What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Problems with #8 dimples cracking

georgemohr

Well Known Member
Hi all,

This is part T-1005B. The task is to dimple the #19 holes for a #8 screw. When I examined the part, the hole was already final sized (actually it was a bit larger than #19) so I didn't drill it. And it didn't have sharp edges so I didn't debur.

The dimples were created with a pneumatic squeezer using Cleveland's #8 dimple dies. The first dimple below shows the crack at 12 o'clock. Note that all of the cracks happened at the location of a dark spot on the metal. The picture of the undimpled hole shows this mark with more clarity. I believe this is some effect from laser cutting, perhaps?

Always, now that the part was toast I figured I'd experiment. I tried deburring and then also slightly overdrilling the next few holes, but both experiments cracked in exactly the same way.

Finally I decided to try drilling my own hole in the same material, deburr, and dimple. These dimples did not exhibit cracking. See the photo of the second dimple.

So my questions:

1) Is this part somehow flawed due to forming the holes?
2) What can I do to avoid this cracking?
3) Is it feasible to repair a crack like this by stop drilling a small hole in the radius of the dimple?



Thanks!
George
 

Attachments

  • dimple1.JPG
    dimple1.JPG
    39.8 KB · Views: 296
  • dimple2.JPG
    dimple2.JPG
    11.6 KB · Views: 1,612
  • dimple3.JPG
    dimple3.JPG
    34.2 KB · Views: 323
Last edited:
I’m not there yet, but on your drilled holes, were they in “new” metal? Just thinking you are on to something with that dark spot. I assume where you drilled, there wasn’t a dark spot. I’ll look at that piece in my inventory when I can, but might be a few days.
 
shooting from hip

I am just shooting from the hip here, but I did stay at a Holiday Inn Express last night.

When this happened to me, the original hole was too small and the dimple die was a tight fit in the hole. I would make sure the male die fits in the hole, snugly but still fits in, before dimpling. If not, something is not right.
 
Last edited:
I’m not there yet, but on your drilled holes, were they in “new” metal? Just thinking you are on to something with that dark spot. I assume where you drilled, there wasn’t a dark spot. I’ll look at that piece in my inventory when I can, but might be a few days.

Yeah, no dark spot on the native material that I drilled myself.
 
I am just shooting from the hip here, but I did stay at a Holiday Inn Express last night.

When this happened to me, the original hole was too small and the dimple die was a tight fit in the hole. I would make sure the male die fits in the hole, snugly but still fits in, before dimpling. If not, something is not right.

I did check the holes as per the instructions to "final drill #19". The holes in this part were already oversized based on that instruction. They slip-fit a #18 drill as delivered from Vans.

One of my failed experiments was to oversize drill to #17 first, but that didn't help either.
 
#8 holes are more prone to cracking. I would try a hand squeezer to see how that works for you. I have not had any #8 hole cracked but I debur them even if they are to the size.
 
Hi all,

This is part T-1005B. The task is to dimple the #19 holes for a #8 screw. When I examined the part, the hole was already final sized (actually it was a bit larger than #19) so I didn't drill it. And it didn't have sharp edges so I didn't debur. . .

. . . So my questions:

1) Is this part somehow flawed due to forming the holes?
2) What can I do to avoid this cracking?
3) Is it feasible to repair a crack like this by stop drilling a small hole in the radius of the dimple?

And it didn't have sharp edges so I didn't debur

My answer resides in this statement. Don’t assume edges are good! Deburr every hole.

Answering your questions:
1) not flawed, not perfect either. Don’t take shortcuts, debur every hole.
2) yep, second question, same answer, debur every hole.
3) yes. Not by stop drilling but by deburing the hole after dimpling. It can be done.

Of course this is just my opinion about how to build an airplane, but as some wise men said once upon a time in the past:

“Here me now and believe this later!”
https://www.youtube.com/watch?v=7Mk1nykjnYA
 
Last edited:
I’ve found that the crispest #8 dimple requires a c-frame and hammer.
IMHO Squeezers or DRDTs don’t cut it.
As mentioned above though, the hole needs to be pristine beforehand or else…
Drill/ream/debur. Whatever it takes.

My technique was:
drill to final size ( this dosent leave much of a burr if it’s just upsizing)

Scotchbrite both surfaces to remove any obvious burrs
One twist only! of the single hole twist debuting bit by hand.

This one:

https://www.cleavelandtool.com/collections/drilling-and-deburring/products/3-hex-deburring-tool

This will result in a clean enough hole to dimple #8

Maybe worth asking vans if it’s acceptable to go to #17 before dimpling.
I think your success rate with untreated punched holes will be low.
 
First, I would ream the holes.

Debur carefully.

Second, I have a "professional" dimpling machine. I came with some complicated heating elements and controls for more exotic material. The operator's manual contains a wealth of knowledge.

Consider heating some test strips to 200-300 deg. as practice
 
Thanks to all that added feedback thus far.

For those that mentioned proper deburring, note from my original post that I attempted several methods of both deburring and oversizing the pre-formed holes and all of them cracked. The only holes that I could get a good dimple formed were the ones I drilled myself in areas of the material that where not pre-drilled from Vans.
 
Last edited:
I really struggled with cracked #8 dimples. What works for me -

- Ream all holes (even if it means going to #17 sometimes)
- Lightly debur with a sharp bit
- Polish the holes right through with a piece of scotchbrite pad wrapped around a small drill bit, chucked in a drill.

I haven't had a dimple crack since I have used this process.
 
I really struggled with cracked #8 dimples. What works for me -

- Ream all holes (even if it means going to #17 sometimes)
- Lightly debur with a sharp bit
- Polish the holes right through with a piece of scotchbrite pad wrapped around a small drill bit, chucked in a drill.

I haven't had a dimple crack since I have used this process.

I will try the scotchbrite polish idea, that sounds nifty! I'm out of Vans pre-drilled holes in this stock though, so hard to say if it would work here or not..
 
Last edited:
Possibly too much pressure?

Just a thought. When I use a pneumatic squeezer I test the die spacing with a sample of the same thickness. I make sure the sample squeezes fully/firmly but no more. Is it possible you are applying too much squeezing pressure? Every thickness requires adjusting the number of washers under the die shanks to match.
 
Out of curiosity, what would happen if this weren't caught and construction continued unaddressed?

In my estimation, nothing of consequence. These might be the least critical dimples on the entire airframe :) But as a possible learning experience, much more valuable.
 
In my estimation, nothing of consequence. These might be the least critical dimples on the entire airframe :) But as a possible learning experience, much more valuable.

We agree George, but if builders are concerned there are a couple of options but first a bit of deeper explanation...

With enough years of experience and testing we have learned that there are specific situations where dimple countersinking final size punched holes is totally acceptable, and others where it is not.

The primary instance where it is not is in cases where the hole is sized to be dimple countersunk for a #8 flush screw (as in this instance).
Based on that, quite a few years ago all parts containing holes intended to be dimpled for flush screws had their production drawings updated to specify that the holes be punched .161 diam and then the plans updated to specify that said holes be final drilled #19 and deburred well, before dimple countersinking.
The purpose of punching under size is because the final drilling produces a better edge finish on the interior of the hole which greatly reduces the likelihood of cracking from the material stretch that occurs during dimpling.

After doing some research it was discovered that the T-1005BC file has the screw holes punched at .166 instead of .161".

We aren't sure how it went undetected for so long, but it has now been corrected.

The following is an engineering standard that we use for remedial work on #8 screw dimples when needed (as already mentioned, it is not considered necessary for this particular part, but we leave the up to the builder).

In instances where cracks have occurred on this part and the crack is not more than 1/3 of the flange width of the dimple, they can be stop drilled with a 1/16" drill bit as long as the edge of the stop drill hole wont extend beyond 1/2 of the width of the dimple flange.
For cracks that are small enough so that a stop drilled hole would be right adjacent to the inner edge of the dimple, a small round jewelers file can be used to remove the cracked material.


Alternatively, the parts can just be replaced but I am not certain what the time line currently is for when a new production batch will be produced with the updated hole diameter.

I hope that helps
 
Last edited:
This is extremely helpful Scott, thank you.

What do you make, if anything, about the propensity for the cracks to begin on this discolored spot inside the hole?
 
Hi all,

Just a follow up on this. I received a replacement part from Vans. It had the same apparent discoloration in the same spot. That part also cracked in one of the dimples at this exact location. The crack wasn't bad, so I just filed it out.


Regards,
G
 
Same Cracking Issues

Working on these parts I'm also having difficulty. Will order new and try drilling them up to #17.

To me it almost looks like these are laser cut and not punched? All four of the #19 dimpled holes cracked. Using a pneumatic squeezer and Cleveland dies.

They were already final size holes so when I drilled them, it didn't take anything off and left the rough manufactured edge. It looks like there's some sort of dross or defect where the laser starts and stops, creating a weak spot. Like a tiny weld pool. You can see it in the #19s below that haven't yet been dimpled.
 

Attachments

  • IMG_6803.jpg
    IMG_6803.jpg
    368.2 KB · Views: 211
  • IMG_6815.jpg
    IMG_6815.jpg
    314.1 KB · Views: 186
  • IMG_6814.jpg
    IMG_6814.jpg
    326.7 KB · Views: 152
  • IMG_6809.jpg
    IMG_6809.jpg
    308.6 KB · Views: 207
  • IMG_6805.jpg
    IMG_6805.jpg
    340 KB · Views: 203
Last edited:
I worked for a stamping co. As a metrology lab inspector. And have seen those tiny burs on holes. They are caused primarily by dull punches. Sometimes by bits of material leftover from the previous series of stampings in a progressive stamping process. Not much you can do other than running a remmer through the hole prior to dimpling.

Hope this helps
RD
 
I also see something else that concerns me. The plate nut attachment holes are dimpled instead of countersunk. The plate nut won’t sit flush when riveted.

The attachment flanges on the plate nut are not designed to mate with dimples. There is a male (the backside of the dimple) but no female side on the plate nut.

In your picture, the dimple for the screw is supposed to nestle in the K1100 plate nut. If the attachment holes are dimpled instead of countersunk, there will be a space the height of the plate nut above the piece it is attached in the nestle. The screw will pull the sheet and the plate nut together but adds a tiny bit of stress on the hole that should not be there.

Since you are concerned about the crack in the hole dimpled for the screw, the plate nut attachment might also be a concern.
 
Not certain if this is what you are referring to, but the two forward-most nutplates are k1100-08D (D for dimple) in which the 3/32" holes for the nutplate attachment are already dimpled. So they mate fine with the dimpled skin. Hard to be certain from the OP's close up photos, but it looks right to me.

-G
 
If nut plate is dimpled, no issues.

I must need to get out more often because I haven’t see those before.
 
Last edited:
Yep. I dimpled the nutplate wings with a close quarters die set. I think the pre-dimpled ones are a little more expensive? Not shipped with the Vans kit anyways.

I've also read that some guys use oops rivets and a shallow machine countersink to attach nutplates vs dimpling. Haven't tried that yet.
 
Oops rivet

If you use an oops rivet, you don't even need to use a microstop countersink tool. Just a few turns of a manual deburring tool makes a big enough countersink for the little head of the rivet to fit flush. Then you don't need the dimpled nutplate.
 
If you're drilling to #19, I would consider reaming with a #19, then deburring the edges of the hole. Drills remove metal by a combination of cutting and tearing. This generates stress risers in the cylinder of the hole.

I submit those risers express themselves as cracks when dimpling. There are other techniques, including preheat to above 250 deg. F that you might experiment with.
 
Leading edge splice strips

I recently dimpled for #8s on the splice strip between the outboard leading edges and the tanks and they turned out great. Those I reamed to #19 and carefully deburred. Out of an abundance of caution I hit the holes with scotchbrite and used some light oil on the parts and dimple dies. I'm using a C-frame where I can for these. No problems. I think those skins and the splice strips are punched parts.

I'm not an engineer but believe its the way those tank bracket shims were produced with a laser with final size holes. IMO they have a weld pool like area where the laser starts and stops that creates a hard or jagged spot more prone to cracking. I used a reamer on the replacements and drilled them up to #17 to clear the rough edge and they dimpled OK.

Maybe there's varying degrees of quality for laser cut parts but I would personally be OK with match drilling (reaming) undersized holes, but it seems like punched parts win so far.
 
Back
Top