A few Thou Under?

I have had to make a number of new parts for motor bikes, bicycles, and other items, which require me to turn down a diameter to die cut a thread. Not knowing any better I usually start with the overall size of the thread, and continue reducing the diameter until the die will start to cut, very hit and miss, and time consuming. Being a relative novice I have bought a number of books that hopefully will keep me from making too many mistakes. I have read most of the books at least twice hoping to find the definitive formula that will tell me what the exact diameter should be for any given thread.

Tubal Cain in his Model Engineers Handbook on page 4.11 touches briefly on the subject by stating 'a few thou under' Stan Bray in his otherwise excellent book, 'Basic lathework', ignores the problem completely and goes strait into screw cutting via the lead screw. So, is it all guess work, or is there a formula? or do I now assume that the same 'few thou under' is OK for a 3/8 Whit thread and a 6 BA.

I can live with my mistakes, but I am frustrated by the time that is wasted, so any advise will be very much appreciated.

Thanks

What dies are you using - split or solid?

When you die cut a thread the metal can deform plastically and so some will push out to fill the in the peaks of the thread if you start with a slightly undersize diameter. There is no definitive value of how much under as all metals, even with the same designation, vary due to tolerance in composition, heat treatment and storage. Some also work harden as well just to add another issue. It is sometimes easier to start a thread is the end of the bar is tapered slightly over about 5 pitches of the thread.

Martin C

For many years I have always read the tables and then gone straight to dummy run to test!...and never been afraid to change the dimension according to the need and function. Also, the tables such as Zeus don't (usually) differentiate between materials let alone the function for the thread. For clock-making I always work for nice fit, nexgt for good appearance and finally for function unless there are good reasons to change this scheme.......

...........As the years have passed I have annotated my various tables with my own observations and remarks according to use etc. As a basic rule; I ALWAYS test any important thread especially one I have not cut for a while or in new materials. The time spent in testing is NEVER wasted in my experience.

Always use nominal diameter, as Tubal Cain, may be a little under. If you thread bright drawn bar, ie 10mm with M10 die the thread will work. If you measure threaded bar, it will be under size on the od.

Threaded bar can be shockingly undersize, .010" on 3/8"x16 tpi and still fits right.

Malcolm,

Read a previous one under "General Questions" and "Threading and the Tables", see 'Hopper's posting on 31/3/2020@23:53:33. I think this about as good advice as any. Note that all threads should ride on the flanks and not interfere with root/crest. Soft metals will be squashed/deformed rather than cutting cleanly and some of the 'spare' metal could choke up the root/crest void, making things a bit tight, hence the 'extra' 5 thou clearance As an example,I have just turned down1/4" brass rod (all I had suitable) to 3/16" and put a 2BA die down it. Too tight, boy did it squeal! General rule of thumb, if metal makes a loud noise, something not right - investigate and correct, which of course I didn't!

Note, .6403 x Pitch applies to BSW/BSF/BRASS26T/ME32/40/60TPI, but I think ME60T is no longer in use. Metric, BA, Unified have thread depths and flank angles to different to that of the Whitworth 'group'. 

HTH,

John

Edited By DMB on 02/04/2020 22:13:35

I normally use nominal size; but I'm not a fan of split dies and avoid using them where possible. Obtaining a rattle free fit is a bit of a crap shoot. I prefer to screwcut or use Coventry die heads, where it is easy to finely adjust the finished thread diameter.

Andrew

I use a table from Maryland Metrics that has the maximum and minimum size of the threads. (extract shown below)

Paul.

Downloaded from **LINK**

The outside diameter is probably not your problem here. A die will usually cut bar that is the full nominal size, or even a bit over. It will then cut a full form thread as per the teeth on the die.

To help get your die started, try putting a large 45-degree chamfer on the end of the workpiece to help get the die centred and started. And make sure you use the end of the die with the tapered cutting teeth to start. The other end can be used to bring a full-depth thread up hard to a shoulder but if you try starting a cut with it, things will be tough.

And make sure die is held square to the job, guiding with the end of the tailstock barrel if in the lathe or by careful checking by eye if in the bench vice. Line the die stock handles up with a horizontal surface like a line in the brickwork on the wall behind etc. Draw a horizontal line on the wall if you have to and line up to that.

However, reducing the OD can reduce the amount of force needed to cut a thread with a die so is sometimes worth doing for that reason. Rule of thumb? Something like about 5 thou undersize for threads between 1/4" and 1/2" diameter maybe. Bit more for larger threads, say up to maybe 10 thou on a 1" diameter. And down to a couple of thou on 1/8" diameter. All in proportion.

Also, on larger threads, say like threading a piece of 1" pipe or the like, a small lateral hacksaw cut on the chamfer on the end of the job provides a notch for one of the teeth on the die to start cutting into and away you go. Old pipe fitter's trick and it works well, especially on 2" pipe etc that can be tough to get started.

Hopper

Some good comments and guidance in your post.

One thing that I have used but nobody has mentioned, is to make the part that you are threading too long and then reduce the end of the piece to the root diameter of the thread for the thickness of the die being used. Chamfer the step from the root diameter going back up to the tapping size. This reduced portion will provide the die with alignment while starting to cut the thread and can be cut off after the threading operation is finished. Care still needs to be taken to ensure that the die is square to the workpiece as the thickness of a die does not guarantee perfect alignment using this method.

Martin

The other answers, and the books mentioned, all concentrate on threads from a home-workshop perspective, which is exactly appropriate to me, where I usually reduce diameters slightly before cutting threads to make the job easier. The advice may not be appropriate to Malcolm. It's right for non-critical threads, which most are. They only have to be 'good enough' for less demanding situations and are made relatively loose of relatively inexpensive metal. They're not as strong or vibration resistant as close-fitting threads made of a strong alloy.

Some bike threads are critical. Strength and resistance to heat or corrosion may all be required. Not good to have steering or brakes fail because of a weak thread failure, or for the engine of a performance bike to come off at speed!

The quality of thread needed can often be guessed from context: number plate fasteners are bog standard ordinary, whilst pedal bolts are special. Bolt heads are often marked to indicate their class, but I'm not sure if this applies to old bikes, especially pre-WW2. In the modern system no marking on a bolt head means ordinary. On US steel bolts the number of radial lines increases with increasing specification. Metric steel bolts are numbered 8.8, 10.9 and 12.9 as they go up. Otherwise, the maker's original specification or plans may be needed to identify thread quality.

How much this matters depends entirely on the purpose of the repair. Simply made threads on a classic motorbike restored for undemanding display purposes are probably acceptable. The same threads on a superbike taking part in the Isle of Man TT would need to be done properly, whatever the specification calls for.

Lastly, it may be impossible to produce a high-specification thread with a lathe or die. It's because cut threads are weak compared with rolled threads and inaccurate compared with ground threads.

Dave

Usually turn the raw material to the nominal size, and smear plenty of lubrication onto it.

If a deep thread, sometimes take a roughing cut, followed by finishing, with the die adjusted as below.

Almost always, when using split dies, I adjust so that an existing thread feels just "right" in it. Where possible, a commercial bolt; and a good quality commercial nut as a gauge on the thread afterwards, before going for the finished product.

If the die looks to be filling with swarf, back out and clear before restarting.

Howard (fat fingers and afterthoughts)

Edited By Howard Lewis on 03/04/2020 12:31:14