Re: Precision level needed or not?

Not between centres, but holding your test bar in the chuck or headstock taper.  If you do it between centres, even if the tailstock is set dead centre to the headstock, it will follow the twist in the bed so give a zero change of DTI deflection at all longitudinal positions.

If it wobbles when rotated or is otherwise off centre in the chuck, no matter, just average the max & min readings.

I actually prefer the DTI method, as no matter how small a cut you put on, it will deflect the bar more the further you go from the headstock.  Use as long a bar as possible and cut a diameter near its centre as well.  No need to pay lots of ££ or $$.  Any old offcut or scrap will suffice.  Just spend time getting the 3 diameters right, which would probably be done easiest betwen centres, but then change to the chuck to use it.  Diameter should be proportionate to length for stiffness.  Thick wall tube is quite acceptable.

On the subject of the giant pendulum, I admire the initiative.  The longer the better of course, but what it must absolutely have is rigidity of the point of attachment at the top, no wobble.  Make it long enough and you can use it for measuring the earth's rotation!  (See Foucault's pendulum.)

For users of boning rods (parallels placed across the bed) you need good eysight and a large depth of field, so bright illumination plus room to view from the end, which we don't all have.  I feel they might be more use eliminating or averaging out twist on a flat bed rather than a V bed, as the sides of the Vs rather than tops are the bits that wear.  However, as a comparative test on a V bed, they could be used as a measure of wear of the Vs.  Any further thougts on that?


On Friday, 14 June 2019, 02:32:29 BST, Stephen Bartlett via Groups.Io <tower.op@...> wrote:

I like the plumb bob idea, but have a question.

I have never done this but would it work to put a mandrel known to be
the same diameter at both ends on centers and then indicate the surface
at the two ends with an indicator on the carriage set at one end and
then the other?

At the very least it might be quicker than taking cuts and adjusting
twist until the ends are the same?

Steve Bartlett

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