Lathe testing
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I'm doing a review of the Grizzly G0937 combo lathe/mill on the [email protected] list.
The purpose of this post is to document how I'm testing the machine. I'm using a ground between centers MT 2 test bar from India which I bought on ebay for a very nominal price. It has proven to be very accurate. The photos show the two basic test setups. The bar cantilevered from the spindle and with the end supported by the tailstock using a dead center. I made 4 passes measuring vertical and horizontal deflections and TIR for both setups. The rings are at ~ 1" spacing from the spindle face. I'm using a 0.0001" test indicator. With the bar supported by the tailstock TIR is ~ 0.0001" from end to end. I clocked it 0.0004" low at 6", however, the tailstock ram has about 0.001" of play in the tailstock casting, so that number is somewhat suspect. I'll retest after I eliminate the ram play. I've adjusted the tailstock position as best I could, but the design is very difficult to get where you intend and keep it there when tightening it up. It was actually within a few thou over the length of the bar which is quite reasonable for an "as shipped" condition. With the end unsupported there is 0.0005" TIR at 1" from the spindle. It is offset to the rear 0.0033" at 8". There is 0.0010" TIR at 6" and 0.0010" droop. This is a 1' 2" axial error of the spindle relative to the axis of rotation. The spindle bearings are 30205 taper roller automotive front wheel bearings. I plan to replace the bearings with angular contact ABEC 7 machine tool spindle bearings to see how those perform. But truth be told, the machine is already more accurate than my skills as a machinist can fully exploit. But I'll enjoy learning how. I don't have an independent means of testing the bar, but I have observed no indication of any measurable error in the bed which I find quite astonishing for a ~$1250 machine including tax and shipping. It inevitably has shortcomings, but the ones I've identified so far are pretty simple to correct. Have Fun! Reg BTW The 7" x14" mini-lathe still sits rather unloved waiting to be reassembled after being subjected to the tender mercies of UPS :-( Between the aggravation and the vicissitudes of old age I just haven't been motivated to work on it. However, I bought quite a bit of tooling for it, so that will happen eventually. |
