Parallel 'turning' a square bar

I've got a bar around 20mm square and 90mm long. I want it reasonably parallel along its length. I dont have a mill, so it need to be done in the 4-jaw of the lathe.
Set it in the chuck, skim the first side. Turn it around, new 'flat' side against chuck jaws, skim to final size determined by how i've set my carriage stop. No problem.
So, now I have 2 reasonably parallel edges at the correct distance apart. I now need to get the third side skimmed and parallel. I can't just rely on putting side #4 against the chuck as this is unmachined yet. I don't have any true reference edge to put a DTI against, so how do I achieve true squareness? Once edge #3 is skimmed, the final side can be skimmed to the appropriate size via the stop.
How do I then avoid ending up with a 'traqezoid' form. This particular job is not super critical in itself, its just a thought that came to me when planning to do it? Have I missed the bl**ding obvious somewhere? Regards Mike
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use a faceplate and one or more angle plates to clamp the bar on its machined sides at 90 degrees to the faceplate. you could skim the faceplate lightly to be sure of flatness, but probably not required.
Dave
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wrote:

Its only 20mm square to start with, so won't have a lot of clamping area. It would need to be at the 'top' of any faceplate and still don't have an accurate reference edge to determine (in this case) when both ends are exactly the same distance from the face plate. If not, just as with using the chuck, I risk getting a bar thats perfectly square in section, but with a 'taper', if you get my drift.
This seems something that should be such a common task that I feel it must be dead obvious, it just doesn't seem to be. Still, good exercise for the old grey-matter! regards Mike
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On Fri, 7 Mar 2008 04:44:24 -0800 (PST), MikeH_QB wrote:

Can you live with three holes through it (crossways)?
One for a bolt, two for dowel pins, bolt and pin it to the angle plate. Skim, reverse and skim to desired thickness.
To make sure your holes are perpendicular, bore them on the faceplate.
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both ends exatly the same distance from the face plate is not important. square to the sides is. as long as your not massively out then bolt to top of angle plate, set approximately square to faceplate. skim. then reverse and use 2 lengths of tube or similar that you bolt to the face plate from behind. skim these in situ so they are the same length. then place face 3 on them, with face 1 on the angle plate, clamp and face off face 4 until at the desired thickness. you will end up with the 4 long faces square to each other. (the ends are a different matter)
Dave
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When it`s clamped to the faceplate,mount a clock on the crosslide and run across the face of it. If you`d told me the finished size I would have knocked a bit out for you. Mark.
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What size do you want it to end up? Mark.
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The simplest thing to do is to check how square it is against a reference square and if it's ok just machine the other two sides like you did the first two. No need to make a rod for your own back if there isn't a problem. However if the first two sides are not truly square to the other two then you need to clamp it to an angle plate by some means. That usually involves using a backplate.
--
Dave Baker
Puma Race Engines
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