The slowest gear on my son's Chester is so slow it doesn't even quote a speed for facing. I assume part of the difference is simply down to the cross slide being on a screw and the carriage on a rack.

Personally, I don't mind too much as I'm pretty sure I would break something if I tried to part off at the same rate as plain turning.

Brian G

Don't think it is an engineering thing. The Smart and Brown Model M has sliding speed is pitch/16 and surfacing is pitch/32 so different but only a factor of 2 from one to the other.

Martin C

Good old engineering compromise I reckon.

Cutting speed is an important parameter. Turning a rod lengthways, it's easy to set a constant speed optimised for material and cutter. However, the lathe needs to cater for slow metals like cast-iron and mild-steel, medium speed free-cutting steels & bronze, and fast cutting metals like Brass and Aluminium. Ideally it should allow carbide to cut about 5 times faster than HSS as well. Having plenty of choice of lengthways cutting speed is a 'good thing', and most machines provide it.

Facing is a different problem because cutting speed changes radically as the tool moves towards the centre. At the outer edge the work whizzes past the cutting point, but cutting speed drops towards zero as the tool moves to the centre point. I guess a lathe's base facing feed rate is chosen to suit the maximum outer diameter it can face. After that, not much point in fretting because the cutting speed inevitably drops at the cutter moves in. Good enough results with a simple gear box. Same gear box deals with parting off, which calls for steady rigidity rather than perfect speed control.

Maybe someone could build a motorised lathe designed to face closer to constant cutting speed by accelerating the tool-post as it moves to the centre. I can think of lots of problems with the idea! As my lathe faces well with ordinary methods, gut feel suggests fine tuning facing speeds is more trouble than it's worth. Good enough is good enough. No facing disasters on my long list of miserable workshop failures!

A less satisfactory compromise on many Chinese machines is minimum rpm. Often screw-cutting under power on these machines is much too fast for comfort! Electronic speed control is great for most purposes, but my dear old mini-lathe definitely needed back-gear. My WM280 with 3-phase motor and VFD is much better managing well enough at 30 rpm, but it's far happier cutting threads in reverse away from the headstock 2 to 5 times faster. Although a WM-280 is 'good enough', having back-gear as well would help on the few occasion's I need lots of turning power at slow speeds. But back-gear is a costly addition and an obvious economy when it's only rarely needed.

Dave

I think you would want to accelerate the lathe spindle speed rather than the feed to keep the surface speed the same with the same (constant) depth of cut set by the gearbox. I think that would be equally worrying to the operator!

Steve

It does on my lathe – Harrison M300. The leadscrew doesn't even turn with feeds selected. The leadscrew only runs when thread pitches are selected. But of course my lathe is industrial with a separate feed shaft running parallel to the leadscrew.

Andrew

Sliding feed on my Rockwell is generated by the slot in the lead screw.

Feed directions are left and out like the Southbend.

I think SoD is getting confused between surface speed and feed rate. When facing ideally one would increase spindle speed to keep surface speed the same as the tool approaches the centre. Indeed on some materials with insert tooling one can see the surface finish change (gets worse) as the tool moves to the centre. On CNC lathes it's easy to use constant surface speed (CSS) when facing, via the G96 code. But the feedrate in terms of distance per rev stays constant. To a large extent surface speed and feedrate requirements are independent of each other on a lathe. Not so on a mill – there it's chip load rather than feed that stays constant with spindle speed.

I'd agree with Clive that the slower facing feeds are down to the practicalities, and cost, of the mechanism within the apron rather than any cutting considerations. My lathe has selectable feeds from 1 thou per rev to 80 thou per rev, with facing feeds half those. Never had the nerve to use feedrates much more than 20 thou/rev. Even with a 3hp motor and a geared headstock the lathe is power limited on high feed rates with sensible depths of cut.

Andrew

The point is he has a 1.2mm/rev sliding feed so to avoid a striated finish tools have to have a wide radius or long flat whereas the surfacing feed is much less to a better finish will be achieved if a pointed tool is being used. yes, you know to put a flat on the tool but a lot of beginners don't.

The reason that the rack is used for sliding feeds is to preserve the accuracy of the leadscrew for threadcutting. Many of us are familiar with a lathe or mill's screws being locally worn. Apart from small 'amateur' lathes, even lathes produced for training or light engineering purposes had a separate feed bar usually driven from the gear train that drives the leadscrew, or a keyway cut along the leadscrew to serve the same purpose. Clive's point above about doing it right, I take it to mean using current technologies of recirculating ball leadscrews or other methods causing less wear of the leadscrew.

Bill

Oh poo, Andrew is right! But as spindle speed and feed rate are geared together on most lathes my point about cutting speed still stands. I hope…

Dave

Clive, looking at lathes.co.uk, the Ikegai A-20 (for example) has some very nice features and seemed to incorporate these quite early on, including the recirculating ball leadscrew. But then it is described as a toolroom lathe.

It almost breaks my heart to imagine what it might have cost new, and after a google-search to see the second-hand prices at the end of its life.

Bill