Centec spindle speeds?

[Andrew TinsleyParticipant @andrewtinsley63637]

I shall soon be collecting a Centec 2B mill, together with the Mk 3 vertical head.

I have the choice of a couple of 0.75kW motors. The 1400 rpm version will give me spindle speeds of 85 to 1400 rpm (six speeds in all), while the 2800 rpm motor will give 170 to 2800 rpm.

So which is the most useful choice of motor. I have heard that things can get a little warm at 2800 rpm, so maybe the 1400 rpm motor may be the best choice?

This is my first mill, so I don't know much about useful spindle speeds.

Regards,

Andrew.

Edited By Andrew Tinsley on 01/10/2017 19:36:43

[Andrew TinsleyParticipant @andrewtinsley63637]

[Simon Williams 3Participant @simonwilliams3]

Go slow!

Ive swapped out my 2800 rpm motor for one that lets me run bigger cutters in horizontal mode. You have got a good top speed via the vertical head anyway so the deciding factor is being able to use slitting saws.

I'vee got an Mk 2 Auto mill which came with a 2 pole motor and had been used for cutting aluminium, but for what I want to do the slower speeds are much preferred. So it does depend what what work you are looking to do, but in general slower speeds have less chance of burning HSS cutters.

So the better answer is do the surface speed calcs for the cutters you want to use in the available gear settings, and choose accordingly. I bet you choose the slower!

Rgds Simon

[Andrew TinsleyParticipant @andrewtinsley63637]

Thanks Simon,

I suppose I was too lazy to do the cutter speed calculations! Well to be honest. I am always a little leary of the cutting speed in feet per minute, which are oft quoted. I suspect that the "industrial" speeds given, are for full flood cooling and pretty deep cuts. I am not really sure how to derate these speeds for amateur use! I tend to be a small cut man on most things. I don't like risking it on the "average" amateur jobs!

So 1400 rpm motor is the way to go. Thanks for that! I usually waste too much time in setting up machine tools and that has saved me half an hours extra work!

Andrew.

[peak4Participant @peak4]

Mine came with a 1425 1Hp 3 phase motor, presumably from new, which seems to do the job fine. I run it off an inverter.

If you get the chance, and can afford it, a 3 phase 2800 with an inverter may give you the best of both worlds, though I've not tried that option myself.

I still need to convert my power feed motor to delta, so I can run that off an inverter too. There is a thread and some photos on this very forum about that job somewhere.

Bill

[Ian S CParticipant @iansc]

It's only when you get to using very small cutters that the high speeds are really needed, although I have done as small as 1/16" with my Rexon mill with a maximum speed of 1100 rpm. I would rather it had a lower minimum speed than it's 90 rpm.

I think you get a smoother running motor with a 4 pole/1450 rpm type over the 2 pole motor.

Ian S C

[not done it yetParticipant @notdoneityet]

Either1425 or 2800 would be better with a VFD. I would prefer to run the motor faster, rather than slower, by altering the VFD, but not by a factor of two.

If you intend using it as a horizontal, you will likely need those lower speeds. Running motors at half of normal speed is not the best scenario – particularly from heating/cooling and motor power output. 3 phase motors are somewhat superior to their single phase equivalents.

[SillyOldDufferModerator @sillyoldduffer]

Depends on what you're going to use the mill for. For example I frequently use mine (not a Centec) at 2800rpm. Higher speeds are good for small diameter drilling, and I have a carbide cutter that loves it! Bigger drills and HSS milling are slower: for HSS I find the rule of thumb rpm = 10000/tool diameter( in mm) a useful guide.

Thus a 1mm drill should be spun at 10000 rpm (yes!) and a 20mm diameter cutter 500rpm. A wide diameter flycutter would be much slower.

The ideal machine would be capable of a very wide range of speeds. But nothing is ever easy.

One thing to watch for on older kit if you plan to increase speed is the bearings. They may not be up for it. For example, a plain bearing starts from rest with metal to metal contact. (Bad!) But provided enough oil is available, the journal soon climbs on to a thin cushion of oil and friction drops enormously (Hurrah!) Unfortunately the effectiveness of the cushion depends on factors like heat, pressure, and oil viscosity. Heat and pressure depend on the design of the bearing and how it is operated. The overall effect is that the internal friction of a bearing increases with speed. If the bearing overheats, the oil gets thinner, perhaps to the point that the cushion collapses and the bearing seizes.

Most often the effect of excessive speed is reduced bearing life, but it's not difficult to over speed a plain bearing to destruction. Bottom line: they shouldn't be operated outside their design limits. Modern kit is usually fitted with roller bearings that are better suited to fast running. Even so, exceeding the manufacturers recommended speeds should only be done with care. At high speed, it's likely the machine will vibrate due to balance problems.

Within the Centec's limitations, I'd look very seriously at Bill's suggestion. A 2800, 3-phase motor + inverter would make the machine much more flexible.

Dave

[Andrew TinsleyParticipant @andrewtinsley63637]

I don't have a problem with high speeds for drills. I have a 3 phase, dual speed motored, high speed drilling machine for that purpose. I am not sure of the manufacturer. But the drill quill is supported by a vertical dovetail slide. It came out of a shadow factory that made Merlin magnetos. It was left there when the place was sold off and I eventually purchased it.

I am a little confused (as always!). I was under the impression that the Centecs came with a choice of 3 phase motors, both 1400 and 2800 rpm, these were supplied from new. So I suppose that 2800 rpm will be a safe maximum?

Looking at some ME books on vertical milling it seems that speeds as low as 30 rpm are recommended for some vertical milling. So Maybe I should go for the 1400 rpm motor and eventually use a VFD to cut the speed back.

As for horizontal milling, I suppose you do the speed calculation for the cutter diameter in the same way as for vertical milling. I am anxious to try out horizontal milling as it seem to be out of favour with model engineers. But then I do have a shaper and I just love to have an excuse to use it, so maybe I shall not be using the horizontal milling as much as I would like! Shapers are just great, I could sit and watch them for hours!

Andrew.

[not done it yetParticipant @notdoneityet]

 

Looking at some ME books on vertical milling it seems that speeds as low as 30 rpm are recommended for some vertical milling. So Maybe I should go for the 1400 rpm motor and eventually use a VFD to cut the speed back.

 

That would be a good decision but do make sure the motor you get can be wired in both delta and star. Some are not. While the top speed rating was 2800 on the Centecs, raising it further might not be a good idea – and that means a VFD was only useful for slowing the drive (and slowing reduces the motor cooling).

 

 

There is always the opportunity, with belt drives, to change the pulley sizes if one really wanted different speeds. But I don't know the input torque rating for that gearbox. It had already been uprated from the 2A, which was fitted with a 550W motor.

 

 

Edited By not done it yet on 02/10/2017 12:01:33

[Andrew TinsleyParticipant @andrewtinsley63637]

Hello "Not done it yet"

I now have 3 phase, so using 3 phase motors isn't a problem, the ones I have can be connected in star or delta. I am a bit reluctant to half the speed on a 1400 rpm motor, cooling always bothers me. I suppose once I get it up and running, I can see if 85 rpm does the job and if not then I can always use a VFD. Suck it and see and don't worry until I have to!

Thanks,

Andrew.

[not done it yetParticipant @notdoneityet]

I now have 3 phase

Ahh, right. Was not aware that you had a 3 phase supply – most of us don't, so have to rely on 220V single phase input VFDs. Yes, there are cheapish 3ph in – 3ph out inverters (but 220V single ph to 415V three ph are rather more expensive). And not all 3ph 415 motors can easily be changed to 220V operation.

I was assuming you had single ph supply and not all single ph 230V motors can easily be speed controlled.

You have choices, but 1425 motor speed is best for you, I am sure.

Often bad news, having to assume!

[Anonymous]

A bottom speed of 85rpm is a bit high for a horizontal mill. If you follow the recommended 100sfm for low carbon steel a 4" diameter cutter should be running at 100rpm. For cast iron, or bigger cutters, you'd need to be lower. My horizontal mill goes down to 30rpm and that is by no means too low. So I'd go with the 1425prm motor.

Forget the VFD, it has no benefit if you have a three phase supply anyway. A horizontal mill is designed to shift metal, for which you need power. Large HSS cutters need to run slowly, so you need power at low spindle rpm. Using a VFD to slow down the motor doesn't give you that.

Andrew

[Andrew TinsleyParticipant @andrewtinsley63637]

Hello Andrew,

Thanks for that input. I assume that the quoted lowest speed of 85 rpm for the Centec is for the horizontal arbour. I am unsure about the geared vertical head, but it appears that the fastest speed is the speed of the motor. Until I get the machine I don't really know. According to Lathes UK. The gearing for the vertical mill is actually in the head itself. So quite where the horizontal speeds come from is anyone's guess, until I can get hands on!

If 85 rpm on the horizontal arbour is too slow, then some sort of speed reduction via the pulley belts look like an answer.

Andrew.

[Dave HalfordParticipant @davehalford22513]

The 2 B and 2A and Automill speed range is 85 to 1400

The only gears in the VH are bevel gears and what goes in comes out unless you mess with the pulley sizes.

I would not run a gearbox at twice the design speed unless you have a spare one

[Andrew TinsleyParticipant @andrewtinsley63637]

Hello Dave,

I understand from reading the Lathe UK site that the Centec was supplied with either a 3 phase 1400 rpm or a 3 phase 2800 rpm motor as standard. If so the Vertical Head has to be up to taking 2800 rpm, otherwise Centec would not have supplied the 2800 rpm motor.

Where I am confused is that the same write up states that there is a six speed gear box built into the Vertical Head. I need to recheck that, as it seems an odd way of doing things and perhaps I have misread the write up on that point.

Andrew.

[not done it yetParticipant @notdoneityet]

Andrew,

The vertical head is belted from the horizontal spindle shaft at 1:1 and the input to output on the vertical head is also 1:1. So the vertical head revolves at the same speeds as the horizontal.

There are twin Vs on the motor to gearbox, for an initial speed reduction but only a single V for the vertical head drive. The 2A only had a single V belt from the motor to gearbox so the change presumably reflected the 33% power increase of the 2B over the 2A. The one horse single phase motor apparently was unable, at times, to start in the highest gear while the machine was cold.

[Dave HalfordParticipant @davehalford22513]

Hi Andrew,

What it says is 'found with' which can mean the parts swappers have been busy like on Simons Automill which was alloy only.

If you bought my old 66 Cortina mk1, when you lifted the bonnet and saw a Corsair 2000e V4 and found the battery in the boot would you assume Ford made some like it?

The Centec's were designed in the late 40's as industrial machines for small work, 3" dia horizontal cutters max. The vertical will happily run a 5/8 or 16mm cutter with a decent depth of cut in steel on1/2hp

Centec speeds are riveted to the mast and the factory documentation does not mention a doubled speed version. If you find a plate with doubled speeds I might agree.

You can use the horizontal to hog out a vertical head spacer for it's self

Don't forget you need to run the motor in both directions

[Clive WashingtonParticipant @clivewashington54052]

I have the 2a with the Mk 3 head and its a great machine. But you should take the time to adjust the head tilt so that its accurately square to the table. Dont rely on the angle scale, mine is about half a degree out and that's enough to mess up a lot of operations.

[not done it yetParticipant @notdoneityet]

Doubled speeds? You mean no 2800rpm motor version?

It is clear, from the specs that these spindle speeds were offered by the factory.

[Simon Williams 3Participant @simonwilliams3]

Posted by Dave Halford on 03/10/2017 20:47:20:

Hi Andrew,

What it says is 'found with' which can mean the parts swappers have been busy like on Simons Automill which was alloy only.

If you bought my old 66 Cortina mk1, when you lifted the bonnet and saw a Corsair 2000e V4 and found the battery in the boot would you assume Ford made some like it?

The Centec's were designed in the late 40's as industrial machines for small work, 3" dia horizontal cutters max. The vertical will happily run a 5/8 or 16mm cutter with a decent depth of cut in steel on1/2hp

Centec speeds are riveted to the mast and the factory documentation does not mention a doubled speed version. If you find a plate with doubled speeds I might agree.

You can use the horizontal to hog out a vertical head spacer for it's self

Don't forget you need to run the motor in both directions

Sorry to disagree, but my Mk2 Automill has a brass plate riveted to the side of the mast showing the speeds appropriate to a 2 pole motor. If it's an after market bodge-on its very convincing. From what I remember of the Centec brochure courtesy of Tony Griffiths the 2 pole motor was a factory option. I get the impression Centec would have made you pretty much anything you wanted to suit your production line. The machine (when I bought it) was immersed in aluminium swarf – I don't know its history but I assume it was bought for a specific purpose. I've robbed the motor off it and popped it on my Myford S7 with a VSD as an experiment – documented elsewhere on this forum – but mainly because the bottom speed (with a 2 pole motor) was too high for a horizontal mill for what I want to do, which is cut gears in steel.

So I'm pretty sure the 2800 rpm motor version was original ex factory . I'll upload a pic' of the gearbox plate in the morning, but I can't get to it just now.

Andrew's original question sought an opinion as to what motor speed was more appropriate to run-of-the mill use. The conclusion seems to be that the slower motor – i.e. 1450 rpm or so – is more use than 2 pole 2800 ish. I'd go with that – the point of a horizontal mill is to run 3, 4 5 maybe even 6 inch dia HSS cutters. The gearbox gives you plenty of speed to run small end mills in the vertical head – it's a very versatile machine. I guess that's why it still commands the price tag it does.

Rgds to all

Simon

[Simon Williams 3Participant @simonwilliams3]

Herewith a pic' of the gearbox plate on my Mk2 Auto mill, as promised:

Rgds to all

Simon

[Dave HalfordParticipant @davehalford22513]

And suddenly I agree.

Though it doesn't mean the gearbox is the same as the slow speed version

[duncan webster 1Participant @duncanwebster1]

The belt drive to the vertical head might well be 1:1, but the bevel gears in the head aren't, so the vertical spindle rotates at a different speed to the horizontal. Mine gets pretty warm if run for extended speeds at top speed, so I'd be wary of running at twice speed. I've had the bearings replaced by a guy who knew what he was doing, didn't make it any better, and I've since read that this is a known issue. The later 2B had the motor mounted underneath inside the cabinet, using a single phase motor caused the sheet metal cabinet to drum, a 3 phase motor made life much more bearable, can now listen to the wireless!

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