I have 2 blake coaxiel center finders. These are the indicators that
have a long metal probe that rubs on the inside of a hole and the
offsetness of the hole makes the center part of the housing go up and
down. This makes the indicator move as much as .5 of an inch. To use
the indicator you turn on the mill and run it at 200 rpm . The
indicator probe move around. you turn the y and x handels untile the
indicator stops moving. the part is then centered. The only problem is
I have two of them and they read about 4 to7 thousands different form
each other. I also have a mitstoyo indicator that fits on it's
percision indicator positioning devise. It also reads different. has
any one used these centering devices and has any one had the same
problems I have had.
I have a Blake clone. I have never checked the absolute accuracy of
readout, but I have no reason to doubt it. Others I have spoken with share
the opinion that it is not realiable as far as using it as a dial
indficator. I would expect any 2 of them to read slightly differently as
the inherent runout of their respective spindles would be slightly
different. AFAIK, this runout is what keeps the "Blake" centering readout
from ever becomming completley quiescent. If someone has more information,
i would like to see it, also.
On Sat, 16 Dec 2006 09:18:46 -0600, "Robert Swinney"
I don't think runout or offset in the Blake is an issue. Here's my
A coax indicator has a stem that goes into the spindle, and then a
pivot and arm that moves a flat (rotatable) disc up and down in
response to radial motion. The location of the pivot is irrelevant
except for scalefactor; it merely converts horizontal radial motion
to vertical axial motion. The only source of error there is if the
movable disc or plate is not square to the stem and free of tilt and
The whole shebang rotates about the center of rotation -- which may
not be concentric with the spindle, but that isn't a fault in the Coax
device. The sensing arm then describes a circle about the axis of
rotation, regardless of geometry between the axis of rotation and the
contact point. If whatever it is touching is both round and coaxial
with the axis of rotation, there should be no indicated motion. When
you then dismount it and mount a drill or cutter, if the actual
spindle is not coaxial with its axis of rotation, there will be some
Another error source is if the axis of rotation is not perpendicular
to the workpiece -- e.g., a mill has not been trammed so the axis of
rotation is perpendicular to the workpiece plane. In this case the
locus traced by the contact arm will be an ellipse on the workpiece
plane so there will be two maxima and two minima per rotation. If the
problem is in the disc being not square with the stem or having some
wobble, there will be one maximum and one minimum per rotation.
Are both using the same length feelers? The feeler length is
going to affect the scaling. So is the angle at which the feeler
contacts the workpiece being centered.
Are you far enough into the range so the travel of the
indicators does not stop allowing the feeler to lift clear of the
surface being traced at any point?
What is the temperature in the shop? My Blake tends to get
sticky when the shop temperature is cold -- even when lubricated with
Starrett "Tool and Instrument Oil".
Does it really *matter*? The purpose of the Blake is to tell
you when you reach a concentric location, not to give a precise
measurement of the amount of runout. This is similar to a DRI (Dial
Runout Indicator) on a lathe -- you only want to get the work centered
-- and the reading of the DTI is a function of the angle of the pickup
lever. (As is the reading of the Blake, for that matter.)
Basically -- this is one of the DRIs mounted to serve for
centering. Again -- the angle of the feeler arm will affect the
accuracy. It is dead on only at one specific angle, which varies from
maker to maker. Try putting some shim stock -- or a feeler gauge --
between the point and the surface (assuming that you can get one narrow
enough to not be confused by the curvature of the ID of the bore you are
checking). If you can't, switch to the OD or some other surface which
you can test, and see how accurate the reading is. Then change the
angle of the feeler from the indicator and see what it shows with the
I've used them -- and I simply consider what you are seeing as
normal -- not a problem. It goes with the basic design of the
instrument. What really matters is whether the three agree that the
same position is truly the centered position. If you get that, you may
have a bent shaft on one of the Blakes.
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