Strengfhs of Fingerioints And Their Profifable Uses

Manufacturers of a wide variety of wood products can make profitable use of a new publication issued by the U. S. Forest Products Laboratory by brushing up on their high school geometry.

That's what the publication is all about -the geometry of finger joints and how it afiects their tensile strength. Finger joints consist of slightly blunted projections cut into the ends of pieces of wood and snugly glued to similar fingers in othcr pieces. The obvious result is longer, more useful pieces.

And if your geometry is right, those joints will be strong enough for a great many uses-in millwork, furniture, coniainers. house trim" even laminated strur:tural members such as beams, columns, arches, and truss members.

Author of the paper is M. L. Selbo, gluing and laminating specialist at the national Laboratory maintained by the Forest Service, U.S. Department of Agriculture. To get a copy, write for "Efiect of Joint Geometry on Tensile Strength of Finger Joints."

Essentially, finger joints are a compromise between butt joints and scarf joints. To make a butt joint, you simply glue to' gether the squared-off ends of two pieces of wood. Unfortunately, such joints are low in strength.

To make a scarf joint that is essentially as strong as the wood itself, a shallow diagonal cut is made through the thickness of two pieces that are to be joined together. This cut should slope about an inch to the foot of joint length-the flatter the slope, within certain limits, the stronger the re' sulting glue joint.

Finger joints are much shorter than scarf joints, hence less lumber is used to make them. Their strength is somewhat be' tween that of a butt and that of a scarf joint-the better designed and glued, the stronger the joint. A well-designed finger joint can develop two-thirds or more of the tensile strength of the wood, Selbo's experimental data show.

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The object in designing a finger joint is to make the taper, or slope, of the fingers as flat as is practical to cut on high-speed machines. The flatter this slope, the greater the gluing area available for bonding fingers together. As it works out, Selbo found, the slope should be about the same as that of a good scarf joint, or about I inch in a length of 12 inches.

Of course, the fingers are much shorter than scarf cuts, ranging from about I to 3 inches. Maximum strength is obtained with a slope of about I:16, Selbo reports.

The shape of the fingertips also afiects joint strength. Tips should be as narrow as possible consistent with production efficiency. The wider the tips, the weaker the joint, since the fingertip parts of joints are in efiect small butt joints.

In his experiments with Sitka sprucc and Douglas fir, Selbo used tips of three sizes, 0.134, 0.090, and 0.045 inch. The narrowest (0.045 inch) fingertip produced the strongest joints. In experiments with oak, Selbo used only the 0.045-inch fingertip.

The length of the fingers also has a definite efiect on joint strength, the longer ones being stronger at any given slope. The longer the fingers, of course, the more wood is required to make the joint.

Selbo data shows how joint strength is related to the three basic elements of ioint design-finger length, fingertip width. and finger slope. By example, he shows how to calculate the approximate effects of each on joint strength and how to determine the best 6nger.joint design for a given purpose.

Reloiler Seminors Scheduled

By Industry Educqlion Center

A schedule of 17 seminars in 1964-1965 for retail lumber and building materials dealers has been announced by the Building Industry National Education Center (BINEC) at Purdue University.

All will be held in the Universityos giant Memorial 'Center at the West Lafayette campus. The program will include five oneweek seminars this year-beginning in October-and 12 week-long seminars in 1965 for personnel in the industry throughout the United States.

"Educational techniques for these programs promise to be unique and provocative," said D. E. Rons, BINEC program specialist. "They will include lectures, workshops, personal consultation, case studies and 'role-playing,' depending upon the specific seminar material."

The 1964-1965 schedule includes two categories: The BINEC Marketing Associate and the BINEC Management Associate programs.

The Marketing Associate program will include a series of three one-week seminars offered in consecutive weeks and recommended for sales managers, inside and outside salesmen, management trainees or middle management personnel for compa- nies of all sizes. Titles and dates for the marketing seminars: ooKnow Your Products"-Oct. 5-9, 1964; Feb. 8-I2, 1965, and Sept. 27-Oct. l, 1965.

"Material and Labor Estimating:Oct. 12-16, 1964; Feb. I5-I9, 1965, and Oct. 4-8, 1965.

"Creative Selling"-Oct. 19-23, 1964; Feb.22-26, 1965, and Oct. 1l-I5, 1965.

Participants who complete' all three of the seminars will receive special certificates designating them as BINEC Marketing Associates.

The Management Associate program in. cludes a series of three one-week seminars for owners, general managers, presidents, general management executives and management trainees whose companies' annual net sales are less than $400,000. Titles and dates for the management seminars: signed to solve growth and expansion prob. lems and is for owners, general managers, presidents and operations vice presidents whose companies' annual net sales exceed $400,000. Dates are Jan.25-29,1965, and Dec. 6-10" 1965.

"Planning and Forecasting for Profit" -Nov. 9-I3,1964, and March 22-26,1965.

Write f). E. Rons, BINEC, Purdue Memorial Center, Purdue University, West Laf,ayette, Ind.

New Leqsing Booklet

The revised edition of its popular study on auto fleet leasing has just been issued by the Foundation for Management Research.

It is entitled ; 't lf,,lpntages and, Disdua.ntages ol Auto Fleet Leasing: A Comparison ol Company Ownership, Salesman Ownership, and Leasing." The revision includes latest cost data on fleet operation, current finance charges, and the results of the Foundation's latest survey of auto fleet leasing practices.

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"Personnel Techniques for Profit"Dec. 7-ll, 1964, and Oct. 25-29,1965.

o'Selling for Profit"-Jan. lI-15, 1965, and Nov. I5-I9, 1965.

Participants completing all three will receive special certificates designating them as BINEC Management Associates.

A fourth one-week seminar, o'Money, Men and Management for Profit," is de-

The study analyzes situations in which it is better for companies to own their auto fleets, to lease them, or to use salesmenowned cars. Precise costs for each situation are detailed.

Single free copies of this study may be obtained by executives by writing to the Foundation for Management Research, l2l West Adams St." Chicaso 3. Illinois.