Taig Mill Z Axis Air Spring Setup

Several folks have asked about the gas spring addition that I made to my Taig CNC mill, and I've been looking for a place to post some things from time to time - mainly so I can find them again once I've forgotten them, but if anything ends up helping someone out, so much the better :-). So here is my first-ever blog post, FWIW. :-)

Below are some pictures of the setup. I've got .nc files for the upper support arms and the lower gas spring bracket, which I'll happily share, if anyone wants them. The vertical steel bars (sides and back) and the pulley holder on the end of the gas spring, I just threw together ad-hoc with some MDI commands in EMC2. This has effectively eliminated any backlash from the Z. (I used to get a little bit of variability, which could mess things up when milling printed circuit boards.) I've got Z backlash set to .0002, which seems to be needed just to allow for a teeny bit of slop/friction in the lead nut. The head itself isn't going *anywhere* in terms of backlash. Note that I am running a 425oz-in stepper on Z. I'm not sure that a significantly-smaller stepper would be able to deal w/ the force from the gas strut, though in reality the net upward force now is probably not much more than the downward force was before the strut was added (ie, from the weight of the head).

The overall setup. Gas spring pushes down on the pulley behind the column (pulley doesn't turn - just an easy/convenient way to spread the load on the cable, and allow for some self-balancing/adjustment), which constantly pulls upward on the head. The gas spring manufacturer recommends keeping the body of the spring toward the top, to promote lubrication of the seals on the piston rod. There is a little stiction after it sits overnight though, so my first move of the day is to always jog the head up a bit, until the stiction frees.

The pulley holder on the bottom end of the gas strut. Just a little block of alu from the scrap box, faced up, and milled with some MDI commands in EMC2. Also, tapped to accept the threaded end of the gas strut piston.

Another view from the side. Drilled and tapped holes in the Taig column to attach the vertical bars. The switch box isn't needed because I use the Sherline DC motor, which has its own switch. The original switch box could be relocated lower on the column. The white block is UHMW polyethelene (or similar - something else from the scrap bin), which stabilizes the gas strut so its not flopping around, and/or putting any torquing force on the upper bracket (which is just some 90-degree-bent alu, screwed to the rear steel bar).

The upper gas strut bracket.

Close-up of the front pulley arrangement. They just clear the stepper motor mounting tube.

The attachment of the cable to the head. Aluminum bracket, which screws into the two unused (at least on my version of the mill) screw holes on the head, above the spindle mounting block. Then two steel plates to clamp the cable snugly. (Not shown, for clarity, is the tachometer pickup that normally is bolted to the mounting plate. The tach will be another post sometime - its needed due to the variable speed Sherline DC motor driving the spindle.)

Gas strut used - purchased from Advance Auto. Who knows what it was originally destined for? The force measures around 30-35 lbs (via my bathroom scale...), and is surprisingly consistent across the stroke, once the stiction frees.

These were on the ends of the strut, but threaded right off. (I'd tried to source the ball joints that these were made to fit, but no luck.) The Advance Auto guys were helpful, if not a bit dubious, as I dug through their gas strut display, pushing down on the various struts, trying to gauge their force. This was actually about the lightest force strut in the needed travel length.

The pulleys came from Home Depot. They were rollers for a screen door. They've got ball bearings in them, and are quite smooth-rolling. We'll see how they have held up to the steel cable after a few years, but the cable really moves very little and very slowly over them, so I don't anticipate any problems.