x2 tramming

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x2 tramming

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krymis:
guys,

i got a real problem. I am pulling out my hair. I am using Rollie Dad's method. I have a pc of 1/2 CRS in a 1/2" collet. It is not a precision shaft by any means but i was told you did not need it with this method. I got the rod in the collet and tightened and ready. I set up my DTI on the column to measure the up and down deviation. The rod holding the DTI is as about as parallel as i can get it while allowing me to adjust the DTI. I set the DTI and rotated the spindle to find the max and min at the collet end. I then reset the DTI to 0 on the midpoint. Then i move the carriage to the top of the column.

At this point i already have a -.002 reading one the DTI telling me that the rod drops that much. Then I rotate the spindle and come up with a min of -.004 and a max reading of .0015. So what do i do with the numbers... do i add the -.002 reading to the min since it drops that far to begin with?

kwackers:
Krymis,

Rollies dad's method relies on the bar being the same diameter at the measuring points (you've checked this right?)

When you zero the DTI at the first point you're simply setting zero at the 'average' movement. When you move to the other end what you're looking for is the average between the two values.
In this case (-0.004 + 0.0015) / 2 = -0.00125.
This suggests that you've moved (towards your DTI??) by -1.25 thou.

The fact that the DTI changes as you slide along doesn't matter. As a thought experiment, imagine doing it with a bar that has a 1 inch bend in it. At the first point the bend hasn't happened yet so your two measurements are zero.
The bar starts bent away at the other end so that when you slide your DTI towards it, it reads -1.000", but when you rotate the bar it bends 1" towards you so your average becomes (-1 + 1) / 2 = 0.
However if the bar has shifted 1 thou away then the first measurement will be -1.001 and the second 0.999 which gives (-1.001 + 0.999) / 2 = -0.001 - which is your 1 thou error.

Hope this makes sense.

krymis:
kwacker,

so that means i need a shim at the bottom of the spindle head to lift the bar right? If so is it telling me that I need to put a .00125 shim in or do i need to half the value to get the shim?

kwackers:
The value of the shim isn't so simple...

The error is the amount the bar diverges between the first measuring point and the second.
Imagine that the first point is 10" away from the 'pivot' point on the head (the point it rotates around when you shim it).
Imagine that the second is 11" away, so you've got 1.25 thou error per inch, but you're ten inches away so this represents a magnification - i.e. the amount to shift the head is a large amount.

The other factor is how far away from the pivot point the shim is. The nearer the larger the effect, the further the smaller.

If you've got a scientific calculator, a reasonable grasp of sine's, cosine's and triangles you can work it out. What you may prefer to do is stick a shim in, remeasure and then best guess...

krymis:
got you. thank you for the explanation. Rollie dad's is not that hard once it is explained like that. I think a lot of people make it harder than need be. And it would not be so bad if there were some better notes on it and less people saying oh it can't work because of this and that.

I am taking the measurements at the max and min of my travels so after i get it dialed in it should be good because the work should always be in the travels correct?

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