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Martin Shaw 112/11/2022 21:52:54
185 forum posts
59 photos

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.img_1709.jpg

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

Bill Phinn12/11/2022 22:08:51
1076 forum posts
129 photos
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 112/11/2022 22:34:11
185 forum posts
59 photos

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 Foster12/11/2022 22:51:27
3630 forum posts
128 photos

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 towers12/11/2022 22:53:24
1221 forum posts
161 photos

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.

ega12/11/2022 22:55:02
2805 forum posts
219 photos

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

Nicholas Farr12/11/2022 23:23:28
avatar
3988 forum posts
1799 photos

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 112/11/2022 23:57:59
185 forum posts
59 photos

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

Hopper13/11/2022 02:44:14
avatar
7881 forum posts
397 photos

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 Charleston13/11/2022 04:55:47
157 forum posts
26 photos

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

JasonB13/11/2022 07:05:37
avatar
25215 forum posts
3105 photos
1 articles

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 yet13/11/2022 07:46:10
7517 forum posts
20 photos

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.

DC31k13/11/2022 07:47:17
1186 forum posts
11 photos

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 Ray13/11/2022 08:16:32
238 forum posts
47 photos

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

Martin Connelly13/11/2022 08:26:51
avatar
2549 forum posts
235 photos

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 Hurley13/11/2022 09:42:42
530 forum posts
89 photos

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

Ady113/11/2022 10:00:40
avatar
6137 forum posts
893 photos

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

larry phelan 113/11/2022 10:03:44
1346 forum posts
15 photos

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.

SillyOldDuffer13/11/2022 10:11:09
10668 forum posts
2415 photos
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 Wheeler13/11/2022 10:13:22
1227 forum posts
101 photos
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?

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