After reading an earlier thread about off-center drilled holes HERE
, I checked my Harbor Freight mini-mill and found that the spindle was not in alignment with the column, apparently not uncommon with the Sieg X2 machines. From what I've read, the only practical way to align the spindle and column is to shim the spindle housing where it mates with its carriage for Y-axis corrections, and to wiggle it about on its mounting screws for X-axis corrections. Either adjustment requires getting to the housing mounting screws, located behind the carriage and hidden by the column. You have to remove the head from the column to get free access to the screws (shown later below). It sounded like a challenge, but turned out to be relatively simple. Here's how I did it. But before I start, the photo below shows what I'll be calling the spindle housing and the carriage that make up the mill head. Your nomenclature may vary. Also, all my measurements are in inches, taken on import tools.
Aligning the mini-mill spindle with the column basically just involves putting a test bar in the spindle and adjusting the spindle housing until the test bar is parallel to the column in both axes. I used E. T. Hackett's alignment procedure except for the choice of a test bar and the measurement method (see the files section of the Yahoo GrizHFMinimill group or at http://www.machinistblog.com/?p=991
for his writeup). Mr. Hackett used an aluminum precision shaft from McMaster-Carr and measured along the length of the shaft after it was turned to midpoint of indicated runout. I used a 7" long, .500 diameter steel shaft salvaged from an old inkjet printer instead, and used Rollie's Dad's Method to do the measurements and alignment (see the link in steamer's post HERE
for more information on Rollie's Dad's method). The beauty of Rollie's Dad's method is that the test bar doesn't have to be perfectly straight, and runout of the chuck or collet won't affect the results either.
If you try this and you've never removed the column and head assembly from your mill before, be forewarned that it's heavy, and top heavy, and can hurt you if you're not careful or can't handle the weight. Also, the head counterbalance arm is spring-loaded, don't let it spring back and whack your knuckles or your favorite dial indicator when you let it loose.
First, I removed the mill column, laid it on a workbench, and removed the motor, the control box, and the electrical box so the head would be easier to slide off the column. I also removed the Z-axis stop and the bolt that connects the spring-loaded counterbalance arm to the carriage. I let the arm swing back and rest against the benchtop.
Next, I did some baseline measurements to see how much out of alignment the spindle was and where adjustments would be needed. The photos below show my test and adjustment setup. The test bar is in a 1/2" collet, but a drill chuck could just as well been used. For the Y-axis checks and adjustments, the DTI point was set on top of the test bar with just enough pressure to get about a .015 reading on the dial. Then, two sets of measurements were taken, one set at the collet end of the bar, and the other set at the far end of the bar. I started my measurements at the collet end. I locked the carriage there and rotated the spindle 360 degrees, and wrote down the minimum and maximum readings from the dial indicator, and then found the average of the two measurements (added the maximum and minimum reading together and divided the result by two). Then, without changing anything about the indicator, I unlocked the carriage and wheeled the head back up the column so that the indicator was at the other end of the bar. I locked the carriage, got the maximum and minimum readings there, and averaged those two measurements. Then I compared the averages. The difference between one end's average and the other end's average was the total misalignment for the distance along the bar. (That's Rollie's Dad's method.) I repeated the procedure for the X-axis, with the indicator point at the side of the test bar, as shown in the second photo.
My initial Y-axis measurements indicated about .0055 misalignment, the spindle axis being tilted downward slightly toward the column. For the X-axis, I couldn't get an exact set of readings because the DTI was hitting its stops as it got close to the bar end, but I estimated that the misalignment was at least .020, more or less, the spindle axis being skewed slightly to the right.
Now, knowing the nature of the adjustments required, I proceeded with the alignment. I did the Y-axis first. I removed the head from the column and loosened the four spindle-housing cap screws (shown in the photo below) so I could put a piece of shim stock between the spindle housing and the carriage. For my mill, the shim needed to be at the bottom of the spindle housing so as to raise the far end of the test bar just a little. I didn't know exactly how thick the shim should be, but since the Y-axis was only off by .0055, I tried a strip of .003 brass shim stock first, figuring that would be a good place to start. I slipped the shim in under the spindle housing where it joins the carriage, tightened the spindle-housing screws, put the head back on the column, and repeated the Y-axis measurements. As it turned out, the .003 shim was a good choice, reducing the Y-axis misalignment to less than .001 over the length of the test bar. The second photo below shows the shim location, just barely visible at the joint. Note that any Y-axis adjustments need to be made first, otherwise any X-axis adjustments that were made would be corrupted when the screws were loosened to insert the shim.
For the X-axis adjustment, I removed the head again and loosened the screws so that the housing could be moved by hand slightly side to side (without significantly moving the shim), and put the head back on the column. I set up my DTI for X-axis measurements again and then moved the spindle housing side to side on its mounting screws to adjust the alignment while monitoring the readings on the DTI. When the alignment was right, I moved the head to the top of the column until I could get to the upper two housing screws from underneath and carefully tightened them. Then I removed the head completely, tightened the two lower screws, replaced the head, and re-checked the X-axis alignment. It took me a couple of tries before I got the bar alignment within .001 of the column axis, because the housing tried to shift a little when I tightened the upper screws. After I was satisfied with the alignment I re-checked both the Y-axis and X-axis alignment to make sure nothing had changed, replaced the motor and boxes, and put the column back on the mill. A final alignment check after the mill was back together indicated less than .001 error in either axis over about 5" of head travel.
I hope the above is of value to anyone considering adjusting their mini-mill. I did a search of the forum but didn't find anything about how to do such an alignment, but if it's there and I missed it, my apologies for the repetition. I'm still a novice on these little machines, and my methods may not be correct and are certainly not the only ways of doing these things. This is just the way I did it. I didn't go into detail on how I removed the column from the mill, the electrical boxes, the motor, etc. either, but if anyone has questions, I'll try to answer. Thanks for reading.