Thread confusion

Hi people,

I am just getting to know my Super 7B.

I have done some basic turning with HSS and indexable inserts and i am now interested in getting to grips with screw cutting. I have Brian Woods' excellent book Gearing of Lathes for Screwcutting and after reading it a few times i am ready to start.

I then had a proper look at the gearing set up behind the leadscrew drive gear guard and noticed the metric pitch chart pinned to the inside of the guard as the photo. So one question that comes to mind, the gears on the diagram - 60T and 63T plus the 50T and the 45T, are they both one piece items and do they mesh together across via the 63 and 50T. Also, are those gear sets even available.

Should i use that chart or just stick to Brian's way of doing it.

Sorry for so many questions but seeing that chart inside the cover has thrown me a bit.

All the best, David.

No doubt Brian will be along shortly, but here is my three penn'orth for the time being:

To deal with the questions you ask one by one:

Firstly you can set up the change wheels any way you like so long as the maths works out and the bits go together, so the set up as per the label you have included is perfectly valid, but does require (obviously) that you not only have those gears identified but also the banjo onto which they are mounted. This is NOT the same banjo as the standard one with fixed centre spacings which is included with the standard quick change gearbox. So your expense factor is obvious.

The gear pairs are splined together by the centre sleeve, which has a key way machined into it. So the 50 and the 45 tooth gears are fixed together and rotate as one assembly, hence giving the ratio change you need. The same is true of the 60 and 63 t gears, they are two separate gears fastened together and rotating as one complete assembly.

What we will call "Brian's" method, as described in detail in his book, has a simpler solution and works just as well as the approximation represented in the gear set Myford drew out in the label you have noticed. However it has several advantages over the "standard" metric kit as you only need a couple of extra gears to do most of the commonly used metric pitches, you don't need a new banjo nor do you need the centre keyway boss. Whole lot of financial happiness there, however the biggest advantage isn't about cost at all, it's that you don't forfeit the fine feed system of the reversible 19/57 gears cluster because you use it as part of the gear assembly. Brian's book has the details.

However you do need a 33 and/or a 34 tooth gear to fit on the first mandrel spindle (below the tumbler gears). Again, Brian;s book describes the details. Now the snag of this is that these two gears are not the easiest of things to find, indeed the 34 tooth one seems to be in national shortage mode at present. But then neither is the metric set, and the special banjo is shades of unicorn poo...

With due deference to the proprieties of advertising on the forum (which is out of order, and I'm not going to do it) I have been making a batch of 34t gears (proceeds to charity) in response to similar questions elsewhere, so if you want one send me a PM and I'll explain further.

And no, I'm not making a profit - I'm not even covering my expenses making them the old fashioned way but if it helps someone try out "Brian's" elegant and simple adaptation, and makes a contribution to charity no-one's the worse for it, so I hope that's Ok with the moderators.

Hope this helps

Simon

Hello David,

How can I follow such lavish praise!

Naturally I would advocate the simpler approach and I really think Simon has covered all the points in it's favour as well as or even better than modesty would allow me to do so.

The big drawback for either method seems to be the shortage of parts various, so I would be very tempted to take up Simon's generous offer----I can confirm from experience that he will make nothing out of the supply!

Happy threading

Brian

Hi Simon,

That explains things very clearly, thanks for your well thought out response. I will be going along the Brian Wood route to threading nirvana and will therefore require at least the 33 and 34 tooth gears.

I will pm you re the 34 tooth gear.

Once again many thanks for your very considered response.

David.

Hi Brian,

Thanks for getting involved. Your method is certainly the easiest / simplest way to go.

May i just ask, with the new drop arm that is needed, are the pins for the gears on the arm made of anything special / hard or just en3b or similar.

Regards,

David.

Hello David,

The pins are made from whatever off-cuts you might have to hand---I would avoid chinesium for obvious reasons. The drawing in the book suggests mild steel. From memory I used bits of free cutting mild steel.

Just to correct you if I may on one point, the drop arm is just a displacement feature that allows the whole banjo to be lowered by 19 mm, gears do not run on it at all. It is a replacement fitting for the standard Myford stud as the bottom anchorage

Regards

Brian

Hi David, good morning. Sent you a PM just.

Brian correct me if I'm wrong but I believe on a Mk2 S7, and with 33 or 34 Teeth your new drop arm isn't needed? If you want to go bigger you need it, my recollection is that a 34T mandrel gear just fits in place and the slot in the standard banjo is just long enough to drop the banjo and line up the transfer gears with a smidgeon of clearance.

I seem to remember you might need to tickle the slot in the standard banjo with a round file, but it should fit together. I've not tried this experiment - mine's the earlier MK1 S7 where it doesn't fit at all and anyway it's irrelevant.

Best rgds Simon

I have no problem fitting the 34t gear on the unmodified banjo on my 70s S7B.

And, credit where credit's due, this is really the Reverend David Hoskins' method back in ME 3955.

cheers,

Rod

Hello Simon,

Yes indeed, strictly speaking you are quite correct, but it starts to get very close to the bone with a 34 tooth gear in place, a round file will ease the situation if it is needed..

I calculated for 35 teeth and more for thread pitch series other than metric and BA [beyond 8 BA that is] and for those cases that really is where the drop arm becomes necessary.

Fortunately it is not a difficult thing to make, mine is permanently fitted and I never now think about so a big gear is easily accommodated, if you can get hold of it in the first place that is !!

You of course have a special set of difficulties with the early gearbox needing the 12 T primary driver, and have found your own way through (along with some help from the late J S) to produce the elegant practical solutions for the partial tooth calculations that are needed to suit the maths involved.

Had I known of it's special needs, before publication, I would have had to give that situation the necessary thought as well. Starting with a clean sheet so to speak, I'm not sure what answers I would have found!

Regards

Brian

Stating the obvious, the changewheels are a means of altering the ratio between the Mandrel and Leadscrew.

This changes the rate at which the Saddle moves as the mandrel turns. So, with a ML7 etc, a 1:1 ratio will move the Saddle at the same rate as the Leadscrew,, to produce a movement of 0.125" for each revolution of the Mandrel..

A 2:1 ratio would produce a 16 tpi thread (0.0625" / rev), a 1:2 would produce 4 tpi.

By selecting a high enough ratio, the Leadscrew can be used for a power feed for turning.

With a selection of change gears, a huge number of variations are possible,as you will see from Brian's book. (Not all of the possible variations will correspond to standard thread pitches, but can be used to produce different feed rates.).

Recently, on a much older Myford, I helped some one set up 20:60/20 (compound gear) 65/20:60 to provide a power feed of 0.00427" / revolution of the Mandrel, as a fine feed for turning. This particular model has a Right Hand Leadscrew, (Unlike the 7 Series which have Left Hand Leadscrews) so the double reduction moves the Saddle towards the Headstock.

Howard

Hi Brian,

Silly old David, of course it is just to drop the banjo so as you say no great sweat on materials.

I will start to look out for the other gears as well so once fitted the drop link can stay there should i need the extra room.

Thanks, David.

Hello David,

I built up a collection of gears over many years on the basis that the various purchases were of odd tooth counts that "must have a use somewhere" so be prepared to grab things as you find them. To test the book properly I had to make a lot as well, but that's another story.

All good wishes

Brian

To my shame I'd couldn't remember the gent's name, let alone conjure up the correct reference. So thank you Roderick for giving credit where it is properly due.

Rgds Simon