Bent thread

[Martin Shaw 1Participant @martinshaw1]

It has become obvious to me that external threading is my bete noir. I was given a Multifix tool post which needs a stud to fix it into the lathe, the attached photo shows the end result.

It really is very simple, a 9/16" piece of EN1APb is turned as necessary, two flats milled for a spanner ,and the ends threaded M10 x 1.5. The first two stages went extremely well and are I think perfectly satisfactory, the threading has gone awry and as can be seen the threaded sections are bent, it's visually obvious in the short thread, the longer one is marginally better. This is the second attempt, the first was no better and for the record the stock was at 9.95mm, the die was brand new from Drill Services and I am using it the right way round.

I feel it must be my technique at fault here which the internet can't correct but apart from the obvious straight at 90 deg starting, can anyone suggest what I'm doing wrong?

I know a single point threading approach is an alternative but it's a big learning curve and I feel yet another cost for the very small amount of threads outwith BA sizes I am likely to do.

Regards

Martin

Edited By Martin Shaw 1 on 12/11/2022 21:54:11

[Martin Shaw 1Participant @martinshaw1]

[Bill PhinnParticipant @billphinn90025]

Posted by Martin Shaw 1 on 12/11/2022 21:52:54:

I feel it must be my technique at fault here which the internet can't correct but apart from the obvious straight at 90 deg starting, can anyone suggest what I'm doing wrong?

What is your technique, Martin? Can you describe it in detail?

[Martin Shaw 1Participant @martinshaw1]

The stock is turned 0.05mm undersize with a thread depth undercut at the shoulder and a small 45deg bevel at the leading edge. It is held vertical in the vice and the die stock held at 90 deg and kept as close to that as poss as it is rotated until it starts cutting. Tne first two or three rotations are taken gently to maintain the squareness with back turns after every half turn forward to break swarf chips away.

Once the die has properly engaged the full length is cut using cutting oil. I would likely take a second pass if the thread is tight in a standard metric nut. It doesn't seem that the thread form is a problem, a nut can be spun on by fingers, it just ends up bent off cventre and the bend appears to be at the point where the threading ends.

I hope that explains things.

Martin

Edited By Martin Shaw 1 on 12/11/2022 22:51:34

[Clive FosterParticipant @clivefoster55965]

Martin

Due to the (relatively) high helix angles coarse threads are notorious for pulling and tilting the die off centre when starting by hand. M10 is quite coarse enough to be troublesome unless you are both good and experienced.

Best answer is to use the lathe with either a tailstock die holder (preferable) or using your ordinary diestock with a suitable rod held in the tailstock chuck to push the die squarely against the work. Rod, or thick wall tube if the thread is long, needs to be almost as large as the hole in the back of the die stock. If using the diestock you will also need to arrange something to hold the die stock handles against rotation whilst still permitting things to slide.

Pull the chuck round by hand or, if you have a decently low speed do the job under power.

For M10 you will probably need extra leverage on the cuck to turn it by hand. Sung fitting open ended spanner on a jaw is better than the chuck key!

Clive

 

Edited By Clive Foster on 12/11/2022 22:51:57

[bernard towersParticipant @bernardtowers37738]

Personally if its something as important as a tool post stud it would be better scewcut even if you do it to about 85 to 90 per cent depth then finish off with a die die for depth and form.

[egaParticipant @ega]

A component like this should be screwcut and, optionally, finished with a die.

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, I agree with Clive, if you do it in the lathe, and if you use a die, use a tailstock diestock, or a squared end rod or tube pushing on the die and not on your hand diestock during starting the threading operation, and get the die started the full thickness at least before finishing with your hand diestock. You will need to advance the tailstock barrel at the same time as turning the lathe chuck though, to keep a square pressure on the die. I very often put the die in my hand diestock, the wrong way round, so to speak, so the full width of the die can be pushed with a blank end Arbor in the tailstock.

Regards Nick.

[Martin Shaw 1Participant @martinshaw1]

My thanks to Clive for the explanation of why it was going wrong, I'm gratified it's not entirely me although I take his point, I am neither good nor experienced. I have a tailstock die holder which has never been entirely successful. I could never understand why until very recently some separate bits of information came together, and here's where I embarass myself. I did not realise there was a right and a wrong way round to start a die and now I know this it explains why often as not threading was at best a mess. Armed with this vital information I now realise that the tailstock die holder along with other contributors most useful tips I have half a chance instead of none. I'll do a couple of test pieces tomorrow before I remake the stud and post the results.

Thanks everyone, I'm a lot more at ease with this now.

Kind regards

Martin

[HopperParticipant @hopper]

Definitely do it in the lathe with the dieholder or the tailstock pushing on the die/holder to keep it square.

Make the OD much smaller, to make life easy for yourself. I would go down to at least 9.9mm or a bit less, maybe even 9.8, for a 10mm thread. Remember, you only need 65% thread engagement before it starts to significantly affect thread strength. So struggling along tying to cut a full thread form with a die is pointless.

Make the chamfer on the end at least as big as the thread depth before you start.

Edited By Hopper on 13/11/2022 02:46:04

[Alan CharlestonParticipant @alancharleston78882]

Hi Martin,

Are you using a die nut (a solid hexagon with a hole through the middle with cutting teeth) or a split die which you can start the thread with a shallow cut and take a number of passes while gradually increasing the depth.

Die nuts are not really meant for fully cutting threads but are used to finish a turned thread as described by ega. By cutting the full thread all at once, the force required is large and can result in bent threads.

Even with a split die and cutting the thread using multiple passes, you should follow the procedure outlined by Nicholas to keep the die square to the work.

Regards,

Alan

[JasonBModerator @jasonb]

You may find the part rotates in the chuck once the die is on a couple of turns particularly if it's an old chuck so may need to move it to the vice but at least the thread will have started square with the tailstock die holder

[not done it yetParticipant @notdoneityet]

Bernard and ega have the best options. If you must just use a die, start longer and turn a taper to aid alignment.

Also, if screw cutting on the lathe, concentricity is likely an issue if using a 3 jaw chuck. A 4 jaw independent is likely awkward for a beginner but a collet holder should avoid any concentricity problem.

[DC31kParticipant @dc31k]

If you absolutely had to do it with a hand-held die holder, here are are couple of ideas:

Make the part you want to thread longer than its finished length. Turn enough of it down small enough so it is a snug fit on the minor diameter of the die, the length being enough for the full depth of the die. This allows the die to locate centrally over the stock and will help stop it tilting. If you have enough material, make the chamfered part that increases from minor to major diameter much longer (maybe twice the depth of the die), so it gives the die a very good lead in. It is wasteful of material but not as wasteful as scrapping the whole part.

Find a tube that is larger than the OD of the thread. Hold in a(n unplugged) drill press (you might need to drive a wooden dowel in the end of the tube so it fits a 1/2" chuck). Load the quill handle (elastic strap) so the tube presses on the die. Turn the handles of the die holder.

You can also take inspiration from electrical conduit thread cutting dies: they have a parallel portion on the holder that goes over the OD of the conduit in advance of the cutting die which centres and guides the die. This works for conduit as it only comes in two sizes but some have an iris mechanism which make them a little more versatile.

See:

https://img.ffx.co.uk/r/700x-q85/tools/AK317DH.jpg

https://www.tlc-direct.co.uk/Images/Products/size_3/COSDG.JPG

Anything and everything you can do to keep the die and material to be cut in the proper relationship will help. The setup needs only two free degrees of movement – rotating and moving in the direction of the thread. Wobbling around freehand in space will make it difficult to produce good threads

[Tony RayParticipant @tonyray65007]

Can I ask why single point thread cutting is going to cost you money ?

[Martin ConnellyParticipant @martinconnelly55370]

Good quality cutting oil is a must.

Martin C

Forgot to mention that I think single point screw cutting is cheaper than buying dies, buying or making tailstock die holders or hand die stocks. One tool will do lots of threads when single point cutting so is a cheap option.

Edited By Martin Connelly on 13/11/2022 08:30:15

[Mike HurleyParticipant @mikehurley60381]

All the above posts contain very sound advice. Its well worth persevering with cutting threads in a lathe as the results will be far more predictable than using manual dies. It does take take practice though – like everything in life!

regards

[Ady1Participant @ady1]

Single point most of the work then finish with the die to tidy up

[larry phelan 1Participant @larryphelan1]

As has been pointed out, single point thread cutting is simple enough and cost effective. One tool will cut many different threads, and cut them well, no bumps or bends.

For something like that, the last thing I would use would be a stocks and die, too much messing around.

Give it a try in the lathe, you might be surprised.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Tony Ray on 13/11/2022 08:16:32:

Can I ask why single point thread cutting is going to cost you money ?

Costs real cash if a beginner has to buy a grinder and HSS or carbide inserts and a holder, when a one-off job can be done with a die.

I've long since coughed up the cash for single point AND dies because:

• Single point removes most of the metal in a perfect helix,
• If the rod is bendy, single point can take several light cuts, and then
• A die is used to finish off, doing hardly any work following a straight pre-cut helix, only shaping the root and tip and paring the flanks to size.

Martin's technique sounded good to me, and I suspect either or both of:

• A poor start with the die not at the required 90°. I find it difficult to start threads accurately by hand and use a mechanical aid whenever possible. Lathe and tailstock, or mill and vice, pillar drill and vice, or home-made tap and die stand.
• The die is in poor shape. They work best when new and are fairly easily blunted by swarf mincing, over-sized rod, work-hardening metal, or the cut wandering off-axis. I've done OK with mid-range taps and dies, but the 'too cheap' variety are unreliable – a mix of OK for light use and too poorly ground or heat-treated to be worth having.

Dave

[Nick WheelerParticipant @nickwheeler]

Posted by larry phelan 1 on 13/11/2022 10:03:44:

As has been pointed out, single point thread cutting is simple enough and cost effective. One tool will cut many different threads, and cut them well, no bumps or bends.

For something like that, the last thing I would use would be a stocks and die, too much messing around.

Agreed; two common, short M10 threads are hardly worth setting up to screw cut. I still can't understand why anyone would start by screwcutting and finish with a die, instead of just using the die for the whole job.

And I wonder why Bernard requires such precision from something as basic as a toolpost stud?

[Ady1Participant @ady1]

It's wonky because trying to start it with a die is rarely successful for straightness, plus you have to use a lot of force to cut 100% of the thread

So above 6mm it tends to be a lot easier if you get the lathe to do 80% of the work and the die does the last 20% kinda thing

The bigger the job, 8 10 12mm, the less successful a die will be at the start

If you want to do a straight start with a die then put the job into the lathe chuck and hold the die with the tailstock barrel when you wind it on

[Martin Shaw 1Participant @martinshaw1]

A lot of very useful info here and proof if it were needed that agreement between engineers about the way forward is often not forthcoming. There has also been raised some questions regarding my options which I will try and answer.

I was an electrical engineer before retirement so mechanical skills are being acquired by practice on its own, which is not to my mind entirely the best way of so doing. The workshop is primarily for enjoyment and to keep my mind active, activities that become overburdened with stress tend to get left by the wayside, however otherwise beneficial they may be. I rarely have any need for threads bigger than M6/0BA and mostly tapping at that so I have never identified a need for learning the skill of thread turning on the lathe, dies in stocks usually works and will so going forward now I know a bit more about things.

The M10 die and 1" stock cost me £14.58, quite enough for two threads. I have no doubt that a suitable holder and insert may well cost a similar amount however such an approach requires me to give up a significant quantity of spare time to learning the skill to an adequate level. It may be useful but I have loads of other activities which I consider more important. I am never going to know everything about mechanical engineering, or indeed a lot of other things but you have all kindly given me enough info to hopefully enable me to sort this problem.

I'll give it all a try on some suitable scrap and let you know.

Kind regards

Martin

[Tony Pratt 1Participant @tonypratt1]

Posted by Nicholas Wheeler 1 on 13/11/2022 10:13:22:

Posted by larry phelan 1 on 13/11/2022 10:03:44:

As has been pointed out, single point thread cutting is simple enough and cost effective. One tool will cut many different threads, and cut them well, no bumps or bends.

For something like that, the last thing I would use would be a stocks and die, too much messing around.

Agreed; two common, short M10 threads are hardly worth setting up to screw cut. I still can't understand why anyone would start by screwcutting and finish with a die, instead of just using the die for the whole job.

And I wonder why Bernard requires such precision from something as basic as a toolpost stud?

Totally disagree, a tool post stud is a vital part of the work cutting and tool registration setup, full marks to Martin for realising making crap is not the way to go. As regards using an M10 die straight off, it seems easy but not likely to give the best of results.

Tony

[Martin Shaw 1Participant @martinshaw1]

Let me be totally honest, if the threads had been straight in the first place, the matter would not have reached the forum. The turned section is a nice touching fit in the toolpost, the threads just hold it down. Obviously I want to do the best job I can hence my request for help to which the response has been most generous.

Martin

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