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The ExperimentalAero DRDT-2 (grey) and the Avery C-Frame (red). |
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The business end of the DRDT-2. |
Tools - DRDT-2
DRDT-2 Dimpling Tool from ExperimentalAero
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In early 2006 I decided I wanted realize the lifelong dream of building an aircraft. After many months of research I decided on a design than includes flush rivets, thousands of them, which led to the purchase of many new tools including the one reviewed here.
A number of years ago a mechanical design engineer named Paul Merems decided to create an alternative tool for dimpling, the process of creating a small countersunk area for the flush rivet to sit. The accepted method to dimple aluminum used in the experimental building field was impact c-frame tool. The c-frame can be used for both dimpling and riveting, so the DRDT-2 does not replace the standard c-frame for all functions. There are two main reasons for why someone would want to create a different method for dimpling. The first is the desire to create dimples with a slightly less level of noise, a c-frame is very noisy and requires hearing protection, and considering that thousands of dimples are required in an aircraft designed with flush rivets, this is not a minor consideration. A second reason is repeatability. While a very experienced and careful builder can make excellent dimples using a c-frame, it takes a great deal of care and a lot of 'feel' to be able to accomplish this thousands of times consistently. Other important factors were ease of use, that it be a relatively safe tool to use, and not cause excessive fatigue on the operator. After a number of refinements, Paul Merems has developed the DRDT-2 and sells it directly via ExperimentalAero as well as through a number of companies involved with the experimental aircraft industry.
Being a first time builder I don't have the experience of creating thousands of dimples using a c-frame, so found the idea of using a tool that can produce consistent results with a novice at the controls quite appealing, I bought one. The fact it could be utilized while others slept in the house was also a huge benefit in my case. My first real exposure to dimples and flush rivets was a practice kit resembling a portion of a control surface. This being the first thing I had ever done with rivets I really wasn't sure about my method of using of the DRDT-2, but ended up with what I thought were reasonable looking flush rivets. Anxious to get building, I put together the horizontal stabilizer (HS) and thought I had done a reasonable job of the flush riveting, but wasn't really sure as it didn't look quite as good as my practice kit. In order to obtain some sort of baseline of what proper dimples should like, I closely examined installed flush rivets and dimples on a number of aircraft and projects that had been created using the standard impact c-frame method. My conclusion was that I had not produced a smooth as finish as what others had done using a c-frame. At this point I wanted to know what I was doing wrong. An order to Avery Tools and I now had both an impact c-frame, and the newer DRDT-2 with which to experiment with. Within a very short period of time I discoverd I was under dimpling with the DRDT-2 and that was responsible for my result, so I set out to come up with some sort of criteria to measure the quality of the dimples.
The one quality of dimples and thus flush rivets on my HS I was not impressed with, was the distortion of the skins resulting in less than ideal reflections around the dimples. I know that what I did was safe and is no worse than many planes out there, even some certified ones, but I still wanted to know more about the process and how to improve my end result. There were three areas I wanted to concentrate on when making the dimples. The first was how to create the dimple with minimum residual distortion in the aluminum surface around the dimple. The second was after doing the first, how well does the rivet sit in the dimple, and third, the visual appearance of the final flush rivet. I started with four sheets of aluminum of thickness from 0.016" to 0.040" each with about 60 to 75 holes (both for AN426AD3 and AN426AD4 sizes) in which tests could be preformed. Using both the impact c-frame and the DRDT-2, I made a large number of test dimples until I could obtain consistent and good results with both tools, well at least as consistent as I could be using a c-frame with my lack of experience.
The basis for what I considered a properly created dimple came down to using light reflection from a four foot long fluorescent tube mounted over the work area. In the case of an under done dimple, the light reflection tended to bend away from the dimple as the light reflection approached the hole, and an over done dimple had the reflection being bent towards the hole. Again, this is what I used as criteria and I have no authority to do so other than it makes sense me.
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When it came to producing a dimple where there was minimum light distortion, for me it was much easier using the DRDT-2, as consistency in results was not difficult, in fact, it was down right simple. Setting the correct amount of preload is important, but not difficult, I added a simple circular scale that allowed me to add pressure in 1/16 turn increments. To obtain consistent results with the c-frame required a lot more attention more expertise and skill than I possess at this time. Having said this however, if forced to comment on the visual appearance of the dimple and how the flush rivet fits prior to final riveting, I would have to say the c-frame can, in the hands of much more experienced builder than I, produce what appears to be a marginally better fitting flush rivet in a dimple. The dimple seems to be crisper in the transition from the skin surface to the 100 degree countersunk surface. Once the rivet is set though, without knowing which ones were done with which tool, I could not tell the difference even though that theoretically there should be a slight difference given the marginally visual difference prior to the riveting process. To make thousands of dimples with a c-frame, and not under or over dimple any, would be the sign of a truly excellent builder.
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Simple scale added to the mechinism used to adjust the preload. By having
it calibrated in 1/16 of turn, repeatability is easy. |
While there are many factors that affect the amount of preload to obtain a dimple with miminum reflection distortion, such as material thickness, temperature, the dimple dies themselves, and minor variations in the overall tool itself, especially if you had your own frame built, here are a few tips to start with. Once the dimple dies are loaded into the tool, loosen the tightening nut, and adjustment ram so that the handle can be brought all the way down against the stop without the dies touching. Adjust the ram finger tight so that the dies are now touching with the handle in the fully down postion. Note the reading of the scale to some reference, I just use the value pointing away from the structure. For 0.016 thick 2024-T3, I find a preload of about 1/2 turn works well. This is done by lifting the handle up and turning the ram by 1/2 turn in such a manner as to exert that amount of additional pressure when the handle is fully down. For 0.025 2024-T3, about 1 turn works, and for 0.032 and 0.040 material about 1.5 turns is needed. This is only a rough guide and will be different depending on factors noted above, start with a little less and add preload until reaching the optimal amount. Once you have found the value for a particular thickness of material, as long as you use the same dies, work at about the same temperature, the results will remain very consistent. If your workshop temperature can change by more than 10 degrees C during a dimpling session, you had better check your settings every so often.
For my purposes, excellent quality dimples with consistency and quiet operation are standout advantages of using the ExperimentalAero DRDT-2. The cost of the entire tool directly from ExperimentalAero was $340 US plus shipping. A front end only version with plans for the metal structure are also available at a much lower cost for those that want to have the frame made locally and save on shipping cost. The DRDT-2 was more expensive than the standard c-frame, but given the quality and most importantly the consistency of the result, it will be my tool of choice when it comes to dimpling.
More information can be obtained from the manufacturer.
ExperimentalAero
12351 E. Lou Bock Pl.
Tucson, AZ 85749
www.experimentalaero.com
