Model Forum / General / Models / December 2006

No, the bed can be twisted and it can still cut parallel.

Yes - but it isn't very important that the bed is untwisted unless you are
using it for milling work, or perhaps super-accurate pitch threading - and
even then a twist in the bed isn't likely to be that important.

RDM won't test for straightness - but it will help test for parallelness of
cut. RDM is a method for calculating relative measurements to the centreline
of rotation of a lathe along different points of the bed, and these results
can be used to adjust the head to obtain parallelity of cut, so that the
centreline of rotation is equidistant from different points on the bed - but
it can't per se tell whether the bed is twisted, you need a level as well
(or instead) for that.

Imagine a lathe with a very twisted bed, so that it undergoes a quarter turn
over it's length - as long as the centreline of rotation and the axis of the
twist are in the same place it won't affect the accuracy of a facing or
turning cut at all.

It would affect eg the pitch of a cut thread - it would have 1/4 extra/less
turn over the length of the bed - but in practice twistedness doesn't matter
much, as such an extreme twist is most unlikely.

Mark, don't bother trying to level your Hardinge when you are ready to
use it.  They don't need it ;)

If you didn't know the beds just sit (Not bolted) on three conical
rings, and the only leveling you need do it to allow the coolant to
drain properly.

Wayne..

This is quite true. And watchmakers lathes were used sitting on a chair,
having the arms rest on the table. I've seen a photo where the operater sat
on the tailstock's end and nearly embraced the lathe (I would embrace a
Schaublin anyway).

Nick

Cheers for all your replies!

I'd never considered making it higher to make it more precise! Do I
need a really big height gauge as well now? LOL

The bench is made of metal, 25 x 3mm box section. Bolted to the walls &
floor, fair bit of cross bracing, probably more to come.

I shall go for grandad's level to start with - nice idea taping a scale
to it, will do. And RDM for the tune up, as it were.

Also, if anybody ever needs to know, the centres of the feet on an SB
9" are not the nice sensible 6" one would ass-u-me they are. DAMHIKT.

Ed

      This is an interesting point.

     I had the same idea when I made the bench to carry my
Boxford ME10. I reasoned that the bench top should, as nearly as
possible, have the same expansion coefficient AND the same
thermal time constant as the lathe bed.

       With this in mind I made a sturdy wooden bench topped
with three mighty box section steel girders laid side by side and
welded together. This assembly was comparable in both weight and
torsional stiffness to the Boxford bed.

        While this achieved the original aim I hit an unexpected
problem - extreme sensitivity of the hold down bolt adjustment.
Because the welded benchtop was so torsionally stiff, a
fractional thou movement of the bolts resulted in more than a
thou taper turn which made adjustment pretty fiddly. With
patience I got a good result and the setup has remained nicely
stable over several years.

         On reflection I realise that the heavy stiff bench top
is the wrong way to go.

   There is more than meets the eye in the comparitively flimsy
sheet metal stands supplied by lathe manufacturers.Because the
sheet metal top is torsionally so much less rigid than the lathe
bed a relatively coarse hold down bolt adjustment results in a
usefully fine bed twist adjustment. The thermal time constant
mismatch is of little importance because the resultant bed twist
is a result of the spring force of the deflected metal top and
this is comparitively temperature insensitive.

                    Jim