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-   -   What does over dimpling look like? (http://www.vansairforce.com/community/showthread.php?t=156532)

Mort04 01-02-2018 07:52 AM

What does over dimpling look like?
 
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?

1001001 01-02-2018 08:01 AM

Quote:

Originally Posted by Mort04 (Post 1229243)
Iíve completed the dimpling on Empennage skins, I donít have pictures yet because I canít seem to capture what Iím 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.

Mort04 01-02-2018 08:11 AM

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?

LettersFromFlyoverCountry 01-02-2018 08:59 AM

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.

wjb 01-02-2018 09:54 AM

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 .....

az_gila 01-02-2018 10:28 AM

Quote:

Originally Posted by wjb (Post 1229271)
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. :)

John Tierney 01-02-2018 11:18 AM

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.

rvbuilder2002 01-02-2018 11:43 AM

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.

Mort04 01-02-2018 11:55 AM

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 didnít want the skin to be pushed down the hole in the table causing a dent.

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

rvbuilder2002 01-02-2018 12:04 PM

Quote:

Originally Posted by Mort04 (Post 1229290)
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 didnít want the skin to be pushed down the hole in the table causing a dent.

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


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