Originally Posted by
WhiteTiger
I suspect you are looking at surface finish and adjustment issues. These budget mills generally have fairly accurate ways, but the surface finish is lacking, which makes them move stiff and irregularly when properly adjusted. Result: most users tend to adjust them for initial ease of movement which introduces functional (but not felt) slop in the fit.
The horizontal bed surfaces are usually fairly good, but the dovetail rail surfaces can be rough as a cob and since those are where the gibs reference to and ride against, the movement is stiff as a board during break in unless the adjustments are backed off for smooth motion.
The safest, easiest cure I know of for this is to strip the machine down, and oil up the ways with a good cutting oil (I like Marvel Mystery Oil, available at any auto parts) then reassemble just the saddle on the Y axis ways, adjust till you can just push it with effort, and spend a few minutes shoving it back and forth full stroke on it's ways. Then spend about 1/4 that amount of time short stroking just the ends of the ways, taking them a bit out of engagement if possible. When it gets easy to move, disassemble, wipe everything down to clean off the iron particles you probably will note turning the cutting oil greyish, then repeat at least one more time.
Once the Y ways are broken in and final cleaned, reassemble them dry and lock the gib down solid, then repeat the whole thing for the X axis. It's important to lock down the Y axis, as rocking in it can introduce force errors in your self lapping of the X ways and make them come out irregular in extreme cases.
When working the X ways, be sure to hold and support the free hanging end of the table when you are short stroking the ends of the table ways (which has to be done to offset the wear distribution, since the center of the table ways gets double the wear with each full travel stroke.
It's tedious, but it pays big dividends. Took me about an hour and a half all together to break in all the ways on my old bench mill, not counting the scraping of the saddle X axis dovetail surfaces.
Once you have all the surface roughness knocked off the mating surfaces, you can adjust the gibs correctly and not have them compress the high spots of their mating surfaces and give you deflection under load.
Tiger