Lathe Speed – What am I missing out on?

[Blue HeelerParticipant @blueheeler]

G'day all,

I run my lathe always on the pulleys that give me a speed of 420rpm and have the gears setup to give me the slowest auto longitudinal feed (very rarely do I thread on the lathe) and whether I'm turning down brass rod to 3mm to make a bolt or turning a 6" cast iron flywheel.

I use HSS and carbide insert tooling, turn steel, brass, copper, cast iron, aluminium, plastic and get a mirror finish on all materials.

So besides a slower auto longitudinal feed rate what am I missing out on by not adjusting belts for different speeds all the time?

Are SFM guides (which are over my head) just to increase hourly part production in a manufacturing setting and not aimed at the hobbyist?

Cheers,
Jim

[Blue HeelerParticipant @blueheeler]

[JAParticipant @ja]

Jim

From what I read not very much. I, and I mean I, would drop the speed for cast iron.

JA

[HopperParticipant @hopper]

420 rpm is the correct speed to turn 1" diameter mild steel with HSS tooling.

But it is also the speed for larger diameter steel if you use carbide tooling.

Cutting brass or ally at slower rpm than recommended does no harm. Just takes longer.

Mostly optimum cutting speed will give you a good finish in reasonable time without unreasonable tool wear. Go too fast and HSS tools wear quickly. Go too slow and materials tend to tear rather than cut, giving rough finish.

Rule of thumb for turning mild steel with HSS tooling is 400 divided by the job diameter in inches. So 400rpm for 1" diameter, 100rpm for 4" diameter, 800rpm for 1/2" diameter and so forth.

Double these speeds for carbide tooling.

Double these speeds if turning ally or brass with HSS

Halve these speeds for cast iron or alloy steel, tool steel, high tensile bolt material etc etc.

But it's all a guide. Whatever works for you, with your toolbits, on your lathe.

Edited By Hopper on 18/06/2019 04:22:52

[Thor 🇳🇴Participant @thor]

Hi Jim,

On my old lathe I do adjust the belt to get different speeds, low speed when turning large diameter jobs since I get more torque and high speed when turning small diameter jobs or brass/alloy to get the job done a bit more quickly.

Thor

[Blue HeelerParticipant @blueheeler]

Appreciate the replies guys.

The belt set at 420rpm for me makes the lathe sound really nice and its all working at a pleasant speed for me and the finish on whatever I'm turning is great.

I think now I'm just so used to it after nearly 7 years of having this lathe (I must point out a very pertinent fact that I'm in mid 50's and before this lathe was delivered off the back of the delivery truck, I had never even touched a lathe before) when I have changed the speeds to polish something it sounds and feels like a totally different beast and when I go back to 420rpm…..all feels good and normal again.

Thanks again for putting up with my newbie questions, I will be a newbie for life when it comes to my lathe and mill, but love every single second I'm on them making or repairing something for my steam engine hobby.

Cheers,
Jim

[Howard LewisParticipant @howardlewis46836]

If it ain't broke, don't fix it!

If you are happy with your results, stick with it! If you start tinkering, you may well improve matters by running tools at their optimum speed, but you might finish up getting worse results or even damaging the tool.

You may well learn a lot, or get a lot of self inflicted grief.

You are a hobbyist, not on piecework, earning your living, so take the easy way out..

By all means play tunes, but be prepared for some disappointments, from time to time.

Howard

[BazyleParticipant @bazyle]

….after 7 years you should be ready to try screwcutting which will bring you a whole now feeling of accomplishment….. but probably best to start with a mandrel handle and take it even slower than 420rpm.

[not done it yetParticipant @notdoneityet]

I often alter the cutting speed while turning or milling if I consider it is not optimum. It can change an acceptable finish to a much better one or help with chip breakage.

60 years ago, most pistol drills were single speed. Then two speed options became available, then variable speed, and later, battery powered. How many would go out and buy a single speed drill these days? Analogous to lathes and mills, methinks!

[SillyOldDufferModerator @sillyoldduffer]

Jim has found a sweet spot that matches his lathe and circumstances. If I could only have one speed on a lathe, 420 is roughly in the middle, and I could screwthreads with a hand-crank.

Unfortunately, 420rpm isn't a universal answer! Too much depends on the machine, rate of work required, finish, materials, diameters, HSS, carbide, coolants, and what you're doing – drilling, boring, parting-off, turning, delicate work, trepanning etc, etc. A wide range of speeds can be very helpful.

My lathe's slow range is 30 to 450rpm, so I could copy Jim almost exactly. However I actually spend most time in the high-range 150-2500rpm, with most work being done around 1000rpm +- 500rpm. Not unusual for me to exploit the full range available either, 30-100 rpm threading, and 2500rpm at full power – 1500W – for getting carbide in the zone (just about).

Cutting speeds are rated for production work and they aren't the law. Still useful though, especially differences between slow materials like cast-iron, steel in the middle and aluminium/brass generally faster. Also interesting to experiment with feed-rates and depth of cut. The finest feeds don't necessarily produce the best finish, and deep cuts can be a jolly good thing too.

My machine isn't perfect for all situations! I'd really like a lathe that could go faster with more power. Never mind, it does all I need and I expect Jim's does too.

Dave

[Nigel Graham 2Participant @nigelgraham2]

The SFM guides are generally drawn from trade practice yes, but they also tend to assume large, very rigid machines many of us don't have. They can be useful guides, but I regard them as maxima more than optima.

They also assume self-acting feeds to keep a consistent cutting rate that can be difficult to achieve by hand on long lengths or across large diameters.

If you obtain good results with what your are using and doing, stick with that; but there's no harm in upping the speed for small diameters and for drilling small-diameter holes; in free-cutting materials. Use lubricant where appropriate though.

Incidentally when I am turning, I listen to the cutting as well as watching it, especially if I'm brushing on a lot of coolant that obscure the revolving cut surface. it's highly subjective, but I find if the cutting starts to sound a bit harsh, it may mean I am forcing it a bit and tearing the material, or the edge of the tool has begun to dull. Or both.

[Mick B1Participant @mickb1]

I was taught the (SFM x 4)/Dia (Imperial inches) formula back in the 70s – the instructors made clear that it's a coarse approximation and there's a lot of tolerance on the SFM, depending on many cutting conditions.

I used it for my years on the shop floor, and it did mean that on the Myford Speed 10 I used to have, for the sizes of work I tended to do, the lathe was set on the 400-odd RPM pulley most of the time on steel, and the 800-odd on brass and ali.

It looks as if BH has hit on the most generally-useful speed by serendipity. As others have said, it pays to listen as well as watch, and speeds much faster and slower have their place in different circumstances – for example, a form tool cutting over a broad profile works best at much slower speed, and there really is no lower limit for screwcutting; sometimes I've done it by rotating the chuck by hand for short internal threads to a shoulder.

I have to say, that now I've got infinitely variable speed, I go very much by 'that sounds about right' and don't always even know my rpm rate!

Edited By Mick B1 on 18/06/2019 19:58:16

[Anonymous]

Posted by Blue Heeler on 17/06/2019 23:24:38:

I use HSS and carbide insert tooling, turn steel, brass, copper, cast iron, aluminium, plastic and get a mirror finish on all materials.

Good grief, I've obviously still got a lot to learn then. First thing is to stop fiddling with the spindle speed knobs.

Andrew

[Paul KempParticipant @paulkemp46892]

Posted by Andrew Johnston on 18/06/2019 20:35:54:

Posted by Blue Heeler on 17/06/2019 23:24:38:

I use HSS and carbide insert tooling, turn steel, brass, copper, cast iron, aluminium, plastic and get a mirror finish on all materials.

Good grief, I've obviously still got a lot to learn then. First thing is to stop fiddling with the spindle speed knobs.

Andrew

Me too Andrew! I have only been doing this metal bashing lark for 45 years and I still can't get a mirror finish on everything I make. I must be using the wrong tooling or not grinding it proper.

Paul.

[HollowpointParticipant @hollowpoint]

Personally I don't worry about speeds too much. I just take a cut and kinda use my instincts to get a feel for how it's cutting and adjust accordingly. The sound and heat generated are a good indicator too. I can usually get a good finish on most materials.

[Blue HeelerParticipant @blueheeler]

Posted by Bazyle on 18/06/2019 18:06:22:

….after 7 years you should be ready to try screwcutting which will bring you a whole now feeling of accomplishment….. but probably best to start with a mandrel handle and take it even slower than 420rpm.

I can do a thread, but much prefer using a button die in a tailstock holder.

80% of my threads are ME threads.

[Blue HeelerParticipant @blueheeler]

Posted by Andrew Johnston on 18/06/2019 20:35:54:

Posted by Blue Heeler on 17/06/2019 23:24:38:

I use HSS and carbide insert tooling, turn steel, brass, copper, cast iron, aluminium, plastic and get a mirror finish on all materials.

Good grief, I've obviously still got a lot to learn then. First thing is to stop fiddling with the spindle speed knobs.

Andrew

I hope I didn't sound too frivolous.
I have a continuous unlimited free supply of brass to practice on and I keep sharpening HSS till I get the mirror finish I'm looking for. It can take me quite a while of trial and error and even small changes in the cutting tool for me make a big difference. I have to go through this process every time, then for a long time a diamond hone is sufficient.

One thing I can't use to get a good finish is Chinese eBay HSS, I've spent hours upon hours trying to get the same finish from Chinese HSS that I get with good quality brand name HSS.

They've been relegated for use in my milling vice.

[Brian JohnParticipant @brianjohn93961]

Why do so many lathes (like mine) have speed dials that go up to 2000 RPM or 3000 RPM ? When would you ever use such high speeds ?

[Bill PudneyParticipant @billpudney37759]

Posted by Brian John on 19/06/2019 01:49:58:

Why do so many lathes (like mine) have speed dials that go up to 2000 RPM or 3000 RPM ? When would you ever use such high speeds ?

Just last week I was turning some 4140 steel, at over 2,000 rpm on my mini lathe. Yes it was quite vivid, and very exciting. The swarf was coming off blue and smoking. This morning I was turning some 6061T6 Al. Alloy, at about 2,500 rpm.

A bit like riding a motor cycle, more speed + more fun (sometimes!!)

cheers

Bill

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Brian John on 19/06/2019 01:49:58:

Why do so many lathes (like mine) have speed dials that go up to 2000 RPM or 3000 RPM ? When would you ever use such high speeds ?

.

Forgive me if this appears cynical, but I think the simple truth is that it is easier and cheaper to build them that way.

The electric motor is [in the vast majority of examples]a high-speed device …

MichaelG.

.

There is of course, a counter-argument that high speeds are needed for small work,

BUT consider: Some of the best work done by watchmakers is still done with the bow.

Edited By Michael Gilligan on 19/06/2019 06:13:58

[not done it yetParticipant @notdoneityet]

Posted by Brian John on 19/06/2019 01:49:58:

Why do so many lathes (like mine) have speed dials that go up to 2000 RPM or 3000 RPM ? When would you ever use such high speeds ?

Have you ever tried to drill very small holes at 450rpm into steel? Say, 1mm or less! I believe lace bobbin makers often turn at those speeds (even though materials may not be metal).

Simply put, manufacturers cater for the widest range of users with any single model, so it is obvious for them to provide as wide a speed range as practicable. Old lathes, with plain bearings, were limited in rotational speed to around 1100rpm (without copious lubrication) whereas spindles with modern bearings can rotate safely at up to several thousands of rpm.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Brian John on 19/06/2019 01:49:58:

Why do so many lathes (like mine) have speed dials that go up to 2000 RPM or 3000 RPM ? When would you ever use such high speeds ?

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

[Anonymous]

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

[JasonBModerator @jasonb]

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

[Nigel Graham 2Participant @nigelgraham2]

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&quot 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.)

[Author]

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