Help Wanted - Rack operated tailstock specifications.

You still ned the crests of the gear to be fully cut, if the cutter can't pass right through the crests will end up too thick and not fit into the shallow but narrow ends of the rack teeth.

Frank, what was the spec of the gear you ordered?

The gear that I ordered is not is not a close match

Measurements:

Mod 1
25.95mm outer diameter (max) and about 11mm thick.

Not really meant as a replacement but only to ascertain if its feasible bore and fit as described in earlier post. If so then I can put more effort into a better approximation.

In an ideal world I would manufacture a replacement unit but in this world I'm not that skilled (etc).

A few loose ends tied...

I had hoped that the drawings would provide complete information for the pinion's manufacture, but that's all there is. As has been said, it looks like the teeth would have been shaped or planed, not cut with a circular cutter. It seems probable that the drawings were for Cowell's own use, and it was not intended that customers machine the pinion. I don't know whether any machining was required by the customer.

The drawing for the rack also omits the pressure angle, but does give an over-wire dimension. The shaft dia., into which the rack teeth are cut is 0.875" - 0.001 -0.002 (not sure I understand that tolerance, but that's what's written...); wire dia. = 0.083"; Overall dia. = 0.914" +/- 0.001". The rack teeth are indeed cut into the cylindrical surface, not a flat. I suppose this requires one less machining operation, but it seems a rather mean compromise. There was no provision for the pinion-rack backlash to be adjustable, the pinion shaft running directly in the main body casting, and there's not enough meat in the casting for an eccentric bush.

The dial was resettable, so requiring a known relationship to the pinion teeth was mercifully avoided. 24 pinion teeth, 40 rack teeth, so >1 rotation of the pinion possible. 48 divisions on the dial, so graduated in 1/16". Mental agility required...

For those not feeling agile

Rack movement should be 3" per rev.

CP of 1/8" x 24 teeth = 24/8 or 3"

3" / 48 div = 1/16" as Kiwi says

As to strength of a bored out gear it is quite wide which will help a lot. I suppose it really comes down to how big a drill bit you are feeding and how hard you are pulling on the spider.

Bearing surface looks quite large so you may get away with a round or single point cutter running into the solid metal provided all burrs are removed. Single point cutter could also be made to swing a smaller dia than a typical gear cutter

Here's an opportunity to start acquiring the skill.

I take the tolerance on the shaft to mean the shaft should be 0.873" to 0.874". It's a rather old school way of specifying a tolerance, and tends to be frowned upon these days. Ideally a tolerance is specified as zero to a smaller/larger number, or as a plus/minus tolerance around a nominal value.

For the application I don't think backlash adjustment is needed. Presumably it is for a tailstock or capstan so the important movement is always in one direction.

Andrew

Andrew. Backlash might encourage drill bit 'grabbing' in awkward materials, but I don't suppose many machine quill feeds have any such adjustment, so it can't be much of a problem in practice.

Thanks for explaining the tolerance spec. style; certainly odd...

Jason. The mental agility I meant was that required to read the dial, swapping between sixteenths and decimal inches (or mm!).

Edited By Kiwi Bloke on 02/07/2023 11:25:51

Agreed, Andrew

0.874 +0.000/-0.001 would be my preferred way of expressing it

… or in plain English

“seven eighths … less one or two thou, so that it slides nicely in the hole”

MichaelG.

Just a reminiscence -

Many years ago, before I had a computer and when ebay wasn't invented, I acquired a very sad looking cast iron Unimat sl. One of the many things missing was the lever pinion. I could see what was required by looking down the hole. As i was unlikely to find one, I decided to make it. The only dimension I had was the hole diameter. I knew nothing about gear forms, and the only likely cutters I had were for clock wheels. I picked the most likely cutter and a piece of bronze and set out to make what you see in the pics. (with a real one for comparison) It is still doing the job perfectly well to this day.

dave8

Edited By david bennett 8 on 02/07/2023 19:13:44

Edited By david bennett 8 on 02/07/2023 19:15:02

Which begs the question...why not use the existing pinion?

The picture in the very first post of this thread looks to have an eminently reclaimable pinion and shaft, just needs some gentle work with file.

Its for feeding drill etc, not a precision measuring device

Ian P

A good point, and I returned to the photos for a closer look.

The question seems to say the damage causes the rack to foul the barrel but I'd am inclined to think, no it's more likely the damaged teeth jamming in the rack.

Since the teeth are all still there, just a bit chewed up perhaps with raised patches on them (somehow!) simply dressing their profiles with a file may well be all that's needed.

Certainly try that first.