# Drill depth vs. cut tap depth in blind hole

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Awl --

Given a desired tap depth, is there a formula for the drill depth?

Along those lines, here is a ditty I use for tapping. R8, R9 establish the relationship between drill and tap depths in subroutine L200, and get passed to the Z's in the drill/tap cycle, preserving the relationship. This also minimizes the error of changing one, and not changing the other.

I have chosen a drill depth 50% greater than the tap depth. Normally, I would assign these numbers to V variables, and later pass those on to R variables, but with no real calculations, this isn't really necessary here.

Feel free to critique the tapping code, as I have no idea why shit is the way it is, just that it works.

N13L200

N14#R8=.48' tap depth

N15#R9=1.5*R8' drill depth

N16M17

N21M30

N22( MAIN PROGRAM

N31M6T21( #21 DRILL

N32L201

N33G0X0Y0Z3.H21E2M3S1500

N34Z1.M8

N35Z0.1

N36G83G98R+0.1Z-R9F7.5Q0.45(

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PV:

I generally make the cut tap depth 2/3 of the drill depth for blind holes. Essentially identical to your drill depth being 50% greater than your tap depth.

I've got a whole slew of "hooks" to pull threads out of blind holes, from modified saw blades down to paper clips with small hooks on them. Also have a series of air nozzles going down to .050" OD to try and blow them out.

Trapped chips in blind holes is the ONE best reason for switching to a form tap. Then you can run the form tap right down to near bottom, I generally go down to the depth of the outer dia. of the drill's cutting lips. When engineers get crazy with their full thread depth callouts I'll often modify form taps to cut full threads to within .030 of the bottom of the hole (after helical milling the hole to get a flat bottom). That last little bit usually has to be done by hand. There's always the option of thread milling, no problem with trapped chips, and you don't have to drill the hole much deeper than your thread depth. Those puppies seem to cost about 10 times what a tap does though. Snap a couple of those off and you'll be depressed for the whole day. :)

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Crochet hooks. They're available in numerous sizes.

Store them next to the lipstick for sticky collets for a little feminine touch in your tool box. I keep my silly putty for proofing mold engraving in the same area too. :)

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Interestingly, I've found Vaseline to be much more useful than lipstick...

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In that case, store them next to the Vaseline for sticky collets for a little bi-curious touch in your tool box...

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BD:

Well now, that's a real machining TIP that I've never heard before. Someone should start making a list.

Actually, thinking about it now I've got like a .125 Dia. stainless rod that I've modified the end (which resembles a knitting needle tip), that I use for yanking thread chips out of larger holes.

Have you ever found a machining related use for tampons? :)

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Count me out! I've had my fill of lits for a while. :)

New? Or used?

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From James Harvey's book I use a drywall screw. Works even better if you grind off the head and use them in a cordless drill. 1/4-20 or bigger anyway.

Thank You, Randy

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BD:

Yetch! I'll NEVER get that imagery out of my mind now. :(

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Randy:

I made a cork-screw looking chip remover, a pain to make and didn't work all that well anyway. Wish I had thought of the drywall screw idea, sounds like a winner.

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In the oil patch, a box of feminine napkins are usually around for soaking up blood from an injury. As far as tampons, any drive by shooters near your shop? ;)

Wes

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Do you remember the old TV show "Combat"? I was just a kid but recognized the "bandages" they pulled out of their kits. RR

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It works ok but does anyone make them with a left-hand thread? RR

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Randy, How about running the drill in reverse?

Best, Steve

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GD:

Probably wouldn't work well. It would tend to PUSH the chips DOWN rather than pull them up the thread spiral. Think corkscrew in reverse.

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Bob, Running in reverse would LIFT the chips up, but the screw wouldn't self feed into the chips.

Best, Steve

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"Lift" meaning chips/strings unscrewing themselves out of the hole? Probably so. I was thinking about a left-handed screw biting screw into the mess and unscrewing it. Instead of a right-hand screw running backward to unscrew the mess, how about something like a phillips screwdriver bit or maybe a drill blank with a left-hand drill type tip or spade tip ground into it? RR

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GD:

Right after I hit the send button I thought GD was probably joking and I took him too literally.

Now from THIS response it seems likely that while you have me on your hook, you're wiggling it a little to watch me squirm.

But for your "squirm" watching pleasure, I'll continue. :)

If you take a regular right-hand screw and stick your thumbnail in the thread (simulating a chip), and screw it clockwise (like screwing it INTO a hole), your thumbnail will travel UP the screw like a chip would. But if you turn it in reverse (like UNscrewing it) your thumbnail will move DOWNWARD. Now having said THAT, IF you could displace the chips sideways INTO the existing threads you "might" get a lifting assist by the threads themselves. BUT, thread chips are usually crammed down in the hole below where the threads end, so that's an ify situation.

I don't have any access to any chips, threaded holes, screws, or anything else right now to actually test this.

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Bob, Not trying to hook you, but watching the screw flights on a large wood screw it appeared to me that the flights would lift chips out of the hole.

Best, Steve

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