Myford 33t and 34t gears for metric threads

[LambtonParticipant @lambton]

Dave,

Being in pedant mode this morning, I have to insist Eric's opening sentence should be 'No lathe can cut a truly accurate thread'. Never mind if it's imperial or metric.

Of course Dave is correct but we are dealing here with the practicalities of what can be achieved with an existing and probably a bit worn lathe.

The point about a 127 tooth gear is that 127 is prime number and cannot be factorised into anything other than 1 and 127. Dave is again correct that many combinations of other gears can be combined in a train to produce slightly inaccurate metric threads that may be acceptable for a particular application.

I also agree with Dave about over preciousness about striving for total accuracy where it serves no purpose but accept that we should make components as accurately as possible with the equipment at our disposal.

I enjoyed reading Dave's post and accept his is being pedantic in order to get some fundamental truths across.

Thank you Dave and best regards

Eric

[egaParticipant @ega]

Posted by Roderick Jenkins on 28/01/2019 09:51:28:

Posted by Lambton on 28/01/2019 09:32

I cannot understand why anyone would find the need to screw cut BA threads as a good set of taps and dies together with a tailstock die holder will do a very good and accurate job. In almost 60 years of model (and full size ) engineering I have never known anyone screw cut a BA thread.

Eric

I agree. I use dies for anything smaller than M6/0BA but die cut threads can be wobbly so the information is out there for anybody needing it.

Rod

I wouldn't argue with this but wonder why the standard screw thread gauge includes the BA form.

Small dies are expensive and if it were practicable screwcutting the corresponding threads would be attractive for the occasional job.

[Phil PParticipant @philp]

I hate split dies, and have now gone over to using a die head for the smaller sizes that I need.

I do use the 33/34 changewheel trick when screwcutting metrics though. And having a Meek screwcutting clutch is the icing on the cake.

Phil

[clivelParticipant @clivel]

I have also been on the lookout for the 33 and 34 tooth gears for a Myford. The 33t is no problem regularly being available from RDG but after searching eBay for awhile I have concurred that the 34t are scarcer than hen's teeth.

One alternative would be to have the gears 3d printed similar to these Change gears for Myford ML7 & Super 7 lathe. But not having the wherewithal to draw them up myself nor access to a 3d printer I would happily purchase a pair if some resourceful 3d printer owner would like to recoup some of the cost of the printer by offering the gears for sale.

Clive

[Brian OldfordParticipant @brianoldford70365]

Posted by peak4 on 27/01/2019 10:59:37:

Can't see any on ebay at the moment.
Try HPC Gears, who offer 34t 20dp in metal and Delrin.
OK would need modifying to fit, but probably easier than making from scratch.

Bill

Wrong pressure angle.

[peak4Participant @peak4]

Here's a post to offend the purists, and I should say I'm most certainly not a formally trained engineer; well I am, but networks and telecoms.

To expand on my earlier suggestion from HPC gears, my first post was a bit of a rush as I got summoned for evening meal, and never came back to expand. I see the PA has now been mentioned twice three times above; once more whilst I was typing this.

I also bought a pair of 33/34 tooth gears off John Stevenson, and I've no doubt that he will have supplied me with them cut at the correct 14.5° pressure angle.

Whilst I have a non-gearbox Super7 over in Sheffield, so don't need them for that, my gearbox equipped "Myford" over here in Buxton is actually a Warco 720.
The gears John supplied are slightly different in tooth profile than the ones in the rest of the Warco gear train, which I'm guessing might be 20° PA

No it's not ideal and it's possibly a bit noisier than it should be, but I keep it well lubed when in use and don't cut huge amounts of metric threads. (I'd never really thought about the difference between the Myford and its later copy/counterfeit before I ordered the gears from John; I also needed a replacement genuine Myford stub gear for John's 33/34 tooth drivers to plug onto as, from what I recall, the Warco ones were a metric, rather than imperial bore.)

Clearly I wouldn't do it with a situation where the gears were in constant mesh, or inside a gearbox, but for occasional use, it works fine and will still probably outlive me, as a hobbyist.

Note also that HPC supply Delrin gears as well as steel and brass, so I wonder if they would bed in after a while, conforming to the harder steel/cast iron 14.5° PA gears with which they would mesh.

There is after all a pair of tufnol gears in the train anyway for reversing, so adding a Delrin one shouldn't cause a major issue.

None of the above will cause any difference in the actual thread the lathe cuts.

I wonder what John S would have made of my post, sadly we'll never know.

Bill

 

Edited By peak4 on 28/01/2019 16:38:37

Edited By peak4 on 28/01/2019 16:40:38

[ZanParticipant @zan]

The conversion set is a waste of money when the simple purchase of two gears will do to all intents and purpose the same job – provided you have a gearbox! All to often people quote micron level differences in pitch or diameters which are impossible to produce on a crappy lathe like a Myford…… ps I love mine, but it is not a tool room lathe…..

[Mike DonnerstagParticipant @mikedonnerstag]

Zan: I completely agree, though this assumes one can get hold of a 34T changewheel, as I don't have the facilities for making one myself, at least not yet. I sent a message with the specification to HPC yesterday. Let's see what they can come up with.

Mike

[JasonBModerator @jasonb]

Clivel, if you can't buy, draw or print them what about cutting your own, RDG do the 20DP 14.5pa cutters. Same goes for the rest having problems buying gears.

You could probably use a close size Myford gear that you already have as a template to grind a single point tool to if you don't want to spend out on a cutter.

[JasonBModerator @jasonb]

Boston gear list 33T and 34T in 20dp, 14.5 pa with 5/8 bore and 1/8" keyway, should be able to get them via one of their UK distributors. GMB Assoc is the one I have got Boston stuff from in the past.

[Anonymous]

Posted by JasonB on 29/01/2019 07:42:44:

……………what about cutting your own, RDG do the 20DP 14.5pa cutters. Same goes for the rest having problems buying gears.

Steady on there; that's a bit radical suggesting that people make something rather than waffling about it.

Andrew

[JasonBModerator @jasonb]

That did cross my mind Andrew.

I know we are in the middle of that 25week period where people won't go into their workshops as they are two cold, this is closely followed by the other 25weeks where it's too hot. That only leaves them two one week periods in the year and by the time they have dusted off the quorn castings there is no time to make anything

I am also half expecting a debate about how accurately the 34 cuts can be placed around the gear blank!

J

Edited By JasonB on 29/01/2019 08:23:17

[clivelParticipant @clivel]

Posted by Andrew Johnston on 29/01/2019 08:07:32:

Posted by JasonB on 29/01/2019 07:42:44:

……………what about cutting your own, RDG do the 20DP 14.5pa cutters. Same goes for the rest having problems buying gears.

Steady on there; that's a bit radical suggesting that people make something rather than waffling about it.

Andrew

For those of us a little less talented than yourself spending a few minutes on a forum trying to find an alternative is an order of magnitude more productive than trying to cut the gear oneself especially if one has neither the knowledge, the cutters, or indexing equipment to hand. Also given a limited amount of hobby time one has to prioritise what interests one most, and learning to cut gears is not high on my list of priorities.

As for waffle, why waste time reading it when you can leave it to us lesser mortals to wallow in

Clive

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Lambton on 28/01/2019 09:32:19:

No lathe with an imperial lead screw can cut a truly accurate metric thread unless a 127 tooth gear is used. Despite this fact both the official Myford conversion set and the 33, 34 toothed input gear method are capable of producing metric threads accurate enough for all practical purposes.

.

On this cold wintery morning, perhaps we should remind ourselves that most lathes cannot cut a 'truly accurate' thread of any species.

Jason's hint reminded me to revisit this: **LINK**

https://www.model-engineer.co.uk/forums/postings.asp?th=101456

… We can but dream.

MichaelG

[Mike PooleParticipant @mikepoole82104]

Fit an electronic lead screw and sell the gearbox and change wheels. I may have just talked myself into a job.

Mike

[norm nortonParticipant @normnorton75434]

Posted by Mike Poole on 29/01/2019 09:31:36:

Fit an electronic lead screw and sell the gearbox and change wheels. I may have just talked myself into a job.

Hmm.. the electronic lead screw will have to be exactly synchronised with the main spindle rotation.

[Nigel McBurney 1Participant @nigelmcburney1]

I trained and worked for six years as a scientific instrument maker and I never saw anyone screwcut a BA thread or indeed later in my life of engineering ,thousands of BA threads were cut using split dies and they did not wobble,provided a sliding die holder was used,I can recall ,that in our works some 60 years ago only one job ,a steel 1 BA screw was produced in batches of 2k that used the self opening dieholder. on a Ward 2A capstan, I can never understand why BA was ever thought of though I gather that it was instigated by a commitee and we well know the rubbish committees invent. In the smaller tooth sizes I do not see any problems with running mixed tooth angles together at slow speeds and relative short running time.

[not done it yetParticipant @notdoneityet]

Posted by JasonB on 29/01/2019 07:42:44:

Clivel, if you can't buy, draw or print them what about cutting your own, RDG do the 20DP 14.5pa cutters. Same goes for the rest having problems buying gears.

You could probably use a close size Myford gear that you already have as a template to grind a single point tool to if you don't want to spend out on a cutter.

I agree. The cost of a rotary table and cutter would likely be less than forking out lots of dosh for a

‘pukka” fymord set of unnecessarily precise gears that may not get used very often. Even better if you already have a mill.

[peak4Participant @peak4]

Posted by JasonB on 29/01/2019 08:22:34:

……………..

I am also half expecting a debate about how accurately the 34 cuts can be placed around the gear blank!

……………

Easy, use one of the 34 tooth 20°PA Delrin gears as an indexing plate.

I may have mentioned them earlier.

Bill

[Michael GilliganParticipant @michaelgilligan61133]

Posted by peak4 on 29/01/2019 10:37:21:

Posted by JasonB on 29/01/2019 08:22:34:

……………..

I am also half expecting a debate about how accurately the 34 cuts can be placed around the gear blank!

……………

Easy, use one of the 34 tooth 20°PA Delrin gears as an indexing plate.

I may have mentioned them earlier.

Bill

.

I think Jason meant 'how accurately' in the sense of 'to what level of accuracy'

[ 'though one of the joys of the English language is it's potential for ambiguity ]

Yes, there will be the right number of teeth … but

MichaelG.

[KWILParticipant @kwil]

I note all of the above chatter, if you have a need for a BA threaded item and have a rack full of suitable items, fine.

If not, cut out the fuss and just make what you need, screwcut if necessary, its not difficult.

[peak4Participant @peak4]

Posted by Michael Gilligan on 29/01/2019 10:46:19:

Posted by peak4 on 29/01/2019 10:37:21:

Posted by JasonB on 29/01/2019 08:22:34:

……………..

I am also half expecting a debate about how accurately the 34 cuts can be placed around the gear blank!

……………

Easy, use one of the 34 tooth 20°PA Delrin gears as an indexing plate.

I may have mentioned them earlier.

Bill

.

I think Jason meant 'how accurately' in the sense of 'to what level of accuracy'

[ 'though one of the joys of the English language is it's potential for ambiguity ]

Yes, there will be the right number of teeth … but

MichaelG.

I know, hence the devil.

Bill

[Michael GilliganParticipant @michaelgilligan61133]

Posted by peak4 on 29/01/2019 11:41:34:

I know, hence the devil.

Bill

.

My apologies then, Bill

I thought you had interpreted Jason's 'how accurately' as 'by what method, accurately …'

… Which, I suppose, demonstrates my point about ambiguity.

​MichaelG.

[Anonymous]

Posted by Nigel McBurney 1 on 29/01/2019 10:29:54:

I do not see any problems with running mixed tooth angles together……..

I expect you'd need to slightly increase the centre to centre distance of the gears to give clearance, although that wouldn't be a problem to implement on a banjo. The constant velocity advantage of involute gearing would also be lost. So in theory one would cut a wavy thread; although in practice I doubt it would be noticed.

Andrew

[Neil WyattModerator @neilwyatt]

On BA, because their only practical application is fixings, the required pitch accuracy is not very high – unless for some strange reason you want to wind studs all the way into really deep holes.

Neil

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