Myford Super 7 screw cutting gears (metric)

ok this started to be talked about in another thread, so to stop me from taking it off topic too much I thought I would start a new thread, now me being dislexic and dyspraxic learning from books and written bits is not my strong point so you might need to bare with me while I try and get my head around this.

from reading other threads I have worked out that it is possible to cut metric threads, with a quick change gear box on a Myford super 7, by just changing one gear, this gear being the 24 tooth input gear on the tumbler shaft.

so my first question is, is this the gear that is being discussed?

as by my understanding the input gear would be the one at the top, not the one with the arrow, but the only gear I can find with 24 teeth is that gear.

so am i somewhere in the ball park, or have I got this whole thing twisted in my mind and I am going off on a random tangent?

Allan

Yes that's the one- you don't get a 'perfect' thread pitch for every thread, but with a couple of other gears (to swap into that same place) you can do all the metric threads with more than sufficient accuracy for most needs...

You do need to rotate the banjo to get the correct mesh, and if you use a small gear on that stud some lathes don't have quite enough adjustment...

Allan,

​So that you can follow the logic from the spindle down, the spindle gear will be 30 teeth, the fibre gears are those that do the reversing which in turn mesh into the next gear down, also 30 teeth on the tumbler shaft..

​So far the gearing ratio from spindle to the second 30 tooth gear on the tumbler shaft is unchanged. The 24 tooth gear [the one you arrowed], which is coupled to the second 30 tooth gear is what Myford term the mandrel gear and that is the main drive gear down the chain to the gearbox.

​You will have found tables that include gears with 33 and 34 teeth to put in place of that, which together with gearbox changes, will give metric ratios in more than sufficient accuracy.

​I'm sorry to contradict RichardN, the restriction on banjo movements comes with fitting gears of 35 teeth or more, NOT the smaller gears. The clamp slot at the bottom of the banjo limits the movement

Regards
​Brian

thanks both for your replies.

so just getting this into my head, to alter that 24 tooth gear, I need to adjust this bit

there dosnt seem much movement to drop the gears down, but if it is only another 10 teeth it might just do it,

so I need to get the 33 and 34 tooth gears which are about £10 each which is better than the £250 for the conversion kit, the next thing is without me going hunting through the net for something that I don't exactly know what I am looking at, where about's would I find a chart for the settings, using the 33 or 34 tooth gears, and what pitches the QC gearbox will cut.

sorry if I am sounding thick on this, once I can get it all working and can play it will all become clear, but until then and it is all theory my brain dosnt like it too much.

Allan

Allan,

John Stevenson used to sell a pair of 33 & 34 T gears from his online shop www.metoolsonline.co.uk, but had to close the shop, then sadly his wife passed away; don't know if he re-opened it. (Hope I remember it right John)

Below is the gear chart he supplied with the 33/34T pair. (including typo on the BA chart).  Hope he doesn't object to it being reproduced.

Edited By Robbo on 11/03/2017 14:50:49

John's away for the weekend so he may not be able to get back to you for a day or two.

Neil

Robbo beat me to it, but I'll post this anyway - it's the table I use:

**LINK**

How do the top 3 rows read,

The Metric and BA is clear, but haven't sussed the top three rows.

Steve

Hello Steve,

​Surely they are the tpi you can expect by using fine feed [as it says] with a standard mandrel gear of 24 teeth

Regards
Brian

Edited By Brian Wood on 12/03/2017 18:35:01

There's always something to trip you up...

I've spent a while over the last few days making a 33 and a 34 T gear, also a 24 and a 26. Gone to play with cutting metric threads with then this morning, and fallen foul of the fact that the gear A on my S7 is a 30/12 couple. Uh?

Of course the answer in in the archive. I quote (from a thread dated 27.2.2012 called "Myford Quick Change Gearbox":-

Mike,

You are not getting the full story. So before you get in too deep......
In no particular order:
1. The last gearbox of the type you have was Serial No. QC (for Quick Change) 2500. QC2501 was the first "modern" one.
2. It is thought that many early gearboxes were retrofitted in the factory with hardened cogs, but no records were kept. Also quite possibly from some point before QC1950, hardened cogs were fitted as standard.
3. The leadscrew on your none-gearboxed machine has quite probably got a different plain diameter at the headstock end, as in smaller than what you require.
It must fit the hole in the gearbox casting closely for obvious reasons.
4. It is somewhat trickier to align the gearbox with the leadscrew, but it must be done and done carefully or the hole in the casting for the leadscrew becomes worn. The "modern" box is almost self aligning as it goes right through, but it still needs doing.
5. To align the gearbox properly, you need a spacer strip between the gearbox and the top two fixing screws along the bed shear. It is just a flat strip with two holes in, but I have no idea what the thickness is. That is a left over from the earlier ML7 fitting.
6. Your gearbox runs at half the speed of the "modern box" to cut the same thread. That is to say, your output gearing at the right hand end is 1:1, whereas the modern box is 2:1 at the left hand end. There is no chance of you altering the principle to be the same as a modern one as you won't make it fit!
7. A consequence of this that you also need a 12/30T FINE FEED TUMBLER GEAR A1974A/1 as it is termed on the Myford/RDG website to drive the standard fixed quadrant geartrain. The modern box uses 24T/30T and hence does not have to be a combo cog.
8. Do not try and use the Myford modern screw cutting details as in a later manual as you will end up with a times 2 factor as above, but you can do far better none imperial threads than the earlier Myford tables.

9. The guts of the boxes are identical and interchangeable apart from the shaft differences of course

Think that's it.

You have some way to go to get going.

Dennis

And I do indeed have the older gearbox on a S7 of the same vintage.

So, I have discovered that the banjo won't drop far enough to allow me to fit my new 33 and 34 gears, I'll have to modify the banjo slot to do so. Is this opening another can of worms?

Having got the gears to mesh, I can in principle adjust the top lever and side lever selections to choose half the TPI the table above indicates, as my gearbox will be running at twice speed. I've yet to think this through, but I doubt I'm the first - does this work?

Rgds as ever to all, and thanks for the help.

Simon

edited to remove a yellow winky face I didn't ask for....

Edited By Simon Williams 3 on 18/03/2017 13:08:11

Hello Simon,

​My server will no longer allow me to attach pictures or diagrams to postings, if fact it interfers with the process of posting as well.

​So, because of that, I can't post a simple drawing for a modification that WILL allow you to lower the banjo on your S7. It takes little to make and allows all manner of thread pitches to be cut into the bargain.
​

Send me a PM please with your email address and I can send you the drawing that way

​Regards Brian

Brian -

Have sent you a PM as suggested, and look forward to hearing further. Thx Simon

I recently bought the two gears 33 & 34 from John and he said he more. I notice some one has post the chart as well.John

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**