#8 dimple die holes

rocketbob · #1 04-25-2011, 08:20 AM

I'm curious as to why #8 dimple dies require drilling a #19 hole since the holes expand so much once dimpled. I've been drilling with a #20 drill to tighten up the hole slightly, but the holes are still huge once dimpled. I'm very tempted to modify my dies in the lathe to make the pilot smaller so I can drill smaller holes. I pulled one of the tank screws on my Rocket and noticed that the Czechs were cognizant of this and the dimpled holes are much tighter. Anyone besides me ever notice this?

DEWATSON · #2 04-25-2011, 08:42 AM

I have and have often wondered the same thing. I wonder how the Czechs do it.

jimgreen · #3 04-25-2011, 10:17 AM

Is it actually important since the countersunk screw head seats very securely?
I don't think a tighter hole would improve the fastener strength.

DaAV8R · #4 04-25-2011, 10:32 AM

I had to increase the hole sizes beyond #19. At a 19 hole, the skin was cracking within the dimples. I performed lots of tests and could not prevent the cracks without without enlarging the holes. Run some tests and inspect the dimples for cracks. I have read here that many others encountered the same problem.

vlittle · #5 04-25-2011, 06:33 PM

Quote:

Originally Posted by DaAV8R

I had to increase the hole sizes beyond #19. At a 19 hole, the skin was cracking within the dimples. I performed lots of tests and could not prevent the cracks without without enlarging the holes. Run some tests and inspect the dimples for cracks. I have read here that many others encountered the same problem.

What he said!

V

rocketbob · #6 04-26-2011, 08:23 AM

I did some research and found that the reason for the large pilot hole is to prevent cracking due to the amount of stretching that occurs with the #8 dimple dies. Its very likely the dimpled holes on my rocket were done with a hot dimpler, which uses heated dies to prevent stress fractures.