Consider the possibility that you have a little slop in your headstock. The
larger radius tool will push the stock more out-of-center giving you a taper
which is larger at the headstock.

"Wayne" wrote in message
om...
I'm so very confused.

I have 3/8" shank holders with HSS inserts. I have 1/32 and 1/64
radius inserts.

I was testing the cutting straightness of the lathe with a
1" steel bar over 5". The results I got were 2 thou bigger
at the headstock than at the free end.

I then had a professional in to look at my issue. He said the
bit I was using wouldn't give me proper results. I was using the 1/32,
and he said it needed to be at the biggest 1/64. He ground a
HSS blank with a knife edge. Then he tested using 1" aluminum.
He then aligned the headstock, which he said wasn't that far off.
(On a previous visit he straightened/levelled the bed).
When he got done it was straight within a few thou over 4" or so.

After he left I tried my test again using the 1/64 bit. With the
steel bar it was still bigger at the headstock, but only by .001.
Using the aluminum bar it was off by .0005.

On both bars after cutting toward the headstock, I left the bit
where it was, and reversed the feed toward the tailstock.
More material was cut off. When I measured them, it was only off
a few thou.

I don't know how it could be larger at the headstock, and then
come out straight on the 2nd cut. And what good are indexable inserts
if they don't cut straight?

Once again I'm so confused.
Wayne