Diamond Wheels Not Flat Nor Round (true concentrically)?

[RickKrung]

I tried out some drill bit sharpening last night with my set of three Tormek diamond wheels.  I approached it all cautiously, not wanting to put too much pressure on the diamond grits.  I tried to let the weight of the DBS-22 drill bit holder jig dictate the pressure, but it wasn't that easy.  Holding the jig and sliding it back and forth took some grip, on both the bit holder and the sliding plate.  Grinding went pretty fast and took less grinding to achieve the desired results than the SB or SG stones did previously.  I took some pix in hopes of sharing how it all went, but the photos were really crappy (out of focus), so I bagged them and will try again and do a better job with the photos. 

I found the contact behavior of the drill bit on the grinding wheels curious.  As I traversed the drill jig back and forth, the drill bit made contact with the grinding wheel at the edges first and more than at the middle.  At first, this variable contact was more on the right side and not at all at the middle or left, so I thought it was misalignment of the USB.  But, as I did more, it became apparent that it really was just at the sides, as though the middle was "lower" than the sides.  I take that to mean the wheel is not flat all the way across.  At least not as flat as the stones were after truing, as there was full contact all the way across with both the SB and SG previously.  I have wondered if there was something about the way I was holding/using the jig, but as mentioned, this didn't happen with the stones. 

Also, the wheel seemed to not be concentric in rotation.  The drill bit would contact on one part of the wheel and not on another as the wheel rotated.  I have not repositioned the wheel to see if the inconsistent contact went away and I have not run an indicator on any of the diamond wheels - I'm not thrilled about the notion of grinding a flat spot on my indicator needle.  There are ways of putting something sacrificial between the grind wheel and indicator, but I have not done that yet either, to see just how much these amount to.  When/if I do so, I think I'll try spinning the wheels on the shaft, without them being locked in place by the nut, to see if it is in the wheel or maybe due to the miniscule gap between the wheel hole and the shaft.  I have not attempted to measure the wheel hole, but I can to that fairly precisely. 

I am not sure there is anything wrong with the wheels.  I am just noticing a behavior that I didn't expect.  I'm not sure how much difference it makes to grinding anything.  It sure doesn't seem to matter to knives.  I do wonder about planer blades, however, as they need to be very uniform and precise.  What I can say is very qualitative: it has the "feel" that something is not right, so the experience of using the diamond wheels is inferior to the stones. 

Also, the wheels are not exactly the same diameter.  The DC and DF wheels are very close, 250.47mm for the DC and 250.45mm for the DF.  The DE wheel is 250.12mm.  I don't think it matters to most uses.  For knives, the difference in wheel diameter makes only 0.1º angle difference.  The only thing about drill bit sharpening was that when it came time to use the DE wheel, the jig had to be run down a tiny bit to get contact.  I'm not suggesting there is anything wrong with the wheels, just observing.

Rick

(P.S. How is that for "devilish details? 

[Ken S]

Rick,

Interesting (and perhaps disturbing) post. On page 156 of the handbook, there is a paragraph, "The Stone is not Running True" which lists the radial and axial runout operating tolerances of the grinding wheels. Presumably this was originally written for the SG. The radial (up and down)  tolerance is + or - 0.2mm (total 0.4mm or .016"). The axial (sideways) tolerance is + or - 0.5mm (total 1.0mm or 0.04")

The handbook advises using the TT-50 truing tool if use has worn the stone out of true or if a tighter tolerance is required. This seems like good general grinding practice with any wheel, wet or dry.

One of my pet peeves with the otherwise valuable handbook is that it has not been updated to include any more than the briefest mention of any grinding wheels since the SG. The handbook should include whether or not the tolerance range is the same for diamond wheels. If it is different, the correct tolerance range should be included. Also, with diamond wheels, it is impossible to true them. The handbook should list return procedures for repair or replacement.

As a machinist, Rick, you know about accuracy tolerances of measuring tools. Even laboratory grade gage blocks are not "exact". Even they have a tolerance range in millionths of an inch and must be retested frequently. I would expect tighter tolerances from a three hundred dollar diamond wheel than I would from a general purpose inexpensive hardware store wheel. I don't know what reasonable tolerances are for diamond wheels. Unlike matrix (the "Original Tormek wheels") they cannot be user trued. The factory trueness is as good as it gets. Tormek should list and explain the tolerances. As you note, your wheels work fine for knives. We can make allowances if need be for the diameter differences with Wootz' applet.

At this point, Tormek has not said much about the diamond wheels except for a very brief marketing video. My limited experience using the diamond wheels has been positive. They cut very well. Since they can not be user trued, they may or may not be suitable for precision drill bit sharpening. It is no shame if this work is beyond their intended design, however, if it is, it should be clearly mentioned in the promotional material and handbook.

I hope support will weigh in on this.

Ken