Leadscrew backlash

[dgapilot]

I've tried to esearh all the various forums for adjusting backlash on the acme lead screws on our machines. Most all of them just say loosen the clamp and play around with them until you get the backlash down to an acceptable amount. For the axis along the length of the lathe bed, that would seem pretty easy. I'de like to add that if you loosen the clamp cap screws, then pry the clamp open so the double nut can slide freely back and forth inside the clamp, then adjust the spring pressure to where you feel comfortable. Crank the lead screw or slide the carraige so the nuts are inside the clamp. wiggle it a little to be sure that the nuts are loaded on either side of the thread, then tighten the clamp nuts. Be sure they are tight on both nuts befor you move the lead screw.

For the cross slide, it is much more difficult. First off, I would have thought that there would be access to the clamp screw. NOT! You have to remove the table assembly and basically take the lead screw assembly out of the table. I had a particularly difficult time removing the CNC pulley from the end of the leadscrew. Broke a puller, finally with penetrating oil, a little heat and some wedges got it to come free. I had some corrosion that had made it's way into the pulley shaft interface.

As you disassemble the table, be sure not to loose the thrust bearings on the crank end of the lead screw. Mine had some burrs on the shaft and the outer race of the inside bearing didn't want to come off the shaft. The clamp block is mounted on the lower portion of the table slide assembly. I had to remove the gibs to get clearance to get the leadscrew out with the clamp block still attached. Again, loosen the clamp screws, and pry the block open so the double nuts slide in and out freely. Then set your spring tension and slide them back into the clamp block. The hardest part of this is guessing at the alignment for when the assembly is reinstalled in the table.

While you have the leadscrew out, clean and grease those thrust bearings befor reassembling it. Put all the pieces back together and check your backlash before putting the table back on the machine. Better to find out now that you didn't get it quite as tight as you wanted so you don't have to pull the table again.

Once the table is reinstalled adjust the gibs so the table slides, yet the rock back and forth is minimized. There are sever other threads on this so I woun't go into any more detail.

Hope this helps some of you out there. I know I sure would have liked to know this before jumping in and pulling the table off the machine 3 times trying to figure it out!

David

[smallblock]

The later Tri Power and Patriot machines have the bracket on the top slide, so its adjustable in place. On early machines like yours, the clamp bolts on the bracket are in line with one of the T slot grooves on the table. With some measuring, you can find the location and drill 2 holes in the bottom of the groove and access the clamp bolts with an allen T handle wrench. Its a good idea to tap the holes for a plug, so stuff can't flow down onto the screw. On the thrust bearings be sure the race with the larger diameter always goes against the fixed bracket.

[dgapilot]

Thanks for the idea! If I don't go to ballscrews right away, I think I'll do that.

David

[instructor37]

On our Patriot machines, the X and Y tables have twin brass nuts held in a 2 piece bracket. One of the nuts has a flange with 2 set screws which press against the bracket. By loosening one side of the bracket clamp which holds the flanged nut you can turn the set screws and force the nuts away from each other. This puts contact on the leading and trailing edges of the screw. We got the lash to zero, but the friction was just too much, so we backed it down to about 0.002" and the tables move nice and free. On the Z axis, there is only one nut,but the lash is very small- about 0.0015. However double check the bracket that goes around the quill- we found some play in it which resulted in another 0.010" lash. It needed the bolts removed and flat washers put under the heads and that tightened it right up.