Die defeated by mild steel

I was trying to make part of a mini lathe toolpost by turning 16mm bar to 12mm and threading one end M12 and tapping the other M6.

I turned the bar and tapped it easily enough but the last operation was to thread the M12 screw.

After 5mm the die bound up like a nut tightening on a screw. At first I thought maybe the steel had work hardened, thought I was pretty certain it was just mild steel I had picked it out a scrap bin. I took a blowlamp and heated it till the metal turned black and let it cool thinking that would anneal it. After that it still would not let itself be threaded.

Anyone know what made the die fail to cut?

Oddly i had a similar problem on some silver steel but then I was aware that mine are cheap carbon steel dies I've had for years. I solved my problem (after assorted bog-ups) by partial threading on the lathe and chasing with the die when roughly down to size. (Usually i test my threads with a nut but this time didn't have one of the correct size and didn't want to compromise the tapped part)

Perhaps starting diameter was a little large, especially if the soft steel is extruding a little and not simply cutting. M12 major diameter for male thread is a little less than 12mm.

I suppose it is possible but M12 is a bit big to thread with a die, anything over M8, I screw cut and finish with a die.

Possibly a cheap M12 die with no cutting clearance,or a worn die they will bind on the thread, regarding annealing,steel should be heated to red heat,held at this temp for a while then allowed to cool SLOWLY, mild steel round should not require annealing, drawn mild steel flats and square may need annealing as there is a lot of stress built up when drawing sharp corners through a die. The advantage of screwcutting in the lathe is that the threads are true to the axis of the round bar,they can either be finished to size ,or left oversize and then finished to size by a light cut with a good die.

As the posters above have alluded, cutting an M12 thread with a die needs a lot of heft. Very difficult to do in the lathe - if that's where you are doing it. I suspect that the die is just getting as far as the end of the lead in before the effort of cutting the full depth is too much. I would need to get both hands on a hefty die stock to thread M12 from scratch in steel. Just holding the blank in the vice without twisting is difficult enough. I always partially screw cut anything over 6mm or thereabouts with a coarse thread.

HTH

Rod

Edited By Roderick Jenkins on 03/02/2016 23:29:24

The die was a fairly decent presto, first time I ever cut M12 on a die so it would make sense that it is too big to cut by hand. Lathe I was using only had fine feeds with no srewcutting but I can probably figure something out,

Screwcutting on the lathe also means your thread is perfectly square with the rod, all the die does is tidy things up

Were you leading with the end of the die that has the tapered teeth? Try turning the die around and using the other end of ut tto clean up the thread and get going again then go back to leading with the tapered teeth.

And +1 on make the diameter a bit under 12mm. (about .005" -.010" in old money.)

Using a die on 12mm should be a doddle, in or out of the lathe, if all is set up right. (Try threading 2" pipework if you want a workout.)

Does your die have a split, if so open it out and take two goes at getting to size, make sure the die is the right way round, take say a 1/4 turn, then back off, continue, use a lubricant, even if its only engine oil. Don't rush it.

Ian S C

I always buy HSS taps and dies but I recently bought some new old stock Presto dies thinking I was buying my usual quality. However when they arrived they did not have HSS on them and the package had CS, from that I assume Presto also make carbon steel dies. I will put them to one side and only use them for cleaning up existing threads.

Edited By Chris Evans 6 on 04/02/2016 08:56:20

From 8mm upwards I usually - like others - rough out the thread on the lathe with a single point tool then run a die nut down in my tailstock holder to finish to size. Even doing it like this a 12mm thread that you intend completing in your lathe needs to be roughed out fairly close to size otherwise you'll still be struggling to finish the thread with the die / dienut.

Rik

I wouldn't get hung up on the fact that the dies are made from carbon steel. Carbon Steel is considerably cheaper than High Speed Steel and it is undoubtedly a fact that cheap dies, inevitably made from CS, can be poor quality. In principal, CS dies can be harder and have a finer grain size than HSS. The downside is that they tend to be less tough (will break and chip) and you have to watch the speed at which you cut threads to prevent them getting hot and over tempering - not usually an issue outside of production use. From a premium supplier like Presto I would expect that, for amateur use, CS dies would be excellent.

That's how I see it, anyway,

Rod

As Hopper said, 25mm conduit and 2" pipe CAN be threaded by hand, if there is no other option and you have the correct tools to do it. I have done a lot of this, hard work! Never had the same success with mild steel bar, even with a long taper to start the thread, it's difficult to start it straight. Old fellows in days of yore, when men were men, mighty thews etc, did the job by hand but used a die holder with a guide (similar to a conduit die holder). It IS possible but there are much easier ways to do the job. Tallow is the best lubricant for this if you want to try it the hard way.

It's the depth of thread, not the diameter of the tube, that is the issue. I have no problems cutting 40 tpi direct on to 1/2" steel, 12 tpi or 1.75mm is a different matter.

Rod

Ever measured commercial metric threads? On smaller sizes -M4 to M8, I always turn o/d before threading to 0.1 lower in diameter, 0.2 would be good for M12.

Regards Ian.

I would recommend prestos' range of split HSS thread cutting dies. I once owned a set of non split carbon dies and theyre only good for the bin to be honest because you cant adjust the overall diameter, theres no start to the thread basically. I know they cost alot for what they are but they definitely work.

I don't have a particular issue with carbon steel itself, it's a pretty durable steel but i suppose it could blunt quicker than HSS and thus have a shorter lifespan, just make sure you get the split ones or you'll be cursing and swearing for england.

Michael W

It could be that the curls of swarf from your first attempt jammed in the die, because you forgot to turn back half a turn to break the strands.

Cheers, Tim

It would be a good piece of advice since plunging in with a die without turning back can make small threads look awful,

Michael W

You can split unsplit button dies with a narrow cutting wheel on a Dremel, you could even do it with a 1 mm thick disc on a disk grinder.

Ian S C