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Copyright © 2021 Quality Machine Tools, LLC
PM-1340GT v8 2021-04
ALIGNING THE LATHE
The most important attribute of a properly set up lathe
is its ability to “machine parallel”, to cut a cylinder of uni
-
form diameter over its entire length. In other words, no
taper.
Leveling of the lathe is a part of this, see Section 1.
Equally important is the alignment of the center-to-cen-
ter axis with the lathe bed, as seen
from above
. [Verti-
cal alignment is nowhere near as critical, rarely a cause
of taper unless the lathe is damaged or badly worn.]
How to align lathe centers
Practically all lathes come with some means of offsetting
the tailstock, typically for taper turning. For routine oper
-
ations, the offset must be zero, Figure A.
The scale usually provided on the tailstock is not re-
liable for precision work
— think of it as only a starting
point. What follows are two methods for aligning cen-
ters, one quick and easy, the other more precise.
Quick method
This method works
only if the centers are in new condi-
tion, sharp and clean.
1.
Carefully clean the taper sockets and the tapers
themselves. Install the tapers.
2. Move the carriage left as far as it will go, then slide
the tailstock left to touch the carriage.
3.
Lock the tailstock (this is important — unlocked to
locked can mean an offset of several thousandths).
4.
Advance the tailstock quill to bring the centers togeth
-
er.
5.
Place a scrap of hard shim stock or an old-style dou
-
ble-edge razor blade between the centers, Figure B.
6.
Advance the tailstock quill to trap the blade, then lock
the quill. If the centers are aligned, the blade will point
squarely front to back. If not, adjust the tailstock off
-
set by a series of very small adjustments.
7. If the range of quill motion permits, check the blade
alignment at various extensions of the quill. There
should be no appreciable variation.
Figure B
Quick alignment check
Figure A
Center-to-center axis
Precise method
This method uses a precision ground steel rod at least 10" long. Look for 3/4 or 1
inch "drill rod" with a diameter tolerance of ± 0.001" or less.
Straightness and uniform diameter are both important
(absolute diameter is not).
1.
Set the rod in a collet chuck, or independent 4-jaw chuck, with the outer end
about 1/2 inch clear of the chuck.
2.
Use a dial indicator to check for runout. If using a 4-jaw adjust as necessary for
minimum TIR (aim for 0.0005" or less).
3. Center-drill the end of the ground rod.
4.
Reverse the rod, re-adjust for minimum TIR, then drill the other end.
5.
Set the drill rod snugly between centers, as Figure C. Lock the tailstock.
6.
Set a dial indicator on the cross slide (to eliminate vertical error use a flat disc
contact point, not the usual spherical type — if not available, machine a cap to fit
over the contact point you have on hand).
