Questions about lathe power feeds.

My lathe (a generic Far Eastern 12x36) has surfacing (X-axis) and sliding (Z-axis) power feeds. According to the plate on the headstock the gearbox allows a range of surfacing speeds from 0.011 to 0.276 mm/rev and sliding feeds from 0.053 to 1.291 mm/rev.

I don't understand why there is a factor of (roughly) five between the two ranges. Is it perhaps a 'feature' of these lathes resulting from economies in the design of the apron gearbox? Or is there an 'engineering' reason why surfacing feed should be so much slower than sliding feed?

If anyone can shed light I'd be grateful.

Robin.

Edited By Robin Graham on 05/06/2020 21:52:53

Economy of design, I'd guess.

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

On my lathe (M300) the cross feed is half the selected sliding feed. I suspect it's to do with the space available for convenient size gears in the apron.

Andrew

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

My Warco faces at about 40% of the turning rate, I just flick the 3-way speed selector into the next position when facing and get the two speeds approx the same without having to play with the gears.

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

Its normal practice to have the facing feed slower than the sliding feed.

Generally with a mostly common gear train inside the apron. The sliding feed is generated via the rack on the bed and the cross feed via the feed screw. Rack pitch is always coarser than the feed screw so sliding feed will be greater unless the designer opts for more or less completely separate drives. Which adds much complexity for little benefit as the slower surfacing feed works fine when used appropriately.

Worth nooging around on the www.lathes.co website to see typical apron and power drive internals to get a feel for why things are done that way.

Some machines have sliding feed and cross feed directions reversed so when the sliding feed is towards the chuck the cross feed is away from the work. Something that I found seriously annoying on my Southbend Heavy 10. But if the lathe only has a single clutch for both feeds it is a safety feature should the user inadvertently engage the wrong feed during a job. My Smart & Brown 1024 only has a single clutch but bothe feeds are in the same direction and I have been known to forget which is selected. Ooops!

Clive

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

Clive I have never come across a lathe with the sliding feed generated by the rack. Surely it's what the leadscrew is for.

Roy

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

Who wouldn't want a finer feed on the surfacing? Do you want a crappy finish for some reason? The causer feeds are normally only there by accident of the screwcutting gearbox giving the range and marketing put it in the spec to make it seem like there is something extra. It's like the screwcutting range specified - when did you last screwcut 128tpi but hey your lathe does 40 imperial threads so must be better than the one with only 20 quoted.

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

Feed directions are left and out like the Southbend.

Roy

Just about all lathes with a feeds / screwcutting gearbox use the rack to drive sliding feeds via a worm drive and keyway pick off from a feed drive shaft. Sometimes, on lighter machines, the feed drive shaft and threading feed screw are combined by cutting a keyway in the screw. The SouthBend and its clones being a common example.

The only industrial standard lathe I can immediately think of that uses the feed screw rather than the rack for sliding feeds is the Ikegai **LINK** . But that has a ballscrew and a fair few other features suggesting the designer was unimpressed with much of standard lathe design practice and metaphorically speaking said "Stuff it. I'm gonna do it right." Leaving aside the practicalities of weight, size and power supply I think I'd rather like one.

Clive

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

Wot? I use the same feed rates for sliding and surfacing cuts (changing the feed gearbox) and I don't get crappier finishes on facing. With the caveat that sometimes the finish deteriorates at the centre. But that's down to material and surface speed issues, not feedrate. It's rare that I need to face an item right to the centre. Normally there's a centre hole so I'm not worried about the last little bit.

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.

On my Colchester Chipmaster, surfacing feeds are half that for sliding.

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

Bill

The Ikegai has a lot of fascinating detail, and not so detail, differences in design and construction when compared to the usual modern lathe like the Colchester 2000 et al.

Looks to me as if the designer made a list of all the things that seemed wrong to, or just plain irritated, him with "standard" lathes and controls. Then set out to fix them the way he wanted.

Hence the do it right comment.

Something I've been known to say and do at times in my field!

Clive