Slowing lathe RPM

[Y C LuiParticipant @yclui16187]

Posted by John Haine on 10/01/2022 09:11:19:

Posted by Y C Lui on 10/01/2022 02:35:04:

… The set up mentioned in your post uses DC brushless motor. …

If only it were! It's clearly a 4 pole single phase induction motor…..

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

[Y C LuiParticipant @yclui16187]

Posted by SillyOldDuffer on 10/01/2022 10:28:24:

I would approach this job by making a stub mandrel and supergluing the steel disc to it…

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 : 

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

[AdrianRParticipant @adrianr18614]

Posted by Dr. MC Black on 10/01/2022 00:34:55:

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

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

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Dr. MC Black on 10/01/2022 08:54:35:

[…]

It's a Taig Lathe (marketed as Peatol in the United Kingdom)

[…]

I was trying to cut a step in Soft Jaws so that I could mount a steel disk that was too large for the jaws of the 3 jaw chuck as supplied.

[…]

.

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

.

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.

[Howard LewisParticipant @howardlewis46836]

MC,

You have E mails

Howard

[Michael GilliganParticipant @michaelgilligan61133]

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.

.

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

[Dave SParticipant @daves59043]

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

Dave

[Peter Cook 6Participant @petercook6]

Posted by AdrianR on 10/01/2022 16:11:56.

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

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

[Michael GilliganParticipant @michaelgilligan61133]

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.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Dave S on 11/01/2022 21:51:10:

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

Dave

.

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.

[Dr. MC BlackParticipant @dr-mcblack73214]

Posted by Michael Gilligan on 11/01/2022 17:59:23:

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.

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

[Y C LuiParticipant @yclui16187]

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

[Y C LuiParticipant @yclui16187]

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

[JasonBModerator @jasonb]

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.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Dr. MC Black on 12/01/2022 01:45:06:

[…]

I suppose reasonableness depends on one's income!

.

… 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.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Peter Cook 6 on 11/01/2022 22:41:51:

Posted by AdrianR on 10/01/2022 16:11:56.

…

…

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.

…

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

[Michael GilliganParticipant @michaelgilligan61133]

Posted by SillyOldDuffer on 12/01/2022 10:44:51:

[…]

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.

[…]

.

Yes … locking the jaws appropriately is essential

Aside from that, I wouldn’t be too fussed about the intermittent cut … it’s really only a matter of optimising depth of cut and speed of feed.

Typical ‘Aluminium Extrusions’ are made of nicely machinable alloy, and can produce gossamer-thin swarf and a near mirror finish.

Conversely: Throw a blunt tool at deep cut and the results will a wreck.

MichaelG

[Peter Cook 6Participant @petercook6]

Posted by SillyOldDuffer on 12/01/2022 10:44:51:

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.

I would wholly agree. MC and I both have Taig lathes so would fall very firmly in the latter category which is why I suggested he needed to think about slowing the lathe down from its 400+ minimum to do what he was trying to do.

I have stepped the soft jaws on my three jaw very successfully

That is a 90mm lump of aluminium, but I have swapped the induction motor for a DC one, and machined the jaws the other way round ( as inside jaws clamped on a steel rod) with a final light skim when reversed.

[Howard LewisParticipant @howardlewis46836]

With a scroll chuck, there has to be the risk that the impacts from an intermittent chuck will tend to loosen the jaws.

An argument for taking shallow cuts, particularly with a low powered machine.

Howard.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Peter Cook 6 on 12/01/2022 12:29:49:

[…]

which is why I suggested he needed to think about slowing the lathe down from its 400+ minimum to do what he was trying to do.

.

Sorry to labour this Peter, but aside from MC and yourself, I don't think anyone knows [except in the very broadest terms] what he was trying to do.

MichaelG.

[JasonBModerator @jasonb]

I generally try and keep the speed as reccomended and often towards the high side as it helps with the vari speed machines and just adjust feed rate and DOC to suit the job.

I can't imagine the Taig is much different to my Unimat 3 and I happily did a Stuart 10V on that running the two feet of the "a" frame at over double the speeds talked about here for cast iron with HSS and that's an intermittent cut about 3" out from the chuck/faceplate.

As I said earlier the different aluminium alloys have such a broad range of cutting speeds from 400-1200 ft per min even allowing for a light machine and the intermittent cut 400rpm should be fine with a light feed and DOC if using HSS as it's at the bottom of the range. Good old Tubal Cain in his ME handbook for example would have us roughing 4" aluminium at 570rpm and finishing at 760rpm

[Dr. MC BlackParticipant @dr-mcblack73214]

I was trying to cut a step in the Soft Jaws of my Taig lathe so I could cut a chamfer on the outside edge of a approximately 40mm disc.

It already had a co-axial hole through the centre.

I closed the soft jaws around a M8 Penny Washer and (eventually) used a Boring Bar to cut the step from the inside.

The vibration due to the intermittent cut (I surmise) loosened the jaws.

I was only able to cut a step around 3mm deep but I would like it to be about a third of the height of the jaws.

Every web-site to which I refer suggests different Surface Feet/min speeds or so confuses me that it makes no sense at all.

I realise now that I was wrong to ask for speeds at present. What I should have requested was a RANGE of speeds for the different materials.

I have already written the spreadsheet to convert Surface Feet//min to RPM

If anybody is able to explain if there's a EASY way to include photographs in messages on this Forum, I'll do so.

With best wishes and thanks again.

MC Black (Dr.)

[JasonBModerator @jasonb]

If the step was only for a 40mm dia disc then don't worry about the speed 400rpm will be no problem.

Thin penny washer was not teh best thing to use, something a bit thicker would be best.

There is no real need to wast that much jaw depth for one job, 1 to 1.5mm depth would have been more than adequate as a "disc" assumes your steel is reasonably thin so little overhang. Just take light cuts off the steel

As for pictures you have already got one in an album so add some more and then when you get to where you want the picture in teh post click the little camera icon along the top of the box you are typing in, select album and then image and click insert

Edited By JasonB on 12/01/2022 13:40:12

[JasonBModerator @jasonb]

Posted by Dr. MC Black on 12/01/2022 13:27:35:

I was trying to cut a step in the Soft Jaws of my Taig lathe so I could cut a chamfer on the outside edge of a approximately 40mm disc.

Also surprised that you can't fit a 40mm disc into the chuck or do you not have outside/reversible jaws for it?

Also hard to see why you were advised to rough turn from the outside in as that would have meant you start cutting the jaws at about 80mm dia, doing from inside out as is normal the largest dia would be the 40mm??

[Dr. MC BlackParticipant @dr-mcblack73214]

Posted by JasonB on 12/01/2022 13:39:19:

If the step was only for a 40mm dia disc then don't worry about the speed 400rpm will be no problem.

Thin penny washer was not teh best thing to use, something a bit thicker would be best.

There is no real need to wast that much jaw depth for one job, 1 to 1.5mm depth would have been more than adequate as a "disc" assumes your steel is reasonably thin so little overhang. Just take light cuts off the steel

As for pictures you have already got one in an album so add some more and then when you get to where you want the picture in teh post click the little camera icon along the top of the box you are typing in, select album and then image and click insert

Edited By JasonB on 12/01/2022 13:40:12

The M8 Penny Washer was all that was available at the time

The disc was around 20mm thick

I was NOT aware that I had an Album. Can anybody tell me exactly how to find the album so I can apply the advice about clicking the camera icon And how to put photographs in the Album so they can be subsequently selected.

Very many thanks

MC Black (Dr.)

[Author]

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