Help Wanted – Rack operated tailstock specifications.

[Chuck TaperParticipant @chucktaper]

Acquired the following recently:

Unfortunately some teeth on the gear (pinion?) portion are damaged which cause the rack on the barrel to foul – preventing any useful operation.

Can anyone direct me to information re the specifications for the gear teeth?

Thanks in advance for any help.

Regards

Frank C.

Edited By Chuck Taper on 29/06/2023 12:55:44

[Chuck TaperParticipant @chucktaper]

[Michael GilliganParticipant @michaelgilligan61133]

Sorry, not at the moment, Frank … but I can tell you that it is the Cowell design

[ not to be confused with Cowells, who make the little lathe ]

MichaelG.

[Phil PParticipant @philp]

I have the same capstan attachment and have always assumed it was Cowells lathes that made them.

Every day is a school day.

Phil

[DC31kParticipant @dc31k]

The handwheel is graduated. What is the total travel for one revolution? Divide that number by the number of teeth on the gear and it tells you the circular pitch of the gear.

There is a relatioship between circular pitch and diametral pitch.

Measure the OD of the gear. There is a relationship between diametral pitch, number of teeth and OD.

The two methods should arrive at the same answer.

'Gears and gear cutting' by Ivan Law is the de facto model engineering reference work.

Edited By DC31k on 29/06/2023 15:54:35

[Ian PParticipant @ianp]

My goto solution when identifying gears is the HPC catalogue

Link to their catalogue request page

Ian P

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Phil P on 29/06/2023 15:23:41:

I have the same capstan attachment and have always assumed it was Cowells lathes that made them.

Every day is a school day.

.

’ere we go : **LINK** http://www.lathes.co.uk/cowellkitlathe/

I bought my tailstock unit from Mr Mole [N Mole &Co. of Watford] long, long ago … and he made a point of mentioning that it was a local firm that made them.

MichaelG.

.

Ref. __ The tailstock featured a quick-action, rack-driven 2-inch travel spindle, this assembly also being offered an accessory (with a 4-spoke capstan) for a range of small English lathes.

Edited By Michael Gilligan on 29/06/2023 16:15:07

[Chuck TaperParticipant @chucktaper]

MichaelG

Cowell was what I was missing – I knew the item had a designation but there was an void in my brain.

I'm hoping to work out the specification of the gear and possibly find something that can be engineered to replace the damaged piece. Hopefully measurement and first principles should allow me to work these out. The Internet has not (as of this note) yielded anything detailed in the way of exactly what I need without making any effort……etc.*

Although following your Cowell suggestion, this turned up Link – [ https://www.model-engineer.co.uk/wp-content/uploads/sites/4/documents/myfordracktailstock.pdf ] – which is at the very least a read.

Regards.

Frank C.

* DC31k that what I'm talking about. Thanks a mill.

 

Edited By Chuck Taper on 29/06/2023 16:38:16

[david bennett 8Participant @davidbennett8]

Here's a long shot request – does anyone have a Cowell-kit-lathe headstock casting that they might sell? (I have all the other castings.)

dave8

[Nigel Graham 2Participant @nigelgraham2]

Identify the pinion DP etc and it may be possible to find a stock one you can modify to fit the existing or a new spindle, accepting that the existing component is a single piece of steel..

[Kiwi BlokeParticipant @kiwibloke62605]

Sorry, late to the party – had to do some serious excavating to find this. The following dimensional info is taken from an old Cowell blueprint, with my notes, in addition. Imperial dimensions.

The pinion, its shaft and the capstan boss are all machined from one piece of mild steel. Pinion OD 1.034 – 1.031". 24T X 1/8" CP. Wire = 0.083" dia. Measurement over wires = 1.099 – 1.097"

Hope this is enough info. PM me if you'd like a pic of the plan, but beware, the quality is rather poor.

Edited By Kiwi Bloke on 01/07/2023 07:08:22

[Michael GilliganParticipant @michaelgilligan61133]

You are a Star, Kiwi Bloke

an actual specification, to identify the pitch circle !

MichaelG.

[DC31kParticipant @dc31k]

Posted by Michael Gilligan on 01/07/2023 07:12:15:

an actual specification, to identify the pitch circle !

The over-pins measurement is usually used as an inspection method, but it is useful here to identify the pressure angle, which we do not yet have.

Two gears of the same circular pitch and tooth count but differing pressure angles will have the same OD and same pitch circle diameter but will not have the same over-pins measurement.

I cannot find a really handy reference just now, but this one should be enough to start:

Determining tooth thickness of various gear types – Part III

There are also a few online calculators where you enter the information you know and it gives you that which you do not. Searching on 'pressure angle from over pins measurement' might find something suitable.

—

Since the question was originaIly posted, it turns out that the answer was here already in this old thread:

https://www.model-engineer.co.uk/forums/postings.asp?th=98925&p=2

a post by CotswaldsPhil 20/08/2014 17:40:18 confirms the specification as 1/8" circular pitch and 24 teeth.

[JasonBModerator @jasonb]

Bit of CAD to draw the gear and a couple of circles representing the wires and you should be able to confirm the pressure angle.

[Michael GilliganParticipant @michaelgilligan61133]

Excellent point, Jason

That said: my ‘educated guess’ from just eyeballing the photo would be 14.5°

MichaelG.

[Chuck TaperParticipant @chucktaper]

Thanks to all who responded.

The (collective) measurements supplied correspond closely to measurements taken from the item in my possession. So I now have the parameters of the gear and I can take a first pass at some sort of solution.

I intent to remove the damaged gear teeth by turning and to then fit a bored out gear onto the shaft. The options being interference fit, locktite etc

I have ordered an inexpensive gear on eBay with a view to machining it as a test case [ that being to ] bore out the centre hole to an interference fit. This can then be slid onto the turned shaft. If the gods are with me I can then put more effort into finding an item with exact specs.

I have used the calculator at Link 1 – [ http://hessmer.org/gears/InvoluteSpurGearBuilder.html ] to generate a drawing. The only reservation I have is the centre hole – it is rather large in comparison to the overall diameter of the gear.

So I have a new query.

Is this approach rational or is the resultant gear likely to be too weak for the job it will have to do?

Any and all views welcome.

Regards.

Frank C.

Edited By Chuck Taper on 01/07/2023 11:48:43

Edited By Chuck Taper on 01/07/2023 11:48:48

PS – To hell with it let it line break wherever it wants!!!!!!

Edited By Chuck Taper on 01/07/2023 11:49:58

[DC31kParticipant @dc31k]

Posted by Chuck Taper on 01/07/2023 11:48:33:

Any and all views welcome.

If I click on the link to the drawing you have generated, it generates something with 3.195mm circular pitch. To the best of my knowledge, 1/8" is 3.175mm.

If the wall thickness is too small, turn down the shaft 'above' it in the photo and sleeve it back up to fit the casting after the gear is installed.

Not specific to your question, but something at least to investigate before removing any metal:-

The handwheel is graduated. Do the graduations have or require any relationship with the clock position of the teeth of the gear?

It would be maddening to put on the new gear and then find the zero mark does not line up the first time you go to use it.

—

Jason's idea of using a CAD model of the gear to work out the pressure angle is in principle good. No complicated formulas involved.

However, it is not clear how any particular CAD program represents an involute nor if any CAD program has the (snapping) facility to place a circle simultaneously tangent to two involutes. Let us hope the CAD program represents the involute by something that can be offset the radius of the pin.

[Michael GilliganParticipant @michaelgilligan61133]

Of course, the simplest way to check the pressure angle is to make a little gauge [or two, or three, or more] and check the angle of the Vees in the rack.

MichaelG.

[JasonBModerator @jasonb]

20pa looks like it fits the bill in Alibre, generate the gear, draw the wire, tangent to one side of a tooth and then tangent to another, mirror and measure.

14.5pa pushes the wires a bit further apart

[Anonymous]

Given that the CP is rational then the DP has to be irrational. It is unlikely that an off the shelf gear will be found, so it will have to be made.

CP = 0.125 DP = Pi/CP hence DP = 25.1327 or thereabouts, which fits nicely the OD specified on the drawing

It is not clear if the boss below the gear allows a cutter at full depth to pass over it. I wonder if the teeth were formed by shaping?

Andrew

[Nigel Graham 2Participant @nigelgraham2]

Your question of cutter clearance might yield to more drawing.

There is a fairly wide run-out groove but as the rack teeth seem cut into the full cylinder, not a flat, the fall towards the rack teeth ends might clear any pinion gullets ending shallow anyway.

[JasonBModerator @jasonb]

Posted by Nigel Graham 2 on 01/07/2023 19:46:37:

There is a fairly wide run-out groove but as the rack teeth seem cut into the full cylinder, not a flat, the fall towards the rack teeth ends might clear any pinion gullets ending shallow anyway.

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?

[Chuck TaperParticipant @chucktaper]

Posted by JasonB on 01/07/2023 20:22:03:

Posted by Nigel Graham 2 on 01/07/2023 19:46:37:

There is a fairly wide run-out groove but as the rack teeth seem cut into the full cylinder, not a flat, the fall towards the rack teeth ends might clear any pinion gullets ending shallow anyway.

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).

[Kiwi BlokeParticipant @kiwibloke62605]

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…

[JasonBModerator @jasonb]

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

[Author]

Posts