Lathe Backgear

[DuncParticipant @dunc]

I am looking at a couple of lathes (9 & 10 inch diameter capacity) and wonder about the absence of a backgear. Both lathes are described as “gearhead”. The slowest speed available is 115 rpm for the 10 inch machine and 130 rpm for the 9 inch. These are not cnc.

 

I believe (perhaps incorrectly) that a backgear equipped lathe would provide a lower speed (50-60 rpm). Other lathes from this company (a 12 inch, for example) do have a lower “backgear territory” speed (70 rpm) but it is one of the normal range of speeds and not obtained from a backgear assembly.

 

 If this assumption is correct, then how vital is a backgear to machining on the lathe? Given the capacity of the machine would the 115 rpm slowest speed be acceptable? 

 

 

 

 

[DuncParticipant @dunc]

Suitable Speed

[Paul BoscottParticipant @paulboscott25817]

Hello

I have a 10-inch Atlas with back gear that in about 30 years I have only used to see how it works.

With modem tipped and insert tooling the absolute need for a very slow has not been one of my requirements.

It is though handy to lock the main spindle when fitting and removing the chucks.

Paul

[mgjParticipant @mgj]

Surely – it the lathe is geared, it doesn’t “know” or “care” how that gearing is achieved. All that matters is the actual output RPM? You are probably better off having everything in the normal range and not having to engage a backgear  because it is less hassle, (but not much less.

 

115/130 rpm Screwcutting to a shoulder might be a bit entertaining if one is not used to it (which I’m not) Got a brake? At 60 RPM and with a small run out, I freely admit to having trod on the stop pedal smartly (more than once in fact), so that might be a consideration

[wheeltapperParticipant @wheeltapper]

Hi

 I’ve got a Chester Comet and the slowest that will go is 100 rpm and that is so hairy screwcutting to a shoulder I made a mandrel handle so I can cut by hand.

 

Perhaps when the warranty runs out I’ll try doing something about the gearing.

 

cheers

the cowardly side of Roy

[JasonBModerator @jasonb]

You want a nice slow speed for screwcutting and knurling also when you want to machine large diameters, a 9″ cast iron flywheel can start to chatter if the speed goes up too high

 

On a geared head lathe having a back gear is no advantage over a slower gear in the main gearbox. Where a backgear comes into its own is on belt drive lathes where it allows the user to quickly select a slower gear with out the hastle of belt changing

 

Also be aware that the variable speed lathes start to loose torque when machining large diameters at slow speed.

 

Jason

Edited By JasonB on 18/10/2009 20:30:37

[Ian S CParticipant @iansc]

)n my 1324BH Taiwanese lathe I find the back gear essential for some of my work done on a comecial basis,it consist of turning the edge of 160mm dia flame cut discs down to 158mm dia,then boring out the center which is already flame cut to 33mm dia to 35mm,getting through the hard skin is to say the least a bit tedious.I also like the 60 rpm for screw cutting.

[DuncParticipant @dunc]

Further to my original post I was looking around and noticed that some Myford Super 7s offer 23 rpm! There must be a use for these real low speeds. Maybe industry with flood coolants, ceramic inserts and whatnot no longer need it but I am thinking that  there is still a place in the hobbyist’s domain.

 

Both machines use a 1 phase, induction motor. There is a toothed pulley to the initial idler and then a v-belt. The rest is gears. I suppose exchanging gears in the existing train to afford greater reduction is a possibility although this would depend on suitable gears and the space to mount them.

 

Strictly theoretical ( as I don’t have either machine available) would adding an additional jackshaft somewhere in the pulley/belt portions be feasible? Evidently, space – or lack – is a consideration but more to the point is could the torque/power be transmitted by another pulley/belt stage or this would be too much? Are there formulas or whatever to calculate this or is it more a “try and see”?

 

I have cheated so far for threading on my current mini-lathe. While I have the threading gear getting it slow enough – with sufficient torque – is a problem and I have used taps/dies (sometimes not on the lathe). I have seen several suggestions for a handle for “manual revolutions” but have stayed with the formula that works for me – so far.

 

 

 

[mgjParticipant @mgj]

Well I  always use a handle for all die threading – better feel, and quicker too.

 

Smaller lathes often go to these very slow speeds simply because they don’t have the torque available to cut a really deep thread without. Things like Acmes, and the like.

 

OK , are you going to cut any deepish threads – possibly not, but then again, if you need to, its because you really need to!

 

Knurling likes a good slow speed.

 

I’d suggest 60rpm being as fast as one would wish to go in that dept, if oyu haven’t, as most of us havent, single point instant release clutches. (And a good flip out retracting toolholder might be high on the manufacturing list!)

 

How to get there. Well jack shafts and all can work, but its all getting a bit complex?

 

If it were me, I’d cheerfully trade a geared head, for a belt drive, backgear and Norton box, if that gave me a good range of speeds and feeds.

 

Formulae – yes quite easy.  Compare diameters. D1/D2 will give the output RPM and torque increase. 

 

ie you have an input speed off the motor of say 1425rpm. You want to get down to 200rpm. At the mandrel 1425/200 = 7.125  So the the total ratio, using the jackshafts needs to ensure that the final drive is 7.125 x bigger than the motor pulley (effective diameter.) You can get there in one step, or 2 steps, or 10 steps, as long as it all tots up to 7.125.

 

And 7.125 will be the torque multiplication.  Mathematically its not complex, but Id suggest, if you want to do that, you get a short book on gears and gear trains, which will cover the principles and then you’ll be right first time with relatively little trouble.

 

If you can though, just buy the right lathe – you are going to have it for a long time! (I hope)

 

Edited By meyrick griffith-jones on 19/10/2009 18:19:44

[chris stephensParticipant @chrisstephens63393]

Hi Meyrick,

I knew we would meet again. 

Now, rather than go to the bother of making a retracting tool holder, takes hours and hours, no make that days, to make the GHT one. (start to finish on mine was about six years, not constantly at it, you understand) A clever design and a nice tool to be sure but a couple of hours making an adjustable cross slide stop is a much better bet, Works even better with the offset top slide thread cutting method, better still to use with the Tangential threading tool. Had to get Tangential tools in somewhere, didn’t I?

Was going to show above at Sandown, but not now. 

chris stephens

[Gordon WParticipant @gordonw]

Hi, I’ve a chester 8″ x 16″ ,lowest speed is 125 rpm,the first thing I did on it was to rebore a water pump(domestic supply), this is way to big a job really, but it got done, now Iam making a larger pulley for the mandrel, boring out the center to fit on the O/D of existing pulley with 3 “legs” to fix to existing pulley with 3 screws, this will give a low speed about 70 rpm will need a new bigger belt. I also use a hand wheel for screwcutting, much easier and quicker, but I only do short threads, maybe 5 or 10

[mgjParticipant @mgj]

Well I was given my GHT holder – and on that basis, a darned good tool it is.

 

I gather with the tangential ones you can also cut a thread -but you still got the problem of stopping this BEEHEEmoth before there is an almighty jam up.(Even with a tangential tool )

I admire people like Gordon who are prepared to do all the mods, but again, you have to take the countershaft apart to get the belt in place – or so it sounds. Thats a lot if aggro for a screwcutting or knurling job, which should be bread and butter for a screwcutting lathe.

 

And I refuse point plank to drive a lathe manually while screwcutting. Driving a die manually backwards and forwards is one thing, but not a single point threading tool! 

[JasonBModerator @jasonb]

Meyrick, have you not tried using a tailstock die holder under power. I always use to turn it by hand but now I have a lathe that goes down to 40rpm its easy.

 

The main safety points to bear in mind are:

 

Remove the handle so it won’t take your thumb off

Wind in any screws below the body if like mine its double ended

Wind the toolpost well back out the way.

 

You just grip the body of the die holder, start the lathe and slide the holder towards the work, as soon as you reach a sholder the holder will rotate in your hand or just loosen your grip when the length of thread is what you want.

 

Either wind out by hand or flick the lathe into reverse.

 

This is OK for threads upto about 1/4″ any more and its hard to grip plus you want to wind back the die to break the chip. A knurled holder is on the todo list.

 

Jason

 

Jason

[chris stephensParticipant @chrisstephens63393]

Hi Jason and Meyrick,

Thread cutting by hand operated handle is a great way to get the hang of the thing, before getting the confidence to do it under power. I show my “apprentices” * this method first. stops them worrying about b*lls ups too much. The other day I cut a 1 1/2 X12tpi by hand( yes the arm has now recovered, thanks for asking) only ‘cos I had the Myford in bits fittng the VFD. It’s too difficult to set the Bantam up for Imperial threads.

 

If you make a tail stock die holder, make it with a handle that is just short enough to miss the bed of your lathe. The beauty of this is that if you hold the handle between fore finger and thumb and as you come up to a shoulder you can let go and let it spin. I realized I am probably breaking some H&S tom-foolery as there is no guard protecting a spinning object and someone will point out the dangers, no names no pack drill. ( why is there no emoticon for blowing a raspberry)

chris stephens

* translation; any person who actually knows less than me.

[mgjParticipant @mgj]

Yes – can and have with a tailstock die holder. Prefer to do it mandraulically, partly to break the chip, and partly because all my dies and taps are CS. Harder and last longer but I’m wary of localised heat build up at the tips.

 

Probably under power it would be better, but I’m just used to doing it the other way.

 

Love threadcutting though. The LS brake mech calls for a square thread, so I’m looking forwards to doing the actuating arm and nut for that. (And not cheating!).

Edited By meyrick griffith-jones on 20/10/2009 21:44:51

[JasonBModerator @jasonb]

I’ve also got the square thread for my brake comming up, not decided whether to make a tap or small internal threading tool for the nut. Its 1/4″ 16tpi

 

Jason

[Peter G. ShawParticipant @peterg-shaw75338]

Hi,

My first lathe was an unmodified Hobbymat – lowest speed of 250rpm. Screwcutting at this speed was, well, frightening. So I made a mandrel handle. I also bought the Essel Engineering slow speed adaptor which gave a slowest speed of 63rpm

 

When I bought my second lathe, I looked at all the specs, including speed, and realized that the more expensive lathes, eg Myford, did indeed have the ultra low speeds of 15rpm or thereabouts whilst cheaper lathes were limited to around 100rpm. Furthermore, the Boxford I would have liked (still do, but the weight & size defeated me) goes down to about 40rpm.

 

Next I started looking at speeds & materials and discovered that the worst case situation requiring these very low speeds was large diameter cast iron.

 

Eventually, I bought the Warco 220 lathe, slowest speed 125rpm, on the basis that a slow speed adaptor would soon be available. It was, but compared to the Essel kit was very messy to use so I returned it. The end result is that for screwcutting I now use a mandrel handle, and I, as yet, have no need to turn large diameter cast iron.

 

Incidently, there is one advantage of a mandrel handle: there is no inertia from the motor (as long as the belts are disconnested) hence one can stop a lot quicker!

 

Regards,

 

Peter G. Shaw

 

 

[mgjParticipant @mgj]

It may be we are pushing this screw cutting thing a bit far?

 

The advantage of screwcutting a thread is really that one can guarantee it is straight. Mostly the pitches we cut taps and dies are in fact available. So a compromise solution may be to singlepoint screwcut (by hand) to a few cuts to make a start, and then use a die to finish- because the die or tap will naturally follow .

 

Yes, one is losing some flexibility, but its like all of these things –  90% of the result for 50% of the cost?

 

I’m alright on this one, because both machines have proper screwcutting gearboxes and a good range of feeds – but then I don’t have the space problem that many have.

 

The only thing thats really important in this discussion is that people go into the showroom with their eyes open, and they understand the tradeoffs?

 

————–

 

Jason – I shall make a tap. Good idea. Years ago I built a Quorn. I have never used it to a fraction of its capabilities, so it needs a surprise. I shall screwcut some silver steel and then make a tap. That’l be a good evenings exercise, and 2x the satisfaction deforming a bit of bronze.

Edited By meyrick griffith-jones on 21/10/2009 18:54:37

[Author]

Posts