Turning small diameters

Hi, I am a novice, considering 2mm railway modelling, and considering buying a lathe. I have browsed the web a lot and would get some sort of Chinese
lathe, maybe a Sieg C3/ C2, maybe smaller e.g a Sieg C1 or even C0 (or equivalents in terms of size and price from other makers). Price would be less than 500, but more importantly weight would be 50k or less, as I want to be able to move it.
The thing I don't understand is : what are the criteria for being able to turn small diameters? Obviously 3 inch diameter in the prototype is 0.5mm 20 thou in the modelled world, and lots of cylindrical things on railways are less than 3 inch diameter.
If lathe size doesn't matter, then I would get one at the top of my budget/weight range, as I would use it to make some more normal sized stuff, like tools for modelling etc.
The few references I can find on small diameters with mini lathes seem to consider 1mm dia or so to be a major achievement, but there isn't much out there that I have found. I guess the diameter is irrelevant without the length - well, I suppose if the diameter in the protoype is 3", the length is not likely to be more than 10 feet, so that's 20mm = say 3/4 inch long, on 0.5mm = 20 thou diameter.
Maybe it's not possible to turn to this size, and anything more than 1/4 inch long has to be fabbed from drawn wire (though clock and watchmakers turn much smaller than this on longer lengths, don't they?).
I assume the material would be brass - maybe a different material makes small diameters more achievable. I guess that speeds would need to be higher, assuming that proper turning needs the correct number of feet per minute past the tool tip - but does this mean speeds beyond the range of ordinary mini and micro lathes (which seem to peak at around 2500 - 3000)? I assume supporting the work must help a lot - do the steadies you can get for these lathes work on diameters this small? Maybe it's all to do with the cutting tool size/shape/material - is there any reason why an appropriate tool would not work in one of these ordinary lathes?
I assume the tolerances I will need will be no tighter than people seem to get with these lathes (so in the 1 thou range at best, much more if it's just cosmetic), so it seems to me the precision of the machine should not make much difference. But maybe I am missing the point.
In general what I read on the web seems to suggest that just because the parts you are making are small, this does not mean the machine you make them in should be small (as I understand it, little machines may be less well made, whereas bigger ones are for pros and will be better made, which should help - at least, I think that is the logic).
Watchmaking is probably a red herring, their parts are so short and their tolerances are so tight etc - but it's the only example I can think of that demonstrates that tiny parts can be turned. Reading up a little on it, I see they all use a hand-graver. Is there something inherent in using a hand tool that makes smaller parts easier to make, or do they just do it like this because it's a craft thing? It isn't obvious to me that the (very expensive) little lathes they use are inherently any more suitable than a mini-lathe, though again maybe I am missing the point. Watchmaking seems to be a vast and esoteric subject, but the few references to turning techniques that I can find seem to suggest that their turning speeds are if anything quite slow (500-1500 or so) but maybe I am getting this wrong.
Hope someone can give me a clue.
Chas
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Chas explained :

That and smaller can be done and even hallow tube on a C3 (also known as a Chinese 7x14), with the right tools and technique - but not by me yet. The lathes do need to be tidied up a little to get best performance out of them. You can get them for under 350 via ebay.
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Any clue on what the right tools and techniques are?
Yes, I might well go for the ebay 7x14 that is actually from the US Big Dog , which I think is not a Sieg but very similar. But if someone tells me I'd be better off with say, a Taig, for the small stuff, I'd get something like that.

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Turning long (relatively) thin pieces requires some skill, the correct tool, and a little luck. High spindle speeds are useful, IIRC I rarely ran my unimat below the top 2 speeds (2500 and 4000 rpm). small diameters have low surface speeds, so going flat out is not usually going to fast. Generally ignore carbide, for small diameters you want the minimum cutting force, to reduce deflection, and for that using HSS, or CS if you really get into it, is best. Sharpen the tools with more top rake than normal, and hone them to a sharp edge. An easy way to asses sharpness is to run the tool *lightly* over you thumbnail. you should be able to shave a tiny thin curl off. Brass is usually cut with no top rake, but I found that a sharp tool worked for small diameters as long as the lathe is not 'sloppy'. The cutting force trying to move the tool into the work that is generated by the top rake is small compared to the force required to move the crossslide generally. Take small cuts, and be patient, its very easy to spend a couple of hours getting a tiny part done. Its also very easy to scrap it on the last cut (DAMHIKT...) Use 'free cutting' variants of metals, in the real world there is potentially a strength loss issue here, but in tiny scale it will not matter, and every help you can get will make it easier. Make temporary steadys from bits of hardwood. parquet flooring is useful for this. On small parts you can help steady by just holding a piece of hardwood against the part. Make sure your tools are on center height, *EXACTLY* as there is less scope for error, if you are a little low your part will catch, bend and then run over the top of the tool, to high and you will increase the cutting force considerably, assuming it will cut at all.

The cons of a little lathe (C0 / unimat etc) are that you very quickly run out of size and power. The myford size (3.5" center height) has been around for a while, and so I guess it must have proven to be a reasonable compromise wrt size vs weight. A general truism is that you can do small work on a large machine, but not the other way round. A not quite on topic illustration of this are the rc helicopter parts I used to make. They weighed about 0.5g, and would fit comfortably on a penny, but I made them using a 1700 kg 10x50 (IIRC) knee mill. As for specifics a chinese 7x lathe will work just as well as a more expensive lathe, but you may have to 'tweak' it a bit to get it working nicely. A friend of mine had a Taig, and did a lot of good work on it, but he also had to tweak that, as it has no leadscrew. He has recently 'upgraded' to a Seig C3 (Arc Euro Trade is just down the road from here), and seems very happy with it.
I started with a super adept (tiny old lathe), but quickly got a Unimat, which I made a lot of small parts on. Works fine, and is still in the cupboard for the occasions when a smaller lighter and faster (4000 rpm) lathe is appropriate. TBH though it hasnt had a lot of use since I got an L5, and then recently a CVA. Most of my stuff, turning wise anyway, is just nicer to do on the larger lathes. You can take large roughing passes, and then finesse the last bits out, where as the smaller Unimat doesnt have the power to do more than a finishing pass every time. The C3 sizes are somewhere in between IMLE.
HTH
Dave
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Dave, Thank you very much. Very helpful. So I should probably look for a machine with a reasonably high spindle speed. These seem to be the smaller ones, I suppose it's easier for them to make a small one run faster. I suppose that narrows it down in my budget bracket to a Chinese 7x (max 2500 rpm), a Unimat 4 (max 4000 rpm) or a Taig (max 7000 rpm)
Obviously you know what you are doing - so would you be confident about making a brass piece 0.5mm dia, 20mm long, on something like your Unimat using your accumulated skill and tooling?
I suppose the exact on-centre tool setting must be quite an issue at small sizes - 1 thou high/low on a 20 thou dia piece is out by 10% of the radius, which I guess is a lot. So I suppose the more precision engineered the lathe, the easier it is to achieve this. But I don't see any conclusive evidence for any difference in precision between the machines in my budget bracket.
Chas
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wrote:
Chas,

Talking about tool holders for modeller's lathes, have a look at the toolholders for the Fonly lathe in this article.
http://www.btinternet.com/~two.mm/articles/fonly/fonlypt1.htm
You will probably want to do free hand turning for items like chimneys and domes and these tool holders let you do it very well. You would have to supply a flat surface for them to sit on but this coould be as simple as a piece of metal plate on the cross-slide.
I started on a Super Adept which was treadle driven (converted Singer sewing machine table, so speeds for small diameters were not too high. For very thin work, the travelling steady was my finger :-).
Another way of turning thin work is to take one cut fron a larger stiffer piece. This minimises the work deflecting from the tool, but you do need a lathe with a bit of grunt to take the cut.
Also for small diameters, you can use a tailstock mounted tool which combines a cutting edge and a steady - i.e. the body of the tool has a bore which is just clearance on the machined diameter and the cutting part of the tool is right at the front.
Jim.
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Jim, Many thanks.

I get the general point, which seems to be that small diameters are more about the skill of the lathe operator than the features of the machine.
I have seen descriptions of the cut-in-one-pass approach, and it makes sense - but mostly these seem to be of the order of 1.5 - 2mm dia.
Yes I have looked at length at the Fonly lathe. I kind of assumed that it's a way of getting some sort of lathe for next to nothing, and whilst I am cheap, I'm not that cheap. On topic, one of the things that put me off about the Fonly, which I still don't understand really, is that it is ungeared, driven from a multitool. So a Dremel, as a good example, runs from 10000 to 33000. They don't all go quite as fast as this, but part of the point of multitools is the substitution of speed for torque. It really surprises me that this vast difference in speed from normal lathes doesn't totally mess up the cutting process. Obviously it works, but it destroys my confidence that I understand how lathes work and the logic of lathe feature selection!
Chas
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No problems. I assume you are a usenet noob? Try not to top post (answer at the top to a question) it *REALLY* winds up some people, and makes it harder to follow the topic. Ive mixed my answers into your original questions, but a full post at the bottom is also used...

Whilst fast top speed is useful I wouldnt get to hung up on it. (My L5 is about 1000rpm) Are you only ever going to do small stuff? (the answer to this is no incidentally, machining is addictive...) I found the unimat limiting in its smallness, the taig is better in this respect, but the 7x chinese is better again, for no more real bench space. If you want it to go faster I would venture that turning a new pulley set is a simple lathe job ;)
Dont forget that the lathe is just the start of the expense, you will also need to budget for tooling, chucks / collets / a bench grinder to sharpen toolbits (assuming you dont have one), metal to turn into scrap etc etc...

Turning 0.5mm x 20mm I could do on the unimat, on the harrision, and on my CVA. however no matter what the machine Id reckon on probably scraping 2 out of 3 if I was having a bad day. This thread ( http://www.rcgroups.com/forums/showthread.php?t 1121 ) shows a picture of some brass parts I made, IIRC they are 1mm in diameter, about 5mm long, but they have a 0.5mm hole cross drilled in the middle. I used the unimat and my *big* TOS mill to make them, as I could have the unimat sat on the mill table and it made the various ops easier. I wouldnt limit yourself to brass either. use the appropriate metal for what ever it is you are making. FC steel turns really nicely for instance, silver steel can be hardened which makes axle point bearings run nicely etc. guide bars from old printers are really nice metal to turn, some sort of precision ground steel.

Setting the center height has very little to do with the quality of the lathe. Pretty much any new lathe will hold the same tolerances at the price level and size we are talking. Some may 'feel' nicer, or have a better finish and paint, but paint doesnt make parts. A quick change toolpost makes getting cock on simpler, but I dont have one for my unimat, just a large selection of shims.
Dave

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On Thu, 12 Nov 2009 05:51:00 -0800 (PST), dave sanderson

But be careful! Whilst superb to machine it is I think plated, probably nickel. I made some pillars for a job recently and bunged them in with the rest of the job for blacking. The areas that I had machined blacked ok, the original diameter did not!
Back to the subject line. When I first fired up my Viceroy (10" swing) I played with some tools and happily turned a length of 8mm printer bar down to 1mm (over about 10mm long). Pleased with that I then drilled it up the middle with a 1/64th drill, even more pleased with that I shoved it on the mill and cross drilled it again 1/64th.
The max speed available for the Viceroy was 1300 odd rpm so I woul not worry too much about a high speed capability. As has been stated sharp HSS on centre tooling is a must! Richard
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Dave,

OK, no more top posting. Yes, a noob. I suppose I'm used to email chains in reverse order, but of course this isn't email.
> ..but the 7x chinese is better again, for no more real

Hmm, I'd have thought the maximum speed of these things was set by the balance of the rotating chuck and associated stuff, and maybe the bearings, so I think I'd be a bit worried about just gearing it up to double or something. Maybe an experiment for when I'm really skilled and confident, in about 2019 or so.

Just what I wanted to understand, thanks again.
I'm getting more confident now that some sort of 7x is the right compromise for me. Tiny things beyond my turning competence will just have to be made some other way. There's always wire.
Chas
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Ridiculous.
There's no reason why any should get wound up by the positioning of a reply.
Surely it is the content of the reply which counts?

Ridiculous.
There's no reason why any should get wound up by the positioning of a reply.
Surely it is the content of the reply which counts?
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Content is only of full value if people can follow it. It is always helpful to make it easy for people to see (1) question and (2) answer, or (1) assertion and (2) response, especially as threads can get quite complicated.
Of course, if this doesn't bother you, or you have nothing of value to say*, go ahead, no-one is likely to shoot you.
*I find there is a fairly strong correlation between this and posting under a fake identity.
David
--
David Littlewood

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I wasn't wound up by it at all. It makes sense to reply at the bottom. It's just us email types who have got into the habit of reading everything upside down, which is, after all, daft. Dave, ignore him, I am grateful for any advice.
Chas
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I wouldn't disagree with any of the advice you have been given, but maybe I can add a few comments from the point of view of one who uses lathes of 3.5" (Myford ML7), 2" (clockmaker's), and 1.75" (watchmaker's) centre heights.
The 3.5" lathe is fine for general small scale engineering work involving a compound slide taking sizeable cuts where finish does not need to be the best, but would be awkward to use a graver on because all that cast iron gets in the way. It can be done of course, but why make things more difficult than they need be. There is also the difficulty of maintaining concentricity unless you invest in collets.
I use the clockmaker's and watchmaker's lathes exclusively with collets, turning sometimes with a compound slide and sometimes with a graver and T rest. The graver gives a finer finish because there is more feel than a screw fed tool, though it requires more skill in its use. If made exceptionally sharp and held at the correct angle the graver can take off two tenths of a thou, even from fairly tough blue steel, and leave a surface that only needs a bit of Simichrome for a mirror finish. Naturally these lathes run faster than the Myford, 5000 rpm on my smallest machine, though I rarely use top speed. The other advantage is the alignment of the tailstock is generally much better than the larger machines. I drill 0.5mm holes with carbide bits and only rarely break them. That would be impossible unless the tailstock is spot on.
Cliff Coggin.
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Chas used his keyboard to write :

You are starting with a thicker material and even if the lathe centre is a little off, it will turn it down to wherever the centre is. The error (assume 2 thou)only becomes a problem if you take it out of the chuck and reposition it in the chuck 180 degrees from your initial position, where upon the off centre amount will be doubled (4 thou). So you mark piece and chuck, if you intend taking it out and putting it back.
Another way is to use an independent 4 jaw chuck which allows you to centre the piece - each jaw can be independently adjusted.
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Harry (M1BYT) (L)
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wrote:

I think you are confusing tool center height with spindle center. You are correct that stock set off from the spindle center will behave as you describe, and as you say this only matters if you want to remove and reposition the stock. However setting the tool cutting edge on center is a must for thin work. If the tool is not at center height then the stock will either try and climb over the tool, or be forced away from the stock by the non cutting front face.
A tip that an experienced machinist told me once for fine finishing is to grind a *ROUND* tool square on the front edge, half thickness (like a D bit), but with a hook chip breaker and some (5 degrees or so) front clearance. Then make a holder for the round bar so it ends up approximately on center height. Rotate the tool 45 degrees in the direction of travel and use it to take *very* fine finishing cuts. because the tool presents a flat edge that is perpendicular to the horizontal from the center height it is always going to be on center height. Its harder to describe than to make honest.... if its not clear imagine a stanley knife blade held so the sharp edge is at 45 degrees from vertical, and pointed towards the stock, and towards the headstock (assuming R to L travel).
Dave
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dave sanderson pretended :

Yes I was, sorry..
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Harry (M1BYT) (L)
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Chas was thinking very hard :

You will no doubt develop those after lots of practise.

Hugh or Harry(something) as he calls himself on ebay? That was who I bought mine from several months back. He is good to deal with and very helpful if you do find problems. Be warned that these are just as they come from the Chinese factory and need some time spent on them to get the best results.

I'm sure the 7x14 will suit you and it is well supported with information. Look up the 7x12 mini-lathe yahoo group.
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Harry (M1BYT) (L)
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Harry,

Maybe. There's also another one, who also does Amadeal or something, who says he has a relationship with Big Dog who supply the Real Bull clone. I am watching an Arc C3 auction that still has a couple of days to go with interest just to see how high it goes.
Or Arc or Chester or Axminster who do the Sieg C3. Or Warco or Clarke who do the C2. Honestly, I'm not much hooked on any particular supplier, and for the sums involved probably wouldn't pick the cheapest of all the clones. Maybe I'll pick based on colour, red is nice but green has a bit more of the authentic machineshop about it ;). Just kidding. Really I'd go for the one with the most accessories, it's not going to be a problem to spend as much again on bits of metal to screw onto it if i'm not careful. It's like my bout of home colour developing in the 70's - spend weeks choosing the right enlarger/lens and then spend 3 times as much on tanks, thermostats, colour sensors and what have you. And at the end of that you find that though you have a dark room that looks like a laboratory, you are actually practising an art not a science. Suspect something similar might happen with this.
I have joined that yahoo group and looked through the postings, though yahoo groups have a terrible search system, and much of the folklore on these machines goes back at least 10 years. But I'll persist putting in the key words and seeing if anything useful pops out. And most web forums are dominated by Americans, some of whom get obsessive about rubbishing any non-American manufacured goods, which makes it hard work decoding the truth sometimes.
My real bible is the Frank Hoosier minilathe.com site which I think is brilliant and by itself is a factor in whether to buy a Chinese 7x. Plus his example has encouraged others, so there's piles of stuff out there, by both good and novice machinists.
Chas
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Chas used his keyboard to write :

They all do seem to be accepting the 7x12 and its variants as a pretty good compromise between cost and build quality.

There are loads of private web sites offering information on the Chinese lathe I'm stilling finding them.
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