Slowing Down the 920 Lathe – Issue 219

[Paul BarrettParticipant @paulbarrett57424]

The article in MEW 219 about slowing down the 9×20 lathe with a countershaft is interesting and useful to these lathes. Having owned one for several years I have found it (after several mods) a very accurate and desirable lathe.

However, I went the 3 phase and inverter route almost imeadiately for several reasons. Whilst I appreciate the cost involved will be significantly higher than making a countershaft (apart from the time involved in making it) it far outweighs the impracality of a single phase motor and transforms the machine.

Firstly there is the initial start of a single phase which is all or nothing. If fitted with a largish chuck and a fair lump in it the machine can rock sharply when starting and with quite a bang. Not so with 3 phase/inverter and a smooth ramp up.

The chuck can also come undone if you start it in reverse again with a heavy chuck/work piece. This can be fixed by fitting a locking screw on the chuck backplate.

But the biggest problem is with the belt tensioner. It is fine when turning forwards as the tensioner is on the trailing side of the belt. But if you happen to be turning/screwcutting in reverse and with quite a heavy cut the belt will slip as the tensioner is now on the drive side and the tension is not enough to cope with the load.

By fitting a 3 phase I dispensed with the intermediate shaft and just used the v belt with the 3 speed pulleys. I fitted the motor in the original motor position and spaced it so it does not need a tensioner. I very rarely change the pulley ratio as the inverter covers all speeds and I can run it down to 5 hz (about 30 rpm) with not much loss of torque. I appreciate the initial cost is high (around £150) but I feel it far outweighs the limitation of a single phase motor.

The other essential mod to do is to fit a tumbler reverse. This is essential if you want to do lots of screwcutting cause at some stage you will need to cut a L/H thread. I used it to cut a new M12 x 1mm cross slide feed screw for the lathe as I made a new hand wheel and graduated it as direct reading and fitted thrust bearings as well.

Other mods I have done is a improved top slide mount. Again essential as the original mount does flex a bit.

If you want to stick with a single phase motor then fitting a counter shaft will be an useful mod but if you can go the 3 phase route I believe you will ask yourself why you didn't do it sooner.

I am not bothered about drilling holes or modifying a machine that maybe cannot be returned to original state. As far as I am concerned if it improves the machine it can only be better than original and after all, these machines are built to a price.

Just my opinion.

[Paul BarrettParticipant @paulbarrett57424]

[Russell EberhardtParticipant @russelleberhardt48058]

One more advantage of the three phase and inverter approach is the improvement in surface finish due to the lower vibration. The torque produced by a single phase motor varies with the angle of rotation. The torque produced by a three phase motor is almost constant.

I agree with you that the 920 is a good machine. I had one as a second lathe for a few years but had to sell it to make room for something else.

Russell

[Derek DroverParticipant @derekdrover32802]

Also agree… after a little "fettling", these are actually very capable machines, and can be picked up at a very good price on the second hand market.

First thing I did with mine was replace the topslide mount.. a simple enough job which makes a big difference. 2nd – rear toolpost for parting off. 3rd, thrust bearings and a new leadscrew/nut for the compound slide.

Am still thinking about how to do the tumbler reverse…

Del.

[Ian PParticipant @ianp]

I am in total agreement with Paul's comment about using a VFD.

On thinking about the VFD subject I have never heard of anyone who has ever fitted a VFD regretting it. Most of the reluctance to go the VFD route is because people think its treading into the unknown, and its just not!

The 920 article was interesting but seemed a lot of work for little improvement.

I am always fascinated by the prices fetched, and the interest in, things like 'The Dewhurst Reversing Switch'. I get the impression that a Myford is only complete when it has one! (its not a bad switch but there are better ways)

Ian P

[Gordon WParticipant @gordonw]

When and if my standard motor gives up I will get a VFD. My solution for slow running was to make a pulley to fit over the existing pulley on the mandrel, ie. the bore of the new pulley is the same as the OD of existing pulley. 4 holes tapped in existing pulley with 4 bits of flat bar fixed to the new pullley. A longer belt is needed, this was my only expense. Gives about 1/2 the original speed.

[Author]

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