Drummond round bed and VFD

The long term project (along with a respectable garden and a tidy garage!) is restoring to active use an early Drummond round bed lathe.

One of the shortcomings of the design is a lack of back gear - so does a 3 phase motor and invertor/vfd enable this to be worked round?

As Manuel in Fawlty Towers said 'I know nothing' so any info or suggestions greatly appreciated.

Thanks,

Nick

Main limitation will be relatively low torque transmission of the simple flat belt drive to the lathe spindle. No problem in getting it to turn reasonably slowly via VFD drive but the belt will probably slip on anything other than modest cuts on larger material. Probably enough torque for screw cutting up to an inch or so diameter depending on material but may well be unable to cope with normal turning. These lathe were originally made for treadle drive so low power, light cuts and uber sharp tools were the norm.

Were I to do the job I'd sacrifice originality by replacing the flat belt pulley drive with multi-vee ones which have much greater torque transmission capabilities. Use a two speed countershaft with the VFD drive to avoid the need for silly low motor speeds. Personally I'd not use a VFD. For a rather similar application I made a back belt drive functionally equivalent to a back gear system which worked well when combined with a two speed countershaft. This was in pre-VFD days so was the only affordable option then. I'd be unsurprised to discover that an inexpensive VFD system would be of similar cost nowadays if you already have suitable motor.

Multi-vee pulleys are relatively easy to make. Especially if you have access to another lathe. If not taking it steady with the flat belt drive will get the job done. Or fake up a drive from a better pulley either held in the back end of the spindle or fixed to chuck or faceplate. Plenty of effective one time use field expedient concepts around.

Clive.

The lathe already has 3 step v pully so is no longer original anyway.

Thanks

Apart from the above just make sure any really low speed cuts are not too heavy and prolonged - or watch your motor winding temperature. The motor cooling fan, running slowly, is not very effective.

They generally don’t recommend less than half motor speed for heavy loads or lengthy running.

I seem to recall that there were third party back gear conversions in the round bed's heyday. I suppose that Clive Foster's point about the capabilities of the drive belt may still apply, but back gear would be more in keeping than a VFD.

VFD on a round-bed sounds like waxing a dirt floor. Especially if you already have a good functioning single phase motor. Might be more in keeping to make a double-reduction pulley between the existing motor and countershaft to slow things down.

There is no motor or countershaft with the lathe at present, so I was exploring all options.

As to the value of the task - of course it is not going to be the equivalent of a more modern machine, but then again I just like to see things put back into a position where they can do what they were originally intended to do. Plain stubbornness on my part I suppose, which probably explains why there is so much half working junk about the place!

Thanks

.

Here's inspiration for you, Nick **LINK**

https://www.model-engineer.co.uk/sites/7/documents/drumm-4-man.pdf

MichaelG.

Superb! Why don't manufacturers describe their products like that today?

I just love looking at old publications like that, I know some of it is nostalgic and maybe a desire to return to a much simpler world, but even then I would have thought that turning on a lathe wearing a white shirt would bring wrath from the domestic authorities and get the stiff collar and tie! Does anyone here know the date of this publication?

Phil

The round bed was introduced in 1923 if I recall and had clearly been in use long enough for some serious users to have aplay with it,...so, about 1925-6...?

A couple of years back the Romford Model Club exhibited a very nicely restored 'round bed' at Ally-Pally show and several Epping Forest Horology Club members were very taken with it and remarked what an ideal clockmakers lathe it would be with a suitable modern electric motor.

The text shows that the 5 cyl radial engine was shown at the ME exhibition in 1924, so Bob has probably got it spot on.

If one reads further down - the 18 cylinder radial engine won a prize in 1930, so this publication was In the 1930s at the earliest. A read on Lathesdotco might shed some light on the models illustrated in that booklet.

.

It took me a while to realise that you were reading a different document: **LINK**

https://www.model-engineer.co.uk/sites/7/documents/drumm-lathe-work.pdf

MichaelG.

Michael,......Thank you soo much for posting that link/document, such superb vintage work. We have a lot to live up to. I particularly liked the 4.7inch QF gun model designed to fire .38 cartridges......can you imagine that now and what would happen next?

Possibly a court appearance and a custodial sentence?

Although the roundbed was originally introduced in 1908, the long bed version of the Drummond was announced in 1925 so the document was after that at least, and before 1943 when production finished, probably before WW2.

Edited By Nick Clarke 3 on 24/11/2018 22:35:52

Edited By Nick Clarke 3 on 24/11/2018 22:43:04

Getting back to the original question the issue boils down to a sensibly economic way of getting a suitable speed range. Assuming the three step Vee belt pulley already fitted to the lathe is retained and that a matching pulley can be found for the countershaft its fairly easy to evaluate what is needed when a VFD controlled motor is used.

Typically a three speed and back gear plain bearing lathe of that sort of size and similar pre WW2 to early post WW2 era would have a specified speed range covering 600 to 650 rpm. Speeds would probably fall somewhere in the 40 to 700 rpm bracket split into maybe 200 to 700 in direct drive and 40 to 150 in back gear. Actual speeds on offer varied with make but, so far as I'm aware, those ranges cover most machines.

Consider a pair of three step V belt pulleys as giving spindle speeds of double, equal to and half the countershaft speed. So for 700 rpm output the countershaft speed needs to be 350 rpm and for 40 rpm it needs to be 80 rpm. A 4 to 1 range which is technically possible using a VFD with a single speed drive but likely to be somewhat unsatisfactory in practice unless a relatively large motor is used.

It seems to be generally accepted that a VFD controlled motor speed range of ± 1/3 rd nameplate speed is almost invariably gives more than adequate performance for all normal purposes using a motor of appropriate power for the job at its nominal nameplate speed. The further you move outside this range the more likely it becomes that compromises in work capability or motor size will be needed. Home workshop types generally don't work their machines that hard so odds are the compromises won't bite too quickly but some engineering calculations to verify things may be needed. Usual issue is lack of power at lower speeds as VFD drives are essentially constant torque at lower speeds so actual power falls when running slower. Belt and gear reduction drives give higher torque at lower speeds. Normally dealt with by using a larger motor and offsetting the speed range so there is more variation at the high end that the low end. Larger motor means larger VFD so it all costs more.

In this case I'd use a two speed belt drive from motor to countershaft allowing a smaller motor to be used whilst remaining within the "always satisfactory" ± 1/3 rd nameplate speed range. Consider a VFD controlled 900 to 2000 rpm from an ordinary 1440 (nominal) rpm motor. Using figures from third paragraph top countershaft speed of 350 rpm at 2000 motor rpm needs 5.7 to 1 reduction and bottom countershaft speed of 80 rpm at 900 rpm needs just over 11 to 1 reduction. No point in getting hung up on exact numbers. Round numbers of 5 to 1 and 10 to 1 will do just fine. Just tweak the VFD control bit further. A 1/3 rd HP motor should do well enough.

Larger pulleys can be expensive to buy. Scavenge the drive shaft, housing and pulleys from an old spin drier to use as primary input drive giving a substantial speed reduction with a set of smaller pulleys up to the countershaft proper to give the two speed secondary. Use the spring loaded tensioner pulley off a car auxiliary drive belt system to tension the belt so no need to faff about when shifting speeds. Nice if you can use same belt for both speeds but two are no great hardship. Two belts worked fine for me once.

To an old school types like me (and Hopper) a VFD system sounds as if it ought to be expensive technology when compared to old school belt, pulleys and single speed motor. But if your scavenging sources are limited costs are probably similar these days.

Clive.

Amazing what they did on that old round-bed lathe in lieu of using a milling machine. -- And even made their own milling cutter to boot!

Thanks Clive Just the kind of information that I was looking for - although as to whether the project is sensible or economic??? Very few of my others have ever been!.

Nick