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Concerned about under dimpling, Please let me know your thoughts

ctremie

Member
I have a DRDT-2 , with dimple dies from ISham (Planetools.com), I have the drdt-2 setup to flex about 1/8th of an inch at the bottom of the stroke. But I am concerned about the dimples looking under dimpled. Thoughts?

Different dies maybe>?
 

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I most definitely see under formed dimples.
It could be caused by the dimple dies, but in my opinion, the DRDT-2 is always going to produce dimples of lesser quality than other methods.
 
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Just need more pressure on the DRDT. Get some scrap alum and make practice dimples in it and adjust until there is no dishing around the dimple.
 
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Straight on they look good but when you get a distance away they seem to distort, is this normal
 

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Dimples

The dimples near your hand look better but still under. The ones at the top are very under dimpled.
I see no burnish marks. You may want to try some Cleveland dies.
I prefer a C-frame.
 
They are certainly under dimpled or not nearly as crisp of a dimple as they should be and I am with Scott 100%
You seem to be at the early stage of your build so still ways to go. It would be worthwhile to spend the $$ and time on a good dimpler and a good table/platform for it.
 
I’m FAR from any kind of expert. How do your dimples compare to the examples in the standard aircraft handbook or in the FAA Inspection and Repair Alterations book? I’m trying to reconcile why all of the books seem to prefer a dimple that only just accepts a flush rivet flush to skin while the forums always seem to prefer a deep dimple…?

Trying to learn myself.
 
They are certainly under dimpled or not nearly as crisp of a dimple as they should be and I am with Scott 100%
You seem to be at the early stage of your build so still ways to go. It would be worthwhile to spend the $$ and time on a good dimpler and a good table/platform for it.


So is the DRDT-2 and Isham Dimple dies not good? I also have it mounted on EAA workbench. What is your recommendations
 
Under dimpled

I agree with the poster that they are under dimpled. Period
Hit it harder, set tension more, what ever,
I'm in the camp of needing better dimpling.
These do not look good.
My three cents worth. Art
 
Dimples

Just need more pressure on the DRDT. Get some scrap alum and make practice dimples in it and adjust until there is no dishing around the dimple.

The pneumatic squeezer, although convenient, does not provide enough force for a proper dimple. Re-dimple with a c- frame dimpler and you will see the diflection around the dimple go away. Or in your case, as mentioned, better adjusment on the DRD2
 
My method for determining - hold the piece so a straight flourescent light reflects thru the dimple. If it looks distorted like a magnetic field drawing- it is UNDER dimpled. If it distorts & looks like a volcanic crater - it is OVER dimpled. If the light reflects thru as a straight line - repeat 14,000 times!

& my choice is a C-Channel with lots of practice.
 
My method for determining - hold the piece so a straight flourescent light reflects thru the dimple. If it looks distorted like a magnetic field drawing- it is UNDER dimpled. If it distorts & looks like a volcanic crater - it is OVER dimpled. If the light reflects thru as a straight line - repeat 14,000 times!

& my choice is a C-Channel with lots of practice.


Hey Ralph, Might these be over dimpled?
 
I did my entire build with a DRDT-2 and Avery dies, and after adjustmemt saw no evidence of under-dimpled holes. I found the device should take some pressure to squeeze and “spring” back after applying the pressure. The whole point of it having that long arm is to allow you to give it enough pressure to produce good dimples.

I’m not flying yet, so who knows: all those rivets might fall out when I take off. But I believe the tool can produce good dimples.
 
I used a C-Frame for most of my dimpling and a pneumatic squeezer when possible due to location. I built my 4x8 foot work table with six 4x4 legs and 2x8 frame under the plywood and MFD top. It is very solid and allows for a good crisp hit without any bounce or deflection while using the C-Frame. I bought all my riveting and dimple tools from AVERY when they were still in business. Very quality dies and tools.

Deforming metal properly takes correct force and suddenness to properly upset the metal and conform with the shape of the dies. My point is if you aren't able to apply the necessary crisp, sharp blow to the tools, it will be very difficult to create a proper dimple. Using a C-frame requires a very solid table, or surface, so the energy in the hammer blow is not deflected from the work and gets absorbed in the table or work surface. This deformation is best accomplished with one crisp hammer blow with the correct force based on the metal type and thickness. Well made and properly shaped dies will also help.

Roberta
 
Cleveland "Spring-Back" dimple dies and the DRDT-2 worked just fine for me. - Its hard to over dimple, you just about have to be trying to do some damage. But, its easy to under dimple.
 
Test coupon

Pictures below

I use a DRDT-2 for large sheets, a CP214C clone for close quarter work. Built and flown a few RVs now using these tools and methods and none have had a failed rivet -- YMMV of course.

Dies are Cleaveland CAT 426-3, cleaned and polished.
 

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Alright! Cleaveland Dies ordered and Overnight! will know and update tomorrow, about to break this handle trying to add more pressure! lol

Thanks guys!
 
I have never used a DRDT-2 but I would guess the speed at which the metal is upset is as important as the pressure. I know my pneumatic squeezer was very quick and a hammer blow on the C frame is quick. I would think the speed that the DRDT-2 is operated could influence dimple quality.

Roberta
 
DRDT-1 and cleveland dies

Another user of the DRDT2 and Cleveland dies.
I really don't like the wack-a-mole C-frames; seems too primitive and too easy to miss and get the hole in the wrong spot. JMHO.
But the DRDT2 needs to be set so the frame warps a little and leaves the donut rings.
 
Another example

I have never used a DRDT-2... I would think the speed that the DRDT-2 is operated could influence dimple quality.

Doesn't appear to make a difference -- slow (#1), fast (#2), C-frame (#3)

The C-Frame dimple (#3) is a tad distorted because the dies aren't aligned quite right.

Or course, you would need to look at the cross section of the material with a SEM or similar device to determine the granular structure...
 

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DRDT-2

I've been using the DRDT-2 with Cleveland dies and have it adjusted so the top arm of the c-frame flexes upward roughly an 1/8th of an inch when fully engaged.

It leaves clear witness marks on both sides of the skin and overall am satisfied with the crispness of the dimples.
 
When I look at the photos in post #1 I see a series of "dots", for lack of a better description, around the dimple. What you should see is a faint smooth ring. Not sure if it's the dies or under-dimpled. The DRDT doest take a good push - certainly, the DRDT must be bolted to the table or the back end will lift up. If it's not and it's not, you're not pushing hard enough.
BTW: my opinion: C frame is better in the hands of a pro. DRDT is better in the hands of an amateur. I used the DRDT!
 
When I look at the photos in post #1 I see a series of "dots", for lack of a better description, around the dimple. What you should see is a faint smooth ring. Not sure if it's the dies or under-dimpled. The DRDT doest take a good push - certainly, the DRDT must be bolted to the table or the back end will lift up. If it's not and it's not, you're not pushing hard enough.
BTW: my opinion: C frame is better in the hands of a pro. DRDT is better in the hands of an amateur. I used the DRDT!

Yes mounted and sturdy!
 
I use a DRDT-2. I set the dies up so that I see about 1/8" of flex in the body of the unit when I full depress the handle when there is no material in the way. I'm using springback dimple dies. When I'm finished, I usually see a ring around the dimple about 1/2" in diameter and when I look at the reflection in the metal, there is no distortion except for where the metal was deformed to create the dimple. I've been satisfied with the final riveted results. I've never used a C-frame so I can't compare.
 
Groundhog day

This is another groundhog day thread. The same thread pops up every year....mostly the same answers....mostly defence of product choice...it's all in the archives.

As previously submitted this excellent and informative video from Cleaveland Tools comparing the DRDT and C-Frame sums it all up. In essence both tools will do the job but the C-Frame will do it a bit better.

https://www.youtube.com/watch?v=_mpbsQF1eu0

The reason the C-Frame produces crisper, more defined shoulders at the dimple is purely because it exerts a dynamic pressure as opposed to the DRDT which exerts a static pressure. It's that simple.

If the shoulder of the dimple is not well defined....ie. a curve rather than a distinct change in angle ....the rivet will set slightly high. This is easy to test for....just run your fingernail across the factory head of the set rivet. If you get ANY catching on the rivet whatsoever your dimples are not well formed.

This is not a structural issue...it's largely a cosmetic issue. So in the end it just comes down to how fussy you are. If you're very fussy go C-Frame....not so fussy and the DRDT is fine.
 
The reason the C-Frame produces crisper, more defined shoulders at the dimple is purely because it exerts a dynamic pressure as opposed to the DRDT which exerts a static pressure. It's that simple.

I don't think that is the reason. I expect it is the peak pressure applied. The C-frame hammer impulse likely produces a higher peak pressure than the DRDT.
This beast produces as good a dimple as either the C-frame or DRDT, and it is not dynamic. But it certainly applies a lot of pressure, consistently.

I'd be interested to see the pressure that the DRDT can apply, which is limited by the flex of the beam vs. that of a typical hammer blow. BTW, I have all of the above, and use them all.
 

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When discussing dimple quality, saying that a hole is under dimpled is not meaning that the dimple is not formed enough to properly accept a rivet. It could mean that, but it would be a severe case that I think even most beginners would detect on there own.

When using the under dimpled statement, I am talking about the condition of the skin in the immediate area surrounding the dimple. As already mentioned by others, evaluating this is best done by analyzing the reflection on the skin surface. Looking at the reflection of florescent light tubes is a good way to evaluate the condition of the skin because they are long straight objects.

A properly dimpled skin will have a reflection that is pure and undistorted except for right at the point of the dimple. An under dimpled skin will have distortion in the reflection within about a 1-1.5" diameter circle (depending on the degree of distortion) area centered over the dimple. This can be seen when looking at the reflection in some of the OP's photos.

A good way for a beginner to learn to detect this is to dimple (with the vinyl removed) a single hole out in the middle of a large skin and then lay it perfectly flat on a table and look at it with your eyes positioned so that the reflection of an overhead light tube aligns parallel and over the top of the row of holes where one is dimpled. If it is a good quality dimple, the quality of the reflection in the area directly adjacent to the dimpled hole, will be similar to the reflection at all of the other holes that are not yet dimpled.

Try this with a thinner (.020) skin and then a thicker (.032 or .040) skin and see what the difference is in the results with the method you are using to dimple in the middle of skins.

Looking for scuffing in the zone right under the footprint of teh dimple dies is often used as a benchmark for a properly completed dimple. It is a good start

There are a lot of variables that can have an influence on what the final result is.
- What dimple dies are being used.
- How large the hole is that is being dimpled
- What tool the dimple dies are being used in.
- The technique of the user
- How thick the material is
- Adjustment of the tool (how well the dies align with each other, how much force is applied to the dies, etc.)

It may not be obvious, but the thicker the skin is, the more force that is required to get a nicely formed dimple. Some tools may do just as good of a job as others, on thinner material, but then not so well on thicker ones.

Same goes for hole size. The bigger the dimple, the more force that is required to properly form the dimple. In some combinations of dimple size and material thickness, it may not be possible to get a well formed dimple with certain tools.

An additional factor that should be mentioned is that in situations where skins were dimpled by a builder to the best they could be done, the processes and techniques used in riveting the skins can turn something that would have been beautiful (in the eyes of RV builders anyway ;)), to something that looks like a mess. Point being.... that perfectly dimpled skins does not in itself assure a perfect finish when done riveting.

Because of all of the above, besides only using high quality dimple dies, I prefer to use a C-frame tool for any dimple that is on an exterior skin and will be visible on the completed airplane. The reason being that it is capable of applying the needed force regardless of skin thickness or dimple size.

For dimples on any sub structure I use what ever method is most efficient and convenient for the location as long as it looks like the dimples are being formed reasonable well.

This response is not meant to say that only certain tools should be used.
I fully understand that there are specific benefits to dimpling with a compression tool. A big one is when noise is a major consideration. I do not agree that it adds insurance against punching extra holes. I have seen it done at an equal level regardless of the tool being used

I will finish by saying that I have talked with thousands of builders over the years, and looked at lots of finished RV's, and have learned that everyone has a different eye for what is good and proper. There is nothing wrong with that, because in the end, as long as a builder is satisfied with the finish quality they achieved, that is what matters (as long as all of the work at least meets acceptable standards). A high level of quality in dimple finish is not an airworthiness factor. If the dimple it self is fully formed, the work done meets acceptable standards.
Having said that, I have looked at a lot of RV's where the builder was convinced that the dimple finish was as good as was possible, when to more experienced builders, it clearly wasn't.

So a lot of different tools and methods will work. Some will work just as well as others, depending on the variables (dimple size, or skin thickness) involved in that particular situation. But in certain situations, I think there are methods that are superior, but each builder has to decide what process provides a result that meets their standards

This is my opinion based on a lot of years doing this as a daily vocation. I have tried pretty much every tool and method that can be used (some that the average RV builder has probably never even thought or heard of). Not everyone may agree, but hopefully it is helpful to some.
 
Redimple

Scott
Ive been asked and never tested.
Is it possible to redimple an under dimpled hole to correct it?
 
Wow I can almost see the blood running down the bottom die of that yellow beast when the brain forgets the foot v/s finger are in the wrong places.
But on the under dimpled dimples. Scott is spot on. Some dimples may be good, But, just look $#Q^# ugly.
Time to hit them again.
Art
 
Something new...to some.

DRDT can make nice dimples with the right force. Cleavland true 'spring back' dies helped me quite a bit.

How about the underlying dimple? Geometrically you can't use the same dies on each layer and expect them to nest properly. Try it, dimple two pieces and see if they nest! Sure, things will smash together quite a bit with soft skins but is that the 'best practice?'

In addition to high quality top dies, we can consider sub structure dies for the layers below that require the shape not of the concavity on the skin that hold the rivet head, but the convexity of the bottom side of the dimple (it's a different size and angle).
 
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DRDT can make nice dimples with the right force. Cleavland true 'spring back' dies helped me quite a bit.

How about the underlying dimple? Geometrically you can't use the same dies on each layer and expect them to nest properly. Try it, dimple two pieces and see if they nest! Sure, things will smash together quite a bit with soft skins but is that the 'best practice?'

In addition to high quality top dies, we can consider sub structure dies for the layers below that require the shape not of the concavity on the skin that hold the rivet head, but the convexity of the bottom side of the dimple (it's a different size and angle).

We do not endorse or specify the use of sub structure dimple dies in our construction manuals for a very specific reason.

When a .098" diam. (#40) hole is dimple counter sunk, the displacement of material due to the shape change, opens up the hole diameter to right near the maximum limit specified by the Mil Spec.
If an even bigger dimple is formed on the same sized hole, the hole diameter is opened up even further.

All static structural testing was done using test articles that were built using the techniques detailed in the construction manuals (standard dimple dies used for all dimples).

Using substructure dies could conceivably be making the structure weaker rather than stronger. Not saying that it does, because all the testing we have ever done was as described above. The only way to know would be to test it.

Just one of the many double edge swords involved with making things "better" when building an RV. A builder of course has the latitude to deviate as they choose, but they should always keep in mind that they own any negative influence that might result, when they do so.
 
Using substructure dies could conceivably be making the structure weaker rather than stronger. Not saying that it does, because all the testing we have ever done was as described above. The only way to know would be to test it.

The deformation of the lower layer is going to happen regardless right? Certainly the rivet head has enough compressive strength to demand that other materials strain in tension. So is whether the substructure is stretched before or after the riveting of real concern? I don't think it is avoidable either way and a proper nest before the rivet expands and locks the relationship between the layers seems to make sense. Seems like making the additional stretch subsurface before the material is work hardened is an advantage. At least it did and was industry standard when this kind of dimpling entered aircraft construction.

A fun video by Walt Disney for those who haven't see it: https://www.youtube.com/watch?v=RtpkFJj08CQ
 
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Not from what I have seen, but if it did, wouldn't it negate the need for forming a bigger dimple in the underlying part?

(I don't think the difference is so much in size as in shape. The skin thickness at the center of the dimple must be less as we stretch that material away from plane. Consequently the angle of the lower layer dimple is the base 100 deg plus approx 10 degrees owing to the additional cone effect of the sheet getting stretched toward the center.)

If the concern about using a sub structure dimple is because the original hole would get bigger, then consider that using a larger angle as is done with substructure angles stretches the material LESS not more. This would allow the stretched hole of the substructure dimple to be smaller than the surface dimple NOT larger. A benefit rather than a concern.

If the sheets are laying against each other after riveting then the dimples would have to have nested, I think that's intuitive.

To nest dimples we have to accommodate the underside of the dimple in the topside of the next layer. We can do this before riveting in a known manner or hope that it fully happens during riveting in a way that we don't control. The need to form a bigger angle in underlying layers is only ever negated if we lack clamping pressure during the drive and the rivet sets before the sheets come into contact.

The chance of an un-nested set seems much more likely when we build in a bad nest. Again, do a couple of good dimples in moderate stock and see just how much gap needs to be taken up by rivet compression.

If we intend to work the underlying shape during riveting rather than in our preparation we are intentionally work hardening that material twice.

You can actually see a difference in sheets combined with nesting dimples vs those combined without nested dimples. It just comes out 'flatter.'

So if the lower layers must accommodate an angle greater that the original surface (100 deg), and they must accommodate the new angle if the sheets are to nest, then why not do it ahead of time and ensure mated sheets before riveting? If it begs testing then why not do that? It would be a pure guess on my part but I would tend to think Lockheed did that already.
 
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Before you get concerned about the negativity toward the DRDT above, pretty much my entire -9 was done with it and I never had troubles with not getting a nice crisp dimple. Nor did I ever see a difference when I used my impact c-frame. I'm a big fan of the DRDT.

Now, that said, I have no knowledge of Isham dies, but I did use Cleaveland dies which, according to Wally Anderson of Synergy Air, were among the best.
 
When a .098" diam. (#40) hole is dimple counter sunk, the displacement of material due to the shape change, opens up the hole diameter to right near the maximum limit specified by the Mil Spec.
If an even bigger dimple is formed on the same sized hole, the hole diameter is opened up even further..

I had to edit my last post and it probably got overlooked.

A substructure dimple isn't really 'bigger' it is a different shape, specifically a bigger angle. That bigger angle means less stretch, a hole much closer (smaller) to the original not bigger.
 
I assumed that is what you meant, and that is what my answer was a reply too.
Riveting does not reform the lower dimple to make them nested together in a net fit.

I think something's getting lost here.

I take two 1" wide strips of .024 or other similar stock. Drill, dimple with the same die and stack. Lots of wobble between sheets, can't get the sheets to mate. Then I drive a rivet and if I get good rivet compression along its shank then the finished stack appears to be very close to mating. Something had to change in those dimples for that to happen.

If I do the same with a sub structure shape on the lower sheet the sheets mate completely before dimpling and look better after driving. How is this not a good thing?
 
I think something's getting lost here.

I take two 1" wide strips of .024 or other similar stock. Drill, dimple with the same die and stack. Lots of wobble between sheets, can't get the sheets to mate. Then I drive a rivet and if I get good rivet compression along its shank then the finished stack appears to be very close to mating. Something had to change in those dimples for that to happen.

If I do the same with a sub structure shape on the lower sheet the sheets mate completely before dimpling and look better after driving. How is this not a good thing?

I have done test samples were a rivet joint was sectioned and inspected. It showed that riveting does not reform the lower dimple to produce a net fit between the two (even tough to your eye it may look like it makes it fit better But as i mentioned previously, if it did, that would be another reason for not using sub structure dies.

I already explained how it could potentially be a bad thing (maybe you missed that post), but that I can't say for sure because we have never test it.

We have tested using the same sized dies so there is no debate on the strength using that method.

With that being the case, I see no point in using sub structure dies and adding in an unknown factor, when there is no clear benefit (in my mind anyway), but it does add a potential unknown factor.

Apologies to to everyone else for the thread drift (though I guess it is somewhat related to the general theme of the discussion)
 
IT WAS THE DIES!!!

UPDATE!!!!!

CLEAVELAND DIES CAME IN TODAY AND BANGED IT PERFECTLY STRAIGHT!!

IT WAS THE DIES!!



Thank god i figured this out early before putting anything else together!!

Thanks everyone for your input!!
 
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