Max speed for an 8 inch 4-jaw?

I'm turning some parts on a 12x36 lathe using the 8 inch 4-jaw that came with it. The mass of the workpiece is negligible compared with the chuck, but the geometry is such that whilst one pair of jaws is balanced the other pair isn't - one of them is pretty much flush with the chuck body, the oppposite one sticks out about 1.75 inches. I'd like to run at maximum speed (2000 rpm) but haven't dared to so far - highest I've got to is 700. I suspect the setup is far stronger than I give it credit for, and I'm hoping someone who knows will tell me not to be a wimp and go for it, but I don't want to learn the hard way!

Rob.

PS - no idea if the chuck is cast iron or steel (suspect former) and there are no markings giving max rpm. Lathe spindle has 38mm bore, so bearings must be fairly chunky. R..

From Machinery's Handbook, a solid rim spoked CI flywheel would be good for about 2,400 rpm at 8" diameter.

A 4-Jaw chuck has a weaker structure than a flywheel, plus once tightened up on work you add to the bursting stresses, so I would be cautious...

That said, Grizzly's 8" 4-Jaw manual says 3,000 rpm with the caveat "The maximum speed listed above is ONLY
possible with the chuck jaws and the workpiece in complete rotational symmetry. The workpiece weight must be within the limits of the lathe, and the workpiece mass must be of equal density throughout to prevent centrifugal imbalance or radial runout—even if a tailstock or other support is used for additional support."

Bison give much more detail. For a cutting force of 30 newtons (just 3 Kgf) they say 4,000 rpm(!) but this drops to 1,000 rpm at 350 newtons.

If it was me I would be nervous of going over 1000 rpm with anything that wasn't perfectly balanced and a light cut.

I quote from the manual for my Bantam:

IMPORTANT

Take careful note of speed limitations when using faceplates: 18 in faceplates should not be run at more than 410 rev/min and 10 in faceplates at not more than 1220rev/min.

Derate/re-rate by the ratio of the squares of the speeds gets you a limit of 1900 RPM for 8 ins dia. My sphincter tells me that this is far too fast and I wouldn't go above 1000 rpm for 8 ins, and that's assuming the load is balanced. If not derate by at least a further 2:1. If the whole machine dances across the floor then derate further.

And don't stand in line with the chuck!

Rgds Simon

Too right, that thing could kill you at those sizes.

People might think this is just a rare happening but I know of someone who was killed trying to turn a very large piece of wood that was thrown out.

I would seriously consider staying well away from the stated maximum speeds, you're not quite sure under what conditions or work piece they were using with it. I don't really get this obsession with speed, there's no reason why it wouldn't turn at a slower speed.              

Michael W

Edited By Michael-w on 29/05/2017 00:01:44

If the OP were to state the actual mass of the item to be turned, the real offset from the centre line, dimensions, material, density, and more - he might get a more specific reply.

I would not countenance making a judgement on the information given. Rotating devices are dangerous enough without stretching close to mechanical limits. I note that a 4" item would have a surface speed of around 2000' per minute. Just those two off-centre jaws will have a considerable (non negligible) effect on the balance of the chuck.

My 12" faceplate is marked as 1200rpm maximum, but interestingly the 18" faceplate is marked 1050rpm.

As for the OP's question, if you can feel the lathe vibrating due to out of balance then the speed is too fast. Of course that assumes the lathe is bolted down properly so it isn't going to go walkies.

Andrew

+1 on don't go fast with an off-balanced job in a four jaw. Could be disastrous. Also not good for your headstock bearings.

Can you tell us more detail of what the job is, and why you want to spin it at such high rpm? The panel may be able to offer some suggestions for alternative ways of approaching the problem then.

If, for some reason, you absolutely must use high rpm, it might be safer to bolt (rather than clamp) the job securely to a faceplate and put a suitable bob-weight on the faceplate to achieve near-perfect balance (weight also firmly bolted to the faceplate). Even then, observe the already posted limits on faceplate rpm.

Or it may be advisable to bolt or clamp the job to a vertical slide or angle bracket on the cross slide and spin your boring bar, reamer or whatever tool you intend to use, at the high rpm in the three-jaw or collet chuck.

Its a bit like using a boring head the chuck max speed will usually be above what you can run at without excessive vibration.

One option is to add a suitably heavy packing piece between the work and the jaw nearest the ctr to compensate for the out of balance of the part, you can see I have added a bit of steel bar to this setup between jaw and mounting plate though still ran a bit below what I would for plain bar.. (do makesure any packing/balace weights are secure) Large 4-jaws often have tee slots between the jaw slots so you can bolt on weights.

And a very poor photobucket video of it here

Hi, this **LINK** may give you a guide to the max speed of your chuck. As has been said that the job is unbalanced, then the speed needs to be reduced and the more unbalanced the lower the speed but not in a linier sense. Your headstock bearings may well be chunky, but a too higher speed with an unbalanced load can soon damage your bearings, which may not be apparent in the short term. Unbalanced loads at high speed can become unstable without any warning. Hopper's faceplate suggestion may be a better option.

Regards Nick.

On my 18 in dia 4 jaw chuck is stamped max speed to be used 800 rpm but on the 24 in dia faceplate is stamped max speed 900 rpm

It depends on the chuck itself. Quite a few 300 mm 4jaws have 800 rpm stamped on them. But some steel bodied chucks have 2500 rpm stamped on them. Me, I would keep under 800 rpm anyway and only run as fast as the balance allows. Some stuff can only be run slow safely.

Neil

Thanks for your replies. In the past I have I have underestimated what machines are capable of (eg painstakingly taking 0.25mm cuts when the lathe is capable of 10x that and more without batting an eyelid), hence my question. At 700 rpm there was no noticeable vibration, but my 'sphincter sense' told me that going any faster would be pushing it unless I had advice to the contrary. Hence the question. From what's been said it seems 700 is about as far as I should go.

The workpiece is actually a rectangular piece of teak in which I want to gouge a flat bottomed 60mm diameter off-centre hole with with a pillar in the middle of the hole. So the workpiece mass and cutting forces are unlikely to be problematic. It was the imbalance of the jaws and the possibility of the chuck bursting that worried me. Other ways to do it I know, but the 4-jaw was on the lathe so I thought I'd give it a go, then fell to wondering how fast I could go. Actually 700 will be more than enough when I think of surface speeds.

Thanks again, especially to those who have quoted from manuals etc. All good stuff which contributes to my eddication.

I realise that I should be consigned to the sixth circle of hell for turning wood on an engineering lathe, but needs must when the devil drives....

Rob.

The difference in speeds quoted may depend on the grade of cast iron, and some face plates may be made of steel.

My diesel mechanic nephew has a piece of a fly wheel from one of the (petrol) Bedford buses that he used to service while doing his apprenticeship. The bus was in service as a school bus when the flywheel exploded.

Ian S C