Lathe Speed - What am I missing out on?

Two main reasons, carbide and small diameter work.

A metric rule of thumb for HSS cutting speed in mild-steel is 10000 / diameter in mm.

The most common need for high-speed on my lathe is drilling. Applying the formula suggests:

1000 rpm for a 10mm drill,
2500 rpm for a 4mm drill,
5000 rpm for a 2mm drill,
10000 rpm for a 1mm drill,
20000 rpm for a 0.5mm drill

Not essential to cut at high-speeds, but the benefit of speed becomes more obvious with small drills. At high-speed they are less likely to break and stay sharp longer. Most noticeable if you need to drill a lot of small holes in something nasty like fibre-glass printed circuit boards.

Similar advantages turning small diameter work in a lathe; high rpm makes the job a bit easier.

Carbide works moderately well at ordinary HSS speeds, but it really comes into it's own worked about 5 times faster and harder than HSS can cope with. Lots of power, high rpm, high-feeds, and deep cuts. Modern CNC machine tools often run at high-speed - up to 20,000rpm or more.

For ordinary workshop use I guess most of us work below optimum speed and it doesn't matter. The best speed is the one that suits you! Pays to experiment because there isn't a simple answer - it's a compromise between the capability of the machine and its operator, the tool he's cutting with, and the size and type of material being cut. In my workshop, the size and type of material varies almost continually as I work through a build. Loads of variation compared with most commercial work. As there's rarely a formal specification, quite often I can't be bothered to change belt speed, or select the perfect cutting tool. When it matters I take much more care, including whacking up the rpm.

Dave

One of the problems with descriptions like "mirror" or "good" finish is that they are subjective and determined by a standard deemed acceptable by the user, which may be different to somebody else. To me a mirror finish means you should get a perfect reflection from the work surface, just like looking into the bathroom mirror.

For a mirror to act as a mirror, ie, mostly specular reflection, the surface finish must be significantly smaller than the wavelength of light. That implies a surface roughness of around 0.1 to 0.2µm. I get about ten times that when turning, which is roughly in line with the range of surface roughness expected, according to Machinery's Handbook. I don't even get a mirror finish on my cylindrical grinder. Better than turning, but not mirror according to my definition.

A look at this thread may be instructive, certainly for carbide inserts the surface speed can be critical depending upon the material ("sticky" steels are among the worst):

https://www.model-engineer.co.uk/forums/postings.asp?th=51900

While surface speed may, or may not, be important, there are also considerations based on work size, as mentioned. The smallest drill I've used on my lathe is 1/32", can remember what spindle speed but probably 1700rpm. On the other hand turning a 16" flywheel would be pretty exciting at, say, 420 rpm:

Even at 85rpm it's pushing carbide inserts.

As and when I get my standard Pultra instrument lathe running the top speed will be 6000 rpm; the special high speed version went to 10000 rpm.

Opening up the question in a wider sense would anybody only run their mill at a single speed irrespective of cutter size or material? Across my three mills I've run cutters from 30 rpm to 24000rpm.

That's enough pot stirring for the moment; last but one day at work so the countdown has commenced!

Andrew

The other thing about using more feeds and speeds is you would could up your productivity 10 fold.

You say you run at 420rpm and the use the finest feed. Now I have 3 easily selected feeds for any gear train that I am running and that usually does for me, one is twice as fast as the finest and the other twice that so I can feed at 4 times the fine rate. I mostly keep my variable speed machine in the lower of the two speed ranges so can get from around 65 to 1150rpm which is not far from 3 times as fast as your "fixed" speed.

So to rough out a small diameter component I could be turning a 3x and feeding at 4x which means the job takes 12th the time it would take you.

I expect to now get the usual posts that say "it's just a hobby, whats the rush" well that's fine for the retired types who have all day to potter about but for those still working or who like to complete more than one project per decade working at a good pace is an advantage.

Have a look at my post on the bottom of this page to see how finish changes as speed and feed increase

I seem to have less difficulties with high speeds as low ones. There are times I want the machine to run ate less than its available speeds.

I've fitted 3-phase conversions to the Myford 7, Harrison L5 and BCA jig-borer, will be fitting one to the Myford VMC Mill; but these systems want the motor to run fast so even on the lowest pulley or back-gear sometimes the ML7's speed dial is verging on the yellow.

The problem is greater on conventional machine like my Meddings bench-drill if I want to use something like a hole-saw, or even its largest twist-drill (1/2" in steel. The answer there is an old, very care-worn Progress 2G drilling-machine, dedicated to slow-speed work, but I've to find room for it yet. At the rate I'm going the workshop won't have room for me to do any work in!

When it's the Denbigh horizontal mill's turn for returning to service, that will have a conventional single-phase motor but large reduction, possibly by gears as well as belts. (I bought it with a ramshackle confection of motor, old car gear-box and improvised chain-drive on an angle-iron tower straddling the mill itself.)

I got a mirror finish once saw a old man looking back at me scared the life out of me

That's why I haven't got round to fitting a mirror in the newly refurbished main bathroom yet!

Andrew

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Sorry to labour my earlier point, Dave [and I note your remark] ... But I think it fair to say that your 'rule of thumb' gives maximum, rather than necessary, speeds [optimum being a term with too many factors for my brain to compute]

Watchmakers habitually use drills, and turn work, smaller than 0.5mm diameter; sometimes at very low speeds comared to those listed, and have done so for generations.

What matters most is getting an appropriate balance of speed and feed ... Small drills work well at low speeds, provided that you can feed them at appropriately microscopic rates.

MichaelG.

Thanks for the clarification Michael - worth emphasising again that the published figures and calculators are aimed at industrial users whose notion of 'optimum' is tuned to the economics of large scale machining. The main cost items are metal-removal rate, tool-life and power consumption. In a large production run it's well worth balancing these. Also other factors unlikely to apply at home like overtime!

I don't bother with optimising. Rule of thumb cutting speeds indicate a starting point, they're not essential. Mostly I work slower than recommended. Carbide is perhaps an exception; I've found good finish with carbide is most easily achieved by violent cutting at recommended rates. HSS seems less fussy.

Dave

Agreed, had to machine the thread off a 50mm dia high tensile bolt. Going at about 700rpm with carbide the interrupted cuts until the thread cleared were going to shear the tool post right off the crosslide. Switched to a CBN insert and cranked up the speed to 2100rpm and the process was way smoother. The only downside it the burning chips flying all over the place!

Edited By ChrisB on 19/06/2019 21:20:50

Edited By ChrisB on 19/06/2019 21:21:12

Sometimes just changing the speed a little will improve the cutting, reduce chatter or change the surface finish.

My car has a six speed box, I don't stay in third all the time.

 With carbide inserts, you have to take deep cuts with inserts designed for steel or the surface finish suffers. Using inserts designed for aluminium to finish steel allows very small depth of cut, useful when you are creeping up on the required size. They are sharp like hss.

Edited By old mart on 19/06/2019 21:44:54

Hi Gents,

So, This is a total newbie follow-up question I guess...

I am turning around 10mm Brass. (I'm trying to make a small whistle, spinning top, etc etc.) all with some small stock to get in some practice.

From the posts above, and from what I can find online, I should be turning at >2000+ rpm?

Is that correct? I'm still a little nervous about spinning my lathe that fast, and have mostly been using it at around 200-300 rpm for similar work, with horrible results. I get a crappy finish, often the thinner the brass stock can bend quite visibly instead of being cut, and it can make a nasty grinding noise.

I have carbide tools, and have the extra sharp mirror looking inserts, which work great on alu at around 300-400 rpm. (Although that was thicker ~30mm alu stock!)

All this cutting speed stuff is very new to me, and I'm starting to think that my previous efforts were a lot more luck than skill. ha!

Thanks in advance.

Del

If those inserts really are sharp, I'd guess you're off centre height, probably high. I think I could get a pretty nice finish on any kind of 10mm brass I know of with sharp HSS tools on centre at those speeds, although I'd normally run about twice as fast at that diameter, even with HSS. If you get rubbing and deflection, there's usually something more than just speed that's wrong.

As Mick says it sounds like your tool height is wrong, any error will show up more as the work diameter gets smaller.

Fine to run at 2000rpm but you won't see a lot of difference running at say 1000rpm while you build up confidence. 2-300 is too slow.

LittleToolShop's calculator suggests 3000rpm for HSS on 3/8" brass and 10500 rpm with carbide. Although those speeds are for economic industrial production and needn't be applied religiously in a home-workshop, they're a strong hint that 200-300rpm is too slow!

Assuming a suitable tool-tip, the three factors that have to be balanced to get good finish are rpm, depth of cut and feed-rate. They vary with material and diameter of the work but there's usually a fairly broad band of conditions that will produce acceptable results.

HSS is more forgiving than carbide, but make sure it's sharp, especially if the depth of cut is low. Took me a long time to realise feed rate is important too. Feeding too slowly risks leaving a fine screw-thread effect, whilst feeding too fast can cause tearing. A shaky hand can produce both effects on the same pass.

Try taking the speed up to at least 1000 rpm and taking a few moderately deep cuts, say 0.3mm or 10thou whilst experimenting with the feed-rate. Best results from power feeding, but good results aren't hard to get by steady hand feeding, it takes a bit of practice though.

Carbide works best at speeds and cuts well beyond anything HSS can do. To get full benefits from carbide the cutters are distinctly blunt, and it pays to attack the job with vigour, provided the lathe can cope that is! One trick is to use sharp carbide cutters intended for Aluminium on other metals. The sharpness makes them behave more like HSS.

Don't be disheartened, it takes a while to learn how to get the best out of a lathe. I've been turning for several years now and still get it wrong now and again. Two common beginner mistakes, mine was pussy-footing, the other is angry gorilla with a headache. I use sound quite a lot to judge; seems to me cutting in the right zone has the lathe audibly working for a living without being flogged. Practice, practice and success will follow...

Dave

PS I failed to spot 'nasty grinding noise' = see Mick and Jason's identification of tool-height as the cause!!!

Edited By SillyOldDuffer on 15/08/2019 15:11:08

There is another, longer winded solution, which might help those with less rigid machinery, or no suitable cutters.

Set the lathe up to screwcut the relevant pitch, and work with the thread rather than effectively intermittently across it.

Bill

Edited By peak4 on 15/08/2019 21:10:48

Good morning,

Ok, so last night i was turning ~15mm brass.

I upped the speed and I checked the center height of my single cutting tool I am using at the moment. It was very slightly high. Like <1mm high. I re-shimmed it and got it to where I think is bang on.

The result was much better, so thanks for the advice.

One thing I did notice, (I am trying to make a whistle btw, like this one Whistle pic) that once i'd bored out the tube section, it would ring like a bell when cutting, giving a very rippled effect of the cut. A bit like what you would see if you look really closely at a record. I did the remainder of the cuts with a finger pressed against the brass, dampening and reducing the ringing. The brass only protruded about 10cm from the 4jaw chuck face!

Thanks in advance.

Del

If it rings like a bell, this shows you are exciting it at a resonance and a sure indication you need to change speed. The advantage of a VFD (variable speed) is that it may only take a small change to go off resonance and hence improve surface finish.

I would hope that your <1mm would be considerably less, perhaps 40x less (I work in thous from time to time.)

Changing speed is a good idea. I'll up the speed again to the next pulley.

Also, yes it was a fraction of a mm. I didn't really measure it, as I knew it was off. I've some new (to me) measuring tools arriving over the next few days, so I'll update you once they arrive.