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I always recommend the indicator be used on the table, never with anything between the two. While it is generally acceptable to use a precision object, if there are minor anomalies in the table surface, you risk false readings, defeating the purpose.
I also recommend you draw file the table surface before running an indicator. Properly applied, nothing is removed from the table, but any projections are removed. They can be the source of vises mounted wonky, so it's good shop practice to hit the table occasionally. Don't use a new file. They're too sharp and have the potential to remove parent metal from the table surface. One that has been used is generally very acceptable. Draw the file sideways (no file handle needed) with the palms of your hands. Not a lot of downward pressure--just enough for the file to glide along easily. When you hit a projection, you'll know by the slight resistance to movement. Go over that spot until you feel nothing. Again, do NOT press too hard, otherwise the file is inclined to cut where it shouldn't. The table of you mill should feel smooth when you run your hand across. It it does not, it needs to be addressed.
As far as application goes, if you're using a DTI, you should have the contact extended parallel with the body of the indicator to minimize error. If you have a long travel type indicator, that can be a better choice, although not necessary. Pick up the table by only a thou or two, not more. That way the indicator will glide on and off the edges without difficulty, even if it addresses the grooves from the side. If you can make a setup whereby the DTI can be mounted such that it trails rotation instead of being sideways, all the better. In that case, turn the spindle such that the indicator always trails. I recommend you not touch the indicator when taking your readings, even if you feel you are not making a difference. What you are looking for may be so small that a tiny amount of error can be added, again, defeating the purpose of the test.
Shimming. Before you do any shimming, it's a good idea to dismantle the machine so you can examine the base and mating surface to insure that there isn't something tilting the column. It may have been assembled with crud between the two surfaces, or some dings that prevent the column from seating properly. What I'd do is dismantle, draw file both surfaces to insure there are no protrusions, then reassemble and sweep the table with your indicator. At that time you can be assured that you are reading the true condition of the machine. You may find you must shim the entire column to achieve the amount of tilt required, due in part to, perhaps, not being able to buy the required shim sizes. You may have to use a half thou, which may or may not be at your disposal, but perhaps one that measures .0015" may be. Anyway, shimming is a last resort kind of approach, so make sure everything that can cause the problem has been eliminated first.
One thing to remember----any applied shims should be directly under the bolt(s), so you don't risk cracking the mounting surfaces. You are dealing with cast iron, which is not known for its tensile strength. If you must, make U shaped shims that can slide in from the exposed surface, so when the bolts are tightened the pressure is applied directly to the shim, not along side the shim.
As far as the spindle being parallel to the column, you may be able to run an indicator held in the spindle, running on the vertical column ways. Extend the spindle to check for variations. It's important that the indicator not be able to rotate, otherwise you get false readings.
Harold
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