Stud bending on threading

I am a complete novice working my way through 'Lathework A Complete Course' by Harold Hall and am presently making the tailstock die holders. I have encountered a problem manufacturing the die holder screws in that, having knurled 10mm mild steel bar (of unknown spec) and turned down a length for threading M5, the studs end up bent after being died down. The turned section is perfectly straight after turning, and I am taking great care not to apply any side pressure to the die holder whilst threading, so am struggling to understand what is causing the bending, and what to do differently to avoid it.

Ensure diameter is size to slightly under, if using a split die use the centre holder screw to open the die up, rotate the die onto the part with a little end pressure and use plenty of lubricant, then rotate the die down the part backing off every couple of full turns to break the swarf. The bending can be caused by forcing the die down the part it one go, this jams the swarf and can twist smaller diameters showing up as a bend along the length. Practice with a larger diameter thread, say M8, I regularly die down to M2 with little trouble.

Thank you Howard, I turned down to 5mm for M5 but die is not split type, so diameter is the likely cause. will try again tomorrow with slightly under size diameter and clear scarf as I go. Steve

Here are two M8 threads I made recently (on the left; originals on the right). All I can say is that die thread imparts a considerable twisting moment - think of how hard you have to turn a die, and consider that force is being applied at a radius of a few millimetres. Fortunately in the case of these t-nuts I don't have to look at them, but I'd be interested to hear any tips on getting them straighter as well!

Glad it's not just me Calum, I have some very similar looking studs!

Make them in two pieces: the larger knurled part, and some M5 studding loctited into them. It's quicker and easier to do them this way, uses less material and makes for a stronger part.

if you are threading in the vice your chances of a straight thread are pretty slim, much better to put the die holder against your work in the lathe and bring the tailstock to bear against the diestock, turn the chuck by hand whilst advancing the t/stock. obviously all this becomes uneccesary when you have completed your die holder!!!

Bin it; dies that are not split are either die nuts intended to clean up threads, not cut them, or they tend to be cheap and nasty. If the latter the die may be poorly ground and will never cut a proper thread. What is the provenance of the die? I don't use dies very often but have never seen the thread bent afterwards. I always use split dies.

Andrew

Dies are from a new tap and die set purchased on line, will invest in some split dies. And yes, currently threading in the vice. Thanks for the suggestion to make in 2 parts Nicholas, hadn't considered that - it certainly would have used less material! Thanks to all who have responded, I have ways forward to progress now. Steve

Normally I do Metric threading at 90% height of the nominal tread height (top to bottom).
For a M5x0.8 mm bolt I turn the stock down to 4.91 mm. For a M5x0.8 nut I turn (drill) the hole to 4.22 mm.
That about 0.09 mm less in diameter makes a huge difference in the threading force needed.

According to the data sheet in my threading tool set, I should use a 4.2 mm drill or 4.9 mm stock!!

Edited By Huub on 26/04/2022 23:10:24

Use your newly made tailstock die holder and substitute standard hardware store screws to make its own screws. Hold off on buying new dies until you try them out in your tailstock holder. They are probably good enough for general work.

And there was me thinking that good quality solid dies were made to cut a thread to a specific tolerance such as 6g. You can't do that with split dies as tolerance varies depending on how much you tighten them.

I doubt many would class Dormer as Cheap and nasty

The unsplit dies I have all happily cut threads on various materials including stainless and all are cut on nominal size material eg M5 thread straight onto 5mm stainless rod. 

But it is possible that you do have a cheap nasty unsplit die rather than the fairly inexpensive (lot less than Dormer) and not nasty ones I have, 

Edited By JasonB on 27/04/2022 07:49:22

Hi Steve, 5mm wouldn't take much effort to bend, especially when doing it by hand in a vice, for a start you need to make sure you start with the die square to the work and not to apply upward or downward pressure on one or the stock handle, to get the die to start, you need to apply equal pressure to both handles at the same time and once it has started for a few turns, you shouldn't need to any downward pressure as such at all. I've got some cheapish non-split dies and they cut an acceptable thread for general purpose items, but I do use split dies for most and more important jobs.

You would be better off using a tailstock holder as Hopper has suggested, myself I very often start the thread with the piece still in the lathe and using my hand stock and die, but with the stock the wrong way round, so to speak, and put pressure directly on the die with a blank arbor in the tailstock and turning the chuck with a good fitting spanner on the chuck jaws, don't use the key hole that tightens the jaws though, as this can lead to fracturing them.

Regards Nick.

Edited By Nicholas Farr on 27/04/2022 08:48:23

The answers have already been covered, but as is often the case, I suspect a combination. Possibilities:

• Difficult material that tends to extrude or work-harden rather than cut.
• Die is of the closed type intended to clean up existing threads rather than cut new from scratch. (All my dies are inexpensive closed and they cut new threads OK, but that doesn't guarantee your set is OK!)
• Blunt die.
• Inadequate lubrication. (Always use cutting fluid)
• Failing to break swarf my reversing a quarter turn every full turn or so.
• Die wrong way round
• Die not started straight - it helps enormously to chamfer the end of the rod
• Die not held straight - this is difficult to do by hand! So difficult I avoid threading unless the tap or die is held straight mechanically: 'great care' doesn't work for me.
• Rod diameter too big.

In addition to the chamfer, it's very helpful to reduce the diameter of the rod slightly below theoretical. For M5, the minor diameter is about 4mm and the thread height 4.9mm, so slimming down the rod to, say, 4.9mm reduces the amount of work the die has to do and creates room for squishy metal to extrude, and for swarf to get out of the way. Don't overdo it because slimming the diameter reduces thread strength, but not as much as might be expected.

Health and Safety note. Cut threads are never used for safety critical fastenings like wheels, brakes, or wings. Cutting introduces several weaknesses such as stress-raising edges. Most problems caused by cutting are avoided by rolling threads and the compressed metal is actually stronger than the original.

For ordinary purposes, home-made threads are fine, even if their strength is deliberately reduced to favour taps and dies.

Dave

Edited By SillyOldDuffer on 27/04/2022 09:46:40

I just knew someone would pop and say they have solid dies which work fine. Since the OP said he was a novice I was keeping it simple. Solid dies are not that common; the only ones I have are total crap, bought as a set online when I didn't know any better. The whole set (taps and dies) is unusable.

For external threads I tend to screwcut or use Coventry dieheads. I don't have the means to check threads to 6g and don't feel like buying go/nogo gauges, so I adjust to suit the mating part. This is what I expect a home made external thread (1/4" BSF) to look like:

The question about provenance has not been answered. If we don't know the quality of the die all other suggested solutions are irrelevant.

Andrew

It's interesting that rolled threads are used at the high volume cheap end of the market, but also in some critical applications. Note critical, not accurate; grinding is mostly used for accurate applications.

Andrew

It doesnt matter if you have the best die in the world. If the job is done in the vice by a beginner wobbling the diestock about the place, a drunken thread will result. But using the lathe tailstock and or a die holder could well result in a straight thread even with a cheap die. It's worth a try .

I was given a tap and die set ! It didn't take me long to find out WHY I was given them ! One die the thread was so far out it made something that a waiter could have used as a corkscrew. Drink the wine, the thread may look straighter after a couple of glasses ! Most points have been covered by other members ! Good luck, Noel.

I have actually made the 13/16 holder (see pic) albeit with slightly bent screws. Based on the considerable advice above I will make the die holder support next and then use the die holder to thread new adjuster screws for this holder and then the larger (1 inch) holder. I will also reduce the turned diameter from 5mm to 90% as suggested. In terms of prominence, I purchased a tap and die set from a prominent dealer and have been very happy with the way the taps cut, the dies appear to be good quality so I think operator error rather than die quality is the issue. Many thanks to all who have taken the time to respond and help me on my way with such great advice, this forum really is a fantastic resource for a novice. Steve

I will also reduce the turned diameter from 5mm to 90% as suggested.

I think you either did not mean that - or you need to read that post again.