Problem tapping small holes in hard material

Red_SC · Feb 15, 2019

I’ve been having an issue tapping a 6-32 hole in Glock slides for mounting RMR red dot sights. I’m not sure the hardness of the steel, everyone quotes the melonite hardness but not the hardness of the steel underneath after the melonite has been machined off. It feels pretty hard when tapping by hand, however.

I’m drilling the holes 7/64 for a 64% thread depth. I’d like to tap on the CNC, but every attempt so far has broken the tap. I’ve used several brands of HSS taps; I haven’t tried carbide yet because I wouldn’t be able to mill out a broken tap. I don’t think it’s a good candidate for roll tapping because of the hardness.

So far I’ve been using a tap guide in a Bridgeport to tap them very carefully by hand, starting with a taper tap and finishing with a bottoming tap. That works but is slow, if anyone has a tap or process that might work better I’d appreciate the input.

partsproduction · Feb 15, 2019

Glock slides use an nitrided surface, tenifer, I think it's very hard,like 60 Rc. In theory if you break through the few thousandths of an inch of that with one of those little diamond ball points in a dremel you should have normal tapping below that.
I once had many broken taps trying to tap scope mounts on a Ruger Mini-14 stainless cast receiver. I finally had to grind the top 1/3 off the OD of the tap threads, running that in carefully.
Afterwards I simply followed with a normal hi tech modern tap and MolyDee. Those were 6-32 I think.

13engines · Feb 15, 2019

You don't mention if you have CNC equipment available. If so, you should be able to thread mill this using a carefully programmed carbide thread mill.

michiganbuck · Feb 15, 2019

partsproduction said:
Glock slides use an nitrided surface, tenifer, I think it's very hard,like 60 Rc. In theory if you break through the few thousandths of an inch of that with one of those little diamond ball points in a dremel you should have normal tapping below that.
I once had many broken taps trying to tap scope mounts on a Ruger Mini-14 stainless cast receiver. I finally had to grind the top 1/3 off the OD of the tap threads, running that in carefully.
Afterwards I simply followed with a normal hi tech modern tap and MolyDee. Those were 6-32 I think.

Agree with a small grind bevel....I used to use a hand HSS chamfer bit in a screw driver handle or T handle so with a few turns I could see if the part or surface was too hard for a tap. liked to keep the tap length about the same length as the drill length, drill and tap at the same location not drill two holes then have to move back to first drill spot, tried to avoid hand and bench vice tapping, used tap ease or spindle oil, checked my drill size to be sure full size. plus a few other tricks but have to log off now to do some other things..

Red_SC · Feb 15, 2019

Not my picture, but this is what it looks like. The Tenifer has already been machined away when I’m threading. I’m trying to do it on CNC, but the tap is breaking.

Red_SC · Feb 15, 2019

partsproduction said:
I finally had to grind the top 1/3 off the OD of the tap threads, running that in carefully.
Afterwards I simply followed with a normal hi tech modern tap and MolyDee. Those were 6-32 I think.

So 2/3 of the thread depth was cut the first pass, and you finished the thread on your second pass? I didn’t think about that, that might work. Run the ground tap in the CNC and chase it by hand.

13engines · Feb 15, 2019

Red_SC said:
https://www.atxarmory.com/v/vspfiles/photos/RMR-Cut-2.jpg

Not my picture, but this is what it looks like. The Tenifer has already been machined away when I’m threading. I’m trying to do it on CNC, but the tap is breaking.

Sorry Red. I corrected my previous response. I meant thread mill, not thread tap. Thread milling should work no problem.

Red_SC · Feb 15, 2019

13engines said:
You don't mention if you have CNC equipment available. If so, you should be able to thread mill this using a carefully programmed carbide thread mill.

I could do that. I use 1”+ thread mills regularly but I haven’t run one that small.

Terry Keeley · Feb 15, 2019

Red_SC · Feb 15, 2019

13engines said:
Sorry Red. I corrected my previous response. I meant thread mill, not thread tap. Thread milling should work no problem.

Which type of thread mill would be preferable for this small of a hole? I use the first type exclusively for the large holes I'm usually dealing with, but since the second type is only cutting one thread at a time it may be less prone to breakage.

G.W. SCHULTZ TOOL Thread Mill, 0.125" Shank Dia., 3 Flutes, 0.218" Shank Length, Uncoated - 54TG50'|'TMM 6-32 - Grainger

SCIENTIFIC CUTTING TOOLS Thread Mill, 0.1875" Shank Dia., 3 Flutes, TiAlN - 4PEL7'|'SPTM098A - Grainger

Red_SC · Feb 15, 2019

Terry Keeley said:
Moly Dee

I need to pick some of that up too, I've been using Rapid Tap.

13engines · Feb 15, 2019

Red_SC said:
Which type of thread mill would be preferable for this small of a hole? I use the first type exclusively for the large holes I'm usually dealing with, but since the second type is only cutting one thread at a time it may be less prone to breakage.

The smallest I've done is 10/32 and used the full thread type where you do it in one pass. I think with the very small moves the tool/machine will be making, you'll have a better chance at producing a good thread with the one that cuts all the threads at once. That is unless you have a machine in great condition with all the slow speed backlash and the likes accurately compensated for.

I usually do course threads in two passes, with about 65-70% thread depth in a first pass and finish with a second.

They make thread mills in sizes smaller than 6/32. I doubt they'd bother if they simply broke all the time where likely no one was using them.

As I'm sure you know, they're a lot higher priced then taps that's for sure, but once you've dialed it in you'll be good for many holes with one tool. And removing broken taps will be a thing of the past. Broken thread mills are way easier to remove.

kazlx · Feb 16, 2019

If you already have experience threadmilling, it should be pretty straightforward to threadmill them. Plus if you break a tool, it won't be stuck in there.

Singtoe · Feb 17, 2019

which hole is the tap breaking in?

Froneck · Feb 17, 2019

I work with the 1911. My son is one of the top pistol shooters in the country. I use the fine thread rather than NC so you might want to try 6-40. Another possibility is to call Engineering of Tap manufacture. I one time had to Tap thousands of a component made from 1/2 hard 4140 with 3/8-16. Every supplier and salesman had the tap that would work, none did! Yes a few holes then tap broke! Eventually I called another supplier and gave him the details. Best advise I ever got! He said he was a salesman and actually knew nothing about machining and if he had to get an answer for a customer he would call the engineering department of what ever item was needed. He gave me the phone number of his tap supplier, I called, asked for Engineering. Explained my situation, was told what tap to use, got the taps and problem solved! I have done that with quite a few cutting tool problems and always their solution worked!

aarongough · Feb 17, 2019

Thread-milling it on your CNC will be fast and safe. As one of the other guys said thread-milling has a large advantage in that if you break a tool it's not stuck in the hole!

I generally find single-point thread mills a little delicate, particularly in small thread sizes. You need to make sure to take multiple passes and you should be fine. Maritool sell both single form thread mills and multi tooth thread mills, I've used both with success for quite a while.

edwin dirnbeck · Feb 17, 2019

aarongough said:
Thread-milling it on your CNC will be fast and safe. As one of the other guys said thread-milling has a large advantage in that if you break a tool it's not stuck in the hole!

I generally find single-point thread mills a little delicate, particularly in small thread sizes. You need to make sure to take multiple passes and you should be fine. Maritool sell both single form thread mills and multi tooth thread mills, I've used both with success for quite a while.

I have NEVER used a thread mill.Just a question ,not a critique.Why would a single point thread mill be more delicate than a multiple tooth mill.I realize that a single point thread mill LOOKS LIKE A FLY OUT ON THE END OF A TOOTHPICK, but surely their is much more breakage force involved with the entire length of the thread being generated in one pass when using a muliti tooth cutter. Edwin Dirnbeck

aarongough · Feb 17, 2019

edwin dirnbeck said:
I have NEVER used a thread mill.Just a question ,not a critique.Why would a single point thread mill be more delicate than a multiple tooth mill.I realize that a single point thread mill LOOKS LIKE A FLY OUT ON THE END OF A TOOTHPICK, but surely their is much more breakage force involved with the entire length of the thread being generated in one pass when using a muliti tooth cutter. Edwin Dirnbeck

Yeah I actually had the same thought go off in my head when I was writing that... I have used the single tooth thread mills in very different situations than the multi-tooth versions, so it could just be that I got my speeds/feeds/stepovers less correct with the single tooth versions!

mhajicek · Feb 17, 2019

I regularly threadmill with Harvey's tipped off single profile cutters in 17-4 H900 and Ti6Al4V-ELI down to 2-56:

Harvey Tool - Carbide Thread Milling Cutters - Single Form - For Hardened Steel

eaglemike · Feb 17, 2019

OSG makes a line of taps for hard materials that will do what you need. I used some on older Witness frames that were nastier than the material you have after removing the surface of the Glock slide. They worked fine, as long as I did my part.
The advice above regard switching to 6-40, or even 6-48 is very good advice. You don't have to use the screws that came with the mount.
Good luck.

Problem tapping small holes in hard material

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