i keep breaking taps

robert102653 · Feb 4, 2016

this has been a continuing saga for me. i have a hole that needs to be tapped M6x1. the chart calls for a 10 mm hole. i bought OSG taps. I drilled the hole. The steel is the only unknown because I bought it at the local salvage yard. No markings but soft enough to file and it turned quite nicely. I used carbide insert tooling. the fist hole i did the tap, a spiral threaded one,
( the kind that shoots the chips out the back end of the tap), sailed in on my slowest speed on my sb 9a with the tap held in the tail stock in a Jacobs chuck with the lathe in back gear. Nice and slow as I said. Plenty of tapping oil, tap magic is the brand. Then suddenly it snapped. that's number one, and I cant get it out because it snapped deep in the hole. the second time different hole it didn't make it even a half inch before it snapped off. What in the hell am I doing wrong? :mad5:

robert102653 · Feb 4, 2016

i meant a 5 mm hole but now on a different chart i see that it should have been a 5.5 mm hole so that's my trouble i cant freaking read

Rob

DanielG · Feb 4, 2016

5mm is correct for M6.

How deep are you trying to tap? 1/2" for an M6 is over 2D. The biggest factor I've seen in tap breakage is not tapping straight. A deep hole will exacerbate the problem if your drill wandered.

Have you tried tapping by hand? M6 is easy enough to do and you'll be able to feel it tapping, which will decrease tap breakage.

janvanruth · Feb 5, 2016

tailstock misaligned
a drill will flex and wander off, a tap will break

dullvinci · Feb 5, 2016

Even with a spiral tap which pulls the chip to the outside, if you go too deep the chips could jam in the flutes and clog the tap. As soon as you see that the chip stops coming out freely, stop and reverse to break the chip. You may even have to back all the way out, put more cutting fluid on the tap and go back in. You did not say how deep you are tapping, but past some unpredictable depth, the tap runs dry, also causing breakage. If for no other reason, the tap should be backed out to get the fluid to the bottom.
Good luck and keep making chips.

n2zon · Feb 5, 2016

The chuck could be off center even if the tailstock is on center. I've tapped down to #8 and #6 screw sizes on the lathe (a SB13) without a problem using a spring center on the taps and center drilling before drilling to size.

I can't tell what the problem is, but maybe you can start by eliminating some possibilities. Can you check the chuck center by turning a point on a piece of stock (carefully centered in the chuck) and then putting it in the Jacobs chuck to use as a dead center to turn a test bar?. Then, if it's off, do another using a known good dead or live center in the tailstock to see whether it's that or the chuck? Just a thought.

John

Tony Quiring · Feb 5, 2016

If using back gear the torque oil substantial so you will not likely observe any signs of trouble until too late.

Options are to just engage back gear on the spindle but leave the rack in normal resulting in full neutral and disconnected spindle from drive.

Then just use hand power on the Chuck to rotate work while grinding tap in by tail stock.

Frequent backing out to break chips and add oil will help.

Lots more work but less than removing broken tap.

Dobermann · Feb 6, 2016

We had lots and lots of trouble with broken taps in stainless steel where I work part time. We asked our CNC machine shop for some advice since they were doing stainless projects with threaded holes for us. The advice was to use spiral point taps (so-called gun taps) with a black steam oxide coating and a really high-sulfur cutting oil. The stuff is very, very dark and smells like somebody dug up the bottom of a swamp. I don't think we've broken a tap since following their advice. That said, I do like the advice given in a previous post which suggests running the spindle by hand so that you won't twist off the tap if it gets into trouble.

dullvinci · Feb 7, 2016

One thing I forgot to ask in my previous post. Are you tapping with the tailstock clamped down? With smallish taps like that 6mm, I always let the tailstock "float" free because it is really hard to match the quill feed rate to the tap pitch with a small tap. I even wipe and oil the ways under the tailstock if it's at all sticky. It also lets you safely reverse the tap under power.
We still do not know how deep the drilled hole is in relation to tap depth. Gun taps are my favorite tap, but not for a small hole tapped to bottom. The chips pack at the bottom of the hole if you don't drill deep enough. That nasty black cutting oil is great stuff and is available at any hardware store as pipe threading oil.

robert102653 · Feb 29, 2016

i have since solved this problem by drilling the proper sized hole now i can power tap withthe tailstock loose and it goes like butter. like i said before sometimes i just have to slow down and read the tap chart correctly lol

tailstock · Mar 1, 2016

When tapping is critical keep a piece of aluminium round stock around and do a test run, this can save money and time figuring out problems. :codger:

Screwmachine · Mar 1, 2016

robert102653 said:
i have since solved this problem by drilling the proper sized hole now i can power tap withthe tailstock loose and it goes like butter. like i said before sometimes i just have to slow down and read the tap chart correctly lol

But are you using 5mm or 5.5mm for the drill? 5 is the correct drill. If you are going 5.5 then I believe you that they are threading like butter, but have a look at your thread, it will be a spiral scratch mark down the hole.

moonlight machine · Mar 1, 2016

On little taps like that I run the spindle about 600 rpm, hold the tap in the T S chuck with the taper just free. I hold the chuck by hand and run the drum switch by hand. As the tap bottoms or sticks release the chuck, it just spins with the work for a second as I reverse the motor. I'v done thousands of holes that way on the old South Bend. No muss, no fuss, thousands of holes tapped, no broken taps.

Engineeringtech · May 17, 2018

Personally, I don't think it is a good idea to power feed any tap into gummy, tough SS. Too easy to break a small tap. I also wonder why everyone is so hung up on the "correct size" tap drill. Yes, 99% of the time I use the tap drill recommended on the chart. But there is no "correct size". It's not always necessary to get 75% thread engagement in tough stainless steel. You could go up to a 13/64" tap drill and still have 65% thread engagement. I bet the tap would be a lot less likely to break too.

Gobo · May 17, 2018

65% thread is perfectly acceptable for the vast majority of applications.

kitno455 · May 17, 2018

I don't think anyone is 'hung up', that's why this thread died two years ago...

allan

Cole2534 · May 18, 2018

I'm not a fan of tapping too slow, it's never worked well for me. If it's a through hole I run it nearly as fast as a drill. On a blind hole with some relief in the end I mark the tap for depth with a scale and a pen. When it's at depth I release the keyless drill chuck with my right hand, allowing the tap to spin freely then I back it out by hand.

FWIW, this is on a gear head lathe with not much of a brake.

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