Holding the Workpiece to Flycut in a Unimat

Lindsay G
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Holding the Workpiece to Flycut in a Unimat

Postby Lindsay G » Thu Jan 09, 2014 1:14 am

This is only the 10th addition to the Lathe section - does this reflect a move away from creating bits from scratch, or that those that use lathes have little need to post topics for assistance? Well, I certainly don't fall into the latter category. Sum total of usage has been a few cylinders for gas lit coaches, hardly taxing stuff! A masterclass given by Terry Bendall to the East of Scotland group left me with several tools needed to turn many a thing, increased my knowledge 1000%, and my enthusiasm to use the lathe - it was a very, very, worthwhile day. Unfortunately said enthusiasm didn't translate into any worthwhile use of the lathe - until now.

With no intention of shying away from breaking new ground, everything has been dusted off to turn chimneys, domes, etc.. However, I've nearly stumbled at the first hurdle and wonder if I'm going about things the right way or if there might be a far simpler way of doing things to date. So, this is a cry for help or reassurance from the gifted ones.

The first 2 items to be tackled are a chimney and a dome - no point in jumping in at the shallow end, what? And the first thing that needs to be done is to use a fly-cutter to turn the seats for them. I have the fly-cutter, courtesy of the masterclass, but how do I hold the workpiece on a Unimat? Terry's series in Snooze suggested clamping the piece but the only lathe clamps I have only fit into the milling table which I can't fit in a usable position without fouling the cross slide handle, but even then they'd be too low. I tried a machine vice clamped to the cross slide but it was too low. After much brain-draining, I called Jim Summers who pointed me to Guy Williams' book sitting less than 6 feet from me at the time! This suggests holding the workpiece in the tool post - which I had completely overlooked. However, the tool space on the tool post is only 10mm whilst I've got a 12.5mm bar to turn - just a bit more than an interference fit.

First thought was to clamp the bar against tool space but despite having dozens of clamps from those 5' long down to teeny weeny ones, I don't have a small enough and strong enough clamp to withstand turning. With the right clamp this must still be an option but, without the tools to hand, I'll move on. The only other solution I could see was to turn down part of the bar so that it fitted into the tool post and leave some bar over that size as the base of the dome is c12mm. The workpiece now fits in the tool post but it is lower than the centre line of the flycutter. So, I tried a few different shim sizes under the tool post which was trial an error stuff and ended in error until I realised it could be achieved more logically. So, a few (well more than a few) more shim thicknesses were prepared; the workpiece was turned down over most of its length, faced off, marked centrally with a centre drill, and gripped in the tool post ; a centre put in the 3 jaw chuck; then shims put under the tool post until it aligned with the centre. If that's confusing the image should tell the story :

flycutting setup.jpg

Anyway, I then went ahead and rearranged everything on the lathe to fly cut the workpiece. I used a set square to ensure the tool post was square to the cross slide, ensured the workpiece lay against the back of the tool space over its whole length and got cutting. So much for trying for precision by sound means, the cross cut wasn't square so I readjusted by trial and error then got cutting, which went fine...nae spot on...a beautiful cut bang centre.

However, did I really need to go to all these lengths to get the workpiece held in position or have I over-complicated matters with a far easier method that I've overlooked? Remember, I am talking about a solution with the Unimat, with other lathes the clamping solution is a lot easier.

And did I get something wrong in trying to line everything up by supposedly sound means which obviously failed, or is it just as well going for this stage by trial and error?

Lindsay

P.S. The base may have been fly-cut beautifully, but I knackered the dome at the final stage of turning - ah well, if at first.......
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Terry Bendall
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Re: Holding the Workpiece to Flycut in a Unimat

Postby Terry Bendall » Thu Jan 09, 2014 8:45 am

Lindsay G wrote: have I over-complicated matters


Not really Lindsay. :) Regardless of the type of lathe, when doing this job by the method you used, it will always be necessary to pack up the work piece to the correct height and that will be a case of trial and error. The lathe you have does not have a top or compound slide so quite a lot of packing will be needed. A better way would be to use the Unimat milling table in place of the cross slide supplied which will have more T slots that can be used for fixing things down although I notice that this caused problems as well. One solution to this would be to beg, borrow or buy a fairly large chunk of rectangular mild steel that could be fixed down where the tool holder goes and use this as a home made milling table. This would of course mean that you need to drill holes in it which may be beyond the capacity of the equipment that you have. You made need tee nuts and clampting bars. It is possible to buy these but making your own is cheaper and good practice. I showed how to make T nuts on page 22 of S4News 162 and some ideas for clamping bars are shown on pages 10 and 11 of S4News 165.

Another way of finding the centre of the bar is to scribe a line using the tip of the cutting tool ofter facing off the end and before the work is taken out of the chuck.

A better way (but more expensive) of milling the seating is to use a vertical slide with the work piece held in a machine vice bolted to the vertical slide. These are available for the Unitmat 3 and other small lathes. see https://www.emcomachinetools.co.uk and http://www.chronos.ltd.uk/cgi-bin/sh000 ... cal%20slid Such things of course are expensive but may be worth while if you want to do such jobs frequently. Even better would be to use a vertical milling machine but the cost issue is even more significant.

As you have found, many things are possible on the lathe and half the fun (?) is finding out a way to do it. Next time it will be easier.

A useful type of clamp for machining operations is the Toolmaker's Clamp, examples of which can be seen at http://www.chronos.ltd.uk/cgi-bin/ss000 ... &ACTION=Go This type of clamp has the advantage that it has a parallel grip and can be firmly locked to withstand the forces involved. A useful thing to make yourself if you are that way inclined.

Terry Bendall

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John Bateson
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Re: Holding the Workpiece to Flycut in a Unimat

Postby John Bateson » Thu Jan 09, 2014 8:52 am

The only other solution I could see was to turn down part of the bar so that it fitted into the tool post and leave some bar over that size as the base of the dome is c12mm.


This is the way I do my chimneys and domes, but I also have a quick change toolpost where the height is adjustable (Conquest Lathe). I also produce a dumbell sort of workpiece because I tend to produce things in small batches, the larger ends do the domes and the inner part is usually just right for the chimneys.
254 Dome Original 8B8C8F.JPG


The fly cutting is done accurately by putting a mirror behind the work piece to judge the first very fine cut - safety glasses as well of course - and this is usually good enough. Towards the end of the cut it is possible to measure the flat parts that are left on the work piece quite accurately and adjust if needed. The piece in the drawing usually gives me 6 domes and 3 chimneys.

While the final shape is done using a hand tool, on my etches I usually produce templates with the chimney and dome outlines which makes the final shaping rather easier than just using the eye.
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Slaving away still on GCR stuff ...

Lindsay G
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Re: Holding the Workpiece to Flycut in a Unimat

Postby Lindsay G » Thu Jan 09, 2014 1:13 pm

Many thanks Terry and John.

It's reassuring to know that I'm not too far off beam, and I've picked up a few more things from the two replies.

I've got several clamps, bolts, and T nuts for securing items to the milling table but the height was a problem in any case. However, I'll purchase a couple of different sizes of toolmaker's clamps as I'll be able to clamp wider diameter items to the side of the tool post using the tool space to keep the round item in place. The vertical slides look the business but made me cough at the cost. I can't see me doing any more than a handful of items to justify them (altho there is the wastage factor!). Good tip regarding the centre line as the tool is already in place rather than setting up the centre drill, probably easier to see for setting the height as well.

The mirror trick is a good one - I found myself leaning over the lathe looking at how things were going upside down - not to be recommended with a tie on and the machine still running. I wished I'd though of the dumbell to get more out of what I was doing at the time - altho' the principle was the same in getting the dome from one end and the chimneys from the turned down area. Wish I'd had the guidance notes to hand before attempting the first dome - I thought I may have to finish things off by hand (confirmed since by Jim Summers as well) but tried with the lathe switched on and I've now got one goosed dome at the bottom but perfect at the top. The hand tool (another of the items made at Terry's masterclass) was a real joy to use and resulted in a good finish even for a first-timer. I'd wondered about fettling a template for the dome, and now think I'll do so particularly since I've got 3 or 4 domes of the same design to make.

That's 2 evenings expended and all I have to show for it is a ruined dome and a bar fly-cut at both ends ready to be turned into a chimney or two. However, I'm actually heartened by what has been achieved, and as Terry says it's fun finding things out.

Thanks again,

Lindsay

John Palmer
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Re: Holding the Workpiece to Flycut in a Unimat

Postby John Palmer » Thu Jan 09, 2014 2:06 pm

Lindsay, I see you're the proud possessor of an original Unimat 3 rather than one of the Taiwanese clones, and I also see a bolt protruding from the mounting point of the vertical milling/drilling column. This suggests that you may have such a column and milling/drilling head - if you haven't I can't recommend it too highly as an add-on to the Unimat if you can get one. I reckon my machine spends at least as much time being used as a drill press as it is as a lathe.

If you do have the vertical column you may also have the fine feed attachment, which makes the Unimat an effective light duty vertical milling machine. In that case I can't see any reason why you shouldn't use a fly cutter with the machine set up in the 'vertical mode'. No need then for packing up the workpiece on shims to match it to the spindle axis. Instead the piece can be readily clamped in the standard Unimat machine vice, and on the whole I would prefer clamping it in this way to holding it in the tool holder.

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Tim V
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Re: Holding the Workpiece to Flycut in a Unimat

Postby Tim V » Thu Jan 09, 2014 4:36 pm

It was seeing the work done on John's Unimat that encouraged me to buy one, a decision I have never regretted.

I used the method from the Guy Williams book (the Ian Allan version was better for scratchbuilders). The mandrel I made to mount the dome/chimney on was small enough to fit the standard tool holder.

I see you have put the centre in the chuck. It does not go in there, take the chuck off and put the centre in the lathe mandrel. That way any discrepancies in the chuck will be eliminated.

I think there is some lathe work on my workbench thread, though there are some on this forum who are anti lathes.....
Tim V
(Not all railways in Somerset went to Dorset)

nigelcliffe
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Re: Holding the Workpiece to Flycut in a Unimat

Postby nigelcliffe » Thu Jan 09, 2014 5:37 pm

Can I suggest reading the 2mm Scale Association Website ? Particularly the "Fonly" articles written nearly 2 decades ago.

As the Unimat is only slightly larger than a Fonly, its simple to take some of the ideas and adapt them for use on an Unimat. The "rocking" holder for flycutting, drawn in "part 2" of the Fonly articles would work as a simple way of adjusting for height, rather than messing for ages with shim packing. It can be made from a few bits of MDF or Plywood, a few nuts and bolts and a bit of rail as the rocker.

http://www.2mm.org.uk/articles.htm


- Nigel

Terry Bendall
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Re: Holding the Workpiece to Flycut in a Unimat

Postby Terry Bendall » Fri Jan 10, 2014 7:16 am

John Bateson wrote:The fly cutting is done accurately by putting a mirror behind the work piece


If the lathe is small enough, as the Unimat3 and my Cowells lathe are, and is not fixed down, turn the machine round on the workbench. ;) John I think has a larger machine so that may not be possible.

Tim V wrote:take the chuck off and put the centre in the lathe mandrel


That works if there is a tapered hole in the mandrel and the centre is tapered. From the picture it looks like Lindsay's centre is parallel.

John Bateson wrote:I also have a quick change toolpost where the height is adjustable


These are very useful if you can get one to fit the lathe. It is ideal for doing this job but again there is a cost. The larger ones can hold four tools which is even better.

Lindsay G wrote:The hand tool (another of the items made at Terry's masterclass) was a real joy to use


Pleased to hear that it worked. :)

Terry Bendall

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Tim V
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Re: Holding the Workpiece to Flycut in a Unimat

Postby Tim V » Fri Jan 10, 2014 4:07 pm

Terry Bendall wrote:
Tim V wrote:take the chuck off and put the centre in the lathe mandrel


That works if there is a tapered hole in the mandrel and the centre is tapered. From the picture it looks like Lindsay's centre is parallel.

Terry Bendall

Sorry Terry, I know what I'm talking about, as I have a Unimat 3 :!: The Unimat centre is parallel, and fits in a parallel hole in the mandrel. I am concerned that this picture shows an incorrect way of holding the centre.
Tim V
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Terry Bendall
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Re: Holding the Workpiece to Flycut in a Unimat

Postby Terry Bendall » Sat Jan 11, 2014 8:10 am

Tim V wrote:Sorry Terry, I know what I'm talking about, as I have a Unimat 3


Thank you for the corrrection Tim. I don't own a Unimat so had not appreciated that. They have obviously found a way of making a parallel hole hold the parallel shank of the centre sufficiently tightly to be accurate.

Terry Bendall


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