Really Silly Question - rpm facing off large diameters

SO....... this feels silly to ask but, If facing a large (say 60mm) diameter, the RPM will be fairly low for mild steel, say 200-250rpm? - but what happens when you get closer to the centre.

A few of my thoughts;

Would you increase speed if you had the option (motor with a speed controller perhaps?)

Would you stop and change speed via belts - perhaps maintaining and average of the speeds for a final pass?

Would you just slow the feed more and more as you get closer to the centre (this could get impractical if facing 4 or 5" above diameters)

I really don't know, but I would welcome some guidance or shared experience,

Many thanks,

Russ B

sorry the title is supposed to read "........facing off large diameters" 

Edited By Russ B on 16/09/2014 14:58:32

Hi Russ

Ideally a constant surface speed for the particular material is the ideal. That means you will be speeding up as you approach the centre to maintain the constant ideal surface speed.

If you have variable speed than it is easy to turn the speed up as you proceed. Some newer lathes even have this as a built in facility.

If you are looking for a nice finish carbide tooling can be a problem tearing rather than smooth cutting as you near the centre of the work and the RPM is below optimal. I find that a nicely ground high speed steel tool can give a better finish near the centre.

Stopping and changing speed can be done but you will always get a small line in the finish where you stopped.

Regards
John

Edited By John McNamara on 16/09/2014 15:14:42

For 60mm just run it at the rpm you suggested and face it at that with HSS tooling. You can usually face ok at below the optimum surface speed but not too much above it. I have never bothered with upping the rpm toward the centre. One nice cut should do the job all the way across.

The problem with upping the RPM is that you have to increase the feedrate by the same amount to maintain the same surface finish. Virtually impossible to do accurately on a manual lathe.

Neil

Thanks for the input, it was just one of those little things that I could place. Hopefully in the next year I'll get to spend some time watching or working in the manual machine shop at Uni - just watching someone experienced do something very very simple will teach me a lot without any thought or spoken words or really any concious thought

"monkey see, monkey do"

I don't understand that. Surely if the lathe is set to face at, say, 4 thou/rev then that's what it will be irrespective of spindle speed? So if the spindle speed is increased (to maintain the surface feet per minute) as the tool moves towards the centre of the work then the feed will increase in sympathy, while keeping the same feed per rev.

Regards,

Andrew

Hello all

When facing, provided that the spindle speed used is somewhere in the right ball-park, the surface speed of cut will be higher than optimum (for the material/tool/cut/feed/lubricant etc combination) at the outside diameter, and lower than optimum at the centre. At some radius, it will be like Goldilock's porridge. At this point, the surface finish will be the best optainable, and will be visible on the faced surface. This is called "Whittaker's Ring" and well known to older manual machinists, who in practice modify the feed rate by feel and sound to obtain the best result over the whole face. Bit like riding a bike.

Works for steels but not very apparent with free-cutting materials, don't know about non-ferrous.

Jim

Andrew

I think maybe Neil is referring to feeding across the work face manually, not on auto feed. So if you increase the rpm you have to turn the feed screw faster to maintain the feed rate in thou's/rev.

Nigel

Nigel's right, I was thinking of manual feed and ignored auto cross feed which will speed up with the increase in spindle speed.

Neil

My way: do it at the highest speed the tool and machine will stand at the largest diameter. High speed steel can still cut with steel chips coming off blue if the cut isn't too heavy. If you change a belt speed half way you are likely to end up with an obvious blemish. When you can't continuously change speed reduce the feed rate as the diameter is reduced. Colin

It takes a brave man to part off with auto feed...ime still at the sea bed in terms of my know how but ide never try it.

EDIT...Ahh just re-read and its facing off not parting.

Edited By roofer on 17/09/2014 18:15:24

Just me I guess, but I wouldn't bother to make any compensation except perhaps on a final, finishing pass - and then the cut certainly wouldn't be too heavy.

And I have continuously variable speed (but not power cross-feed .... I wonder how many here actually have that).

Oooops, didn't think of that.

To answer Bandersnatch, I do have power cross feed, and listed it as an essential feature when drawing up a specification to decide which type of lathe to buy.

Roofer: I almost always part off under power feed. Mind you it divides the professional community as well. I have two friends both of whom are professional machinists. One swears by parting off under power and the other would never do it.

I can't get my head round the need to change the feedrate as the surface speed changes? If I understand correctly the idea is that as the surface speed reduces the feedrate should be reduced as well. Does this apply to normal parallel turning too, and if not why not? Logic would dictate that if I turn a given diameter at a certain rpm and feedrate per rev then if I double the rpm I should also double the feedrate per rev? Does anybody do this?

Regards,

Andrew

Until modern cmc times,there must have been tens of thousands of lathes,centre ,capstan ,autos for example where engineers and operators had to live with problem of variation in cutting speed when facing,some materials are more forgiving to tools so at the start of cut a 10 to 20 %increase in the correct surface speed is acceptable,particularly with free cutting materials,to save time going right to the centre pip,a centre drill hole if permissable can be used to run out the tool. On castings,which required facing ,non functional areas would be relieved and left as cast ,so the outer diameter could be faced at the correct speed ,the tool wound over the relieved area quickly and the central area machined at a higher speed,a design feature to reduce machining to the minimum,when turning with carbon steel tools was slow in Victorian days, Castings were not made with fancy shapes just for the fun of it,Tougher steels need more care and patience,start too fast and the tool will wear quickly so by the time its run on to the centre ,the face area may not be flat as well as having a poor finish ,just be patient its a hobby not a production engineering challenge.

I rarely alter the feedrate when turning along the lathe, generally it stays in the same gear giving the same feed per rev whether the spindle is going at 50 or 1000rpm, maybe up the rate when roughing but not often.

Same with facing select a gear that gives the same cut per rev as turning (yes I also have power cross feed) though I do wind up the wick as the tool gets nearer the centre.

Think the Hardinge lathes had independant feed speed from the spindle speed.

Edited By JasonB on 17/09/2014 20:44:36

The spindle speed will have an effect upon the cutting speed in feet per minute (in pre Napoleonic measurements) . However the reality is that this is only critical when facing large diameters. 60mm, regardless of the machine being used, is simply NOT large diameters so it is not necessary to alter the spindle speed when facing.

Now in my early days on my lathes in the Arc, we turned/faced cast iron parts and mild steel components from material of the order of 3 or 4 feet diameter. Now that constitutes "largish" diameters and it was necassary to alter the spinle speed ast various stages to maintain something like the correct cutting speed. At 3 feet diameter asp[inle speed od about 12 rpm woould give you a cutting speed of around 100 feet per minute.

I don't doubt it Andrew - but from your postings here over the years I don't think your shop could be in any way described as typical in this milieu.

Given the reputation of the bandersnatch I suppose I am lucky to have merely been put in my place; but apparently that place isn't on this forum.

Andrew