Turning small diameters



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Chas,
Dave Sanderson has given you lots of good advice, which I will try not to repeat.
You don't say how *long* the very small diameters are to be. This is important, as workpiece deflection is one of the biggest problems with turning very narrow parts. Varies a bit with material stiffness, but once you get to more than a couple of times diameter the finished part will taper (wider at the outboard end) too much to be any use. As Dave said, you can use a steady, but the conventional type with fingers will almost certainly not close down far enough (you could try making the fingers more "pointy"). Probably a better solution is to use a rose bit (think of an end mill with a hole up the middle) or one of these small diameter turning tools:
http://www.chronos.ltd.uk/acatalog/Myford_Lathe_Compatible_Acessories.htm l
This has advantages: unlike a rose bit, which has to be home made and only works for one diameter, it can work for a variety of diameters (you have to make a bronze insert for each) and it is ready made (and it is quite cheap). It also allows the turned part to have indefinite length.
Even a steady at the end won't help if the turned part is more than a few diameters long - the middle will deflect away. The above devices overcome this.
I suspect the use of hand gravers has in part to do with the fact that not all the turning is parallel, with some curved sections etc, but I'm not a watchmaker, ever. Also, AIUI, the tolerances in watch/clockmaking may not be what you think, I believe sloppy (= very low friction) fits are the norm.
The other issue you need to consider is maintaining concentricity. Very often you will need to work on the end few mm of the part, by pushing most of it into the chuck (to minimise deflection) then to work on the next bit you pull it further out, and with most chucks you will be lucky to have it concentric with the bit you turned first. The best solution is to use collets, which are much better at maintaining concentricity - they are pretty standard on watchmaker's lathes for that reason. This is something to look at when selecting a lathe. The best and most convenient system I have used is that on Myford's 7 series lathes, which has 2 MT collets held in place by a screwed nosepiece. Not cheap, and I suspect from what you say that the Myford lathe option will be outside your price range anyway. You can get MT collets with threaded rears which are held in with a drawbar, work for most lathes with an MT headstock; never used these. Collet systems using adapters for ER collets are an alternative, though in my experience the concentricity straight from the box is not as good, you may need to true up the taper on your lathe, not for the faint hearted.
On the question of lathe size, then up to a point go for something larger than your immediate needs. It is, as you say, always possible to do small work on a big lathe, and not possible to do big work on a small lathe. I say "up to a point" as big lathes tend to be much more awkward to do small work on. I have a Myford S7 (3.5" centre height) and a Harrison M300 (6.5" centre height) but would always prefer to do small work on the Myford (the collet system is part, but only part, of the reason for that). In addition to looking at what collet systems work on your potential lathe, it is very helpful if it has, or can be fitted with, a finely graduated leadscrew handwheel, like the S7. Something I really miss on the M300. Thus means you can position the cut to within half a thou, which is difficult to do with the topslide (as (a) the graduations are too small and (b) you have to set the bloody ting to turn exactly parallel in the first place).
For fine work you need to ensure your slideways are smooth but without a trace of slop; this is one area where a cheap lathe will need fettling. A topslide lock (you will have to do this yourself) will aid security. For getting really fine cuts with a topslide (as opposed to one of the above devices) set the topslide to a fine angle so that 1 thou on the dial gives 0.1 thou of cut (but don't forget to allow for the extra movement towards the headstock, DAMHIKT).
One final thought: tool shape for turning narrow parts should be knife types with a sharp corner, not round-nosed. The latter will cause much more deflection force than a knife type; the latter should have a very, very fine radius to reduce stress raising in the sharp corners, but no more. (This is because almost all the cutting force with a knife tool is parallel to the axis, towards the headstock.)
Hope this helps. If you give more detail about the things you want to make, I'm sure the collective expertise here will be able to add more. I myself have just re-activated my interest in RM after a gap of 30 years, investigating recent commercial N gauge stuff, but have not yet got involved in re-engineering any of it.
David
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David Littlewood

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Forgot to say, you'll have to scroll down, the thingy is right at the bottom of this very long page....
David
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Dave,

I thought this might be a problem.

The link seems to be broken but I found the Chronos page and tool, thanks. It's obvious I'm going to have to get used to some very precise and careful setup - at first sight it looks like even a minute tailstock alignment problem will just break the turned part as it goes through the hole. Obviously yet another skill to add to the very long list.

Ah. This makes sense.

This seems to be available for Chinese 7x lathes. e.g. http://www.littlemachineshop.com/products/product_view.php?ProductID07&category‡4479994

Hmm not sure whether this is available, even as an accessory, on any of the lathes in my size bracket. No photos really show the hand wheel properly. Google searches (hampered by the difficulty of knowing what search terms to use) don't find me anything. On the basis that if it isn't advertised, it isn't there, I assume I won't have this. Anyway, I don't really understand, I'm sorry. Is this used after repositioning the stock in the collet? Or for positioning for second and subsequent passes? Bit lost I'm afraid.

It definitely helps a lot. Can't be more definitive really. I was just looking for general guidance on the relationship between 2 choices I want to make - deciding to model in a tiny scale, and buying a lathe to, amongst other things, assist in the modelling. Which I have received.
Chas
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You can get similar things from UK suppliers.

Some have it (Myford S7, Unimat, the Sieg C0 and C1) but many don't (Myford ML7, Sieg C3, lots of other Oriental lathes, my Harrison M300). It is extremely useful for moving the carriage in precisely controlled increments to apply a cut of known length. See the top picture:
http://www.lathes.co.uk/myford/page2.html
It's the handwheel at the bottom right.
Otherwise, you have to use the topslide (which may or may not be set to turn parallel, depending on what you last did with it) or use power feed with a stop, having measured the length previously by some other means. Whatever this means is, it is unlikely to be more precise than the large 1 thou graduations on the big handwheel.
You can buy add-ons, for example
http://www.sherlineipd.com/handwhls.htm
but you would perhaps have to work out for yourself how to fit it. You could also buy a blank handwheel, eg
http://www.arceurotrade.co.uk/Catalogue/Machine-Spares/Machine-Handles
...but then you would need to graduate it yourself. (Make a graduating head and you will learn more about precision machining than you could imagine.)
One feature of lathes with fitted leadscrew handwheels is that they (usually) have left-hand leadscrews so that the direction of turning the handle is the natural one. Lathes without often have right-hand leadscrews, which means that the wheel (if you fit one) turns the wrong way. One can get used to pretty well anything, but it helps if all the handles work in the same way. I have been contemplating fitting a handwheel to my M300 with a sun/planet gear to reverse direction, but I doubt I'll ever do it.

I suspect that if you buy a tiny lathe to do tiny bits, you will soon be frustrated. I started out with a Unimat 3, but found it fairly feeble and soon got pissed off with it. (It also got through drive belts with distressing frequency, usually breaking my last one on a Sunday.) Consider buying something second-hand that's got more capability. A few years ago, the common advice would have been "buy a Myford" but today it's hard to justify that if you are not seriously keen - the far east stuff has got so much better and is much cheaper than a decent second hand Myford. Of course, Dave Sanderson had a nice looking Harrison L5 on homeworkshop.org.uk on Monday at a good price, but that might be a bit big for you.... (He was too much of a gent to mention it here so I'll do it for him. Of course it may be sold by now.)
Another question you should ask yourself is "am I ever going to want to cut screw threads (much superior to using a die). If so, be sure it can do this (the C0 can't, AFAICS). Are you likely to need imperial and metric (the C3 is only one or the other, depending on which you buy). Some (eg
http://www.warco.co.uk/WM-180-Variable-Speed-Lathe-6D064B881E.aspx
seem to be able to do both.
I started out using dies, but once I had experienced the delight of a well made screwcut thread I was hooked.
BTW, you may have had trouble with the Chronos link because the last letter wrapped to the next line. Sometimes you need to copy/paste these things.
David
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David Littlewood

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Dave, I guess this is getting a bit off the small diameter topic, but I don't mind if you don't mind being my unpaid tutor.
The graduated lead screw stuff: So I assume you are making the general point that being able to accurately position the lead screw and hence the feed position is a good thing, rather than something specifically to do with small diameter turning - yes? So, I was being dumb and thinking about the saddle feed handwheel. I see the point. And I see that many other lathes do have a leadscrew handle, and it is graduated.

Thanks for the pointer. The Harrison looks really nice, at a glance, but it's just too big.

Yes, that seems to be a consensus, both in this thread and elsewhere on the web. Now that I am clear that there's no issue with small diameters and a somewhat larger machine, I am sure I will plump for a bigger one.

Well, that comes into the same category as manual exposure on a camera. It's hard to say exactly when you will want it, but it seems like the sort of thing you ought to be able to do. So I would count this as a criterion. I have no idea whether I want imperial or metric.I suppose I would go for metric, aren't most threads metric now in the UK? This site suggests metric is most sensible http://start-model-engineering.co.uk/machines-and-tooling/minilathe /, largely based on better availability of stock. But I must admit, if you tell me that I need an M6 nut, I do a quick conversion into imperial so I can visualise what it means. Same with lengths, and weights. It's an age thing. I no longer convert new money to old money (but I can if you want - I was working on the counter in Boots on D-day as a holiday job as a lad and developed the knack of instantaneous conversion for all the old dears who were confused. I do sometimes bewilder shop girls by expostulating 'Fifteen and thruppence for a bar of chocolate - you must be joking'. Now there's a useless skill - and I could have been paying more attention in metal working classes).
As I see it, if I make both male and female threaded parts myself, I can pick whatever I want. If I have to match to someone elses thread, I have to know in advance whether it will be metric or imperial, and I have no idea, so I might as well just gamble, I suppose. It seems a bit gadget-conscious to insist that my lathe must be able to do both, when I don't really know how much threading I will ever do. But thanks for the pointer to the Warco, which seems to be the same lathe as the Chester DB7. It's probably just a bit heavy and a bit expensive.
Some machines seem to offer a metric to imperial conversion kit which includes a 100:127 gear. LittleMachineShop offer a kit for the 7x that has a new lead screw - but only metric, presumably the Americans all buy the imperial version of the lathe. Maybe at a pinch the equivalent imperial 'kit' could be made up from spares.
Chas
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That's a large part of what newsgroups are about.

Yes, and also to be able to get very fine control when turning up to a shoulder (without relying on the topslide). Best method, I find, is to remove most of the material stopping a few thou short of the required shoulder, then do a clean-up of the shoulder to the desired position, then take off the last thou or so on the diameter. Others may have a different approach.

I agree; my Myford is imperial, but my Harrison and milling machine are metric, and I would for preference always use metric. My milling machine has a DRO, so I can switch between the two in a second, but I suspect a DRO is much less useful on a lathe. Those DROs which are add-ons to the handles are IMO a con - they don't give an absolute measure of position, and still suffer from backlash problems - though to be fair I've never used one.
(BTW, and back to your original question, I think you would find a milling machine about as much use as a lathe for modelling work. Be sure to get one which retains position as you move the head or knee - many of the smaller oriental types have a round column with no register, so if you move the head you lose position.)

Screwcutting internal threads is something requiring a bit more expertise - it's the same job, but backwards and with your eyes shut (to paraphrase Ginger Rogers).

Very likely, though check the banjo has room for the changewheels required.
david
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David Littlewood

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Dave, Many thanks for all that.

This thread is quite long, and since you have opened the topic of mills, I have started a new thread on that topic, where I refer to this point
Chas
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No problem; I injured myself somewhat a few days ago and am spending far more time than I would wish sitting on my bum at the computer.
David
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A|so consider a decent second-hand model - often better value if you know what you are buying. Saw this just now:
http://www.gandmtools.co.uk/cat_leaf.php?idr46
(No connection). The MD 65 was very well regarded - I think it may have gone out of production, but there is quite a lot of literature on mods and improvements.
David
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David Littlewood

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I think that one has gone already, the link doesn't bring anything up. But I assume it is a Hobbymat - there was one on ebay and I looked up what lathes.co.uk had to say, and they were complimentary - unusually they thought the combined milling attachment that came with it was very good for the money. Their general view seemed to be that the overall quality was much higher than than the price, because it was communist E Germany government subsidised.
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