Thread cutting problem

[JasonBModerator @jasonb]

Which brings us to another possibility – How slack is the die in the holder and what is it's OD. I've seen similar issues where a metric OD die was being used in an imperial holder which means the die sits off ctr as the adjusting screws push it to one side of the holder.

Not usually a problem with imperial threads as the dies all tend to be imperial but you never know, metric threads can be had in both metric and imperial ODs.

[HopperParticipant @hopper]

Posted by Buffer on 29/11/2020 17:04:44:

thanks again to everyone who has taken the time to reply with advice.

I did what Andrew suggested and cut a thread perfectly on a piece of 3/16th brass. So there is nothing too wrong with the die it would seem.

I managed to cut a thread in steel but only by taking the bar down to about 0.18 but it didn't look very nice. It seems that despite me pushing the die towards the chuck it just wouldn't start pulling itself forward along the bar and it appears to just grind the steel away until it can get going and then it pulls itself along.

Testing your die on a piece of brass does not tell you anything much about its ability to cut much tougher steel. Test it on a piece of 3/16" steel in the vice by hand. It sounds to me like your die is not sharp or not correctly ground from the manufacturer. It happens.

And you should not have to take the bar down to .180" to get the die to work. A 3/16" die should thread a piece of 3/16" (.187) bar with no trouble. Something is wrong there for sure.

Like Jason, I would put SOD's money on the problem being a duff die.

[HopperParticipant @hopper]

Posted by Buffer on 29/11/2020 17:04:44:

To speed this up does anyone know any easy way to work out when you can engage the leadscrew for a 40tpi thread with a 4tpi leadscrew?

On the other hand: Holy cow, dude. If your lathe is big enough to have a 4TPI leadscrew, it's probable that the tailstock is too heavy to get pulled along by that tiny, tiny 3/16" x 40TPI thread with a thread depth of 16 tiny thou, even with you pushing it. Sledgehammers and mosquitoes come to mind.

A sliding-type die holder might be in order. Or a regular manual die handle pushed up against the job by the tailstock barrel under handwheel pressure might work better, so you dont have to try to move that massive tailstock mass.

[Martin ConnellyParticipant @martinconnelly55370]

There is also something I wrote about the threading dial here Album 46824

Martin C

[Rod RenshawParticipant @rodrenshaw28584]

Andrew, I guess so.

I use an Arrand tailstock dieholder which also has very little float but it has three hex headed screws to hold the 2 halves of the holder together and these can be left slightly loose to allow the die to line itself up. One half (end?) slides on a mandrel that goes in the tailstock taper and the other holds the die. By experiment I find some dies cut well enough with the screws tight and some do not. Perhaps I should buy some better dies and pension some of my older ones off!

Jason. I read somewhere about someone who had approached this aspect by fitting an additional pinching screw diametrically opposite the 3 normal screws and then used the 4 screws together to centralise any dies which were very loose in the holder. Sounds like an idea which might help in some cases. Or I suppose one might try a slip of shim opposite the pinching screws to take up some of the slack.

It's odd how tailstock dieholders work well, straight out of the box, for some workers while others struggle.

Rod

[Anonymous]

Posted by Rod Renshaw on 30/11/2020 10:50:12:

Jason. I read somewhere about someone who had approached this aspect by fitting an additional pinching screw diametrically opposite the 3 normal screws and then used the 4 screws together to centralise any dies which were very loose in the holder.

It's interesting to note that the die holders for both the Britan and Arc systems use four screws as described. But in both cases the dies are a snug fit, no more than a barely detectable shake. So I can't see the fourth screw being much use for aligning an off centre die.

I'm not a fan of split dies. Where possible I prefer to screwcut or use Coventry dieheads.

Andrew

[JasonBModerator @jasonb]

Up until recently I use a bit of shim on a couple of 25mm OD dies as my holder is imperial but did break the rule of a lifetime and actually made a metric holder after a bought a few more 20 and 25OD dies from Uncle Ketan.

I would imagine the 4th screw would also allow for the odd occasion when a smaller 25 or 20mm die was being used in a 1" or 13/16" holder where they would not be such a snug fit.

[BufferParticipant @buffer]

I just did what Hopper and a few suggested and tried cutting a thread on the bench by hand on a 3/16th steel rod and I got the same problem. So I have filed the die in the round filing cabinet in the corner of the room. Just to be sure I turned the same bar down a bit and cut a perfect 4mm thread on the same material. So that's that as some said duff die.

[HopperParticipant @hopper]

Frustrating! It pays to buy quality tools — but even that seems to be no guarantee these days.

[Martin DowingParticipant @martindowing58466]

Posted by Buffer on 29/11/2020 17:04:44:

thanks again to everyone who has taken the time to reply with advice.

I did what Andrew suggested and cut a thread perfectly on a piece of 3/16th brass. So there is nothing too wrong with the die it would seem.

I managed to cut a thread in steel but only by taking the bar down to about 0.18 but it didn't look very nice. It seems that despite me pushing the die towards the chuck it just wouldn't start pulling itself forward along the bar and it appears to just grind the steel away until it can get going and then it pulls itself along. So I did what I didn't really want to do and set the lathe up to screw cut. In the end I managed to get a very nice test piece cut on the other end of the brass rod so later I will try it on the steel.

It is common for die to cut well on brass of nominal diameter but on steel only on slightly undersized bar.

From my experience 2-3 thou undersized steel bar is getting threaded very well.

I have M10 die where manufacturer have stated that steel bar to be threaded must be 9.85 mm of diameter!

Of course die must be pressed square. Proper holders are best but some shabby settings employing pressure of tailstock barrel for squaring while starting your thread also can work in absence of holder.

[Phil PParticipant @philp]

Buffer

Just as an experiment, go retrieve the die from the bin and see if you can make any impression on it with a file.

I had a similar experience with a new (cheap) die a while back, it was made from something other than the HSS that was written on it and was quite easy to make a nick into with a needle file, so it was probably as soft as the material I was trying to thread.

I bought another new (expensive) die and had no further problems.

Phil

[John RutzenParticipant @johnrutzen76569]

I don't usually bother with a tailstock die holder, just bring the tailstock chuck up against the back of the die stock and apply slight pressure as you turn it by hand.

[Anonymous]

Posted by John Rutzen on 01/12/2020 08:29:23:

….bring the tailstock chuck up against the back of the die stock and apply slight pressure as you turn it by hand.

I used to do that but the toolpost/cross slide always seemed to be in the way. So I bought a tailstock die holder for the few times that I use dies.

Andrew

[JasonBModerator @jasonb]

I used to do the same when I first started out but it's so much easier and quicker to thread under power so a tailstock dieholder is what I use now, just gripping the body with tommy bar removed works for me up to about M6 or larger if a fine thread.

[Author]

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