a bad blake coaxiel center finder

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. Don Huseman
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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.
Bob Swinney

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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 reasoning:
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 wobble.
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 offset there.
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.
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    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 new angle.

    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.
    Good Luck,         DoN.
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