Questions about lathe power feeds.

If you have a battery drill or screwdriver you can often make a coupler to the lathe handle, top or cross. Try it, see if it is any use for you then see if it gets needed so often it is worth making a permanent solution.

Many older lathes had a splined or keyed shaft at the rear driving a the cross feed screw through a dog clutch via worm and wheel or bevel gear pair. The dog clutch control being bought out to the front of the machine over the bed. sometimes inside the saddle casting. Drive to the shaft was taken off the spindle via a drop gear and banjo in the same manner as was used for the screw-cutting feed.

I've had dealings with a Mellor lathe **LINK** which has that style of power feed drive and is of similar size to a Myford. The system works well but, despite being simple in concept, there are an awful lot of bits involved.

I think a better, more modern, approach would be to hang a stepper or DC motor off the back side of the saddle with a similar right angle drive and dog clutch arrangement. The dog clutch could either be operated directly with a rod over the bed, Mellor style or electromagnetically via a solenoid or servo system of the type used in radio controlled models.

A rod gets in the way a little and limits the the tailstock movement. I'd go for servo operated dog clutch and stepper motor. I imagine the necessary control electronics can be obtained ready built at very reasonable prices these days. A suitable right angle worm or bevel drive gearbox may be the trickiest thing to source. Small ones with integrated DC motor drives are readily and inexpensively available but such units don't seem powerful enough.

Hanging the motor straight off the back is inelegant and looks likely to be hard to arrange as you need to provide clearance for the rear of the cross slide. Overhung length could be considerable too.

Clive

Edited By Clive Foster on 08/06/2020 13:33:45

I've just been making a 1/4" hex to 1/2/" sq coupler to let me drive the leadscrew on my little Perfecto shaper - an opportunity to deploy the Quick Step Mill and GHT headstock dividing attachment:

I have checked today and was surprised to find that the surfacing/cross feeds are less by a factor of four ie they are one quarter of the sliding/longitudinal feeds. This results in cross feeds of between 0.01" and 0.0005" per spindle rev.

This information is not given on the gearbox plate although I now see that it could be inferred from the lathes.co.uk information pack which I have.

Am I right in thinking that gearbox plates generally do not state the cross feeds? My pre-PXF Myford obviously does not and I note from the PXF manual that "For most purposes the rate of power cross traverse may be considered as being the same as the corresponding longitudinal saddle feed though it is actually 0.9472 x the longitudinal feed."

Someone clearly "did the math"!

Thanks for replies to my questions, and also for the interesting general discussion of lathe power feed. The reason I asked is that I am trying to make better use of carbide insert tooling than I have been, and thought I should try to understand the whole business of speeds and feeds better than I do.

It seems that it is normal for surfacing feeds to be slower than sliding feed for a given gearbox setting, but not by as much the factor of five on my machine. I'm glad that there is no suggestion that surfacing is fundamentally different turning - that would have worried me! It seems that it's just fitting gears into the available space.

The comparison with Andrew's M300 was especially interesting as (according to the late John Stevenson who installed the machine for me) the Far Eastern 12x36 lathes are (loosely perhaps) based on the M300. There are clearly big differences though - Andrew's machine has a factor of two rather than five between the sliding and surfacing feeds.

This is the plate on my lathe's headstock:

A somewhat bewildering array of possibilities!

I had a look at the lathes.uk article on the M300 and am now wondering if if the difference is to do with the external gearing between the spindle gearbox and the drive box. On my lathe it looks like this:

That's with a 48T on the output from the spindle box and 24T on the input to the feed box - the upper half of the chart above. The alternative is to reverse them, which slows the feed by a factor of four. Maybe this is something to do with it - it seems different from the arrangement on the M300 as far as I can tell from lathes.co.uk.

It was also interesting to hear that Andrew's lathe is 'power limited' at 3HP (if I remember right - apologies if not). My own machine is 2HP, so even more so, but I've never felt close to slowing the motor. So perhaps I can push harder.

Robin.

The difference between yours and a M300 will be in the apron gear arrangement not at the head end assuming you have a leadscrew and feed shaft along the front, do you have an exploded view that could be compared to one for a M300?

This is the threading/feed plate from my M300:

The gear train seems to be slightly different, although the last two gears also need to be reversed to get the coarser threads and feeds. The note bottom left clearly states facing is half sliding. Looking at the exploded diagram of the apron in the manual there is a 5 tooth low helix skew gear which might be involved in the feeds. That would be expensive to make. That could explain why import lathes use simpler alternatives, but leading to a higher ratio.

Some other points to make are:

In light of a recent thread on an odd 7mm and 1.1mm pitch thread it's interesting to note that 1.1mm pitch is listed.

In the manual the chart is extended to allow cutting DP and Mod worms using a 56 tooth gear in the drive train rather than 44 tooth.

My lathe is definitely power limited. With a decent cut I can hear the motor slowing as the cut comes on. I've pushed it far enough to stall the motor more than once. Calculations showed that I was trying to remove about 3 cubic inches of steel per minute. So the old rule of thumb is about right.

Andrew

Thanks (belatedly) both.

Jason - I no longer have the manual which came with the machine, which was pretty useless anyway (inaccurate even when comprehensible) but somewhere I have a much better Grizzly manual which JS gave me. Grizzly sold the same machine as a gunsmith lathe. I'll try to dig it out, but I'm pretty sure that there was nothing as exotic as a low helix skew gear in the apron. I suspect that might well be the reason for the difference.

Andrew - thanks for the pic of the M300 threading/feed plate. For comparison this is the threading chart from my machine:

1.1 mm is conspicuous by its absence - C2/M2 would give 1.125 mm which wouldn't have fitted in the box

Although the tables only show changing over the 24/48T gears to go between ranges (the change between the 120/127 T idlers is purely to do with clearances) the lathe came with some extra, undocumented, change wheels, so maybe it can play more tunes if I can figure out how to use them.

Interesting that your 3HP machine is power limited - I have never slowed the 2HP motor on mine. Assuming the machines are similar in rigidity (they are similar in weight, but that may not be the same thing) I can push harder.

As I think I said earlier, the reason for trying to clear this stuff up in my mind is that I want to get the best I can out of carbide insert tooling. Surprisingly for me - I'm hard put to remember something that happened 9 minutes ago let alone 9 years - I remembered Andrew's Cambridge Turning Trials . Not very relevant to me at the time as I had only a small benchtop lathe, but I'll be looking at those experiments again now that I have a similarly sized machine.

Robin

Edited By Robin Graham on 13/06/2020 22:32:26

Edited By Robin Graham on 13/06/2020 22:33:39