in the direction of the X axis and work its way towards the back of the machine having started at the front. A 180 degree raster angle is very similar to the 0 degree raster angle, but it starts at the back and works its way towards the front. The image below illustrates the difference between these raster angles.
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What is a stepover? The stepover is the amount that the tool moves by after each raster pass. The smaller the stepover the more accurate the machining will be, the bigger the stepover the faster the machining will be and your design will require more hand finishing. The images below illustrate the difference between a large and small stepover.
The image above on the left shows a 15% stepover and the image above on the right shows an 80% stepover. The area shaded in yellow is the material that will be removed and the area in red is the material that will not be removed. As you can see, the higher the stepover the less accurate your machining plan will be and the more hand finishing will be required to finish your design. The Manufacturing Centre will use 2 machining plans, a raster roughing at 80% stepover and a raster finish at 15%.
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Car Manufacture - Design Guide
What is a stepdown? A stepdown is the maximum depth that the cutter can go to when removing material. For balsa the cutter can go to 300% of the tool diameter, as we are using a 6.35mm tool, this means that we can remove 19.05mm of material for each pass. For the New F1 in Schools billets a stepdown of 100% should be used, this is 6.35mm. As the billet is machined from the right and left sides we need to cut to the centre line + the tool radius. This is 32.5mm + 3.175mm = 35.675mm If each pass is 6.35mm deep, then there will be 6 passes in total as illustrated by the image below
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Car Manufacture - Design Guide
What is the maximum length I can make my car body? The billet will be held in the fixture as illustrated in the image below
There is an aluminium mandrel inserted into the cartridge hole and an aluminium rectangular fixture supporting the front of the billet. In order to avoid hitting the front of the fixture your design must not be any longer than 205mm
What is a tailstock? A tailstock is usually used to apply support to the longitudinal rotary axis of a workpiece being machined. A lathe centre is mounted in the tailstock, and inserted against the sides of a hole in the centre of the workpiece. A tailstock is particularly useful when the workpiece is relatively long and slender. Failing to use a tailstock can cause "chatter," where the workpiece bends excessively while being cut. In this application, if we were to machine past the front of the car on the right and left hand sides of the design there would be nothing to hold the billet in the fixture as illustrated in the image below. When the red section is removed the billet will be loose in the fixture.
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Car Manufacture - Design Guide
If you do not add a tailstock to your design the manufacturing centre will have to leave the end of your design in the billet, as illustrated in the image below. The green section will not be removed leaving the end of your design still inside the billet.
You should extrude either a rectangle or a circle from the end of your design to act as a tailstock, as illustrated in the image below. The red section is the tailstock, by extruding this from the end of your model the manufacturing centre are able to machine your whole design.
What size should my tailstock be? The tailstock should be at least 10mm x 10mm when viewed from the front and should be extruded at least the tool diameter past the end of your design, so 6.35mm beyond the front of your design.
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Car Manufacture - Design Guide
Why is 29mm the Maximum height of my cartridge hole? The manufacturing centre are using the F1 in Schools official billets, the cartridge hole is predrilled in these billets so that they fit the F1 in Schools Car Manufacturing Fixture. The cartridge hole is 29mm from the bottom of the billet as illustrated in the image below.
29mm
As the image above illustrates, it is possible to have the cartridge hole lower than 29mm, but for every mm lower than 29mm you must deduct 1mm from the maximum height of your body which is 50mm for a 29mm cartridge hole height. It is not possible to raise the cartridge hole above 29mm as that would involve adding material to the billet which cannot be done.
What Tolerance will my car be manufactured to? The Denford Router used by the Manufacturing Centres has a repeatability of +/Â 0.2mm, as your body is machined from both sides this is a limit of +/Â 0.4mm The machining profile used for finishing is a 15% raster finish, as 15% of 6.35mm is 0.95mm your part may be oversized by this amount either side of a feature. This means that the tolerance you should expect to have your car manufactured to is:
+2mm Â0.4mm
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Car Manufacture - Design Guide