"Phil Teague" wrote in message
om...
(Ken Strauss) wrote in message
om...
Many small lathes quote impressive but incomplete accuracy numbers in
their specs. For example, the Prazi SD400 (see
http://www.emachinetool.com/new/cata...?ProductID=180)
claims a "spindle accuracy of 0.0002". What would be a reasonable
expectation of total accuracy? Say the variation in diameter along 12"
between centres?

I don't think .0002" is necessarily exceptional for spindle runout. My
Chinese 12 x 36 is at least that good. The question of variation in
12" between centers would probably depend more on how well your
tailstock center is aligned with your headstock and how accurate your
centers are. You should be able to turn something between centers to
the accuracy of your spindle TIR which in the case of the Prazi should
be .0002".

Center line-up probably is the biggest practical factor, although it
shouldn't be hard to get them perfectly lined up, assuming the lathe has
normal tailstock adjustments.

In a cheap lathe another factor is straightness of the bedways. A little sag
or vertical arch to the bed will have little effect, but a curve in the
X-axis will have a pronounced effect. This is easy to detect by turning a
sample piece and charting its actual diameter every inch or so along its
length. Straight taper is a tailstock alignment problem; anything else is a
bedway problem.

Ed Huntress