Slowing lathe RPM

I would use the following values, for HSS tooling:

Cast iron: 60fpm for castings, higher for continuously cast

Stainless steel (austenitic 3xx series): 80fpm

Low carbon steels: 100fpm

Brass: 300fpm

Aluminium alloy: 600fpm

However, none of these values are critical, and there are other factors that need to be taken into account. I don't calculate an exact value, but just do a rough calculation in my head. In general HSS tooling is more forgiving than carbide when it comes to the range of cutting speeds. There are no right and wrong answers. Even those in the professional literature are only guidelines.

Andrew

I often use glue! Horses for courses maybe?

Dave

Rather than faff about with pi and inches after looking up a materials Surface Speed in feet/minute, I prefer to start with a simple easy to remember approximation that does away with all the hard sums, hurrah.

It is:

RPM = 10000 / diameter in mm

This happens to be about right for HSS cutting carbon steel. Thus a 25mm diameter steel rod would be spun at about 400rpm, and 5mm diameter at 2500rpm.

Other materials are simply proportionally faster or slower than ordinary steel:

• RPM for cast-iron and stainless is Half steel RPM
• RPM brass is one and a third steel RPM
• RPM Aluminium is twice steel RPM

If using carbide rather than HSS, multiply the HSS result by 4 to 6. (If your machine can deliver the necessary power and speed. Otherwise, as fast as it can manage.)

Be prepared to experiment. Much depends on what you're doing and the limitations of your set up. Starting with the estimated RPM, adjust depth of cut and feed rate for best results. If you get problems like chatter, poor finish, or overheating, play with RPM as well. Don't expect a hobby lathe to work at industrial production rates! With HSS it's usually OK to reduce RPM a lot. Carbide run too slowly and with insufficient depth of cut may not produce an acceptable finish.

I run my machines so they sound loaded but aren't labouring. Sounding OK and producing an acceptable finish are more important than sticking religiously to the estimated RPM. The estimate is a good starting point, not a guarantee.

Dave

With regard to slowing the motor, if it is an induction type (Rather than a brushed or brushless DC type    the answer is NO. Induction motor speeds are determined by the supply frequency and the number of poles.

If your motor runs at 1470 rpm, it sounds like a 4 pole induction motor, so your speed reduction has to me mechanical.

the obvious way is to either fit a smaller diameter pulley to the motor, or a change one on to the countershaft.

Once you have decided what is the lowest speed required for the lathe mandrel, you will know the overall reduction ratio between the motor and the Mandrel.

The arithmetic is simple.

Say, you want a minimum Mandrel speed of 250 rpm instead of the present 435 rpm.

So the overall reduction ratio is 1470 / 250 = 5.88:1 ASSUMING that the motor pulley is 2" diameter, driving a 4" pulley on the countershaft, the countershaft will revolve at 735 rpm. so the ratio between the countershaft and the Mandrel has to be 2.94:1 In which case with a 5" pulley on the Mandrel, you would need a 1.75" diameter pulley on the countershaft.

These will not be the sizes for your lathe, but shows the principle involved.

You measure the existing pulleys, and decide which would be the easiest one to obtain / make, taking into account availability of belts, and range of adjustment available.

Basically, the simple you either reduce the size of one of the driving pulleys, or increase the size of the driven pulley until you get to about the size of ratio that you require.

Bear in mind that you may have problems (belt wear / eventual breakage and range of adjustment ) if the driving pulley is too small,

The other side of the coin may be that your "wished for" driven pulley might be too large to fit inside the guards, or fouls on something.

So you might have to reiterate your choices and calculations until you reach dimensions that are practicable.

HTH

Howard

Edited By Howard Lewis on 10/01/2022 15:32:52

I didn't express it clearly but I meant the OP will need to replace the existing motor with a DC brushless type to realize his "plug and play" solution of varying the speed steplessly by simply turning a dial.

Edited By Y C Lui on 10/01/2022 16:08:32

I haven't tried the super glue method but I think that should work quite well in the OP's case because the workpiece has a relatively large surface area.

For the OP's reference there are Youtube videos showing the method used by professionals and amateurs on big and small machines : 

https://www.youtube.com/watch?v=r6DCvtcU8_M&t=110s

https://www.youtube.com/watch?v=0svTlISjr3s

Edited By Y C Lui on 10/01/2022 16:17:26

Maybe the correct answer to this question would be for why did he suggest that you need a slower speed?

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I referenced this image on p1 … but [with due credit to Jeffree/Peatol/Taig] thought it worth posting here:

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In attempting to machine the soft jaws … Surely MC is trying to use the machine as the manufacturer intends

… ergo, the only issues are related to technique.

Fond as I am of ‘wax chucks’ … I can see no good reason to glue the workpiece.

MichaelG.

MC,

You have E mails

Howard

I have never previously noticed these for sale as spares: **LINK**

https://www.soigeneris.com/blank-soft-jaw-set-3-jaw-1051

Seems very reasonable [U.S.A.] pricing.

MichaelG.

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Edit: __ and these look useful too !!

https://www.soigeneris.com/full-circle-soft-jaw-set-3-jaw-1052

Edited By Michael Gilligan on 11/01/2022 18:03:05

Also known as pie jaws, available for most sizes of chuck.

Dave

As the person in question who suggested slower speeds (and I make no claims to be an expert) I was sharing the numbers I use on my Taig. I normally use numbers close to those suggested by Dave (SOD). At 100mm he suggests 10000/dia in mm for steel and twice that for Aluminium. For a 100mm Aluminium that would be 200rpm.

I find it interesting that Andrew's numbers agree with SOD for 100mm (4" ) dia steel (100rpm), but the multipliers for brass (3 vs 1.5) and aluminium (6 vs 2) are so different. I will try cutting a bit faster in aluminium, but the Taig's minimum speed of 400+ rpm limits cutting pieces of steel to around 1" diameter.

MC's original issue was exacerbated by the interrupted cut (20 - 22 impacts/sec ) on a 100mm diameter. I suggested reversing the jaws and cutting the step close to the centre as if they were inside Jaws then only taking a final skim with them back as outside jaws.

Edited to get rid of icons

Edited By Peter Cook 6 on 11/01/2022 22:51:52

Thank you for clarifying the matter, by giving us more detail, Peter

I, for one, was focussed on the brief statement in MC's opening post:

"A more expert chap has suggested that I need to machine Aluminium at slower speeds."

... Which I took to be general advice about machining 'Aluminium'.

MichaelG.

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Thanks for that, Dave

They were new to me ...

They are presumably just slices off a convenient extrusion; but ... not having such an extrusion readily to hand, I thought the price very reasonable.

MichaelG.

The UK price for a set of jaws is £23.00 including postage

The Taig Toolpost is $5.10 but £25.00 (including postage) in the UK.

I suppose reasonableness depends on one's income!

MC

The OP mentioned that the prime purpose is to turn a steel disc that was too big to be held by the original chuck. If the steel disc is too large, say, more than one inch in dia, then the lathe will need be slowed down ( or more importantly, has the torque increased ) in order to do the job.

There is no info about the shape of the disc or the kind of cut that need be made but holding this kind of workpiece by clamping the outside is not workable if the cutting is to reduce the outside diameter. Even if it is workable, soft jaw will be an expensive solution at least for me if it is just used for making one piece of parts.

For me, I will seriously consider using glue, either super glue or epoxy.

Edited By Y C Lui on 12/01/2022 04:08:47

This is a way to reduce the speed to 178 RPM and increase the torque at the same time  : https://www.micro-machine-shop.com/taig_motor_mount.htm

Edited By Y C Lui on 12/01/2022 06:59:43

Could use a **GT insert to turn the steel then no need to bother about reducing the speed

That is assuming the disc being already round needs it's OD turning or facing and not just a hole bored in it.

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… or more particularly; depends on the value assigned to one’s time

i.e. what it would cost you to make instead of buy the item

The U.K. prices that you mention do put a different light on it though.

Thanks for sharing that information.

MichaelG.

Andrew and I both emphasise that the estimated RPM are guidelines. We agree Brass and Aluminium cut faster than steel, which is the important thing.

My rule of thumb gives an RPM that's in the right ball-park, but the actual sweet spot is determined by a combination of your tool's rigidity, power, torque, and speed range. It's necessary to experiment.

Andrew, who is a professional, knows each metal and alloy has a surface cutting speed that optimises removal rate, power consumption and finish. This is important in the cut-throat world of manufacturing where inefficient production methods are commercial suicide. But optimum cutting requires heavy, powerful machinery. The lighter gear owned by most amateurs can't cope, so the operator has to compromise. Makes sense for anyone owning hefty equipment to start with Andrew's recommendations, and equal sense for anyone owning hobby gear to start with mine. They're both good starting points.

Back to MC's problem, it was the interrupted cut that caught my eye. Interrupted cuts have to be slowed down, and are often troublesome.

MichaelG suggested faulty technique. Where the soft-jaws clamped rigidly? If not, they will move in the chuck causing extra shock in addition to the interrupted cut. I'm no expert! However, I'd grip a disc in the chuck holding the jaws firmly open about 10mm and then cut from the inside outwards. Reduced RPM, feed-rate and depth of cut. Also, check the cutter is still sharp - it's taken a beating.

In defence of glue, a stub-mandrel avoids the interrupted cut problem altogether.

MC's end problem is how to hold a steel disc, not how to cut soft-jaws. Another way is to superglue a short length of rod to the steel disc and hold the rod in the ordinary chuck jaws. An advantage of using a stub-mandrel (coloured blue below) is the full depth of the disc can be turned because the chuck jaws are behind it.

Soft jaws pictured below the stub-mandrel: both methods hold the steel disc. I'd try glue first because it's easier. Soft-jaws if superglue fails!

Dave