Myford super 7 Drive

I am thinking of converting one of my Myford Super 7scto a toothed belt drive from the countershaft to the spindle, The plan is to power it with a 1.1kw 2 pole motor on a VFD, normal Vee belt from motor then a 1 to 1 ratio toothed belt to the spindle, reason for a 1.1kw motor is to compensate for loss of power I get from using a .750 watt motor at low speeds.

Any recommends or advice would be appreciated as am filling in my time as am self isolating.

OK, I'll kick off an argument...

Why a toothed belt? Even a link-V-belt can transmit enough torque for a not-very-rigid, little lathe. Poly-Vee belts run more smoothly and cooler than simple V-belts and can transmit more torque. Making the required pulleys is easier too - a kit is even available. Retaining the option of altering motor : spindle gearing by belt-swapping is still a good idea for a VFD drive, to retain good low-speed torque.

Considering that Myford used to go for a 3phase 1/2hp motor or 3/4hp single phase motor then I feel that 1.5hp will rather over motor the lathe and in the event of a disaster could do some serious damage. I take your point that you would like a bit more grunt at slow speed but as you will already have the pulley system is it really too much trouble to slip the belt across to a more suitable setting. I run my Myford with an inverter and a 1hp motor and even though it is a flux vector drive it will still be under powered at low frequencies. I find that often the need for slow speed is because the job is a large diameter so a decent cut is going to require a lot of torque, even with a flux vector drive which will perhaps be able to keep 100% torque down to single digit frequencies it will be no match for the torque that is multiplied by the mechanical speed reduction and back gear. The variable speed Myfords still keep a high and low speed belt reduction. A VFD is a joy to use but it still cannot do the impossible. I suspect that even your large motor will stall at low speed with a decent cut on a large diameter job. A toothed belt will eliminate the belt slip that can save damaging the lathe but what will break next? An inverter can current limit but the large motor will introduce more inertia into the drive so a crash at high speed is going to break something. A poly V conversion will offer slick belt changing and a smoother drive less inclined to slip, Hemingway have a kit to make this conversion.

Mike

If hoping to run the motor at low speed and high power, consider the reduction in cooling as well. Likely better than DC drives as few, if any, have come on here crying that the’ve burnt out their three phase motor.

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Amen to that

Poly-Vee is the way

MichaelG.

I want to be able to change speeds without stopping it.

I am still going to use the 2 step pulley on a V belt from the motor to the countershaft, By using a 1.1 kw motor will give me a better torque range than a 1hp motor which I currently have fitted, I am not making large components as I have a much larger lathe. I am working on a project which requires a large number of non ferrous components,I have a virtually new Myford capstan attachment and a multiple tool holding cut off slide. Some of the components need threading so need to be able to change speed, I used to be a tool maker and often tooled up medium to very large turret and capstan lathes.. I just wanted 1 toothed belt pully with a mod on it so that if needed I can still use the back gearing.

This is a conversion I have done. The motor & Omron inverter that is. 1.1 kw. I did it a while back, but I just ordered a new set of original belts from myford. Not had any trouble whatsoever. I did have to change the primary twin pulley to a single as the spindle on the motor is larger. I still have the 4 pulley speed choice though. Mine too has the torque vectoring. But when I run at a snails pace down around 5hz to polish & detail rings , there is no real load demand. Good luck with the mod. Hope it all goes well. I certainly have no regrets. Did get some stick on here for going so large. The thing has not exploded yet.

Steve.

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All fine, if that does what you want

... but Poly-Vee runs sweeter

MichaelG.

All three of my Raglan machines have had both variable-mechanical and electronic speed adjustment on the run.

The Centec has electronic speed control on the run and the choice of discrete gear ratios.

I most certainly change the mechanical gearing much less than without the VFD, but still like to take advantage of the option. VFD (with mechanical speed change retained) would be my choice every time - but tripling the power potential is way OTT, unnecessary and potentially dangerous. Just doubling the power of a properly designed machine is, IMO, OTT. Poorly thought-out design changes are even worse.

Yes, it can obviously be done, but not necessary in most circumstance - and often it will stress the machine, reduce accuracy and likely wear it out much faster (through abnormal wear and tear).

Think again is my advice.

When you say toothed belt are you talking about toothed timing belts or notched v belts. Toothed timing belts run the risk of a cogging effect resulting in finish issues on your parts. Notched v belts would be fine.

Martin C

Toothed timing belts run the risk of a cogging effect resulting in finish issues on your parts

Modern timing belts run smoothly. If that were not the case, many (if not most) of the worlds CNC machines would have surface finish issues from the axis drives, as timing belts are the most popular way of connecting servo motors to ballscrews.

A bigger issue with high speed timing belt drives is noise, generated by air getting trapped between the belt teeth & the pulley grooves. This is more of an issue with wider belts & one recommendation is to use multiple narrow belts on a wide pulley to allow the air to escape more easily. One of my former colleagues used a wide timing belt on a milling machine spindle drive. The motor was around 15Kw, so the belt was quite wide & large pitch. The noise was horrendous - like being stood next to an air raid siren & ear defenders were a must while operating it. The customer was not happy but, fortunately, the drive had been designed around standard Taper lock pulleys & replacing the timing belt solution with 3 groove B section vee belts transmitted the required power in near silence.

I have not had good experiences with Poly vee drives. Higher power than model engineer size machines admittedly, & only chosen due to the theoretical ability to transmit the required power at short centre distances, but the belts wore out in short order. Moulded cog raw edge wedge belts would be my choice here.

Could be fun trying to unscrew a Super 7 chuck after putting a 1.1Kw load though it.

Nigel B.

Edited By mgnbuk on 28/03/2020 10:58:42

I all ready run a toothed belt drive on a BT30 vertical milling head which I have designed, It used a 25mm wide belt, runs quiet as a mouse using a 1.1kw motor.

After reading all of your suggestions and comments I have decided to stick with my 1hp motor and try out a toothed belt and have a look at a polly Vee as well , although my large lathe runs on a polly vee at the moment but am not that impressed with it.

I thank you all for your comments and suggestion and will see what sort of success I have.

Ian L