The Edge tool is useful but I think you could replicate one with some careful machining. Note, important bits are the ground steel ends with center drills on outboard ends. Those OD's are what you are indicating on. The aluminum in the middle is basically a carrier, you are not indicating down that part. I'm 99% sure its a section of tubing which is cheap. You could in fact make a longer tube & increase the accuracy of tailstock offset on that (geometric) basis alone. The only criteria is that the indicator set up on your carriage but register on both the HS & TS collars without disrupting the indicator itself.
The other methods of turning test bars are either a continuous cut down cylindrical stock. Or you undercut the middle portion & then only cut the end 'dumbell' bosses for the accurate/measuring part.
The way I would make an Edge clone:
- turn 2 very identical steel collars parts with 1) center drill 2) collar boss 3) tubing ID fit in one lathe setup setting so they are all concentric. (This is a bit of chicken & egg because it presumes your tailstock for center drilling is reasonably accurate already)
- turn the tubing ends square & dress the ID bores also in one setup
- locktite the collars into either end
- with the assembly between centers, skim off a couple thou from the collar boss OD on HS end only. Lock the carriage on this setting. Flip the bar end for end & repeat. You have now accomplished making 'mostly' identical dumbell ends. (Clue: look at the video where they are grinding this feature). The caveat is if your lathe is a bit inaccurate to begin with, it might affect the accuracy of your test bar. But we are confining most of the error to cutting very close to the HS, not involving chucks & minimizing TS affect. By buying a ready to use bar, that has been done for us already. Choose your poison.
To use, you insert the bar between centers. Lock down the TS (important). Zero reference DTI off the HS collar, traverse down to TS end & compare that reading. Its that simple. No cutting involved, just measuring. Note there are 2 separate measurements which affect taper turning or off center drilling however you want to look at it.
- in/out across the lathe bed. Initially set the DTI ball at 3-oclock position. If HS measures 0.000" and TS is out toward operator 0.003" then jack the TS back in until it reads 0.000" but do this with the indicator in place because the jacking / set screw will likely have an affect on reading.
- up/down where TS is higher or lower than HS spindle. Set the DTI ball at 12-oclock position. I've heard most TS are factory set 1-2 thou high & isn't quite as bad as in/out from taper turning perspective. Supposedly this is to allow for wear. But I dont quite get this. My lathe has been 2 thou high since day-1.
But also realize that before the TS is zeroed into position, the lathe bed should have no twist and the HS must be aligned to the lathe bed. That's a different subject, but supersedes TS alignment.
