Re: Lathe Spindle Runout Question
In 2009 was going to make a lighter 7mmRemMag rifle for myself on a Brazilian VZ24 action with a Lothar Walther 1300 taper barrel.
I decided I would first practice with my new lathe on a Brazilian 1908 action and a Rem700 take off barrel. I cut off the threads with a parting tool, cut the threads, and started reaming. But I forgot to de burr the cut off. The reamer started out with a wobble and never lost it all the way to headspacing. The chamber had .005" of run out.
The LW barrel got dialed in with a gimbal in the 4 jaw with a .0001" Interapid test indicator, and straightened out with a spud and spider, so it was not just concentric with the lathe, but parallel in the chamber.
When I took the rifles to the range, they would both shoot 0.75" 3 shot groups.
Like fear of a nuke bomb turning into fear of nuke power, I had turned fear of eccentric ammo into fear of eccentric chambers.
"Gauging Bullet Tilt" By A. A. ABBATIELLO 1960 ~ 1968
<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">MATHEMATICAL SOLUTION
A laterally displaced center of
gravity moves through the rifle bore
in a helical (screw) path. The pitch
of this helix is the pitch of rifling,
and its radius is the lateral displace-
ment of the center of gravity. On
leaving the muzzle, the center of
gravity continues in the direction it
had at that point. For example, if it
leaves at top of the bore and rifling
is to the right, the departure will be
to the right. The bullet travels ap-
proximately 2l.5" in a 24" barrel,
making 2.15 turns in the 10" twist
of rifling. The number of turns
shows the orientation on emergence
compared with that in the chamber
before firing. The angle of emer-
gence is that angle whose tangent is
2 pi times the lateral displacement
divided by the rifling pitch. For
.004" point displacement and I0"
rifling pitch, the tangent is 1/8(2·pi)
(.004)/l0 and the corresponding
angle is 1.1 minutes.
The displacement on target from
this cause is proportional to the
range and can be obtained without
noting the angle. For example, ,004"
point displacement gives in l0"
rifling pitch, so far as this mecha-
nism goes, a target displacement at
100 yds. (3600") indicated by the
proportion .00l· pi /10=X/3600, from
which x =1.1".</div></div>
Notice how the tilted ammo must be inserted with random rotational orientation in order to get larger group sizes?
That does not happen with tilted chambers.
The bullets get flung off center, all in the same direction, and all into the same tiny group.
What does it all mean?
You will not be able to measure the difference in accuracy with two rifles, one chambered on lathe with .0020" run out and one with .0005" run out.
I would still pay for a lathe to have the smaller run out... but not because it will make rifles more accurate.
Also, I like to measure spindle run out statically. I put the dial indicator or test indicator on the spindle and then apply a radial force. Then I reverse the direction of the force. I do this to determine when to replace bearings, and when I am setting the pre load on new bearings. Sometimes I can get a complete fix, by reversing the axial orientation of a worn preloaded pair of bearings. If only one side of the inner race and one side of the outer race are worn, then use the other side.
