Myford Super 7 screw cutting gears (metric)

Thank you John,

Steal away, the publicity is very welcome!!

​Brian

The mystery deepens! - or at least my confusion is ripening.

The input gear train of my QC g/box looks like this:

xxxxx

Looking down to see the hidden gear looks like this:

xxxx

So a 12T gear drives an idler (57T), which drives a 19T/57T combination. The 57T drives the wide 72T of the g/box i/p shaft. Hence the gearbox runs at HALF the speed of the lathe spindle. OK so far.

Inside the cover of the gears is a label:

xxx

BUT (if I've got my sums right) the centre distances of the fixed studs of the banjo don't correspond to those of the proposed gears on the label.

Measuring the lathe banjo, the centre distance between the two fixed studs is 1.900 ins, which works OK for two gears as fitted of 57 and 19 teeth. The centre distance of the second pair - the second fixed stud and the g/box I/p shaft is 3.225 ins, matching the tooth count of 57 and 72 teeth.

The label shows to substitute a 60 and a 44 tooth gear pair on the first two studs, so the centre distance will be 104/2/20 = 2.6 ins, not 1.9 ins as on the banjo.

The second pair, on the second stud and the g/box shaft are 52 and a 60, so 112T total, so centre distance 112/2/20 = 2.8 ins. The banjo centre distance is 3.225 ins.

Have I misunderstood something?

Thanks as ever, Simon

Yes for metric you need a new metric banjo which doesn't have fixed centres.

**LINK**

The whole idea of the 33 / 34 gears is to do away with all this and still keep the reversible fine feed which you can't do with the Myford official setup.

Ah-Hah!

Thank you John, I think you've just explained the missing link.

So for my elderly S7 with the half speed gearbox drive, I need a 17T gear to bring pitches of 1.0, 2.0, 3.0 and 4.0 mm within the setting range of the gearbox. EG for 1.0 mm pitch I put gear T as 17T, then set the gearbox in the 36 TPI slot.

For 1.5 mm pitch I put gear T as 34T, but now the gearbox output to the lead screw is running at twice the expected speed, so I set the gear selector to twice the TPI in the table above (= half the rpm) to bring the travel speed of the lead screw back to where it should be. So I set the gear ratio for 48 TPI not 24 TPI, so the lead screw runs at half speed.

In my 1.00 mm pitch example, I can't use the 34T on the tumbler, as this would mean I needed to set the gear box for 72 TPI and it doesn't go that high (slow). I can do 2 mm pitch with either a 17 T gear (and set 18 TPI) or use the 34T gear, but then I set the gear box for 36TPI.

And so on. What larks eh? I'm off to make a 17 t gear.

All this started from wanting to cut a nut to "fit" a 2 mm pitch screw - the buttress thread on the tail of my U2 collets for the Deckel grinder. I was curious to see if I could get any nearer than cutting 13 TPI, which, crude though it is, actually worked quite well.

Thanks all as always

Simon

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

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

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.

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

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

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

**LINK**

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

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

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

Dennis

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

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

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

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