Lathe Options Metric or Imperial

i have recently purchased a Milling Machine which is set you with metric measurements and I would when money permitting get a Laithe to accompany it. It has been my intention to get a laith with metric measurements.

But it has been mentioned to me that could be a problem when trying to cut threads as most threads are still in imperial setup even if the heads of the bolts are set up for metric spanners and sockets. Although some/a lot of automotive bolts now have metric thread sizes.

Is it correct that a metric set up Laithe is going to have issues cutting an imperial thread?

All advice is much appreciated.

Hi Jamie,

Any self-respecting lathe will be able to cut both metric and imperial threads - it is just a matter of selecting the appropriate gears.

That is the basic simple answer.

A bit more complex is that essentially there are three types of lathes for screw-cutting purposes - the so-called training or plain lathe, non-screwcutting gearbox or changewheel lathe, and screwcutting gearbox lathe.

The training or plain lathe generally does not have provision for gear cutting.

Non-gearbox or changewheel lathes uses a system of gearing between the mandrel and the leadscrew to cause the cutting tool to travel at an appropriate speed to cut the thread. Different pitches are catered for by changing the gear ratios. These lathes usually have sufficient gearwheels to allow the lathe to cut the majority of threads one might desire to a suitable accuracy, in any threadform, and to any length. There is usually a chart of some description detailing the required gearing for the pitches shown, but other pitches can be cut by calculating the gearing required. The big problem is the accuracy required as the ratios to cut an imperial thread on a metric machine, or vice versa, may not be exact, but for short threads will generally be satisfactory.

Screwcutting gearbox lathes have a gearbox which allows the selection of specific pitches without any need to change the gear wheels. Any ratios not covered by the gearbox will need one or more gear wheel changes. As will the selection of pitches for anything other than the built-in form. By that, I mean that a metric lathe fitted with a screwcutting gearbox will need a (translation) gear change to cut imperial threads, and vice versa. Again, there should be charts available to help sort it out.

For what it's worth, I have a metric changewheel lathe. According to the screwcutting chart it can cut 23 metric pitches and 29 imperial pitches.

My personal thoughts are that you should go for the metric lathe as everything is gradually being changed over to metric.

Hope this helps,

Peter G. Shaw

It's not impossible to cut metric on an imperial lathe and vice-versa, just a bit more awkward.

It's usually only bolts/studs into aluminium blocks that sometimes have imperial threads these days, as they are less likely to strip.

On older cars all bets are off.

Neil

I have a foot in both camps, my larger lathe is Metric and my smaller Myford S7 is imperial, does not matter a damn if you fit a Digital Read Out, you can have both.

My WARCO mill is metric. I got fed up waiting for an ordered WARCO metric lathe but they had an imperial in stock so I had that. No problems between swapping between the two as I was in the industry when things started swapping over from metric to imperial so I got used to using both.

All of the kit in my workshop is a mixture of systems and most in here who have been "fettling" for years will probably be no different. If an inspector from BSI ever came snooping round her he'd have a fit - an interference fit?

Rik

Re On older cars all bets are off

You can also get a set of thread gauges which will help you to identify the pitch. I have imperial, metric and BA.

Cheers,

Peter G. Shaw

Cutting threads is not the only problem. Metric machines have metric holding bolts and spanner sizes. Inch lathes have Whitworth if old and Unified if newer or from USA. It is important on a mill as eg the 2 morse taper comes with either a 3/8 Whit holding screw or an M10 holding screw, and while they are not interchangeable, the Whit one will go in several turns before jamming solid ...

My recommendation would be to stick with one or the other, and get a digital caliper to ease the swap from one dimension system to another.

Cheers

Edited By Tim Stevens on 14/02/2015 17:11:51

Edited By Tim Stevens on 14/02/2015 17:12:07

My imperial lathe takes M10 hold down bolts on the cross slide same as my imperial mill uses M10 on the table, Mill also has a M12 drawbar.

Unless you are going to be cutting a lot of long threads I would not let the threadcutting setup of the particular lathe influence your choice.

Edited By JasonB on 14/02/2015 17:17:14

You are unlikely to cut more than a couple of dozen threads a year, and probably most modellers do less than half that. So not worth fussing either way.

What is significant is the cross-slide and top-slide screws and whether they are metric or imperial. There are many lathes now that are fitted with "dual" dials claiming to handle both scales but one will be either awkward to use or frankly wrong.
Or it may have one dial that is a 'kludge' eg 1mm screw pitch, dial with 10 major divisions (fine that's clear) but 4 subdivisions because that is roughly '1 thou' but puts you in danger of turning a good fit into a sloppy one. Ok they are 0.025mm but if you have a metric mindset you want 0.02 or 0.01 so you are forever estimating.
So I sugest deciding if you have a metric or imperial mind and concentrate on getting those two slides in the right system AND make a point of checking the lathe really does have that pitch and not a kludge dial which the vendor does not even realise.

My lathe & mill are both metric simply as most equipment & fasteners are metric these days, albeit, as stated there is a lot of imperial stuff still kicking about, & will be so for many a year.. I still have a Snap On 1/4" drive socket set from over 40yrs ago, plus imperial allen keys, loose & clipped sets, around in my gear from yrs long gone.

George