Problem tapping 5/8 x 18 in 4140

I am taping a 5/8 x 18 hole for a breech plug on an in-line black powder project. I drilled a 37/64 hole with the barrel in the lath using the steady rest and running true to the bore. Barrel is Green Mt 50 cal, 4140 steel. I started the tap in the lath using the tail center to keep aligned. My tap was purchased from a local screw supply house. I have a plug tap that measures .630 OD. It will not cut more than about 1/2 inch and gets too tight to turn. Good supply of cutting oil and I have to clear the chips 5 or 6 times per revolution on the tap handle. I'm wondering if this tap is over limit in size? I'm ready to call MSC and order one of theirs. But not sure of the H limits. Any help would be great.

According to my Machinist's Handbook (admittedly from 1955) the maximum limit for the major diameter of a 5/8-18 tap is 0.6315 for cut thread taps and .6305 for ground thread taps. (I think it's safe to assume yours is ground.)

Recommended tap drill is 14.5mm or 0.5709". You're drilling 0.578 which shouldn't be a problem and should even give you a little allowance for your tap being right at the limit.

Does not look like the tap is the problem. You might try looking at recommendations for machining 4140 pre-hardened and poke around in the gun-smithing board.

Lewis

The only thing you didn't make clear is how you set the steady. If the steady isn't set on center properly, the tap could be gong into the hole crooked, which would cause something similar to what you describe.

Think about a three tap set: Taper, Plug, and bottom.

Start with the Taper tap.

When it gets tight, back up a quarter or half turn, then advance a quarter turn or so. Repeat and repeat. With tapping experience you will develop a feel for when the tap is starting to bind and needs to be retracted.

Stringy alloys can be tough to tap, I call the above method "Pecking". You will have to fullly withdraw the tap often and clean chips.

After you have the taper tap in deep enough, repeat with the plug tap and "Pecking".

Use a good sulphur type heavy cutting oil. Lots of it! Keep those chips clear.

It your tap is a low priced carbon steel import, may not be sharp or may have incorrect cutting angles/grind for alloys. Try a quality high speed steel tap.

Are the breech plug threads tapered or straight?

I likely would have threaded with a boring bar, as I have a good collection of small bars and threading tool bits. This way I would be able to obtain a good thread fit.

If you are really into building M/L rifles, then invest in a high quality three piece tap set for each breech plug size you work with. Taps get dull also.

You may want to make up a revolving tap holder for the tailstock. Let me know if a photo would help.

I have threaded a lot of 316 and 317L Stainless, your 4140 is a slice of pie compared to threading stainless.

So far I have done what I've been doing for years. The bore was indicated on center on the steady rest with the muzzle in the head stock center. I suspect the tap is import. But before I give up on that one I'll get a taper tap to take some of the strain off the plug tap. I use lots of oil, clear the chips often and break the the chip often. MSC may have to be sending me a better quality than I can get here in Las Cruces, NM. Not a hot bed of tool supply, but the weather's great. Just moved here from the Catskill Mts of NY.

Thanks for your feedback.

I imagine you've got a place for the chips to go, so I'd recommend a spiral point (hey, a gun tap ) and wind it all the way in. I frown on backing up a tap too often because this will often break the tips off some of the teeth. While it might 'feel good' to break the chip, resist the urge until it is really necessary. You can inject new lube (the sulpherized stuff) every half dozen turns or so.

And yes, use a high quality ground thread spiral point plug tap, then you can finish it with a bottoming hand tap.

When you say centered on the steady rest, what does that mean? The steady rest has to be positioned so the center off the work piece is inline with the lathe spindle. You could have zero TIR on the circumference when rotating the part but the part could be wobbling.

I initially have the work centered on the spindle center and the tail center which then makes it easy to adjust the steady rest. And then with the drive dog secured so the piece is held into the spindle I check the bore with a plug and indicator both for wobble and run out. I'm pretty confident in the setup being square. I've used the same setup for chambering rifles for many years.

choppero,
The only thing I'd add/modify to the good advice already offered is to use actual tapping compound made for steel. Perhaps the "good sulphur type heavy cutting oil" is sufficient, but because you're having difficulty you might as well give yourself the best chance. I have the best results with the thick, gooey "Butterfield Tapping Fluid" but have heard that it's no longer available.

SilveradoHauler,
I'd be interested in seeing a pic of your revolving tap holder for the tailstock. I need to make one and would much appreciate seeing your design.

-jmcvo

You very likely have a chineese tap. To tap alloy steel you must get a set of taps, taper, plug, bottom, from a reputable manufacturer and it should be HSS Get an American or European set. You have to have quality to tap alloy steel and still have a useable set when you are finished. Peter

My vote is for a set of HSS taps. I ordered from MSC last night and should see them today or Mon. I rechecked the concentricity of the bored hole and the actual thread cut so far and I'm in good shape there. So I have learned a new lesson for big hole threading...spend the time and money for good taps. Some day I may get setup for single point threading with a boring bar, but for today....taps.
Lots of great insight here...thanks for the tips.
Tom

I've had really good luck with Castrol Moly-Dee in hard tapping situations. Just tapped a bunch of 316 stainless discs about 3/8 thick for PG-16 cable glands, expensive tap, something like 22.5 x 1.41mm. Don't get me started on metric thread standards.Other problem is my holes were undersized as cut on the water jet and the operator cut them off center so I couldn't bore them out to size without making the darn holes oval.

Make sure you get it started STRAIGHT! On the few firearms projects I have done for myself I have bore a short distance to a guide diameter just to lead the tap in, then cut that piece off. Seems to work well.

I suspect your tap is getting bound up by the chips. In normal use the chips follow the flute around to impinge on the thread's minor diameter. Figures. You can turn the tap readily about 1/3 turn which about equal the chip length in the flute then the tap stops. If you force it the chip may buckle and you can turn the tap further or - not. Good way to break it. The tried and true way is to reverse the tap a part turn to break the chip and then tap some more - until the chips lock the tap again. PITA when you want a tapped hole.

Many clever taps have been devised. Time for some "tap-ology" You need a tap that deflects the chips down the flute. The simplest and most accessable is a "gun" tap (spiral point) where the cutting edge of the flute is inclined to the axis about 10 degrees. It deflects the chips so they exit the flute ahead of the tap ("shoots" them ahead - hence "gun" - droll eh what?) and don't jam against the minor diameter. They work slick and clean. In a through hole you can simply run a gun tap into tap drilled work under power without reversing or fiddling.

Blind holes and spiral point taps pose a problem:the chips exiting ahead of the tap may pack in a blind hole. They have to be tap drilled deeper if possible otherwise the tap has to be backed out frequently to clear chips from the tapped hole.

In thinner material like structural steel sections I've tapped holes using a battery drill and a gun tap. A little misalignment doesn't seem to matter to the tap. Buzz them ain and back them out fast as you can pull the trigger.

Then there are spiral flute taps whose flutes are like a drill's. They deflect the chips UP the flutes to exit at the work surface. You can drive them under power into the hole just short of the bottom. Becareful though. If you think a broken regular tap is a hassle to extract, a broken spiral flute tap is a hassle in spades.

Hey! You have a lathe right there? Why not bore the threads?

Forrest,

You've got way more hours in on this than I do, but I believe a "gun tap" is a common term for a spiral point tap that pushes the chips out ahead and "spiral flute" is a helical tap with flutes that carry the chips up out of the holes.

Spiral point is more common and cheaper, so it's a worthwhile distinction.

Lewis

Fc. I looked your assertions up. You're right. Dammit!

I tend to write like when I learned a zillion years ago. Back then it was "gun tap" period. Later it was "spiral tap" not a "spiral point tap." A "spiral flute tap" was called a "helical flute tap" back when I was learning the trade.

So. Sorry if my old terminology is sometimes misleading. Correct terminology is a big concern for me so when I get jumped for screwing up I really cringe.

I fixed my post.

You might add one more to your terminology, Forrest... "cheap crappy chinese tap". That sounds like what the OP is using and likely the source of the whole problem. I have a few brand new chinese two flute endmills I keep as examples of what poor quality cutters really are. The ends are sharpened over 1/8" off center, and these are 3/8 and 1/2" endmills. Totally useless.

I make ML barrels as a side line. Basically, as has been said, you need 3 taps to do the job right. I've got lathes big enough to shove the barrel thru the head, bout the only thing other than that I do different is that I bore the hole after drilling it undersized. Better control of the size & a better shoulder to bottom against with the breech plug.

Paul

Forrest,

They had to change the terminology because we young'uns kept confusing "spiral" and "helix". Both spiral flute taps and spiral milling are examples of helices, whereas a gun tap is the only one that actually does have spirals on its point.

So in theory, you are correct. In practice, someone gave me a Union Butterfield catalog and I was nerdy enough to read it cover to cover.

I had a fun morning when a co-worker wanted to argue with me that the custom made 1" x 22 tpi spiral flute tap he was holding in his hand was, according to him, a thread mill! It didn't end well, for his work or the tap, both of which were quite expensive.

Thread drift complete!

We can talk about anything we want until choppero comes back and reports on his success with new taps. I love it here.