Originally Posted by
extrapilot
For my SX3, I found it difficult to implement a gas spring for 2 reasons. One is- the ratio of min to max stroke ends up limiting travel (that is, the minimum height > max height/2). So, you either get too long a spring and lose -Z travel, or vice versa.
Two is, unless you use 2 (one per side), you will find that the load is asymmetric (twist moment). In theory, not a big deal, but with these mills, you can deflect the head a few thou just with mild hand force on the spindle. And, unless you have very good ways/jib on Z, you will find that, additionally, the head will twist slightly as it moves due to slop in the ways. Z is sensitive to this since the spindle is offset 8" or whatever-tiny stuff you would not worry about in X/Y is a problem in Z.
I went with a counterweight- just a bucket with 50lbs of cement and a couple of pulleys. The really nice thing about that is, you can easily add weight if need be, and by moving the mill fore/aft relative to the pulley, you can change the load plane (i.e. pull up and back, pull just up, pull up/forward). I just made a stupid little U bracket that latches onto the SX3 cover cooling holes- so I can move that fore/aft 4-5" also. Made tweaking out the details a lot easier. I tried the strut, but switched over and have had no problems. Inertia is not a concern here- you are not doing 200IPM rapids with immediate reversals on Z (I hope!)- If you use Mach, etc, just set the accel to a reasonable value and you will not miss a step. Now if I could just get the head to not twist, it would be a hell of a mill for $1200.
Hope the info helps in your decision-
Happy holidays-