Harold_V wrote: ↑Tue Mar 31, 2020 3:39 am
The overall accuracy of a mill begins with the column, knee, and way surfaces. Here's what I recommend.
Begin by checking the gib on the knee. It should be snug, but not overly tight. The knee should be able to just settle by its weight, not driven downward by the screw.
When the gib is adjusted (a snug gib ensures the knee is not leaning downward at the front), you should then dress the table of your mill. Do that by using a single cut or smooth file, one that has seen some use, so the original sharpness is gone. Note that a new file offers the opportunity for metal to be removed where it shouldn't be.
The file is placed at an angle on the table surface, no handle, and drawn by your palms parallel to the T slots, covering the entire table surface, side to side. By doing this, any slight projections (usually caused by clamping on a chip, or from dropping small objects on the mill table) will be removed, leaving the mill table surface untouched otherwise. I do not recommend using a stone for this purpose, as it isn't selective and will remove, albeit very little, metal where it shouldn't. Once you have deburred the table, it should feel silky smooth when stroked by hand. If it isn't, repeat the file operation.
With the table centered on the saddle, side to side, use an indicator mounted in the spindle to sweep a circle. I make sure my indicator crosses the front and rear T slots, so I make the circle as large as I can. I don't like using anything between the table and indicator, as that offers a slight chance of introducing error, especially if one skips the deburring of the table. When adjusting the head, the restraining bolts should be snug, but still allow the head to rotate. If they are not, when you tighten the bolts you most likely will see a change in your setting.
When you place the indicator, it is best applied such that the contact addresses the table such that you can rotate the indicator with the tip trailing. I often don't do that, but to ensure that the crossing of the T slots doesn't alter the indicator setting, I make contact by only a thou or two, so the contact moves very little. I also use the largest diameter tip for the indicator. This allows the indicator tip to address the T slots from the side without losing registration.
The larger the circle you sweep, the more precise will be your setting. New mills are made with the front edge of the table ever so slightly higher than the back, to address settling as the machine wears. If your mill is not new, ensure that you don't leave the table high at the front. I prefer to make it even, front to back, just as I do side to side.
There's no reason why you can't get a head within a half thou sweeping a large circle (about 8"). That degree of precision will yield no issues when moving the knee, keeping in mind we're talking about a milling machine, not a jig borer.
Dialing the top of a vise isn't recommended. If, by chance, the vise top is not dead parallel with the table surface, you will defeat the purpose of dialing in the head. Considering the fact that the large surface is also the moveable surface on a mill vise, it most likely won't be dead parallel.
H