Myford motor pulley key

[Allen Smithee]

I fired up the lathe yesterday (Myford S7B with Newton-Tesla 3ph VFD conversion) to make a fairly simple tubular nut adaptor to fit a spinner onto a particular motor/prop combination. When I fired up the motor it initially stuttered, running at low speed with no torque as if the 3-phase drive had lost synch or something, but when I stopped an restarted it all was normal so I put it out of my mind. This morning I went to make a second one, but the lathe kept doing this low-speed, no torque thing and was making a strange noise. I gave it a good lookover and found that the key that takes the drive from the motor shaft into the first pulley has vanished, and the drive was being transmitted solely due to friction between the shaft and the grubscrew that locks the key, so I think that's probably the problem.

Newton-Tesla don't list spares in their documentation or on their website, and with it being a Sunday I can't ring them today. But I'm assuming that it being a standard B56-format motor with a 5/8" shaft that the key is a standard size or a standard part (it must be to SOME external standard because it engages with the key slot in the Myford first stage pulley).  Myford don't list it either.

Does anyone happen to know the dimensions, specification or part number for this key?

TIA,

AS

[gipetto]

I'm sure i made a woodruff key in the past freehand on an anglegrinder. give it a shot, but hold it in a vicegrips.

[Jasonb]

If it is just a square section key then measure the slot width and buy a length of key steel. Poing to be 5/32" or 4mm on a shaft of 5/8 or 16mm dia. Check it is not a metric shaft as a newer motor may be metric.

Edit looks like they may go a bit bigger with the key at 3/16 but easy to measure

https://www.brookmtl.co.nz/media/attachments/2024/05/29/crompton-greaves_b56-frame-motor-series-brochure.pdf

[Allen Smithee]

I just found this drawing on the Newton-Tesla website, but the resolution is poor - it's not clear whether the key is 4.26mm or 4.76mm. I'm guessing it's 4.76 partly because the keyway on the shaft is shown as 4.78mm wide and partly because it's a standard size (ie 3/16").



I'll give them a call later and as soon as I have conformed the size I'll get some 1/16" key steel from M-machine.

AS

[Charles Lamont]

3/16 square is standard for a 5/8 shaft. There is no need to get a specific spare part. You could make one in mild or silver steel which would probably be perfectly adequate, or buy a length of key steel on ebay (or search the swarf tray and floor for the original one).

[john mills]

Hi
it would be worth checking the actual size both in the shaft and the pulley as if it has been loose enough to fall out it has been loose and could be out of shape ,make the new one to fit   .if you find the old one see that it fits it should not be loose enough to fall out.
working in a matainance work shop with metal working machinery we were often making keys some were quite complex different sizes each side with tapers
and tee heads  but when machines had jam-ups they would get spat out or at least damaged so they were loose.
john.

[Allen Smithee]

Well I had a quick chat with Newton Tesla and being a wonderful bunch of people they are popping a replacement in the post without charge - it may not be explicitly covered in the (10 year!) warranty, but they don't want to charge me for such a small thing.

I have to say that I have been well impressed with this 3-phase conversion to the point where I completely take it for granted. It must be several years since I last moved the belts to do a speed change (the low-speed torque makes it unnecessary) and the top-end grunt means that I tend to cut most things at 2,500rpm with a BCT or insert tool, only backing the speed down if I'm going for a super-fine finish cut with a sharp HSS tool! The whole setup comes pre-configured and you literally just bolt the motor on and hang the control box somewhere. They suggest the controller should sit face-up on the apron where the normal switch goes, but I decided to screw mine to the underside of the shelf behind the lathe so it's roughly at eye level and well clear of swarf, oil etc. It's not cheap at £600ish, but it significantly expanded the capability of my machine!

AS

[Allen Smithee]

...and the replacement key arrived in the post this morning. Wonderful service!

AS