Sieg SC4 variant tailstock.

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Munty Scruntfundle

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Hi there.

I found out the other day (using the razor blade method) the tailstock on the SC4 is very slightly high, probably only 1/4 a mm or so, but it's enough to mess up a 150mm bar by .066 end to end. I tried making a simple slitting saw arbor a few days back which turned out awful, while this taper might not sound like much it means nothing will sit in a collet perfectly. So I need to eradicate the discrepancy. I've tested with the quill in and out, I get the same results within a couple of .001s. So it's level, just high.

The tailstock on the SC4 had no height adjustment, so what's the best plan? I could try to mill fractions off the bottom, but that would mean complete disassembly/reassembly over and over and I'm not sure I'd find a way to ensure I removed the same depth from the flat and the dovetail side. I could gently polish up the surfaces (which are horribly rough) by hand, but I'm not sure I could eliminate any bowing. I could contact Axminster and ask for a replacement, but I'd have no guarantee any new tail would be any better, it may be worse.

Any ideas?

Many thanks.
 

chaoticbob

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Hi Munty. As no-one has picked up on this I'll bung in my two pennies' worth, with the caveat that I'm not a trained engineer. I was however thinking about a related problem recently and made some calculations.
The result was that in this scenario (ie tailstock high by delta_h, but tool cutting at centre height at the headstock), the difference delta_r between the radius of the work at the headstock and the tailstock ends is given to a very good approximation by:

delta_r = (delta_h)^2 / D

where D is the diameter of the work at the headstock end.

What that means in this case is that assuming a workpiece diameter of 10mm and a tailstock vertical misalignment of 0.25mm, delta_r = 0.25^2/10 mm or 6.25 microns on the radius, so the work should turn out 12.5 microns fatter at the tailstock than at the headstock. Not the 66 microns measured.
This makes me think other factors are at play - have you checked out the alignment of the headstock spindle to the ways for example?

Having said all that, in your position I'd return the lathe to Axminster for replacement - 0.25mm misalignment isn't acceptable for a new engineering lathe. I too have an Axminster lathe and it turns parallel between centres to 5 microns over 150mm without my having had to adjust the tailstock at all, though I did spend some time 'leveling' - ie adjusting to take twist out of the bed. So they can do better!

Hope you get it sorted, Bob.
 
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