Myford Super 7 screw cutting gears (metric)

[John Stevenson 1Participant @johnstevenson1]

Yes, correct. you will want a 17T instead of a 34 and a 16 1/2 tooth instead of the 33

[Simon Williams 3Participant @simonwilliams3]

I've got half a tooth in my upper right mandible….

Seriously, if anyone is hankering to follow the details of the label in my Mk1 gear box as above, I can't make the arithmetic work out. Interestingly the kit of bits on ebay just now (John's link as above) doesn't contain a 52T gear, nor yet a 44, and neither refers to a 63.

Ho hum

Thanks John for the messages.

Rgds Simon

[John Stevenson 1Participant @johnstevenson1]

You only need a 17 for the 1mm pitch as the box doesn't go high enough as you say but the standard 24T and gearbox position 56tpi will give you 0.49 which doubled up will give you the 1mm pitch you need.

[Simon Williams 3Participant @simonwilliams3]

Thanks John. I look forward to trying it out.

As a non sequitur (apologies moderators) the Jacobs spindle nose chuck you let me have works a treat. I cheated and bought a D1-3 back plate blank, rather than making one from scratch, but the chuck is in good use and I've got a repeatable run out of about 1 to 1.5 thou swapping collets and work in any combination.. I can do better than that with a bit of judicious bumping, but the chuck is several times better than anything else I've got and I'm very grateful to you for letting me have it.

All the best

Simon

[Brian WoodParticipant @brianwood45127]

Hello Simon,

​John has you pretty much in hand, the logic is all correct as I would expect

For my own interest, your gear train photo does not show a 1:2 reduction gearing tucked in behind the wide 72 tooth wheel which was the case in the later gearboxes. That explains the 12 tooth mandrel wheel.

I assume your gearbox will have a small housing on the tailstock side enclosing gearing. It is not an arrangement I am familiar with and it will not therefore cater for the reduction gearing above.

​You may find some fine pitches unobtainable as a result and I am willing to help, if you need any, to test out other arrangements that might suit what you have to hand.

​Regards Brian

[Nick HughesParticipant @nickhughes97026]

Here's a set of charts that should help:-

**LINK**

[DMRParticipant @dmr]

Just to fill in some gaps on this thread:

Brian, the early gearboxes before QC2501 had an aluminium cover at the tailstock end with 18T(gearbox output)-30T(idler)-18T(on the leadscrew, also at the tailstock end). Fine feeds were the same.

Simon, as you may have realised by now, you cannot fit a 17T cog on the tumbler assembly as it would cut into the keyway, being too small. The metric set on e-bay as advised by John S is for the later gearbox, but the metric quadrant piece in that picture is the same. The tables appearing on this thread will not work on the early gearbox. If you want further help you can PM me, but I have never worked out the various approximations to metrics for the early box.

Dennis

[Simon Williams 3Participant @simonwilliams3]

Well, thanks one and all for the interest and advice, and I guess I really owe the OP an apology for hi-jacking his original thread.

But the rugby's over, Wales lost to France on a technicality, so while I've been sulking in my shed this morning I made this:

The gear on the left is the original 30/12 pair, and the pair on the right is my new 30/17 combination. I made them in bronze so I could soft solder the two parts together, and also when I rummage in the drawer through the accumulated selection of change wheels in a couple of years time I'll remember what this funny looking thing is for. For JS' benefit, when I've made the 16 1/2 gear I'll post a picture.

I haven't tried this out yet, I'll have a play with cutting whole mm pitch threads over the next few days, but I'm confident. What could possibly go wrong?

Regards to all

Simon

[Allan BParticipant @allanb]

Not a problem Simon, it’s been very interesting seeing the difference in the years of the gearboxs, glad my thread gave you an area to get your answers.

Once I get the gears off John I will update on my attempts at metric threads, and also my ventures into multi-start threads.

Allan

[DMRParticipant @dmr]

Very impressive. I didn't consider that. You have clearly mastered some arts.

Dennis

[Brian WoodParticipant @brianwood45127]

Hello Dennis,

​You have filled in my knowledge most helpfully thank you.

I am now left wondering if it might be possible to vary the 18/18 arrangement, with a suitable adjustment of the idler wheel linking them in that little outrigger housing so that a 16/32 pair can be substituted with the 32 on the leadscrew; it would have the effect of converting the gearbox itself to become equivalent in operation to the later versions where that 1:2 reduction is achieved in a 26/52 combination driving the leadscrew from behind the wide 72 tooth gear. It all depends on the relevant shaft spacing of course

​It will spare Simon the trouble of making the 16.5 tooth wheel and at the same time make all the tables that Nick Hughes posted the link to work as shown. Simon can then hang up his 30/12 and 30/17 combinations [nice work on the latter by the way] as curiosities. Having said that I would even pay to see the 16.5 tooth wheel !

​It might be useful to see what others think of the approach outlined.

​Regards Brian

[DMRParticipant @dmr]

Hi Brian,

Short answer – no and no. It is not as you call it a "little outrigger housing". The cog positions are part of the gearbox casing at that end. The 18T is on the top shaft at the opposite end to the modern 26T driver. The idler gear is on an extension to the intermediate shaft which does not come out of the modern box at either end (it's there as a collar only). The guts of the boxes are identical but the casing changed shape. In any case a bigger cog will not fit onto the leadscrew (against the bed) in the relevant position. It's no problem for the 52T at the other end. It's a non-starter, so to speak. No idea how many early boxes still exist and the first one was certainly not QC0002. The aluminium casing covering the cogs was complex and heavy and was always likely to get smashed up by the saddle. Not easy to fabricate/copy. I have given 2-3 people details of it but no-one ever came back to say they had managed to make one.

PM me and I'll send you some pictures if it still intrigues you.

Regards, Dennis

[Simon Williams 3Participant @simonwilliams3]

Having benefitted as I feel I have from so much of the various contributor's expertise, I thought it only fitting to put up a couple of photo's of what is under the cover on the RHS of the old type gearbox we've been discussing in such detail. This is what the diecast cover looks like:

 

 

 

And this is what it covers:

 

 

 

xxx

 

And (blowing of trumpets, fanfares, commotion) here is my first metric approximation. Thst's a 2.0 mm pitch thread gauge.

 

 

Looks like I'm not quite holding it straight, but in reality I couldn't see any discrepancy between the gauge and the thread (even if I didn't take a very good photo!)

Many thanks to everyone!

Simon

(edited for typo's)

Edited By Simon Williams 3 on 19/03/2017 20:57:49

[Allan BParticipant @allanb]

That is definitely different to the gearbox on my super 7.

Very nice thread by the way.

Allan

[Simon Williams 3Participant @simonwilliams3]

That metric banjo kit made nigh on £200!

Now, about this 16 1/2 tooth gear…

If I made another combo gear as 29/16T I'd have introduced a 0.3% error into the approximate calculation.

Following a train of thought from Mr Stevenson who was cutting gears in Delrin but making them oversize to make the tooth shape stronger, I reckon I could make the 29T gear mesh adequately with the tumbler gears. I'm working on the table of actual TPI against metric pitch, but I'm betting it's somewhere handy.

Just checking I've got a dividing plate with 29 on it …

Simon

[Allan BParticipant @allanb]

Now we are getting into calculations and skills beyond my skill set lol, I have started a college course that is supposed to teach me this lot, but unfortunately due to the lack of tutor for 3 months it looks like they are skipping the teaching of gear cutting and gear train calculations 🙁 so I am very interested in how this is going.

Allan

[John Stevenson 1Participant @johnstevenson1]

Simon,

Do you have a set of 20DP gear cutters or just a few?

I reckon we can get the 29T to run well instead of the 30T but need to do a bit of calculation.

It won't be until tomorrow night as only just walked in after a 160 mile drive and a bit wasted.

[Brian WoodParticipant @brianwood45127]

This is now getting very interesting. I have to be out much of the day but will follow developments with keen interest later. Just briefly though, thank you Dennis for the added information, pictures would be good and I'll PM you later

Regards
Brian

[Simon Williams 3Participant @simonwilliams3]

Morning John, Brian

JS – Complete set of 20DP 14 1/2 deg cutters in stock, they're only Garvin ones but all there and almost new. It's the only complete set I do have.

Also confirm I've got a 29 circle on one of my division plates.

If I cut 29 div's on a blank of OD size for 30 T (1.600 ins) how do I modify the tooth depth calculation (and choose a different cutter???) to be sure the otherwise oversize 29 teeth don't jam in the 30 T they mate with. Mind you, this 30 T is home-made (aluminium) as you may have spotted from the photo's, so it's a pound to a pinch of snuff the teeth were cut slightly over deep.

Do I cut 29 teeth to the depth they would be if the blank were the correct size = 1.55 OD less 0.108 depth of cut each side = 1.334 which is the PCD of the root of the 29 tooth gear cut normally?. That's 0.133 depth of cut each side from the 1.6 dia blank.

Brian – I was going to tackle (am just starting) the calculations to see how near the "correct" pitch this gives for the relevant choices. (Is it worth the bother of making the gear?) It's a wet and rainy day, so it's that or the hoovering…

Best rgds Simon

[steamdaveParticipant @steamdave]

Posted by John Stevenson on 18/03/2017 16:25:08:

At the risk of stealing Brian's thunder, may I pass a link on to his new book.

Gearing of lathes for Screwcutting by Brian Wood, published by Crowood Press.

I recently picked a copy up from ARC and it makes for very interesting reading and a lot of work has gone into this publication.

However from Crowoods page you can actually see part of this book and the modification that Brian talks about.

**LINK**

And the typo on the contents page. Should it not be a SouthBEND lathe rather than a SouthBOUND lathe?

Dave
The Emerald Isle

[John Stevenson 1Participant @johnstevenson1]

Posted by Simon Williams 3 on 20/03/2017 09:47:33:

Morning John, Brian

JS – Complete set of 20DP 14 1/2 deg cutters in stock, they're only Garvin ones but all there and almost new. It's the only complete set I do have.

Also confirm I've got a 29 circle on one of my division plates.

If I cut 29 div's on a blank of OD size for 30 T (1.600 ins) how do I modify the tooth depth calculation (and choose a different cutter???) to be sure the otherwise oversize 29 teeth don't jam in the 30 T they mate with. Mind you, this 30 T is home-made (aluminium) as you may have spotted from the photo's, so it's a pound to a pinch of snuff the teeth were cut slightly over deep.

Do I cut 29 teeth to the depth they would be if the blank were the correct size = 1.55 OD less 0.108 depth of cut each side = 1.334 which is the PCD of the root of the 29 tooth gear cut normally?. That's 0.133 depth of cut each side from the 1.6 dia blank.

Brian – I was going to tackle (am just starting) the calculations to see how near the "correct" pitch this gives for the relevant choices. (Is it worth the bother of making the gear?) It's a wet and rainy day, so it's that or the hoovering…

Best rgds Simon

Simon,

Use a No 5 cutter instead of a No 6 and cut to 0.112" deep

The gear blank needs to be the same size as the 30T one but you cut 29 on it.

I'll explain later tonight when I have a minute.

[Simon Williams 3Participant @simonwilliams3]

John – Thank you for the update, I'm very interested to understand the process behind those figures. I think you can guess what I'm planning to do tomorrow!

Brian – Here's my attempt at the arithmetic.

For my old style gearbox, if I replace the 12T gear with a 17 T and select 18TPI, I get 12/17 x 18 TPI = 12.706. Pitch of 12.706 TPI = 1.999 mm.

For 2.5 pitch, I replace the 30/12 pair with a 29/16 combo gear, select 14TPI as per the table above. Now my lathe cuts 12/16 x 29/30 x14TPI = 10.15 TPI, or 2.5025 pitch. Pitch error is +0.1% (contrast error of – 0.2% with 33T gear on new gearbox.)

For 3.5 pitch replace 30/12 pair with 29/16 and select 10TPI. Lathe now cuts 12/16 x 29/30 x 10 TPI = 7.250TPI, 3.5024 pitch. Error is + 0.1%, error on "new" lathe is – 0.2%

So I think we're onto something here.

Obviously multiples and sub multiples of these pitches come out with the same error, so that's OK. Haven't looked at sub 1.0 pitches, probably never going to happen.

Would appreciate your confirmation!

Best rgds Simon

[Brian WoodParticipant @brianwood45127]

Hello Steamdave,

​Of course it should be Southbend, well spotted. The type setter had given the whole chapter the title Southbound lathes, that we did spot and correct but I hadn't realised the contamination had spread to the contents page as well.

​Proof reading a book of that sort after it has been typeset by a completely non-technical person became a huge amount of work. Just for starters, I had every entry in all the tables to crosscheck for transposition errors, there are about 8000 entries in total. There were other errors introduced as a result of ignorance of the subject, formulae being a special favourite. If that is all that remains I don't think it is that bad.

Too late now to do anything about it of course, I do hope though it won't put you off reading the book and getting something useful out of it.

My apologies

Brian

[Brian WoodParticipant @brianwood45127]

Hello Simon,

​I've approached things a different way and recalculated the old style gearbox ratios based on the fact that it does not include a 26/52 [ ie 1/2] reduction ratio to the leadscrew.

​Now that you have made your 17/30 combination gear perhaps you would check some values for me please before I see what other possibilities these manipulations bring.

​For a pitch of 0.5 mm you should use a gearbox value of 24 TPI
​For a pitch of 1.2 mm you should use a gearbox value of 10 TPI

Regards
Brian

[Simon Williams 3Participant @simonwilliams3]

Brian –

Not sure we're on the same page here!

24TPI selected driven by 17 T gear produces 1.5 mm pitch. Equally 10 TPI selected gives pitch of 3.6 mm. If nothing else this is pro-rata from established start point where 18 TPI selected produces a thread of 2 mm pitch as per the table published on page 1 of this thread.

Just to be sure I've gone out to the shed and tried both, can confirm as above.

Rgds Simon

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