For a praactical approach assuming both x and y screws are the same  put a dial indicator up against the table and move the handle 10 turns and take a reading.

13tpi would be very unusual for either thread form on a mill.

A good idea of the pitch could be had from the handwheel dials but don’t trust them 100%.

It is more likely to be 12.7tpi which is 2mm pitch and would be what you find on a metric machine with 2mm per turn of the handwheel.

Metric pitch would point towards Trap form but leadscrews are seldom the standard pitch for a given diameter so may need screwcutting also probably left hand. Ditto ACME, you won’t be able to buy 2mm pitch ACME taps.

Would the “mould delrin round a heated screw” technique work in this application?

Presumably if there is room to get a carrier for the delrin thread section in it should be possible. Delrin ought to be more than up to handling the feed forces involved and a moulded thread will intrinsically have little or even zero backlash.

Even if it doesn’t last as long as a carefully made bronze thread its easy enough to make another. Heck I’d probably make two while I was at it.

Screw cutting acme or trapezoidal threads to accurate size and minimal backlash is a miserable job. Best to have some practice on a more friendly material than bronze before going for the money shot. The smaller the thread the harder the job.

Watching a well practiced expert do such a job from scratch in about 15 minutes did not fill me with confidence as to my ability to manage a good job on my ownsome.

Clive

moonman’s measurements are consistent with a TR14x2 thread, except he hasn’t confirmed the angle.  (30° vs 29° if ACME)

As an Imperial ACME thread of 13tpi is extremely unlikely, I’d risk buying the tap.

The desire to DIY and save money don’t necessarily go together!  In this case, I think the quickest and cheapest way to fix the mill, a current product, is to buy a spare part and just fit it.  No measuring or difficult metalwork.

But maybe moonman relishes the challenge and sees this as a learning opportunity.  It is:  identifying the thread correctly is tricky, Bronze is awkward stuff, and trapezoidal threads are demanding.   Therefore a high-risk of failure whilst developing the necessary skills.  Maybe several attempts to get a good one.  Could get expensive!  Think twice if saving money is the goal.  If building skills is what’s wanted, then go for it!

Clive suggests moulding Delrin around the lead-screw, and I’ve read several reports of success.  Never say how long Delrin lasts though! On the web folk are more likely to tell of marvellous bodges than to confess they fell apart after a few months!  But I’d expect Delrin to be ‘good-enough’, just not as long-lived as Bronze.   Be my first choice if I couldn’t source a reasonably priced spare.

Dave

moonman,

Might your Chester 16V Mill be a copy of some other generic Far Eastern model?  If so, have you tried other suppliers of identical/similar machines?  Depending on the cost of said spare, it may be worth a punt if all else fails.

I think the imperial dials are marked 80thou per turn rather than 79 similar to what is found on some minilathes

I suppose that depends on if the OP has the equipment to modify the mill which often needs a second mill to carve out space for the larger ball nut.

It is possible to buy stock ACME and Trapezoidal nuts from, e.g. HPC Gears. They are sold simply as cylinders with threads through them, so you’d need consider if and how they’d fit (with due modification or carrier) the individual machine.

Just checked the Z axis leadscrew and it is 5/8″ diamiter and other axis are smaller

David

[ for pedants everywhere ]

’exactly’ within the acceptably small rounding-error

MichaelG.