# Eccentricity of a Profile Projector

E

#### etdiyas

eccentricity of profile projector

can anyone help me in determining the eccentricity of profile projector, and what standards are required for this method. We calibrate our profile projector using a standard glass scale and only the x and y axis are being compared with the standard.

hope anyone could help me on this.thanks

D

#### David Mullins

Hmmmm

Isn't the profile projector 2 dimensional, therefore you only need to check x & y?

R

#### Ryan Wilde

There are two tests that I can think of that might be known as eccentricity in a profile projector, and since I don't know which one you mean, I'll address both. If you mean something entirely different, please describe it and I'll let you know.

MAGNIFICATION ECCENTRICITY:

The most common method that I have seen used for eccentricity is through the use of either a calibrated gage pin or gage ball. Magnification accuracy is checked in the center of the screen, using the glass scale. The stage of the projecter is then moved to place the ball or pin at the edges of the screen in each of the four quadrants (right, left, top, and bottom, or upper right, lower right, upper left, and lower left - I've seen both methods employed). The magnification error found at the center position is compared to the error at all other positions. Make sure you check both X and Y axes for magnification in each point.

If the error is excessive, but linear, the error can be adjusted out. For example, right is low, middle is nominal, left is high. By readjusting the angle of the internal mirror (it's no fun, trust me) you can remove this error. If the error is excessive and nonlinear, you just bought yourself a three thousand dollar mirror, because your's is warped.

COMPASS ECCENTRICITY:

This one requires very specialized tooling to check. Get a nail (a roofing nail works nicely because of the broad head) and make sure the point is very sharp. Position the nail in the center of the screen, so that the point is exactly in the center. Slowly rotate the compass (screen) and ensure that the point stays in the center. If there is any deviation from center, then find the worst deviation, measure its magnitude using the x-y stage, rotate the compass 180°, and measure the deviation directly opposite. Take the mathematical difference, divide by two, and you have your compass eccentricity.

To adjust compass eccentricity you have to loosen the three or four screws that retain the compass and reposition the screen. On some models the retaining screws merely hold an offset screen retainer, so you would loosen the screws and rotate the retainers to set the proper position. Be careful not to rotate the screen in the frame, as this will alter your zero compass reading.

That's it for now,

Ryan

E

#### etdiyas

david,
you are right, but the problem is this when our supplier calibrated one of our profile projector their results has an "eccentricity of center of cross line on screen at rotation". And that was the problem i observed in the other projector when i calibrated it using our procedure (x and y axis) and unfortunately i don't know what is the procedure and standards for this.

E

#### etdiyas

ryan,

the compass eccentricity could be the problem that we are encountering right now on our profile projectors. And thus the nail would be the best tool i can use? Can i use the cross hair glass as a tool?

the problem i observed was this, when the cross hair glass was put on the stage and centered on the screen and when i moved the x axis to the right the line of the scale seems rising same with the y axis but whe i rotated the compass (357º7' reading on the vernier) the problem perhaps was eliminated. Hope you got my point.

thanks

M

#### metrologyguy

I am not sure of your problem but, if you are trying to quantify "Z" error I have a suggestion. Using your reticle (glass I hope) you could position it on top of two 2" or 4' gage blocks. Take a reading and record the results. Remove the gage blocks and place the reticle on the comparator table. You would need to have a small angle plate or something else square to assure you are placing the reticle in the same location on the table that it was in for the 4" reading. Then using only the Z adjustment focus on the same mark on the reticle, take a reading.

If the Z axis is properly aligned, there should not be any significant error.

I hope that was the question you were asking.

R

#### Ryan Wilde

Originally posted by etdiyas
ryan,

the compass eccentricity could be the problem that we are encountering right now on our profile projectors. And thus the nail would be the best tool i can use? Can i use the cross hair glass as a tool?

the problem i observed was this, when the cross hair glass was put on the stage and centered on the screen and when i moved the x axis to the right the line of the scale seems rising same with the y axis but whe i rotated the compass (357º7' reading on the vernier) the problem perhaps was eliminated. Hope you got my point.

thanks
It sounds as if you have two problems, both of which are fixed at the same time.

First problem:
The "eccentricity of center of cross line on screen at rotation" means that the crosshair on your projector screen is (or was) not in the phsical center of the compass rotation. Basically, your glass was not centered in its frame. Using the cams or retainers or whatever you have, it needs to be centered.

Second problem:
From your description, it sounds as though your compass and screen are not aligned with your stage. If the compass has enough adjustment (probably not - there is very little adjustment on the compass) then you can use that, which is simple. I use a nail, you can use the crosshair if you like. I'll describe the adjustment the more difficult method, which is probably the method you will have to use. Set the compass to zero, and position your crosshair at either the far left or far right of the screen. Move your x-axis to place the crosshair at the opposite side of the screen without disturbing the crosshair on the stage. If the crosshair doesn't stay on the horizontal screen line, then loosen the screen and rotate it 1/2 of the error, keeping the compass at zero. Repeat the above steps until the compass at zero agrees with the x-axis motion. This may impact your eccentricity, and you will have to adjust for both at the same time. After you do it a few times, it gets to be fairly easy, but it will take you some time the first time.

Hope that this makes sense,

Ryan

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