screwholes
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Old 04-05-2009, 09:46 PM   #1
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Why do the best gunsmiths drill screwholes slightly under the miner size of the screw tap (say .009 or so) before using the tap?

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Old 04-05-2009, 10:00 PM   #2
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My initial response would be because it gives more purchase to the threads when you tap them.

In the shop, routinely, we drill out #6 screw holes for #8's for better, more beefy screws on the scope mounts.

JD

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Old 04-06-2009, 02:20 AM   #3
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Damn! I saw this thread title and thought I was in for another "camel toe" experience...

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Old 04-07-2009, 06:07 AM   #4
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Why? Because it is much easier to remove metal than to put it back again! The holes for most screws are properly not the screw size, but a numbered wire size. A 8-32 screw does NOT get a 1/8th inch hole, but the # drill size smaller (#29). Dimensions are to the OUTSIDE of the threads of the screw, not to the inside (drill diameter) See chart: Drill and tap size chart - Wikipedia, the free encyclopedia

A somewhat sloppy machinist that does not have numbered drill sizes might use a smaller bit, hoping the tap will ream the hole- but that is a great way to break taps, and they are a b!tch to get back out. Now, ask me how I know that.....

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Old 04-08-2009, 03:24 PM   #5
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most spec for drilling tapped holes are for 75% thread.so a smaller hole will give you 90/100%.I have cleaned many hole with and electric remover.age has got me and there is a name for it.I can remember how to do it but not the name darn.

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Old 04-13-2009, 06:27 PM   #6
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Quote:
Originally Posted by c3shooter View Post
Why? Because it is much easier to remove metal than to put it back again! The holes for most screws are properly not the screw size, but a numbered wire size. A 8-32 screw does NOT get a 1/8th inch hole, but the # drill size smaller (#29). Dimensions are to the OUTSIDE of the threads of the screw, not to the inside (drill diameter) See chart: Drill and tap size chart - Wikipedia, the free encyclopedia

A somewhat sloppy machinist that does not have numbered drill sizes might use a smaller bit, hoping the tap will ream the hole- but that is a great way to break taps, and they are a b!tch to get back out. Now, ask me how I know that.....
Don't need to brother. Been there, done that too, and have the scars and t-shirt to prove it!
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