Paul Kierstead wrote:
1) Turn something rough to round between centers using a spur and live
center
2) Cut a tenon on the head end using a parting tool
3) Remove spur center and replace with Talon
4) Take blank and put it in the Talon, pressing the tenon shoulder
against the jaws
5) Mount Drill chuck (with a drill) in tailstock
6) Drill a hole in blank
.......
In (5) I would expect the "hole" from the live center in the blank to
line up quite close to the drill bit tip, assuming everything runs true.
It doesn't; the drill bit is always off center. The blank runs
*reasonably* true in this scenerio, but not perfectly. The bit is always
off center in the same direction it seems. Of course, with small bits,
this results in the bit flexing toward the little hole the live center
made and gives you a slightly oversized hole. It is a cheap lathe (a
Craftex from busy bee tools, something like Grizzly in the US).
......
PS: The lathe has a rotating head; this is a cause of trouble, as even
tightened down strongly it seems to be able to be moved by a good bump,
i.e. catch. Could this be a cause of trouble? How would one fix it,
outside of a welder?

Although you don't say how much inaccuracy there is in the alignment
(and even the best machines will have some tolerance) I think you maybe
expecting too much.

First of all, perfect realignment (in my experience) when you remount a
work piece is just not going to happen. In the scenario above, just
think about all the interfaces between the block of wood and tip of the
drill bit. Small innacuracies are bound to creep in so many places and
compound to make a noticeable problem. Secondly, making a lathe with a
sliding tailstock and moveable quill and getting the two axes to be in
perfect alignment is an engineering feat beyond my comprehension. Add to
that a rotating headstock and the engineering compromises that must be
made to manufacture a cheap lathe, and you are gonna be a long way from
perfection.

What can you do? Other than check the basic alignment of the headstock,
your best bet is to be sure that the tenon is is the bext possible natch
for the jaws. Choose the opitmum diameter for the jaws, where they form
a circle. If the jaws have a dovetail, make sure you cut a matching
shape in the tenon. You want the jaws to grip but not compress the wood.
All the cuts should be nice and clean. Close grained hardwood like maple
will give best results.


--
Derek Andrews, woodturner

http://www.seafoamwoodturning.com
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