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What does over dimpling look like?

Mort04

Member
I’ve completed the dimpling on my Empennage skins, I don’t have pictures yet because I can’t seem to capture what I’m seeing. Does anyone have pictures that I can compare to my skins?

I use a C-frame, the dimples look crisp, there is no pillowing around the dimple from what I understand that to be. Looking directly above each dimple there is no distortion, however looking down the rivet line with the skin at eye level, you can see ripples along the rivet line. Only when when light hits it at the right angle. Did I over dimple the skins causing distortion in the metal along the rivet line?
 
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I?ve completed the dimpling on Empennage skins, I don?t have pictures yet because I can?t seem to capture what I?m seeing. Does anyone have pictures that I can compare to my skins?

I use a C-frame, the dimples look crisp, there is no pillowing around the dimple from what I understand that to be. Looking directly above each dimple there is no distortion, however looking down the rivet line with the skin at eye level, you can see ripples along the rivet line. Only when when light hits it at the right angle. Did I over dimple the skins causing distortion in the metal along the rivet line?

In my experience, under-dimpling is more likely to cause the kind of issue you're describing than over-dimpling. Try doing a test piece and purposely under dimple a row. You'll see it. Then re-do them, with more force than you think you need. You might find that you need to recalibrate your eye and hand... Looking at the dimple along a line normal to the surface will not tell you much. Looking along the line with a light source in the background to reflect is the best way to determine dimple quality, as you are finding.
 
Ok, I’ll need to do that and also try to get some pictures. I always thought over dimpling was also a possibility using a c-frame, and I thought I was using quite a bit of force with the hammer. When a dimple is done correctly should you not see any type of distortion what so ever?
 
Dimpling IS a distortion as it's stretching the skin, albeit slightly. If you're using a good set of dimple dies, you're not going to distort it more than the dies will allow.

But, I don't know what your setup for the C-frame is. with mine, I had a table I built around the C-Frame. If the bottom die is only flush with the table, you may be affecting the skin around the dimple somewhat and maybe that's what you're seeing. If you have no table around the c-frame and the skin is hanging down around the sides, that might also have something to do with it.

Just try to have the skin well supported as you whack the dimple dies and you should be fine.
 
I don't believe you can "over dimple" with a c-frame. With a well supported skin, I usually give each hole two serious slams with a dead-blow hammer. Dimples come out perfectly.

One thing with a c-frame though, always be sure that dies are in contact with the skin before wacking; I hold the shaft in place with one hand and wack with the other. There is nothing worse than having the piece jump on impact ... creating an unintended lightening hole .....
 
I don't believe you can "over dimple" with a c-frame. With a well supported skin, I usually give each hole two serious slams with a dead-blow hammer. Dimples come out perfectly.

One thing with a c-frame though, always be sure that dies are in contact with the skin before wacking; I hold the shaft in place with one hand and wack with the other. There is nothing worse than having the piece jump on impact ... creating an unintended lightening hole .....

I prefer two or three less than serious blows with a normal hammer - safety glasses worn to protect against metal/metal chipping.

You can hear quite a difference in tone when the dimple is formed and the whole area of the upper die is on the metal surface. No need for extra blows after that. :)
 
A line of dimpled holes near the edge of a sheet of aluminum will tend to distort the edge of the sheet upwards, due to the combined stretching affect. A simple edge roller or slotted block of hardwood can be use to bend the edge back down.
 
As already mentioned, you can't really over dimple using a c-frame tool unless you were using a 5 lb hammer and hitting way harder than anyone would likely do.

What you see is more than likely under dimpling (in my opinion occurs to some extent on about a 3rd of all RV's built to date).

One way to confirm is to make another pass with the tool down one row of holes and see if you note any difference.

A properly dimpled hole is such that there will be zero distortion in the surface reflection (with vinyl removed) beside on the entire area beside the hole after dimpling. There usually is some (amount depends on the sub structure and the riveting process being used)after riveting but if under dimpled it makes the situation worse.
 
I am using a Cleveland C-frame, Table and dies. I was using a rubber mallet as a hammer as I was always told never to use metal to metal while hammering on your tools. I gave it 2 good blows. The die is maybe 1/16 above the carpet on the table, because I didn?t want the skin to be pushed down the hole in the table causing a dent.

When I get home from work tomorrow I?ll try another pass on the skins in question. I?ll also post some before and after pictures for future use. I thought I was using quite a bit of force while dimpleing so I?ll give each hole one more whack.
 
I am using a Cleveland C-frame, Table and dies. I was using a rubber mallet as a hammer as I was always told never to use metal to metal while hammering on your tools. I gave it 2 good blows. The die is maybe 1/16 above the carpet on the table, because I didn?t want the skin to be pushed down the hole in the table causing a dent.

When I get home from work tomorrow I?ll try another pass on the skins in question. I?ll also post some before and after pictures for future use. I thought I was using quite a bit of force while dimpleing so I?ll give each hole one more whack.

The rubber hammer is likely your problem.

We use a hard plastic tipped (replaceable tips) metal hammer in our shop. It still requires hitting quite a bit harder than if a steel hammer were used.

Another big influence is how solid the surface is that the tool is on.
If you put the tool on a concrete floor the force required to get a proper dimple is reduced greatly. When doing large dimples such as for screws (the ones most likely to not be fully formed by lots of builders) I often move the tool to the floor or switch to using a steel faced hammer.
 
The rubber hammer is likely your problem.

We use a hard plastic tipped (replaceable tips) metal hammer in our shop. It still requires hitting quite a bit harder than if a steel hammer were used.

Another big influence is how solid the surface is that the tool is on.
If you put the tool on a concrete floor the force required to get a proper dimple is reduced greatly. When doing large dimples such as for screws (the ones most likely to not be fully formed by lots of builders) I often move the tool to the floor or switch to using a steel faced hammer.

I have experienced this problem early on because I was dimpling on my table and it was flexing. Now I do all of my dimpling on concrete and the dimples come out spot on! Not fun kneeling for the whole process but beauty is pain. :D
 
Scott & Jereme match my experience. I did use a big (3" dia x 4" head) rubber mallet for all my C-frame work, but I swung it like I was driving big 3" framing nails; 1st a medium blow to start the dimple, then a really hard hit to finish it. My work table is a section of maple bowling lane; it looks like a giant butcher's block, but made with 1" maple strips on edge and over 2" thick. The top is ~ 3' x 5', and it's so heavy it's difficult to pick up one end without help. I'd be surprised if one could build a simple plywood work surface stiff enough to allow good C-frame work.

Charlie
 
Scott & Jereme match my experience. I did use a big (3" dia x 4" head) rubber mallet for all my C-frame work, but I swung it like I was driving big 3" framing nails; 1st a medium blow to start the dimple, then a really hard hit to finish it. My work table is a section of maple bowling lane; it looks like a giant butcher's block, but made with 1" maple strips on edge and over 2" thick. The top is ~ 3' x 5', and it's so heavy it's difficult to pick up one end without help. I'd be surprised if one could build a simple plywood work surface stiff enough to allow good C-frame work.

Charlie

That's where the DRDT-2 really comes in handy.
 
Sure, if you're building on a cubic money budget. And if you search the archives, there are numerous posts by people having the same issues with the DRDT-2.

Not saying you shouldn't buy one, if that floats your boat, but it won't get you better results than properly using a C-frame.
 
Sure, if you're building on a cubic money budget. And if you search the archives, there are numerous posts by people having the same issues with the DRDT-2.

Not saying you shouldn't buy one, if that floats your boat, but it won't get you better results than properly using a C-frame.

I agree.

Every dimpling tool requires some level of skill and understanding for good results.

I have seen no indication that the DRDT-2 assures good results. I have seen it produce just as much bad results as any other method.
 
Dimpling

Kyle.
If possible, position the c-frame over the leg area of the bench for solid supoort. I have two EAA benches. I place the benches beside each other and position the c-frame so the business end is over the leg end of the benches. Everything gets clamped so nothing can move from all the beating. Check position of everything often. I use a 2lb HF Dead blow but I know several builders use 2lb steel. HF has a lifetime warranty. I'm on my 4th. Gives you an idea how hard the hammer is driven. Best of luck.
 
Hmm, interesting. All this rubber mallet stuff seems like too much work. I use a 4# steel hammer held just below the head and about a 6-8" swing. More like lifting and let drop than having to swing. One hit does it......... just me I guess.
 
Hmm, interesting. All this rubber mallet stuff seems like too much work. I use a 4# steel hammer held just below the head and about a 6-8" swing. More like lifting and let drop than having to swing. One hit does it......... just me I guess.

Show your work. ;-)

Seriously, I avoided using a steel hammer partly for safety (hate wearing goggles), but mainly because I didn't know what effect it would have on the C-frame's ram. Didn't want it to end up looking like an abused cold chisel. Besides, it's not like I couldn't use the exercise.

Charlie
 
I use a hard hammer

I started with a plastic dead blow hammer from HF, but did not like the results/effort I had to put in to it. So I switched to a standard sized ball peen hammer and I've been happy with that ever since. yes my ram has mushroomed just a very tiny bit, but that is mostly from the occasional "miss" of the hammer on the center of the ram with a slight glancing blow on the edges of the ram being the result.

After using it that way for a long time now, the ram is still in very good shape and will last me at least as long as it will take to finish the fuselage.

+1 on concrete or stiffening up the "strike area" as much as possible
+ 1 using the Cleaveland dimpling table with adjustable legs to really get the dimple height just right.
+1 on supports all around the ram/set of the C-Frame
+1 that many times the wrong height of the dimple die compared to its support surroundings can leave some very odd shapes in the skin surrounding the dimple.

Would also recommend to the OP to try different hammers and also try using different amounts of force with each blow. with my hammer sometimes only 1 hit will suffice, and almost always never more than 2.

Practice with scrap - with different techniques and different tools, until you find the one you like.
 
Any answer to the original question?

I cant seem to find an example of over dimpling. I keep reading over and over its hard/unlikeky to over dimple but in another thread I was discussing my results with a new set of dies and was wondering if under dimpling could be the cause. I went back over some holes and hit them pretty hard with a hardwood flooring hammer and the c-frame on the concrete floor. To me the dimples look crisp but from a distance the whole row looks rippled. I think it may be what the OP was trying to describe. Its very hard to capture the ripples on camera.


From distance:

https://drive.google.com/open?id=1tdozQ-42dKjeIofjHleue_Macam1tR27

Closer shot:

https://drive.google.com/open?id=1KB-nGF6LAjubFUtvrMExdjjTfQlH156v

Ripples when sighted at eye level:

https://drive.google.com/open?id=1oLYalJezt2LPBlJqae9Jk0JBolvXeQAh
 
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I did the same exact thing after reading the responses to my original post. I went over the holes again, the dimples looked the exact same, however once I riveted the skins on, those ?ripples? seemed to have went away, and the rivet lines look great to me. At least with my first time builder eyes...:eek:
 
It's pretty hard to judge something like that when looking pics; you don't know the lighting, etc.

One thought: you do need to keep the skin perpendicular to the die axis when you strike the die with your hammer/mallet. That's probably the most likely way to mess up a dimple, assuming you're hitting it hard enough.
 
Interesting

I did the same exact thing after reading the responses to my original post. I went over the holes again, the dimples looked the exact same, however once I riveted the skins on, those ?ripples? seemed to have went away, and the rivet lines look great to me. At least with my first time builder eyes...:eek:


Thats interesting, the riveting process may have pulled the skin tight alleviating some of the rippling. Im a little hestitant to just try it but I may not have a choice. This is already my second hs skin on the right side. The first one had an unfortunate priming accident and was dented
 
It's pretty hard to judge something like that when looking pics; you don't know the lighting, etc.

One thought: you do need to keep the skin perpendicular to the die axis when you strike the die with your hammer/mallet. That's probably the most likely way to mess up a dimple, assuming you're hitting it hard enough.

Originally they were done on drdt2 with support tables and all that. Setup/aligned good.

I rehit them on the c-frame on the floor. Supported the overhanging skin by wood blocks.

Either method same outcome with the ripples. The c-frame method left a bit more than a witness mark and is even a little galled from the impact
 
What does it look like if you stand a high quality straightedge on the line of rivets?

I tried to do a little calculation on the force it would take to change the thickness of the metal. Here's the spec on 2024 T3:
http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=ma2024t3

Looks like yield strength is around 50,000 psi. I'm guessing the dimple die diameter is around 1/2". Area would be .25 squared, or .0625, times pi (3.14):~.1963 square inches.

.1963 * 50000 = 9817 lbs

If my numbers are correct, it looks like it would take close to 5 tons of force on the die to actually thin the metal.

Now, I'm not an engineer; maybe someone can check my work.

If they are correct, maybe someone can tell us how much force is generated by a medium swing of a 1 1/2--2 lb hammer.

Charlie
 
To ask another question on this subject, has anyone tried to process under dimpled rivets after setting to smooth them?
 
Again, hard to really tell anything for sure over the interwebs; what's the actual gap there? What size feeler gauge will slide into that gap? Experienced builders say my riveting is quite good, and I can see light intermittently along a riveted line. Pretty sure almost anyone will on their plane.

Can you scrounge up a couple of pieces of scrap, and run a line of rivets to joint them? Anything a few inches wide & 6-8 inches long would work for a test. Dimple on the floor with good hard whacks, like you did on that piece, rivet the scraps together, and then ask yourself how it looks. Might help you evaluate whether you're doing it right or not.

Also helpful is to get someone from your local EAA chapter to come over & look at it with you.
 
Look at the skin before dimpling.

If you think you are doing everything right but still see distortion around your dimples, try taking a look at a pre punched skin BEFORE you dimple. You?ll notice the skin is already quite distorted around each hole long before you go near it with your dimple dies, presumably as a result of the punching process at the factory. Much is made of reflections passing straight through the dimple but I don?t see how this is actually possible when you have distortions there before you even start.
 
What does it look like if you stand a high quality straightedge on the line of rivets?

I tried to do a little calculation on the force it would take to change the thickness of the metal. Here's the spec on 2024 T3:
http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=ma2024t3

Looks like yield strength is around 50,000 psi. I'm guessing the dimple die diameter is around 1/2". Area would be .25 squared, or .0625, times pi (3.14):~.1963 square inches.

.1963 * 50000 = 9817 lbs

If my numbers are correct, it looks like it would take close to 5 tons of force on the die to actually thin the metal.

Now, I'm not an engineer; maybe someone can check my work.

If they are correct, maybe someone can tell us how much force is generated by a medium swing of a 1 1/2--2 lb hammer.

Charlie

Ask and ye shall receive! Looking at your reference you've pulled the tensile yield strength for this calculation, but what you really need is the compressive yield. Unfortunately, your reference doesn't give a number for that value. Compressive yield for 2024-T3 aluminum sheet in the typical sheet thickness we use on the RV's is closer to 40 ksi.

It looks like in your original calculation you confused diameter and radius. Don't feel bad. I do this for a living and I still make that mistake from time to time.

Load, P = stress X area
Area = pi * R^2
R = D/2

The compressive yield allowable is the stress in this case.

Therefore,

P = (40,000 lbs/sq. in.) X pi X (0.25 in./2)^2 = 1,963 lbs

So it's a little less than one ton of force to start thinning out the material. If the actual contact area of the die is less, then the force required to thin the metal will change correspondingly.

Your second question is a little harder to answer. It depends on the velocity of the hammer and the elastic response of the two materials being collided. That's getting out of my field of expertise so I won't hazard a guess.
 
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Thanks Art; wasn't sure if I was using the right strength numbers. But recheck your area calc. I used 1/2" for diameter, divided by 2 to get .25, squared and multiplied by pi. I just measured one of my typical dies and it's ~.493" diameter (using a HF caliper...). I think we can get the extra .007" out of the cone for the dimple. :)

Looks like you started with 1/4" diameter. Am I reading it wrong?

Using my diameter & your 40K psi, I get ~7852 lbs.
 
If you think you are doing everything right but still see distortion around your dimples, try taking a look at a pre punched skin BEFORE you dimple. You’ll notice the skin is already quite distorted around each hole long before you go near it with your dimple dies, presumably as a result of the punching process at the factory. Much is made of reflections passing straight through the dimple but I don’t see how this is actually possible when you have distortions there before you even start.

The skins are already dimpled but I think you are on to something. I do recall seeing some distortion already in the skin around the punched holes.

The OP mentioned the problem lessend after riveting. I did not want to take that chance and have it come out crappy so I decided to cleco it all back together. It looks much better clecoed down to the sub structure with everything pulled tight. I guess Ill just go for it and see what we get
 
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Thanks Art; wasn't sure if I was using the right strength numbers. But recheck your area calc. I used 1/2" for diameter, divided by 2 to get .25, squared and multiplied by pi. I just measured one of my typical dies and it's ~.493" diameter (using a HF caliper...). I think we can get the extra .007" out of the cone for the dimple. :)

Looks like you started with 1/4" diameter. Am I reading it wrong?

Using my diameter & your 40K psi, I get ~7852 lbs.

You are not wrong! :D That was my mistake. I misread your initial dimensions.

See I told you I still screw up. That's why we always have another set of eyes on the analysis.

Back on topic now. I have an example of good dimples on my construction log. Check out the entry on 16 December, 2017 for a picture. I'm using a DRDT-2 and I did have to go through a bit of setup to get the dimples the way you see them in the picture.
 
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Back on topic now. I have an example of good dimples on my construction log. Check out the entry on 16 December, 2017 for a picture. I'm using a DRDT-2 and I did have to go through a bit of setup to get the dimples the way you see them in the picture.

From what I can see I think your dimples look like the ones produced by my new cleveland dies. The actual cone portion of the dimple looks small and the radius at the top of the dimple looks a little big. For me, this causes a gap around the rivets (see my thread on gap around rivets if interested).

I went back and rehit them all with my old avery dies to make the rivets fit better. The dimple from the avery die has a larger cone surface and less radius. The rivets fit nice.

I ended up following this thread because after dimpling the same holes multiple times I started noticing ripples and distortion in the skin that is different then the dishing you get with under dimpling. Hence the question "what does over dimpling look like" which is still not answered.

Luckily just as in the OP's case, I think all will be ok when its riveted down to the sub structure (I clecoed everything back together and looks better with skin installed vs free state)

Anyway I may buy a set of cheap dies and dimple some scrap with my 20 ton press to see what over dimpling looks like :eek:
 
From what I can see I think your dimples look like the ones produced by my new cleveland dies. The actual cone portion of the dimple looks small and the radius at the top of the dimple looks a little big. For me, this causes a gap around the rivets (see my thread on gap around rivets if interested).

I went back and rehit them all with my old avery dies to make the rivets fit better. The dimple from the avery die has a larger cone surface and less radius. The rivets fit nice.

I ended up following this thread because after dimpling the same holes multiple times I started noticing ripples and distortion in the skin that is different then the dishing you get with under dimpling. Hence the question "what does over dimpling look like" which is still not answered.

Luckily just as in the OP's case, I think all will be ok when its riveted down to the sub structure (I clecoed everything back together and looks better with skin installed vs free state)

Anyway I may buy a set of cheap dies and dimple some scrap with my 20 ton press to see what over dimpling looks like :eek:

Those are indeed Cleaveland dimples I used. I also bought their substructure dies for dimpling the ribs. Have you tried those and checked the fit?

I'm about to rivet my vertical stabilizer together this weekend. I'll find out then if those substructure dies make a difference.

Edit: Never mind! I just read your other thread and it answered these questions.
 
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