Myford Super 7 Metric thread cutting gears

To continue my saga:-

We've worked out how to play tunes on the gearbox output ratio to achieve a much wider variety of TPI or pitch settings that the original set by altering the input mandrel gear. It's only fair to say that this is the essence of Brian Wood's book, and far be it from me to steal his thunder.

The difficulty we've created for ourselves is that the input (mandrel) gear for the MK1 S7 QCGB is this all in one double gear. The Mk 2 lathe has a demountable driver gear so (within reason) you can fit any standard change wheel to it and play even more tunes on the TPI chart.

So the two compound gears I made were as follows Here is a picture of the standard 30/12 set on the left, and my 30/17 set on the right.

xxxx

I made it out of bronze because it is made in two parts - a 17 tooth gear cut on a stub shaft, then a 30 T wheel soft soldered onto the stub of the smaller gear. Besides , it looks pretty and I can recognise it easily in the drawer full of change wheels.

So the same principle applies to the gear I christened the 16 -1/2 tooth gear, which was a bit of a joke between me and the late John Stevenson. I don't have a photo of it, but it looks exactly the same as the bronze one above except the small end is cut for 16 teeth, and the larger one is cut for 29 teeth, but on a blank of the same outside diameter as a 30 tooth gear. This ensures that it meshes correctly with the existing tumbler gears - there's no adjustment on the centre distance - but runs at the speed of a 29 tooth gear - it's only got 29 teeth so it runs a bit faster (as a driven gear the smaller it is the faster it runs).

So far so good, but where do you buy one? Well, I only know one answer to that - simply there's the one in the picture in a drawer in my shed and I'll make a duplicate if you are interested. If so send me a PM and I'll explain further. I'm not too clear on the etiquette of touting for business openly on the forum - mod's please note I make no direct charge and all of the money goes to charity. It's such a specialist application I don't think anyone is making them commercially but if anyone with the technology fancies their chances be my guest. Indeed if Jesper knows anyone who can do this locally please have a go! Not sure if 3D printing technology is up to this, but some day someone will try it out.

So I hope that answers the questions remaining, if not let me know what's not clear and I'll try and fill in the gaps.

Good luck, and the arithmetic and more details are in the two threads referenced above.

Ooh, and Brian's book goes into much more detail, with pictures and set-up tables and I recommend it.

Best rgds Simon

Hello Simon; Thank you for all your efforts.

I did follow your suggestion and made a thread with QC setting 20 TPI - and the pitch is 20 TPI; So far so good. then I just need a bunch of "metric teeth".

Jesper

Hello again Simon and Jesper,

Until Simon had difficulties in trying to apply the information in my book, I had not appreciated that the earlier gearboxes were constructed as they are. It is only fitting therefore that Simon's work, so well supported as it was by John Stevenson, should take centre stage on the solutions he had made, tested and tabulated. He has done a first class job.

I would though just like to add a note of caution if machining thread pitches that are coarser than 0.125 inches [8 tpi]. The gearing in the Myford gearbox is only 10 mm wide, I am quoting here from memory but they are narrow, and the smallest tooth count gear within is 16 T of which there are three. The strain on teeth in that size, when pushing the gearbox hard, is very likely to bend soft teeth. This was I think the reason behind Myford switching to hardened components from gearbox Serial Number 2501 onwards while at the same time improving the design to carry the leadscrew through to reduction gearing on the change wheel side and dump the 12/30 gearing to replace it with 24/30 instead

And finally, as a little bit of mischief maybe, my fag packet calculations suggest another variant to solve the 16.5 tooth dilemma. A very similar degree of error as 16/29 is found for 17/31 and I wonder if 31 teeth could be cut into a blank sized for 30 equally well. The teeth will be a trifle narrower than the 29T version but that might be an advantage. I don't know if Simon would like to explore that as an alternative, it might be an interesting experiment.

Regards Brian

Unsuspected layers of complexity!

Good point about coarse threads. Even for the later (no 1680) gearbox the manual has the stern warning":

"THE CUTTING OF UNUSUALLY COARSE PITCHES (IN EXCESS OF .125"  EXERTS EXCESSIVE PRESSURE ON THE LEADSCREW AND GEAR MECHANISMS. GREAT CARE SHOULD BE TAKEN SO AS TO MINIMISE THE LOADS IMPOSED."

Edited By ega on 26/11/2019 10:07:18

Edited By ega on 26/11/2019 10:07:40

Vain attempt to remove moronic smiley!

Edited By ega on 26/11/2019 10:08:32

My my my, thank you Brian for those warm words, JS did most of the thinking I just followed his instructions!

As ever your ingenuity in finding an alternative answer amuses me no end. I'm currently planning a house move so my playtime in my shed is somewhat curtailed, but I'll sit and work out what the error is for the useful range of threads doable on the Myford by using a 17/31 cluster as a comparison.

Your point about wrecking the internal gears of the gearbox is well taken - as is EGA's observation of the warning in the handbook. I'd just note that if someone is contemplating cutting coarse threads there is plenty here in the forum about using a mandrel handle to circumvent this problem.

FYI Jesper has been in touch and we have agreed I will supply copies of the 16/29 and 17/30 cluster gears I made for myself directly to him. I'm looking forward to hearing his M62 x 1.25 thread has come out right on the second visit.

And just in passing, I only got involved in this whole saga because I wanted to cut a 20 mm x 2.0 pitch thread in a nut to fit the U2 collets on my Deckel grinder. I was intent on making a U2 variant of JS's ER32 collet blocks for milling a hexagon on something. I'd cut 13 TPI and got good results - the collet closed up and all was well, but my interest was piqued by having read about the 33 tooth gear idea, so I made one but couldn't fit it. JS explained I was barking up the wrong tree, and the rest is history.

I'll publish a error table shortly, it's only a few minutes work with a calculator.

Best rgds Simon

Simon,

I don't envy you with your house move, we are still recovering from the move to our present address 20 years ago!

No, seriously, it was tiring to say the least and my workshop contents took most of the day to shift into the big shed in which they now live. The house content was by comparison mere child's play.

I hope it goes well for you when that day dawns.

Best wishes Brian

Hello Simon,

I ran a few calculations this afternoon based on a combo of 17/31 instead of your 16/29.

The results were rather disappointing and erred unacceptably on the low side so I suggest you stick with what you have worked out

I think the reason is that the error moves to the minus side of the equation for 16/29 which is 0.5517

whereas 17/31 = 0.5484.The 'true' value is of course 16.5/30 which is 0.55 but now the error in minus has more influence hence giving low results. Check it out yourself, I think you will find I am right. I also now have a full table of ratios for the gearbox that you and Jester both have which might be useful to you. I can't send it by PM, my server screws all those attempts up but I can of course by email

Mine is wood_y(at)btinternet(dot)com

Best wishes Brian

Thank you all for the interesting posts - You are really experts.

Brian - I would like to receive your calculations per email; j.crone(at)get2net(dot)dk

Best regards

Jesper

Brian, Jesper, good evening

I've spent a happy few moments with my trusty calculator and come up with the following

Sorry it's a bit small, I've had all sorts of fun and games converting a table in Word into a format the forum can accept.

Obviously my table only shows the pitches where the 29/16 or 31/17 gear is relevant. The error figure in the last column is the pitch error in mm per revolution compared with the nominal "correct" number. Of course we're quibbling about microns here, and arguably the limiting factor is the wear in the lead screw and halfnuts. But that just explains why the traditional 127 tooth gear isn't necessary.

Using a 31/17 gear consistently gives a greater error than the 29/16 gear I chose. I can't take the credit for that as I didn't do the analysis at the time, though it looks like we came to the right conclusion nevertheless.

Best rgds, let's post this and see if it's legible!

Simon

xxxxx

Hello Simon,

When I look back at the calculations for the tables in my book, the pitch values that come from the gearing using a 33 tooth input mandrel gear are on the minus side of "true value"

OK the error is only microns so who cares, they are really quite good enough for most work but that does explain why the 17/31 combination I suggested as an experiment was worse than anticipated by compounding another negative error with an existing one.

Over several turns of a screw into a long nut for example, the accumulated error could result in thread locking. Not a very likely problem I will admit but if there is a better solution, such as 26/29, why bother?

I have sent a copy of the gearbox ratio table to Jesper and will happily do the same for you

As before, this has been an interesting exercise; I wonder how many others owning one of the early gearboxes will have found it useful

Best wishes Brian

Brian and Simon

With great help from Brian and his book and from Simon an extraordinary friendly assistance getting the missing 16,5T and 17T gear as described above, I'm now making metric threads on my Super 7 with OLD gearbox.

Thanks to everybody for useful comments and assistance

Jesper

Jesper,

Thank you for your comments, they are appreciated.

Simon though did the bulk of the work on your situation and it is very satisfying to hear that you are now making metric threads as you had hoped with his help.

If I may, do bear in mind that the OLD gearbox was fitted with 'soft' gears and dog clutch teeth which rough handling could burr the edges on. Heavy thread cutting at pitches equal to or exceeding that of the leadscrew will strain and bend the teeth on the smaller gears. I may be preaching to the conversant of course in which case I'll climb off my soapbox.

Happy threading

Brian

So - Thank you guys for your kind and positve vibes, and it all turned out well if I do say so myself.

Here's the picture I took of the two gear clusters I made before sending them off to sunny Denmark:

As before, I have made the smaller gear on a stub, then a second gear cut as a plate wheel is soft soldered onto the stub. This time I made them out of EN32B, 'cos that's what I had.

Pleased it turned out well, and thanks for the encouragement off camera from Jesper and from Brian.

Season's greetings all

Simon

I have a Myford Super7B with QC gearbox and powered cross slide.

Lathe no SK156253
Gearbox QC155983

I am wishing to make a screwcutting clutch accessory as designed by Graham Meek and to this end I have bought both Graham Meek's book "projects for your home workshop" and Brian Wood's book on "Gearing of Lathes for Screwcutting" - both of which are excellent reference books.

Graham Meek's book shows the attachment REPLACING the standard Myford cluster reversing handle which leaves the reversing cluster permanently engaged when the lathe is used for non-screwcutting operations.

I see pictures of Myford Super7s with a Graham Meek style clutch unit, but with a modified cluster reversing mechanism in the position of the standard Myford design i.e. two levers, one for operating the cluster gears and one for operating the "Meek" clutch. such a unit is illustrated in Brian Wood's book Fig.1.9 where he calls it "Graham Meeks reverser modification"

One thread on this site quotes Ken Willson (KWIL) as being the person to contact regarding possible drawings or details of this mod, but I have been unable to find a link to him.

Please can Graham Meek, Brian Wood, Ken Willson or anyone else give me any help as I wish to create the screwcutting clutch unit whilst retaining the reversing cluster gear unit to enable complete disengagement of the screwcutting gear chain. I would hate running a noisy gearbox drive when it is not necassary.

Has any reader of this thread completed this mod and are there any drawings available with this mod?

David Haythornthwaite

KWIL (Ken) has not been such a regular visitor to the site lately but you stand more chance of him noticing if you start a new thread rather than tack it onto this one, Graham is also more likely to see it.

In the mean time there are a few photos in KWIL's album here I can only claim a little input in so much as he borrowed my 1/16" broack to cut the keyways

Edited By JasonB on 10/12/2019 19:13:26

THank you Jason,

I have now seen the photos in Ken's album. I shall start a new thread as you suggest.

Regards

David

Hello Moderators, here's an oddity

According to the "latest forum posts" the last entry in this thread is from Brian. timed 11/12/2019 09:24:48.

But that isn't what I'm seeing above.

Uh?

Simon

I moved it to David's new thread.