Myford ML7 4mm Screw Cutting

I have a Myford ML7 without a quick change gearbox. It is used for very basic manufacturing of parts to restore classic vehicles, mainly lorries and tractors.

I have a requirement to cut a 4mm thread on a 33mm shaft, however the thread table only goes to 3.5mm.

Simple question, can the ML7 cut 4mm, if so what do I have to do to it.

Thanks.

Hi and welcome to the forum.

According to Martin Cleeve's book "Screwcutting in the Lathe" you can cut a 4mm pitch thread on an ML7 with the following gear train:

35 -- Any Idler -- 25/45 -- 50

The 35 goes on the headstock spindle.It engages with "Any Idler" which is any gear that fits the space. That gear engages with the 25. The 45 is on the same stud as the 25, keyed together. The 45 engages with the 50, which is on the leadscrew.

This combination gives an accuracy of 1 in 8,000, which is fine.

Great.

Thanks for the information. I

will have a go and also have a look to see if I can get a copy of Martins book. The lathe is not used a lot but it would be useful to have more information on other threads than in the Myford table.

No worries. The book is well worth buying and not expensive. Its one of the Workshop Practice series. Has pages of charts for metric threads on imperial lathes with the 8tpi leadscrew like ML7. All using the standard change gears. Invaluable.

Just a point of interest here. As pitch increases the speed of the carriage goes up for a given rpm. This can increase the torque required in the gear train and from the motor and the deeper cut requires more effort as well. This is possibly a good time to consider a spindle crank handle if you don't have one already.

Martin C

As the ML7 leadscrew is 1/8" pitch you will need slightly more than one rev of the l/s per headstock rev. The leadscrew may well be absorbing more energy than the headstock! You should consider using a setup that does most of the cutting with the leading flank of the tool and applying only very small cuts.

If you think that you will want to cut a lot of different Metric threads, the ideal would be to get a 127T gear. They are large,(6.45 inches diameter ) but can be found.

Another alternative is to see if you can find, or make / have made / a 63T gear. (This will be 3.25 inches diameter, but remember that Myford gears are 14.5 PA, not 20 )

This is not quite as accurate as a 127, but many mini lathe owners use a Module version to cut Imperial threads on a Metric Leadscrew machine, or Metric threads on an Imperial Leadscrew machine.

You would need a No.2 Brown and Sharpe type gear cutter to cut such a gear, and, obviously, some form of Dividing Head..

Howard

Hope I'm not teaching granny to suck eggs:

Don't forget that when cutting this thread, because it's metric and not Imperial - with the caveats about it causing high loadings as above - you must not disengage the clasp nuts until you've completed the thread to full depth. Hopefully, you can put in a decent undercut at the end of the thread so you can stop the lathe as soon as the tool reaches the undercut. (Hope you've got a clutch, but loosening the belt tension on a standard ML7 and stopping the motor should do it.)

Then you have to wind out the cross-slide, and leaving the clap-nuts engaged, reverse the lathe so that the tool goes back to the start of the thread, increase depth of cut, and ditto repeato until it's done.

Don't attempt it if you're tired - it needs a LOT of concentration.

Good luck!

I'm not convinced that cutting loads are significantly higher when cutting a coarser thread than the leadscrew like this one. (4mm pitch on a 1/8" or circa 3mm pitch leadscrew)

Surely, a 10 thou cut is a 10 thou cut? What difference does it make if the that cut is taken at a greater helix angle?

And is the helix angle even greater? The OP states a 4mm pitch thread on 33mm diameter. All things being proportional, wouldn't that be the same helix angle on the thread as cutting a 2mm pitch thread on a 16.5mm diameter? Which nobody would be concerned about.

The only extra load might be a bit on the change gear teeth from the ratio being "up" rather than "down" but if you take note of how very little finger pressure it takes on the leadscrew handwheel to take a 10 thou deep cut with a screwcutting tool even at a very rapid rate equivalent roughly to 4mm pitch, it is such a light load it seems unlikely to ever come close to shearing off gear teeth?

Surely, a 10 thou cut is a 10 thou cut?

If the top slide is set to ~30 degrees a 10 thou cut, set by the top slide will only be half that.

My take on it. At full depth cut that would be ‘like’ a 2.5+mm deep cut at over 0.1mm per rev feed. Not many would cut at that depth in steel with a myford, if surfacing? The back gear is likely needed for both operator control and extra power required for the cut, not so much for the advancement of the cutter? So most of the increased cutting load is towards the bed, not the lead screw? But at full depth, the advancement is likely to be much less than the initial added depth (the ‘ten thou&rsquo as these will be ‘finishing‘ cuts. The cutting angles will make a difference, no doubt but not so much.

4mm is only just outside the normal range of the Myford so should be ok, the spiral on the Quorn was regarded as beyond the capability of the Myford in normal screw cutting mode due to the load it would put on the gear train.

Mike

What was the lead on the spiral on the Quorn? From looking at pics it looks something like 1 or 2 TPI or 25mm or 12mm pitch or something along those lines. Would be a pretty interesting job on a Myford. I had always imagined it was a milled spiral but of course back in those days of yore the old boys did everything with the trusty Myford and milling machines were for girly men!

Oh yes they would. Easy peasy. My old Drummond Flagellator and the ML7 do it regularly. It's all in how you grind your tool bits.

.100" deep cut at .004" feed. At 400rpm.in standard mild steel.

Edited By Hopper on 29/03/2020 10:23:49

The helix angle will be similar but the full depth 4mm pitch thread will be removing double the material of the 2mm pitch at double the rate for the same rpm = 4x the load on the lathe.

The spiral on the Quorn column was left to the builder, from none at all to a 1/2” to 3/4” pitch. The professor did mill it with a saddle mounted milling head and a jury rigged slow drive to the lead screw. Not really screw cutting proper, more a spiral milling task. Unless you have a dividing head and drive from the table lead screw then it is quite an ingenious method.

Mike

OK that sounds more like the way to go. Yes the old professor was an ingenious chap as were many of his colleagues back in those days of "make do".

We might be heading back to those days with lots of time to spend isolated in the workshop but limited toys, err tools, available to buy online and get delivered quickly. We may have to go back to doing the job in hand with whatever limited material and machines are already in the workshop for the next few months. Could be an interesting result. Might even be some MEW articles in it along the lines of "Coronavirus Lockdown Jury Rigged Tools".

One way you can allow for the strain on course pitches is to use a wire and pulley and weight (or a bungee) to pull the carriage along and use the action of the leadscrew to act as a brake not a driver.