G0704 Ways Sticking

[Pyronaught]

I'm in the process of converting a G0704 over to CNC and am having an issue with the X axis slide sticking. What is odd is that it is only sticking in the region between 1-1/2" to 3-1/2" on the ways and nowhere else. I would expect it to stick at one of the ends if it was a ball screw alignment problem, but there is no sticking out at the ends. If the gibb was to tight then I would expect the whole range to be hard to move. If I remove the gibb the sticking problem goes away, so it is not a bad section of ball screw. I really don't see how this could be anything other than a defect on the dovetail. Any ideas? It doesn't seem to be a common problem based on google searching it.

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[coherent]

It's normal to have a spot that's a bit tighter. They are machined, but the imports aren't the most precise. Hopefully nothing is bent.
Best bet is to remove the tapered gibs and check the gibs and ways for any mars, nicks or damage. It could just be a chip or something. Check them, clean them and reinstall and adjust them properly. Cleaning and.proper adjustment may fix the issue. You can finish/polish them to make them more accurate and smoother but that's a whole new can of worms that usually isn't necessary.

[Eldon_Joh]

one of the methods i have used for my soft cast iron lathe is to tighten the cross slide gib screws and move the assembly back and forth, lead screw and nut removed.

where the oil film breaks down due to metal on metal contact(and you need to continuously flush oil to the ways while you're doing this) the cast iron will start polishing itself. you can then see these shiny spots and scrape, grind, or file, or use sand paper or a stone to grind them down.

the problem is this process will end up averaging the two surfaces together and you won't know what is going on, other than its tight and yet it also moves freely. this is often good enough, in my case its less than 3 tenths out of straightness in a foot of travel. yes there is a lot of stiction, deal with it.

you really need to scrape one or more of the surfaces perfectly flat before modifying the other surfaces to get the whole system to slide smoothly.


if you want a stiction free system you need a dovetail slide that has the same clearance, everywhere. note i said clearance, not pressure. the gib needs to be adjsted for just a few tenths of slop, so that oil can remain constrained between the metal to metal interface.

the moment you have metal on metal, its going to carry the whole weight of the machine tool, slide, whatever, and your stiction is going to equal the static friction of cast iron on cast iron, which is like .2 or so. so your ball screw or leadscrew has to exceed say, 20 pounds of force for a 100 pound table, and the moment it exceeds that pressure the friction drops to almost zero becuase the oil film built up and your table jerks forwards a few millionths of an inch due to the leadscrew expanding. (not to mention the bearings, or the magnetic stiffness of the servos or steppers)

[Pyronaught]

The jib looks clean. I would expect if the jib were causing it that it would be sticky the whole way rather than just in one spot. It doesn't jam, I just have to turn the hand wheel pretty hard to get it past the sticky spot. I wonder if running the machine back and forth across the sticky zone a bunch of times would wear down the high spot and make it smooth? I could write some gcode that just makes the X axis go back and forth a hundred times or something, with the jib very loose at first and then progressively tightening it.

[Eldon_Joh]

The jib looks clean. I would expect if the jib were causing it that it would be sticky the whole way rather than just in one spot. It doesn't jam, I just have to turn the hand wheel pretty hard to get it past the sticky spot. I wonder if running the machine back and forth across the sticky zone a bunch of times would wear down the high spot and make it smooth? I could write some gcode that just makes the X axis go back and forth a hundred times or something, with the jib very loose at first and then progressively tightening it.

find two rods of approximately the same diameter, take the table off your mill, lay the rods in the dovetail and measure the distance between them.

you will likely find more than a few thousanths difference

[Pyronaught]

I found the problem. When I took the ball screw out to slide the table back and forth to do the oil trick, there was no sticking. It turns out the top of the ball screw nut was not ground down far enough and was rubbing on the bottom of the table as it passed overhead. Since the bottom of the table casting is not machined, there are some pretty big random high spots and that's why it was sticking in some places and not others. In the place where the sticking was the worst it would actually push the nut down so far that the screw would then hit the saddle and rub there as well. I took an angle grinder and ground down the top of the nut and also knocked down some of the high spots on the bottom of the table and now it is working great. The ball screw nut just barely fits under the table, I'm surprised this does not happen more often. Or maybe it does and people just figure it out before resorting to asking for help on the forums.

[Eldon_Joh]

Or maybe it does and people just figure it out before resorting to asking for help on the forums.

imagine the days before the Internet.

[metalfixer]

This also needs to be done on the Little Machine Shop 3990 Mini Mill.
Hard to see in the picture, but I took .060 off down the center on the underside
of the table with a 3/4" end mill. I was lucky to have a Bridgeport mill available.
Larry