Adjusting the X-Axis backlash

The following is a note I sent to the Taig user group on Yahoo

Just wanted to tell about my experience in adjusting the backlash on my CNC mill. I have the extended bed version, and have had an increasing problem with too much backlash for a while now, ending up at about .025, far too much even with software compensation. Now I know the procedure for adjusting it, but I found that when I adjusted the grub screws on the saddle nut that the table would start to jam up before the backlash was acceptable, this was particularly at the extreme ends of the table. If I wanted to make use of the full table I needed to reduce my maximum travel speed down to about 10 IPM otherwise the stepper would start missing steps. I knew that others were talking of 30 IPM, in fact Taig advertises this.

Okay where to begin? I decided to tear down the X axis and see what I could find. After removing the lead screw, I tested the table by itself and found that with the gib adjusted correctly when the table was in the middle I could not move the table closer than about 1.5 inches from either end without force, this meant I only had a usable 9" of travel. I decided to lap the table thinking that this might be the root cause of the problem, lapping and cleaning took about 3 hours. When I had everything clean of grit I lightly oiled the ways and put the table back on. What a surprise, the table slid across the ways with only light finger pressure. I readjusted the gib carefully and now I had a nice even pressure the length of the whole table. This was getting exciting.

I reassembled the lead screw carefully adjusting the saddle nut to what I thought was the right amount of pressure and started to test. Okay, the first setup wasn't that great, I still had lots of backlash, but at least the table moved freely. I started tightening the grub screws in the saddle nut (middle screw loose for now) adding about a 15 degree turn to each alternately and testing the table by running it from one end to the other. This proceeded well and the backlash was improving, suddenly however the table started jamming again before an acceptable backlash was reached (by acceptable I mean that I could still feel table movement by moving the table from side to side by hand) no need for a test indicator at this stage. I loosened the gib on the table and this freed things up. I then went back to adjusting the backlash thinking I'd re-adjust the table once the backlash was correct.

I finally got what I thought was acceptable backlash, but didn't measure it as I decided to run a short program which moved the table from one end to the other 10 times. I ran it but even with the gib fairly slack couldn't get more than 12 IPM without starting to miss steps. By this point, I was thinking that I would settle for slower speeds if the backlash stayed acceptable. After running the program a few times I jogged the table to the middle and tried to move it by hand from side to side as before. My heart sank as there was an audible "clunk" and the table shifted by at least .1 inch. This was going nowhere.

I decided to strip things down again and see if I could find any other indication of why this was not working. My first thought was to take the lead screw and saddle nut assembly to the bench to adjust it there. I took the table off and gently used the middle part of the lead screw to ease the saddle nut out of the bed. Someone mentioned recently that the factory used locktite on this, but there was no evidence of this. I started to unscrew the nut from the screw and noticed that the two grub screws where protruding from the underside of the saddle by about 0.075, "a hah" I thought, this doesn't look right. I checked the bed of the X axis and sure enough there were two matching gouges in the surface. It looked as though what was happening as I tightened the screws to take up the play in the saddle they were becoming seated on the bed before the nut closed enough to take up the proper amount of slack.

Now I was sure I was on to something. I cleaned the screw and the nut and filed about 0.1 off the ends of each grub screw. I screwed the nut back onto the lead screw and adjusted it to a snug but not too tight fit and also made sure that the grub screws were not protruding from the underside. I then re-inserted the saddle nut in the bed, and attached the table. I ran a couple of quick tests and started the fine adjustments over again. I also re-adjusted the gib to an acceptable state and slowly started to increase the travel speed.

To make a long story shorter, I now have .003 backlash and can traverse the table at 30 IPM. As a test I ran the program all the way up to 35 IPM without a miss. I'll happily settle at 30 however for normal usage, but it's nice to know I've got a fair amount of leeway. I also believe that I can get the backlash down further, but I also think the threads in the nut may have been partially damaged or misaligned due to the stress put on it by the over length grub screws. I believe that the main cause of the table jamming was the pressure put on the nut, and also the fact that with the screws biting into the bed the nut was frozen in position. I believe that there needs to be a pivotal movement allowable on the saddle nut to take up any slight variations between the lead screw, saddle, and table.

If the grub screws were the correct length to begin with then I obviously have far too much wear on the saddle nut, I find this hard to believe, I'll run it this way for a while and see how stable things are. If there are problems I'll consider buying a new saddle nut.