Use of small dies

[Gavin RobertsonParticipant @gavinrobertson70663]

Hello, I am fairly new to model engineering and am tackling a PM Research model lathe (my third model from PM Research as I am proceeding carefully re difficulty). Although I purchased a set of UNF/UNC taps and dies from Chronos because PM Research drawings are US imperial I have been having real trouble with the 0-80 UNF die in particular. I suspect for the price the set may not be very good quality. The issue is that the die is very difficult to engage and when it does it produces a useless thread as it is too small diameter – it is a solid die. Having consulted the book specs I then tried an 11 BA split die from another supplier and still got a rubbish thread but then re-applying the UNF die it did then engage and produced a thread though rather poor. There is some distortion but I put this down to my ham-handiness. My question is: If I can obtain another die should I go for a split or a solid die? It seems that Tracy Tools can supply either and the prices suggest could be far better quality than my original set. I am working with cs bar stock but experiments with other materials were equal failures.

[Gavin RobertsonParticipant @gavinrobertson70663]

[SamsarandaParticipant @samsaranda]

Possibly the grade of steel that you are trying to thread, do you know it’s specification? My choice would be a split die as solid dies are usually for tidying up or finishing a thread. Dave W

[Michael GilliganParticipant @michaelgilligan61133]

Everything is critical at that scale

Material, dimensions, quality of the die, lubrication, alignment … you name it, it’s significant

Watchmakers perform miracles: but the care that they take, and the price of top quality dies, is awesome.

Keep practising, and keep notes of what you are doing [right and wrong]

MichaelG.

.

This table might be useful:

https://www.engineersedge.com/screw_threads_chart.htm

and if you want the eye-watering price for a set of Bergeon taps & dies:

https://www.cousinsuk.com/sku/details/tap-die-sets/t37726

Edited By Michael Gilligan on 09/09/2022 19:06:48

[SillyOldDufferModerator @sillyoldduffer]

I suspect the metal – what's "cs bar stock" – and the diameter. If oversized and/or a tough alloy it will put up a fight.

Are you able to measure the diameter to confirm it's not too big for the die? (Unless strength is critical I always reduce the diameter to somewhat less than maximum because it greatly reduces the stress on the die. I always chamfer the end to give the die an easy start.)

Technique matters too. It's important the die be kept centred and at a right angle to the job throughout the cut. Quite difficult to keep straight, especially if the die isn't a good fit to the holder. (In my cheap sets the taps and dies are mostly OK, but the holders are wonky. For that reason I avoid threading by hand, instead holding the die in my lathe's tailstock. Always lubricate and clear swarf frequently.

Cheaper dies are brittle and easily damaged if they encounter anything difficult. Forcing them on to a slightly oversized tough metal at an angle and not clearing swarf every 3/4 turn or so will soon ruin them. Once a die is blunt, bin it!

None of my dies are split. In yesteryear, solid dies were only used to clean up existing threads and weren't much cop at cutting new ones. Split dies allow the operator to control the depth of cut, which is often useful in adverse circumstances. Modern solid dies seem to be made to cut precision threads; they expect the stock to suit the die and don't allow the operator to mess with dimensions. Not difficult to split a solid die with a Dremel, but I've only had to do it once – I slim down the stock instead, which takes the strain off the die.

Dave

[Michael GilliganParticipant @michaelgilligan61133]

Posted by SillyOldDuffer on 09/09/2022 19:19:58:

[…]

Are you able to measure the diameter to confirm it's not too big for the die? (Unless strength is critical I always reduce the diameter to somewhat less than maximum because it greatly reduces the stress on the die. I always chamfer the end to give the die an easy start.

[…]

.

Wise words, Dave … especially when you look at the tolerances in that table I linked

MichaelG.

[duncan webster 1Participant @duncanwebster1]

It is a common misconception that reducing the OD of a male thread or increasing the ID of a female thread seriously weakens the assembly. It doesn't (unless you go mad). There isn't much scope on such a small thread, but oversize will be fatal.

Edited By duncan webster on 09/09/2022 19:40:34

[larry phelan 1Participant @larryphelan1]

It might be a good idea to see about fitting your die holder with a guide bush, to help it stay true on the job.

Think about the holders used for pipe threading.

I made such a holder a few years back and found it so good that I never bother with my "proper" stocks now.

This one takes four different size bushes 6,8 10 12 mm, but could take whatever you want.

Also, I never use solid dies, only split ones.

Just my pennyworth !

[old martParticipant @oldmart]

A split die can be opened up so a partial thread can be produced at the first pass which can have an advantage. The diestock bore needs to be slightly bigger than the die, or you will not be able to adjust the size. The steel used will vary in threadability enormously, and being to measure the diameter to at least 0.0005", 0.01mm is vital with such small diameters, a screw micrometer may be better to use than calipers. Use cutting oil on steel. Tracey Tools are very good suppliers.

Edited By old mart on 09/09/2022 20:02:41

Edited By old mart on 09/09/2022 20:04:07

[HOWARDTParticipant @howardt]

You may find Joe Pie YouTube videos useful. He has made a few small machines and his techniques are useful.

[ChrisLHParticipant @chrislh]

I had similar problems at the start and thought they would all be solved by use of a tailstocl die holder. Well, it didn't always produce a good thread. This was traced to the dimples on the o.d. of some dies being carelessly placed and consequently presenting the die to the work at an angle. My current method is to use the hand die holder supported by a rod held in the lathe tailstock to keep the back face of the die square. The rod should have enough clearance in the die holder to allow the die to self centre on the work.

Of course, this only works if you have a lathe. Sad to say I never had any joy threading by hand and eye myself.

[old martParticipant @oldmart]

Making a small screw I would use larger diameter stock and turn down the shank of the screw first. Then have a metal bush with a hole in it for the larger diameter to fit in against the face of the die to ensure squareness. I would screw it into the die rather than screwing the die onto the work. After the thread was finished, the head diameter and length could be carried out, together with any screwdriver slot. Hexagon flats would be easier to produce before cutting the head from the main stock.

[Paul LousickParticipant @paullousick59116]

I have made some small threads, down to M1.2 and found it difficult to start cutting (by hand and using a tailstock die holder) and found that the best solution was to make the part a bit longer, with a reduced diameter and cut off the excess after the thread was made. The smaller diameter is easily threaded and acts as a guide for cutting the full size thread.

[Gavin RobertsonParticipant @gavinrobertson70663]

Great to get so much useful advice to review and help me decide on a way forward even if a bit more trial needed on my part. Now have quite a lot of advice to work with. Just to respond to some of the questions that have been raised:-

The original CS was that supplied in the kit, the part to be threaded was turned down per drawing and my tables confirmed that PM drawing had specified the appropriate diameter. Of course, what with the scrap I ran out of bar so had to purchase material from another source in order to try again. So valid point for me to consider. When experimenting with the BA split die I reduced the core o.d. per tables and started with the die spread. At the time I had not sourced a UNF split die, hence the experiment. Certainly, these extremely small UNF/UNC dies appear harder to find in UK but I have read good reports about my proposed source so may be worth me following up if I elect to give up persisting with my solid die. Indeed I may now have worn it out or it is just a rogue as I have not had this problem with other sizes in set. So as above lots of useful comment for me to follow up.

[Gavin RobertsonParticipant @gavinrobertson70663]

Postscript: Today I achieved success following some of the advice. Slightly reduced diameter, chamfer on slightly overlong end, used lathe tailstock chuck to push lightly against the die to keep square and get a start. Then progress made by alternately turning headstock chuck and moving tailstock chuck forward to catch up. The start diameter and extra length made all the difference over past attempts.

[Michael GilliganParticipant @michaelgilligan61133]

Keep up the good work, Gavin

MichaelG.

[old martParticipant @oldmart]

There is a difference between die qualities, and small ones are much more difficult to use, practice with different types of steel will find the most suitable type. There is no substitute for having plenty of spare metal to practice on before committing to the actual workpiece. If possible, perform the most difficult proceedure early on so that less time and effort is wasted if it fails. Sometimes it can be better to tap a hole to Loctite in a length of good male thread rather than make a workpiece out of a single piece. Of course, with screws, that would only work if the head was large relative to the thread.

[Gavin RobertsonParticipant @gavinrobertson70663]

I have been reading 'Tabletop Machining' by Joe Martin as I use a Sherline lathe. Found the comments about backing off a die and the potential for swarf to wreck a new thread very helpful. At this very small scale I really can see the problem. Today I did a practice run with the advice in mind. Have not actually tested the resulting thread engagement quality yet, though with a magnifier it looks a bit better than my previous rubbish work. This latest was on just a bit of surplus CS rod of which I admit not knowing its origin.

[Howard LewisParticipant @howardlewis46836]

You are doing the right things.

You are using small threads, and if you are cutting threads on the lathe, by hand, such threads are fine pitch. As such they are incapable of dragging a Tailstock along the bed of a lathe..

The 0-80 ANF has a thread depth of only 0.008"

For fine pitch threads, you either need to use a Sliding Holder for Taps or Dies, to drive the Tap or Die Holder by hand when it is supported and aligned by a centre in the Tailsrtock. The Tailstock barrel will need to be advanced and retracted, at almost every turn, as the Tap or Die is moved forwards and backwards, as the thread is cut. Alternatively, a spring centre, such as a "Taptru" will do this

Using a Sliding Tailstock Die, or Tap holder has the advantage of keeping the thread square to the work so that it is not "drunken". (Also, fewer taps will be broken, since they do not take kindly to being bent! ).

It goes without saying that a lubricant, such as Trefolex, or Rocol RTD should be used, and that the ,tool should be withdrawn frequently to remove any swarf.

HTH

Howard

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