Metric thread on imperial lathe, without reverse.

[Vernonv]

It's been a long time since I cut a metric thread and I can't remember if I have even cut one on my current lathe (which doesn't have a reverse).

Anyway, my question is .... is it possible to cut a metric thread on an imperial lathe, if I disengage the half nuts and reposition the tool at the same location and start on the same number on the thread dial, for each pass?

[bob ward]

I guess so, but your carriage will need to be accurately repositioned each time, maybe use a stop. Do a trial run first if you are working on something irreplaceable.

[mahgnia]

It's been a long time since I cut a metric thread and I can't remember if I have even cut one on my current lathe (which doesn't have a reverse).

Anyway, my question is .... is it possible to cut a metric thread on an imperial lathe, if I disengage the half nuts and reposition the tool at the same location and start on the same number on the thread dial, for each pass?

Vernonv,

It is extremely unlikely that you will get the correct positioning on each pass due to the metric thread not being a multiple of the lead screw pitch, even using the thread dial, unless the pitch chart for your lathe gives you this option.

The are a few possible options:
1. Fit a reversing switch to your lathe motor (usually a fairly straightforward job)
2. Turn the lathe in reverse by hand using a crank temporarily connected to the motor, lathe spindle, or one of the drive gear shafts.
3. Install a metric lead screw and half nuts (this may present it's own set of new problems)

Andrew

[Vernonv]

Do a trial run first if you are working on something irreplaceable.

Yes, I will certainly do that.

1. Fit a reversing switch to your lathe motor

No can do (and I have tried) - the lathe has a clutch and it doesn't work correctly when run in reverse.

2. Turn the lathe in reverse by hand

I was hoping to avoid that if possible, but may just have to.

3. Install a metric lead screw and half nuts

I can't really justify the expense or time to do that.

Any other comments/thoughts anyone?

[Big Shed]

Vernon, I don't know which lathe you have and this may be way off beam, but on the 9x20 lathes a very popular modification is to fit a tumbler reverse.

Here is one of those mods, there a numbe rof others on the net:

Steve Bedair's 9x20 Lathe Site

[Vernonv]

Thanks Fred ... I have an old Nuttall lathe, which has a leadscrew reverse lever already.

I just did a search on here and realised that I asked this very same question some time ago (2010) , but unfortunately I didn't post what the conclusion was. Although I seem to remember that I ended up opting for an imperial thread.

I might have to do some testing tonight and see what my options are in regard to the leadscrew reverse.

[Stustoys]

leadscrew reverse wont work unless it only have one engagement postion and is spinning 1:1 with the spindle(I think)

There is one way that I'm aware of(but have never tired)
You have a fixed starting point for the carriage with the halfnuts engaged. Mark the chuck and the leadscrew at TDC. Take a cut. disengage the half nuts, move the carriage back the the start point. Wait until the marks on the chuck and leadscrew line up on TDC and engage half nuts.(I think lol*)

Stuart

* sure "sounds" easy enough. Though how easy it ends up being might depend on the TPI you are cutting and the TPI of your leadscrew

[Metmachmad]

Please bear with me here, but I must be missing something regarding the pitch of metric threads. Maybe someone can explain it to me (and possibly others also.)
Say you have got a 10mm thread with 1.5 mm pitch you are wanting to cut on an imperial lathe, to convert it to Threads Per Inch (TPI), because thats the way imperial lathes cut threads, by my reckoning that would equate to 25.4mm (1 inch) divided by 1.5 mm pitch = 16.933333 TPI.
What imperial lathes allow one to cut 16.933333 TPI, certainly not mine. Please explain.

[Vernonv]

What imperial lathes allow one to cut 16.933333 TPI, certainly not mine. Please explain.

If you have the right change gears you can certainly get close, if not exact.

[Stustoys]

A 127-120 gear gets you exact
63-60(I think thats the one used) gets you close

Stuart

[Metmachmad]

If you have the right change gears you can certainly get close, if not exact.

Are you suggesting that 17 TPI would be close enough?!?!.

[Vernonv]

Are you suggesting that 17 TPI would be close enough?!?!.

Possibly, depending on the intended application - thread pitch error is a compounding problem i.e. on a short thread interface (thread contact length) it's less of an issue, whereas on a long thread interface it is more of an issue.

However, often you can get the right metric thread from an imperial lathe if you have the right change gears.

[franco]

Vernon,

Some time ago I made an expanding mandrel to fit the outer end of the headstock spindle hole so I could mount change gears on it for direct indexing with the headstock spindle. After I made a 37/47 metric conversion gear I thought of making a handle for the mandrel to wind the spindle backwards manually. This got boring very quickly, so as a temporary measure (several years ago!) I made a quick release coupling to fit the mandrel and use a 13 mm electric drill to reverse the spindle after disengaging the back gear - not the half nuts - so the drill does not have to back-drive the motor and countershaft. As a temporary measure this works OK for the small number of metric threads I cut on this lathe - must get around to making a more permanent solution - one day - maybe..........

Using the tumbler gears to reverse the leadscrew as suggested above to wind the carriage back will not work because the relationship between the spindle and leadscrew positions will be broken.

Frank

[matthew_g]

leadscrew reverse wont work unless it only have one engagement postion and is spinning 1:1 with the spindle(I think)

There is one way that I'm aware of(but have never tired)
You have a fixed starting point for the carriage with the halfnuts engaged. Mark the chuck and the leadscrew at TDC. Take a cut. disengage the half nuts, move the carriage back the the start point. Wait until the marks on the chuck and leadscrew line up on TDC and engage half nuts.(I think lol*)

Stuart

* sure "sounds" easy enough. Though how easy it ends up being might depend on the TPI you are cutting and the TPI of your leadscrew

Stustoys has hit the nail on the head for a quick easy fix for a one off.
I have done this exactly as Stu has said for a one off job, If you had to make hundreds of parts I would be looking for other alternatives, but this way does work, there is no extra expense to justify and no extra 2 or 3 days of modifying your lathe for a 2o min job.
Good luck mate.


Matt

[steamingbill]

Have a look at the calculator in this thread.

https://www.woodworkforums.com/f65/la...ulator-118398/

Its good at showing how close you can get to metric threads.

Ironic - I just realised that it was written by Vernon the chap who started this thread -

There are some other calculators and examples here Screwcutting Programs the text below is copied from that page.


EXAMPLE 2: (Imperial leadscrew, metric thread required)
Leadscrew Turns per inch : 8 (1/4 inch pitch)
No. of teeth on primary shaft gear: 48
No. of teeth on 1^st stud driven gear: 50
No. of teeth on 1^st stud driver gear: 36
No. of teeth on leadscrew gear (driven) : 44
TPI = 50/48 x 44/36 X 8 = 10.185 TPI
Convert to millimeters : Pitch in mm = 25.4 / TPI
25.4 / 10.185 = 2.494 mm pitch
This equates to a 0.25% error against a 2.5-mm pitch.

Bill