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Minimum Hole Distance to Edge

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Sailplane Driver

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#1
I want to tap and drill some 2-56 holes in stainless with very little edge clearance and very little width to do so. I have enough material to hold the screw but I have a problem with countersinking the cap secrew in a aluminum plate that will attach to the stainless. See the sketch. Scan0007.jpg
  1. How close to the edge can you get in aluminum before a clearance hole starts to tear through the side? The hole will be around #27 or 0.144.
  2. How close to the edge can you get in stainless for a threaded hole? I only have 0.134 wide material to work with and I need to offset slightly for the cap screw as noted in 1.
 

magicniner

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#2
1. The answer will depend on numerous factors including the exact alloy you are using and the cutting tools used. If you want it really close you can leave the part over size, drill & countersink, then mill to size and 20 thou will be easy.

- Nick
 

JerryK

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#3
For what it’s worth
The screw thread can be to the slot.
It’s only a tangent point and will not weaken the thread.
But, I would stay back a little.
You can always turn the screw head down a little, and a smaller counter bore
 

Sailplane Driver

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#4
For what it’s worth
The screw thread can be to the slot.
It’s only a tangent point and will not weaken the thread.
But, I would stay back a little.
You can always turn the screw head down a little, and a smaller counter bore
I didn't think about turning down the screw head. Great idea, thanks.
 

Asm109

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#5
If you tap the hole after the slot is in, the tap will bulge the metal and give you a visible screw thread in the bottom of the groove when things get thin. Tap first then cut groove and you can go down to a couple thou with no sign.
 

RJSakowski

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#6
I try to keep at least the center of the hole one hole diameter from the edge. In the case of a tapped hole, , the center would be the major diameter of the thread from the edge. As for the counterbore, I will mill a slot intersecting the counterbore so there is not a zero thickness edge.
 

Wreck™Wreck

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#7
Do not break through, there is no reason that .005-.010" from an edge will be a problem assuming a cut tap, if form tapping you may want to leave a bit more depending on thread lead and material.

Have at it.
 

Sailplane Driver

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#9
Darn, I can't even see anything that small much less space it next to an edge!
I thought about going to a smaller screw size but I don't even want to think about trying to tap the holes without breaking the tap. The 2-56 is bad enough. I broke a 2-56 tap a while back trying to tap aluminum. I was lazy and tried tapping dry. Big mistake which I will not repeat. These little taps are quite delicate. As to spacing it from an edge, that's what DROs are for. There is no way I would try this using a ruler and a punch.
 

Sailplane Driver

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#10
I did the math and this is what I come up with. #29 drill - 0.136 - for screw head countersink. Screw body - 0.086, width of material 0.134. Subtracting half of the countersink and screw body radius from the material width leaves 0.023. Splitting that between the two sides of the material leaves a wall thickness of 0.011. I have an additional 0.006 in the aluminum I can overhang. Should I split the difference equally, add more to the aluminum to prevent the side from blowing out, or add more to the stainless to prevent the threads from bulging into the channel?
 

RandyM

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#11
Side mill the counterbore into a U shape and eliminate the thin sidewall.
 

Sailplane Driver

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#12
That's a DOH! for me. I should have thought of that. Now I can center the threaded hole in the available material. Thanks
 

dennys502

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#13
Shorter version.jpg

This is what it would look like with a 2 - 56 x 3/8.
Are you tapping blind or can you do a through hole?
 

Sailplane Driver

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#14
I was planning on tapping blind. I just burned the end off a #50 HSS drill bit. Looking for something that will drill the stainless.
 

dennys502

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#15
I like carbide but HSS will work. Keep the rpm's down and the feed higher than you would for steel.
Lots of coolant as the stainless will work harden.
 
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