Chinese Lathes

Any particular reason for going slower than 125 rpm?

Tom

I have a Chester "Conquest" 7x12, which is advertised as having an American circuit board and which may be different to the other 7x12 models. At a little over the ~20 rpm slowest speed, perhaps 30 rpm no-load, I can't hold back the chuck by hand, and I can cut 60 mm dia 2 mm pitch threads in cast iron or bronze in about five passes.

BTW I know that a lot of people think Chester suck on customer service, but I always ask for one guy by name when I call, the guy I first dealt with, and I have had no problems at all. Touch wood. I have made several orders, and they have all arrived promptly, when they said they would arrive, all well packed and complete.

Oh, apart from one centre drill which was noted as "to follow", but they sent another by post to arrive the day after I phoned to chase them up about that - they were full of apologies, "oh, haven't you got one yet?" - could be bs, but does that matter?

Maybe just lucky?

I have a Chester lathe with a variable inverter (1 ph to 3 ph). Lowest useable speed is 10 - 15 rpm. Torque is still adequate for cutting large diameter (50 - 75 mm) threads. With practice, I can stop the thing within a few degress - ideal for cutting threads up to shoulders (an exciting job!).

I haven't changed the speed using the belts since I had it.

In message , Tim Christian writes

Or get hold of an old 'Holbrook' and let the micro-adjustable feed clutch take the sweat out of the whole process.

I've got a Chester 920 which I'm very pleased with. I've no complaints about their customer service. I had a couple of problems (one wrong bit sent, one faulty bit), but they were sorted out without any both.

Can't help you with the VS Drive questions, I've never really needed to go slower than 125 rpm (or whatever my lathe goes down to !) The only drawback with the 920 is that there is only 40mm throw on the tail stock.

Cheers Mark

Snipped...

I'm turning some parts on a 4 inch diameter, but the cut is discontinuous and the material is BDMS as an example job (I'm hoping this isn't something I want to do again!) and turning a thread at 125rpm into a shoulder sounds like it would need a good supply of heart pills!

Steve

FWIW, I think your going to spend money on something that you really haven't justified in the examples you've given above.

Tom

Can you expand on that? The original question was about the use of a countershaft and additional pulleys to give the possibility of lower spindle speeds without laying out much cash. The big money is in the lathe and tooling, surely not in a couple of pulleys and bearing blocks.

Steve

Sorry, my memory have thoughts of you using a speed controller.. As for the examples you gave, 125 rpm for 4" is well down on the norm for the cutting speed for that diameter and screwcutting at less 125 rpm is usually reserved for something much larger than you would anticipate ever cutting on that sized lathe. Unless you know something that the lathe maker doesn't? Screwcutting is an eye/hand co-ordination exercise that is developed over time with experience, doing it at abnormally slow speeds detracts from the finish..

Tom

Steve

The Model Engineer (6 June, 4 July and 1 August 1997) included an article on "Modifications for a Warco 918 Lathe" which included a spindle speed reduction down to 40 rpm along with finer feeds and an extended metric thread cutting range.

The reduction was accomplished by an extra pulley block mounted on an extension of the "bracket plate" and driven from an additional small pulley machined/screwed to the front of the intermediate pulley/clutch assembly. I've used the names included in my copy of an old CT-918 parts list but a quick look at a 918/920 drive system will make things obvious.

To be honest it looks quite a time consuming project for the benefit gained and based on the fact that on one seems to have mentioned it to you, not very popular. The only reason I remember it is that it was on my long list of "things to do" and of course, it never got done. I'm afraid I haven't kept the original magazines but I do have a poor quality photocopy I could send you if you do decide to "go that route". The article also mentioned that Axminster used to sell a very similar arrangement for their 9180 series lathes.

I'm afraid I can't help with the DB10G as I haven't seen the drive system to see if it is similar. I must admit that I have looked at the similar Warco version of this lathe which is already fitted with an "American sourced" variable speed drive system that appeared to work very well at Donington. Although it seems a fair bit more expensive than the basic 920, I suspect it is a much more capable machine.

If you are cutting threads up to a shoulder why not use the old fashioned "amateur" method of turning the spindle by hand. I know it is not "cool" and would be hard work for long threads but it is much easier on the heart and has just worked well for me on a couple of ER collet chucks. I must agree with Tom however, the finish at the speeds I generate leaves a bit to be desired.

Please forgive me if this "post" is not correct but it is the first time I've tried. Not helped by me attempting to preview my ramblings which promptly seemed to vanish with a Google server error. Anyway hope this helps.

Best regards

In message , Tom writes

In that case I'll certainly stick to my Holbrook with its micro-adjustable lead screw dog clutch and fool-proof re-engagement!

Looking at my feeds & speeds table, I'm looking at 60rpm for roughing cuts and the cut is discontinuous, ie there are gaps in the cut another reason to go slow. I'm using books rather than experience - so it's slow going, but I don't understand why you think 125rpm is too slow for this task?

Thanks

Steve

Thanks - I'll look up the back issues. yours is a great post, don't worry. The Etiquette for replies is to post at the bottom so this is a bad post!

Best Regards

Steve

So you are using HSS? If so, most MS will machine at least 100+ FPM and usually there is a accepted caveat of around .012" feed & .125" depth of cut associated with those tables for HSS cutting speeds. Your lathe would be challenged to meet those figures, so a slower rate of feed coupled with a lesser depth of cut would enable you to operate at a higher RPM. On smaller and lighter lathes, increasing rpm and taking lighter cuts when the cut is interrupted, can reduce the impact on lathe and tooling. Changing to carbide tooling would make your speed concerns disappear, although then you might more hp, ya can never have enough..:-)

Tom

Agree with your concept but am interested in your "change to carbide" for an interrupted cut. My personal experience (limited)is that for light lathes we need to use positive rake tooling and my current inserts do not last well (at all!) on interrupted cuts, even with very light .005/.010 cuts and reasonable speeds. I know with lots of hp and negative rake tooling this is not a problem in industry but I would be very interested in any recommendation you have for positive rake tips that you have found will stand this sort of treatment.

Best regards

I completely agree with this observation. +ve rake carbide tips with intermittent cuts seldom last very long. Continuous cuts good, intermittent cuts bad.

Inserted cardide toolholders in the smaller sizes can be both fragile and clamping forces limited. I would recommend the acquisition of some brazed tip carbide turing tools for rough and interrupted turning. The bigger the better, even if it means machining the bottom of the shank to set the initial centre height. Rigidity is everything with interrupted cuts. A selection here:

would imagine that autojumbles would be a prime source if the prices new are a bit steep.

Tom

Turned up the speed this morning and the work has progressed much better thank you. I am using 3/8 HSS tools on an ancient (1905) 3-1/2 centre lathe. I've made a new topslide saddle and toolpost and the bits I've been attempting to turn were the T-nuts that hold the topslide to the cross slide! Not sure HSE would approve of using G cramps to hold the lathe together to make the t-nuts, but now its done.

Regards

Steve

It's getting the job done that counts, Steve.

Tom