Novice guide to backlash adjustment

While milling some heatsinks last week, I was puzzled why my workpiece shifted a couple times, even though I thought I attached it to the table very securely. The workpiece was moving a considerable distance -- I could see it, no dial indicator needed. I was taking some relatively heavy (for a taig) cuts, but I thought my setup was very sturdy. So, after re-clamping my workpiece I decided to watch closely and try to see when and how the workpiece moved. I was milling a 0.1 deep 0.25" wide cut slot in one pass, from the outside edge of an aluminum block almost all the way to the opposite side. So I am watching my milling bit approach the block along the Y axis. As soon as the bit touches the workpiece something really scary happens - it grabs the workpiece, bites into it, and throws it more than 1/8" along the X axis! Nothing broke, nothing went flying, only the workpiece shifted once again. It was obvious something was wrong with the X axis. After cutting that slot I stopped the mill, and tried to move the table along the X axis by hand. To my amazement I moved a lot, probably more than 1/8"! That's some serious backlash, I thought, but had to go to bed instead of figuring out what was wrong.

Today I finally got around to disassembling the mill. Loosening the socket head screws that hold the bearing plate against the table is quite a challenge. I twisted my all-metal T-handle wrench out of shape before one of the screws finally gave way with a spark, a popping sound and a tiny cloud of smoke that smelled fairly similar to gunpowder. I've never seen anything like that before. I suspect some serious locktite at work.

Carefully sliding the table off to the left revealed the x axis ways, and the first not-so-pleasant surprise! There is a number of scuff marks on the ways. I am quite sure that I have not done anything to cause them. I've never disassembled the mill before, I've never slid the table so far that it exposed ways, and I can't see how anything would get between the ways and the table. Moreover, there are no marks on the corresponding surfaces of the table. The only guess I have is that this is a manufacturing defect. Fortunately, as far as I can tell it does not adversely affect the performance of the mill, but I am still planning to ask people on the taigtools group what they think about that.

After removing the table I tried to tug on the leadscrew, and that's where the second surprise came -- the nut appeared to be pretty snug. Maybe there was some backlash, but not enough to actually notice. At least nowhere close to the 1/8"+ I was seeing. Just to make sure I took out a dial indicator, and checked. The indicator showed well below 0.01" backlash. Hmmm... what else could be the culprit? I tried to move the bearing plate against the leadscrew and - there it was - 1/8" of movement. Yikes! How did that happen? Well, it turns out that there is a locking nut that holds a steel bushing-looking thing with four holes in it (this is one side of the stepper motor coupling; the other is mounted on the motor shaft, and there are four plastic tubes, looking like thick drinking straws that connect the two). That nut was not tight enough. I could've sworn that I tightened it up when I took the manual handles off and installed steppers. Besides, I think I would've noticed that my x axis was loose while using my mill over the past year or so. But on the other hand it is highly unlikely that the nut would've worked itself loose because it is the locking type with a nylon insert. Maybe I am just too ashamed to admit that I made a stupid but serious mistake when converting the mill to CNC So, lesson #1: Before blaming the leadscrew nut for apparent backlash and taking apart your mill, check whether the bearing plate is tight and snug against the leadscrew. Make sure it is as tight as possible. I tightened the nut till the extra friction in the bearing became noticeable and then backed off about 1/8 turn. I am not sure what would be a good way to tighten that nut because I could find nothing on the leadscrew to hold on to that is not round. Eventually I just used some rags to protect the surface of the steel-bushing-looking-thing-with-four-holes and held it tight with groove joint pliers. Even if it got scratched, the performance of the mill would not suffer one bit.

Adjusting leadscrew nuts is fairly straightforward. I followed Nick's advice to tighten them till they are a bit too tight and then back off 1/4 turn. That worked well. Not satisfied with that, I tried to push my luck and see how tight I could get them without significant increase in friction. Not very far, it turns out. But every little bit, even just a few degrees counts, and makes a significant difference in backlash. I played with that for a while. Tighten one screw, put the mill back together, check for excessive friction, check backlash, take it apart, tighten it some more, put back together, indicate, take it apart, tighten the other screw, put it back together... rinse and repeat many times. After countless times I finally figured out what feels too tight, and what feels just right. I'll try to remember that for future reference.

After finally getting to the point where I was happy with the backlash, I cleaned the mill, oiled the ways with some ATF, put some lithium grease on the leadscrews (I hope that's the right thing to use), put the X axis stepper on and decided to do a test run. It worked well... but only as long as the table stayed a good distance away from the left-most position. As the table came within about 2 inches of the left-most, the stepper would stall. Why??? I took the mill apart again, checked for binding, and found that the leadscrew rotated freely in the nut. So, what's the culprit? It is the same bearing plate again! This is lesson #2, and the one that Nick did not mention, but I think it is fairly important. Before tightening the screws that hold the bearing plate against the table, move the table as far to the left as it will go (for Y axis as far toward the front as it will go). This ensures that the leadscrew is better aligned with the bearing in the plate, and will minimize binding. After putting the mill back together one last time, everything was working nice and smooth.

... well, almost. Do you remember I mentioned those little plastic tubes that look like pieces of a thick beverage straw? I've heard people over and over loosing those, and then trying to figure out where to get replacements. I made sure not to loose them. I put them in a safe place, but after everything was assembled one was missing! I did not want to shop for, and then wait for replacements. After scouring around the house for a replacement I came across... an old toothbrush. Toothbrushes are made of a relatively tough yet flexible plastic, quite similar to the original "straws". It did not take long to make a replacement out of one. The only scary part was using the 3-jaw chuck at the highest lathe RPM. It looks and sounds like a plane, and I can only hope it does not decide to fly away...

That brings me to the third and final lesson: There is no way to perfectly align stepper motors with leadscrews. Therefore leave some room, maybe 1/8" between the two halves of the stepper motor coupling (so you can see 1/8" of those drinking straw-looking things) when all screws are tight. That will compensate for the inherently imperfect alignment, and your leadscrews will not bind, and the stepper motors will vibrate less.