How to dimple for #8 screw without cracks?

So you do a nice job drilling a clean hole for a # 8 screw using a #19 bit, you do a good job deburing both sides of the hole, then you take your #8 dimple dies and dimple the hole only to find that you have small cracks around the ID of the hole.

How do you prevent this?

Was the piece too thin to machine CS? Is it a structural piece or something like an inspection cover? The only ones I have done were machine CS'ed.

Guess I've been lucky but I use the CP-214 pneumatic squeezer and I haven't had any cracking on #6, #8 or other size holes. I use the standard dies from Avery.

Well, for better or for worse I use a slightly larger bit if the skin starts cracking (#18, sometimes a #17). I haven't fallen out of the sky yet.

gpiney said:

Was the piece too thin to machine CS? Is it a structural piece or something like an inspection cover? The only ones I have done were machine CS'ed.

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Yes the side skin of the rear fuse is too thin to machine CS and the tank skins as well for a #8 screw.

w1curtis said:

Guess I've been lucky but I use the CP-214 pneumatic squeezer and I haven't had any cracking on #6, #8 or other size holes. I use the standard dies from Avery.

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I use the same dies and same squeezer as well as a hand squeezer and a DRDT2, all produce micro fine cracks on #8 dimples with a #19 predrill. They are very short small cracks from the ID of the hole to about halfway down to the flat part of the dimple. So small that you must look carefully to see them but they are there.

Jamie said:

Well, for better or for worse I use a slightly larger bit if the skin starts cracking (#18, sometimes a #17). I haven't fallen out of the sky yet.

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I will have to try a larger pre drill to see what happens. The #8 dies tend to really open up the ID of the hole anyway so why not give it less metal to move in order to prevent a crack? The only problem I see with this is that it is then harder to keep the die centered in the hole till the dimple is formed.

Brantel said:

I will have to try a larger pre drill to see what happens. The #8 dies tend to really open up the ID of the hole anyway so why not give it less metal to move in order to prevent a crack? The only problem I see with this is that it is then harder to keep the die centered in the hole till the dimple is formed.

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Brian-Are you using quality dimple dies? I've heard great things about Cleaveland's dies (that's what started their business I was told), pretty good things about Avery's, and not so good things about everyone else's.

I have Avery's dies for #6, 8# and 3/32" and Cleveland 3/32" and 1/8".

Can't tell much difference in the results from using both of the smaller dies.

I tried using a #16 predrilled hole with the #8 and it worked out great! No cracks and I measured the ID of the hole afterwards and it was no larger than one that was predrilled with a #19 so from now on #16 will be my drill of choice for predrilling for #8 dimples!

Just saw this thread after someone asked me why my tank dimples aren't cracked. The only thing I can think is that if you debur to much, the skin around the outside of the hole gets thin and can lead to cracks. Maybe lighter on the deburring here would sovle the problem.

I used a #19 with a 2 by 4 backer, then deburred lightly, (2 light turns with the debur tool). Squeezed with pneumatic squeezer set really tight, and no issues with cracks.

Just a thought.

Use a larger drill bit. The die will still centre and won't have to stretch the skin so much thus no cracks.

Cracks with #8 dimples. Larger drill bit?

Norman CYYJ said:

Use a larger drill bit. The die will still centre and won't have to stretch the skin so much thus no cracks.

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That's an idea worth to try, since I have experienced the same problem with #8 dimples.



Do you suggest a #18 drill bit instead of a #19?
I use quality dies from Avery and they don't show any indication of irregularities and I use a hand squeezer and apply force gently and gradually.
Don't know of any other things I could do to avoid that.

As a general note, the slower you form 2024, the harder it gets. So once your technique is good there's no reason to squeeze slow except to be careful. I'd drill 19 and start the dimple. Then drill 17 and finish it. That gives you a centered dimple. Also, you need to grind the side(s) of one of your female dies because the way you're doing it you're crushing your 3 dimples with your 8. If you dimple the big hole first, then your #3 female has to have one side ground down to clear tha 8 dimple. If you dimple the 3's first, your 8 has to have both sides ground parallel the clear the 3 dimples. Good deep 8 dimples are not easy. If the location allows I sometimes cheat and don't go so deep.

Clean polished holes and WD40

So I have been practicing for the last two evening trying to solve how to eliminate the cracks. I've been on the phone with Vans for their advice, talked with friends, etc. Not to mention laid awake a bit a night. Here is what I have found so far.

For me, it doesn't matter if the hole is #19, #18 and #17, it can crack. That's not to say that going up a size or two on your pilot hole doesn't relieve some pressure, but I found that it wasn't the complete answer.

In talking to Vans support. They were not keen on the idea of using the #18 or #17 pilot hole. They felt this may open the door for the dimple to be off center. They didn't say no, but they didn't like it. I tried it on some scrap just to see for myself. It's not as oversize as I thought. So I squeezed it. Both #18 and #17 cracked.

I did notice that there was a difference in hole sizes after they were squeezed. Its a bigger pilot hole, and after squeezing, surprise surprise, the hole in the dimple was larger too. Seeing that it did not eliminate cracks...I resolved to focus on the #19 holes. What seemed to be the common denominator in all my cracked holes was how well/clean the holes were drilled and the deburring. Its easy to be sloppy with the drill in opening up the holes and its easy to debur too much. My 3-flute deburring tool does not do a nice job when looking under a magnifying glass. The one hole debur tool is better, but its not perfect.

I carefully drilled all the tank skin holes (39 test holes) up to #19. I started #25, #22, #20, then #19 (This process gave me brain damage the whole way...its not a great way to enlarge holes this way) Once I was done I then used a small piece is 600 grit sandpaper on my Dremel, Just a small piece wrapped on the mandrel. I carefully polished the holes (It took a long time to prep the holes) but they looked great. At the suggestion of a friend, I sprayed WD40 on the work and the dies and started squeezing. All 39 holes made beautiful dimples and no cracks!

I'm still waiting for a #19 reamer to arrive and I may abandon the deburing tool completely. As you can see from the pic its not a perfectly clean edge on the dimple, they are some very tiny jags on the edge. I may use the Dremel felt and polishing compound to debur the edges instead.

I am one step closer to solving this.

P.S. I have the Planetools.com dimple die

That...is a really nice dimple.

Would the dimple dies being say 180 degrees f. And the aluminum warmed up via 500 watt lamp help things along. Just a question. Maybe use bo lube while dimpling. Please don't hit me for a bad question...too hard

As was mentioned earlier, there is a fine line shy of being too hot to structurally impair the material. I thought about putting a heating pad on the holes an hour before I dimple them.

I think you found your problem. Jagged edges in the hole will lead to cracks. And 3 flute deburring tools do tend to make jagged edges.

Is reaming up to a particular size the solution then. And or twisting up some 600 grit wet or dry and sanding the hole before dimple. Is there a different angle of screw that may achieve the same thing.i mean less metal deformation equals less possible fractures? I'm only trying to learn

Attempt #2 - Its working

Tonight, 42 test dimples are successful. I repeated the process and tried using the dremel felt tip with polishing compound to debur. It works ok, but not enough that I'll swear by it.

I tried a few holes going slow with the squeezer, they didn't crack. But I did not do enough of them to say going slow worked or did not work.

I do notice a huge difference with the quality of the hole and also the WD40..There's just no question about it. Will bolube work? Well I only have the solidstick type. I rubbed that on the dies, and it didn't work. I had cracks. Maybe the soft paste works? I'm sure other lubes can be used...but the reason I choose WD40 is that its safe for aluminum, its pretty slippery, but most important....its thin and runny. I chose to avoid thicker lubes on purpose to mitigate chances of it causing pressure points, or similar attributes.

Will reamers work?. I'm 100% positive they are the way to go as the give a nice clean hole. I ordered a #19 reamer from ATS on Monday, so I will have to wait for it to arrive before I can test it.

Will just using 600 grit sand paper in the hole, twisting by hand work? Maybe, Maybe not. But there's 148 holes to dimple on the wing tanks and I don't want to prep holes for a week. Nuff said!
With the Dremel, it really polishes the hole, it's the only way that I want to do it. I checked with the 600 grit and you really have to work the hole in order to open the hole up to #18. So I feel good about the choice.

By the way, you can go cross eyed looking at all those dimples...I found these so much of a help

Lube the dies and the area to be dimpled with a little oil.

Just completed all of my #8 dimples this morning. Looked them all over closely with magnifying glass and zero cracks. I'm convinced of several key factors here. First of all, temperature of the metal has little to nothing to do with it. The temp was 50? in my shop during the process and I made no attempt to heat the metal. All holes were drilled with a #19 bit. Ideally I would have liked to use a reamer, but didn't want to wait around for an order to arrive. All holes were then deburred with a single flute debur tool on an electric screwdriver. Yes, I think having the right debur tool is a key element. I then looked at a dozen or so holes with a magnifying glass to convince myself that there were no jagged edges on either side of the holes. I then squeezed the dimples with my pneumatic squeezer. In my mind, having good quality dies is another key point.

i have to agree rough deburring can cause cracks to form. or over deburring

Very nice write up and approach to this issue. A clean sharp drill will go a long ways to making a good hole. Before deburring I feel with my finger for rough edges, if there are none, it does NOT get the process. Aggressive deburring causes more problems then it solves!
Over the last 23 years I have done thousands and thousands of these holes, and in service I have yet to see a crack that has extended from a dimpled screw hole while in service. This is not an excuse for poor workmanship and just knowledge of this issue will make you modify your techniques to a balance between what is practical and absolute perfection.