Backgear for a Chinese 920 lathe?

I am mulling over putting a back gear on a generic Chinese 920 lathe (Warco 918 / Chester 920 style lathe).

This is just an armchair exercise at the moment. Looking on the web, there seems to be only one chap that has done this and he used epicyclic gears from a car automatic gearbox. Not having ready access to such a gear system, my thoughts turned to a hefty mains DC motor that I have (rated 500 rpm and 1 HP).

Now if I could devise a system to change belts from the original 1450rpm motor to the DC motor. This should decrease the speed of the lathe by 2/3rds approx.

Now is there any reason that this would not work as a "backgear"? I usually miss the glaringly obvious in these matters! I appreciate that these lathes are shall we say a kit?, But I would happily do all the necessary fettling and mods such as firmer tool set up, tumbler reverse etc.

If such a motor mod is feasible to give a pseudo backgear, the machine would be mainly used for screwcutting, so maybe it is a matter of simply changing the motor for the lower speed one and forgetting the complications of a two motor set up

Constructive comments please? The lathe is going for a song, so very little to lose.

Andrew.

IMO you're better off getting a machine with a dedicated backgear system

It would be an interesting and challenging construction, one to learn something from. I would have a go at it, especially as the intended lathe is cheap enough to invest in the idea, even if it fails.

Good luck

Brian

Thanks gents,

The lathe is lightly used and 5 years old, cost is peanuts, so I may well have a go unless someone can see a problem?

Andrew.

Andrew - what is the problem that you are trying to solve?

The Emco Compact-8 that I have is my "go-to" lathe. I do have a larger English "1124" lathe, that rarely gets used, the Compact-8 does everything well, and the "issues" it has are easily bypassed.

Threading - I simply spin the chuck by hand, if I am making a short thread. I've learned to keep a leather glove handy for times like this!

I do have a second Compact-8 with a VFD on it, which was a bolt-on change as the motor was a standard frame; I'll admit that this one was going to be CNC'd, but real work as got in the way, and other than having fun with the spindle motor, it has sat for a year, so no experience turning with it under VFD control.

Edited By John Alexander Stewart on 27/11/2017 12:00:40

I have back gear on a smaller lathe but cannot remember when I used it last, VFD control more than copes with most work.

If you just want a lower speed for screw cutting why not just fit a 3-phase motor and vfd?

I had a 920 lathe for a couple of years as a second lathe and found it to be a good lathe except for the speed being a bit high for screw cutting. I had to sell one of my lathes to make room for a cnc mill and chose to keep my UK made Atlas clone despite it being rather more worn but because of the back gear and the number of accesories I had made to fit it. I have since fitted a vfd and now rarely use the back gear.

Russell

The problem, if any, is that the 920 series of lathes (Copy of the Emco compact 8), has too high a speed range to cut internal threads (usually with blind end!). My nerves are not up to it! So it is either a slower speed motor ( which I have) or build a back gear. I suppose a VFD may be a solution, but I would be running the motor at maybe a third of its rated RPM and I don't feel comfortable with that. Cooling being my main concern . I have seen two motors fry doing that sort of thing!

The DC motor looks to be a good bet as it won't cost me anything, as it is on my storage shelf and I would be running at the sort of speed that I want. Doing screw cutting by hand is not on, I have far too much work to even contemplate that! Buying a VDF would probably cost twice what the lathe changed hands for and I really can't see the point as I have a 500 rpm motor to hand that will give me the speeds I want

I don't really mind dedicating the 920 to a screw cutting role. I suppose I am a bit leary of the rather small cross section drive belts, and decent cuts, being enough to do some damage to them

There are a lot of these about so a generally applicable solution would be nice rather than a specific one aligned to one motor. Also you want to be able to get the high speed settings still as you are bound to suddenly need it if it isn't available.
A back gear also delivers more torque for cuts on larger dia but as you say you want it for screwcutting that isn't a problem, and belts can still deliver reasonable grip.
Have you considered the possibility of getting another intermediate pulley into the 'space' at the top rear? (with modified cover)

I have the same problem with my lathe (Warco 220/Mashstroy C210T), minimum speed 125rpm. There is also the problem with inertia meaning that even if I switch off the motor, I have no idea where the carriage will actually stop.

My solution is to use a mandrel handle for short lengths. (Definition of short. Whatever you can cope with or risk!) And then risk it on long lengths. Actually, it isn't too bad on long lengths because I can usually stop it early enough and then finish off by mandrel handle.

Regards,

Peter G. Shaw

Hello Bazyle,

I agree that a general solution would be good, as you say "there are a lot of these about". I shall take a good look at the belt drive set up and see what might be done in the way of extra pulleys to reduce the speed.

Peter, Using a manual handle on the headstock, or even worse, turning the chuck by hand, is a total non starter for me. I have a lot of blind holes to thread and I have a muscle wasting disease that would make me exhausted after the first couple!

The 920 lathe shares this problem with the Emco compact 8, so it would be good to come up with a simple solution. A countershaft of sorts might be an interesting exercise. Without a great deal of work, these machines can be turned into a reasonable lathe, a tumbler reverse is simple to add on and beefing up the tool mounting is also a quick job.

Andrew.

There was a "slow speed attachment" marketed some years back (80's??) that put an extra pulley pair low down in the drive train area and then onto the regular pulleys. I don't have a 920 so cannot comment further.

There was a tumbler reverse published in ME but I don't have the issue numbers to hand - maybe one of the online indexes could help?

Thanks Bizibilder,

No problem with a tumbler reverse, that is easy enough to make. I would be interested in the slow peed attachment. That would save me redesigning the wheel!

Andrew.

Andrew,

I do a lot of screwcutting at reasonable speed, if it is a long job I just check the motor temperature by hand. Never really had to stop. As for fried motors, if that is a problem, mount an external fan and cool the motor by that means.

Edited By KWIL on 27/11/2017 19:36:35

Use an internal threading tool that cuts on the far side of the hole and run in reverse starting at the bottom of the whole so the tool moves out of the hole at whatever speed you fancy

Sounds lateral thinking Jason! Might just need to fix up a tumbler reverse for that. But that is a given anyway.

Thanks,

Andrew.

If you just want to slow the machine for an activity like screw cutting you would be OK. If you need to slow the machine to turn large diameters then the increased torque from gearing down could be useful.

Mike

Andrew,

Sorry to hear about your medical problems.

Bizibilder,

Spurred on by your comment, my homemade index has come up with a slow speed drive by David Berrecloth for a Warco 918 lathe. Published in MEW54, page 51 issue date 01 Nov 1998. That article then refers back to ME Vol 178 No 4043 Page 693 06 June 1987. and continued in the two following alternate issues.

Regards,

Peter G. Shaw

Thanks Peter,

That is a really good lead. I may have the ME articles but certainly not the MEW reference. Can one pay money and get a digital copy from our hosts?

Thanks again,

Andrew.

The MEW article is in the archive, but if someone wants to email me a good quality scan of the article I can put it up on this website.

Neil