Metric gears on Myford big bore lathe

I am turning up some backplates with the big bore 42.5mm x 2mm pitch thread. Instead of the faff of setting up the proper Myford metric conversion gears, I intended to cut metric threads using 33T and 34T driver gears in place of the standard 24T driver. It’s a well-known wheeze that worked well on my previous ML7 and gives a close approximation to the correct pitch.

To my surprise and annoyance, the gearbox banjo does not swing down sufficiently to allow the 33T and 34T to be mounted. The ML7 and big bore gearboxes are identical as far as I am aware. It seems the big bore modification means the gears in the tumbler train have more teeth and this has pushed the train downwards. The banjo pokes out through an opening in the bottom of the gearcase, which is not the case on the ML7/Super 7. Has anyone else noticed this? I could extend the slot in the banjo but it may go beyond the thickened part of the casting. It seems there are a few knock-on effects from the big bore modification.

Andrew, some of the S7's have problems with the bango not dropping far enough and the slot has to be filed out, not all, just some so I can't say what the differences are.

It's usually just the 34 gear but you obvioulsy have problems with both.

Any chance of a couple of pics please ? one end on [ W ] and one from SSW ?

John S.

Here's a photo. Note the larger tumblers gears and the projection of the banjo through the gear case. There is a corresponding slot in the cover. For the purpose of the photo, I have released the banjo so it is fully down. Note the small clearance to the 24T gear.

Success. There was enough meat on the casting to lengthen the slot by another 3-4mm and that has done the trick.

Andrew

OK Fully see the problem which is because of the big bore the spindle gear and the tumber output have had to be made bigger and equal which is the decising factor.

There is an partial answer in that currently I am doing a run of new gears and one is a special at 17 teeth for a guy with one of the very early screwcutting boxes as they had an outboard 2:1 ratio onto the leascrew and his ratios work out greater by two, the numbers on the chart.

He could halve the numbers but he wants the fine pitches he's loosing.

In your case you would select 9 on the box [ 4mm pitch ] instead of 18, there isn't an answer to the 33 unles you can get a 16 1/2 in there.

[ EDIT ] without knowing what's behind the banjo could the stud be moved ?

Edited By John Stevenson on 01/01/2013 12:46:25

John,

I think you will find that the Anchor Pin (stud) is fixed in position by the corresponding hole in the gearbox left hand face.

Kwil,

Just wondered if a new hole could be tapped if that was the method of fixing but Andrew looks to have sorted it anyway

John,

Does the early gearbox have a reversing fine feed cluster gear? On my 70s gearbox the fine feed divides the pitch by 9 (I think!). Might give some more options for the finer threads.

Rod

John & Kwil

Thanks for your suggestions. It was just possible to sort it without resorting to something out of the norm. The threads are now cut. The 33/34T solution certainly makes metric gear cutting much quicker to set up.

Nowadays, I prefer to cut such threads by hand using a spindle drive handle and leave the motor running in reverse at a moderate speed for a quick return without disengaging the leadscrew half-nuts. I find the torque from a 1HP motor at screwcutting speed is frankly frightening if something goes wrong. I haven't fitted an inverter to this lathe yet but running the motor at reduced speed is another way of reducing the torque at the tool tip, so the motor stalls if there is a jam up.

Andrew