Springback Dimple Dies
The History of Cleaveland Aircraft Tool Springback Dimple Dies
Often the question is asked why our dies are so much better than the competition. This is especially true now that all of our competitors claim that they carry 'the best springback dimple dies available'. Our dies are the best in the world for the cold forming process and I will attempt to explain why without giving away any trade secrets that will make it possible to duplicate them. I usually answer the question with the statement, "I will send you a set, try them, send them back if you don't agree that they are the best". I have never had any returned. Van's as well as numerous builders groups have done 'dimple offs' and declared us the winner of each. One competitor purchased the same brand of machine that we use, but still is not duplicating them. Another competitor says that while theirs are the best, it doesn't really matter what dies you use, and there is this second painstaking process you have to go through to get a good dimple. They go on to say that we didn't invent the springback die, that it was invented in 1942 and 'is very difficult to improve upon'. That statement is true, we have "perfected" the springback dies, and believe me it was, and still is a very difficult undertaking which leads into...
In 1984 three friends in Boone Iowa started RV4 kits. Ralph Koger, Keith Campbell, and my father Buzz Lauritsen. They each had keys to the other's shops and would sneak in and circle with a sharpie rivets that were not as good, or other 'flaws'. This friendly competitiveness vastly improved the quality of work that each builder strived for. After each buying inferior tools form the big aircraft tools suppliers and wasting their money they started to replace key items with the best quality they could find. Sid Golden in California was in the business of buying used tools from aircraft suppliers or factories and 'polishing them up' and reselling them to the homebuilt crowd. One set of dimple dies purchased from Sid performed far better than the others. Repeatedly ordering 'the same kind' never resulted in a match. It turned out that this set was a fluke. The dies were passed around and built all three airplanes. Sid even stopped on his way to Oshkosh one year to visit us 'kooks'. The three airplanes went on to win a gold lindy and two bronze lindy awards at Oshkosh in 1987 and 1989, as well as the Wright Brothers Award in 1987 and 1989. That was uncommon for metal aircraft in those years. Certainly something was different. People started calling, 'how do I get some of those dies'. Buzz sent the dies to one of our now competitors to copy, but it was not done to his satisfaction. We tried half a dozen 'precision' machine shops but could not get the same results. Finally Buzz commissioned the help of a machinist who was the head of R&D at a shop that made fuel nozzles for jet engines. The tolerances that they worked in, the machines, and the inspection equipment available finally made a duplicate of 'the fluke'. We were then coached into what machine to buy and had help programming it to produce the dies. Even though the fluke clone worked, it was obvious that very slight deviations to the geometry greatly effected the dimple produced. So we decided to play with the geometry on both the male and females to come up with the perfect dimple. By varying the funnel angle, the funnel diameter, and springback angle by as little as .0002" made a difference. After hundreds of combinations we were able to find the sweet spot. We were under the assumption that different skin thicknesses and the compression vs. impact tools would all result in different die geometries, to our astonishment the best die in the c-frame (most important) worked equally as well in all tools and in most skin thickness. The only one with a different combination was the .016" skin and that was being phased out of the RV models, so it was decided not to offer a 'specialty' die for 016 skins. You can't measure this stuff with conventional instruments because of the minute angles involved so special 'devices and procedures' had to be created to allow us to keep the quality control that we require. Notice in the reference below that Templin and Fogwell found that 1/2deg. could be critical, that is almost .002" across the face of an 1/8" die! But they were working with manual machines in 1942. Ours are controlled to 0.03deg. or 0.0001" across an 1/8" die. Most tool vendors have now copied our dies, but they can't maintain the quality control that we do. As with any of our tools, give them a try and if you are not satisfied we will give you your money back. They can be found on our webstore at: http://www.cleavelandtoolstore.com/products.asp?dept=48
You always get what you pay for, there is no way around it. Good quality tools are a joy to use, poor quality tools lead to mistakes and frustration. Poor dimple dies lead to 14,000 mistakes on your airplane.
Now that I have explained what makes our dies different look for a tutorial on the proper technique in using dies and how to evaluate your results. With just a little education you can have perfectly flat skins with no extra work.
- The following quote is from Page 555 of:
- Technological Knowledge without Science: The Innovation of Flush Riveting in American Airplanes, ca. 1930-ca. 1950
- Walter G. Vincenti
- Technology and Culture
- Vol. 25, No. 3 (Jul., 1984), pp. 540-576
- (article consists of 37 pages)
- Published by: The Johns Hopkins University Press on behalf of the Society for the History of Technology
- Stable URL: http://www.jstor.org/stable/3104204
"Templin and Fogwell, working with 100? rivets, took it as their task to produce a set of dimpling dies that would (1) leave the sheet flat around the dimple after the dimpling operation, (2) form dimples that would nest tightly into one another, and (3) provide a cylindrically shaped hole so that no undesirably sharp corners would dig into the rivet shank. These goals they attained by trial-and-error variation of the geometrical parameters of the dies. They satisfied the flatness requirement by having the dies extend outside the dimple in such a way as to strain the surrounding sheet slightly in an angular direction opposite to that of the dimple; with the removal of die pressure, elastic springback of the metal then caused the sheet to lie flat. The precise amount of strain proved important, and a difference of 1/2deg. could be crucial. They solved the nesting problem by choice of a suitable die pressure and by careful selection of different maximum diameters for the dimpling cones on the male and female dies. The cylindrical hole was obtained by drilling the initial hole in the sheet to a diameter slightly smaller than that of the mandrel on the male die (which served to center the die in the hole, as in fig. 3c); the suitably tapered mandrel then forged the hole to the desired shape in the dimpling operation. The hole could not be made too small, however, lest radial cracks appear around its edge."