SB Stone Flat Spot

[RickKrung]

Still at my friend's (Bill) doing a variety of sharpening, knives, chisel, axe, and wood turning tools.  All have required enough metal removal that I have started with the Norton 3X wheels before cleaning them up with the SB stone and then finishing them with a polish on the SJ wheel.  The knives received stropping on leather as the final touch. 

I had trued the SB stone before leaving home and after eleven knives, one chisel and one axe and several coarse gradings, I was noticing a "tick" "tick" sound and a bit of a hitch in the gitup as the wheel rotated.  On looking and listening closely, I located the culprit.  A flat spot had developed. 

It is difficult to see in the photo as it is only very slightly darker than the rest of the stone, laterally across the stone, at a very slight angle up and to the right.   


I had not run into this before and do not recall reading about it.  I suppose it might be a weak or soft area in the stone that eroded away with all the grinding.  I was able to get rid of it with re-truing the wheel, but I marked the spot on the label on the side of the stone in case it recurs. 

It may not matter once I receive the diamond wheels, but  I hope I still can make use of the SB stone.  I feel sure I will continue to use the SJ wheel for finish polishing, but suspect I will not have much use for the SG wheel as I'm not using it now, only the SB and SJ. 

Has anyone else run into anything like this?  Anyone have any explanation other than a soft spot?

Rick

[wootz]

I see this with wear-resistant alloys, and suppose might be caused by a highly hardened steel - not only a spot or two, but the whole SB surface starts grinding less aggressively.
Rather than truing the stone every time I see this, I refresh/expose the silicon carbide grains with a cheap low-grit diamond plate.

SB is a peculiar wheel in sense of fine-grading - the best I've found is to #400 with a diamond plate, any finer grit didn't work well.

[RickKrung]

Thank you, Wootz.  Interesting.  I do not recall noticing the flat spot while sharpening the knives.  I only recall noticing it when using the stone grader on the coarse side.  I have a grader dedicated to the SB stone and only use the coarse side.  I read somewhere not to attempt grading the SB stone using the fine side.

I noticed before, your use of a diamond plate for grading to 1000 grit.  That must have been the SG stone.  I have not done this, partly because I do not have such a diamond plate (although I have looked at them online).   

It is hard to figure which tool or tools might have been hard enough as none of the knives were of remarkable quality so as to be so hard and as I said, didn't notice it until somewhere in the chisel and axe sharpening.  And as said, it may not matter once the diamond wheels arrive. 

Rick

[Ken S]

If you look at our most viewed topic (by a wide margin), it is the stone grader. The same topic received 130 replies, the second highest in the history of the forum.

We have a problem. Whether or not there is a problem with the stone grader itself, there is definitely a communication problem between Tormek and the users. The present programs educating the users have not succeeded. We need a better video program from Tormek. Sweden, I hope you are reading this.

Ken

[RickKrung]

I just shot myself in the foot. 

I still have no good explanation for how the flat spot I posted about above came to be, but tonight, I know exactly what caused a perfectly straight across the SB stone groove.  I dragged a large knife across it to see how the black marker was scraped off while setting the USB height. 

I've done this before, albeit with less pressure, without causing a flat spot, so I am a bit curious why it happened with this knife.  It is a very thick knife, forged from a piece of rebar.  Due to its thickness, it has a fairly wide bevel. I don't know how much the latter matters, but as soon as I turned the machine on and started grinding, the tick-tick of that groove was evident. 

It effectively ended the session for the night, as it was late and I need to re-true the stone before proceeding.  I had the sense that the groove was growing from the grinding action.  I pointed out to my friend who forged the knife that I'd seen the marker scraped off procedure done before, but in the videos at least, the wheel is rotated rather than dragging the knife across the stone. 

Another lesson learned... or should I say "realized". We'll see how well it has been learned. 

Rick

[cbwx34]

I just shot myself in the foot. 

I still have no good explanation for how the flat spot I posted about above came to be, but tonight, I know exactly what caused a perfectly straight across the SB stone groove.  I dragged a large knife across it to see how the black marker was scraped off while setting the USB height. 

I've done this before, albeit with less pressure, without causing a flat spot, so I am a bit curious why it happened with this knife.  It is a very thick knife, forged from a piece of rebar.  Due to its thickness, it has a fairly wide bevel. I don't know how much the latter matters, but as soon as I turned the machine on and started grinding, the tick-tick of that groove was evident. 

It effectively ended the session for the night, as it was late and I need to re-true the stone before proceeding.  I had the sense that the groove was growing from the grinding action.  I pointed out to my friend who forged the knife that I'd seen the marker scraped off procedure done before, but in the videos at least, the wheel is rotated rather than dragging the knife across the stone. 

Another lesson learned... or should I say "realized". We'll see how well it has been learned. 

Rick

I will be curious what those with more experience with the SB stone have to say... but it's hard for me to imagine that a stone that, quoting from Tormek, "has been developed for shaping and sharpening* HSS and other exotic alloyed steels", can't "survive" a single pass across the stone without damaging it to the point of putting it out-of-service... isn't defective?  Just out of curiosity... how fast is your stone wearing during use?

I'm thinking that contacting Tormek support is in order... hoping this isn't consider "normal". 

[Ken S]

I agree with CB. We have the factory expertise of support. This is a good question for them.

Please make sure your reply is posted.

Ken

[RickKrung]

Thanks.  I will contact Tormek support, once I've gathered a little more information (Pssst...  Did you hear the one about the engineer, the biologist and the lawyer?... ).

I want to true the SB stone again to get rid of the groove and try working with the knife again.  And probably some other knives and other types of things.  I would like to see it develop another problem area.  The two that have shown up were not in the same place on the wheel and do not seem to have come from the same type of use/abuse. 

To that end, I retrued the stone today, in a couple increments, due to time limitations. 

First, here is the knife I was working with when the groove appeared.





To begin, I took a picture of the groove showing wear marks leading in (top) and exiting (bottom) the groove.  Second photo is of the stone diameter measurement: 1245.53mm prior to the first truing.  Please make note of the small nick in the corner of the stone, just below the groove on the near side.  It will be a reference point in what follows. 





After the first truing, the groove was nearly all gone, but I could still detect it as a faint white or shiny line angling from lower right to upper left, beginning just above the nick, right in the middle of the frame.  Stone diameter at this point was 245.32mm, so just over 0.2mm had been removed. 





Second truing eliminated detectable (to me at least) signs of the groove.  The reference nick is barely visible on the left side, middle of the frame.  Stone diameter is now 245.15mm, almost 0.4mm smaller from the beginning diameter. 





I have not tested the trued wheel to see what happens next, I've run out of photo capacity for this post. 

One takeaway is that truing doesn't have to take much off the diameter of the stone.  The truing done here to get rid of the groove was extreme.  The stone was true after the first truing, when only about 0.2mm had been removed from the diameter, which amounts to an actual 0.1mm depth of cut of the truing tool. 

If I have done as many as ten truings in the 5-6 mos. I've had the stone, I'd be surprised.  That could account for 1-2mm of diameter lost, plus the just under 0.5mm today.  Since the stone is 5mm smaller than when new, about half the stone diameter loss may have been due to wear during grinding. 

Does that seem reasonable, from a general use perspective?  I do not have a clue.  Does it speak to a faster than expected wearing, as if it were defective in that manner?  Probably only support can answer that very well, thinking they would have studied this previously. 

Rick

[Ken S]

Rick,

Having my grinding wheel be out of true has snuck up on me at inopportune times. One time was when I was demonstrating Tormek at a woodworking show. I was working by myself, and several people were waiting to have their pocket knives sharpened!

I imagine a bar graph with the top of the vertical axis being completely true, and the horizontal axis being time. With use (the time axis), the point on the truness vertical axis lowers. It continues with a downward trajectory until it is retrued. From that trajectory, ee can calculate the average trueness.

If we retrue frequently and lightly, there will be more and smaller bumps, a higher average trueness. That's why I believe in frequent retruing.

Truing may not be the problem, however, a true wheel easily eliminates one variable.

Ken

[cbwx34]

One takeaway is that truing doesn't have to take much off the diameter of the stone.  The truing done here to get rid of the groove was extreme.  The stone was true after the first truing, when only about 0.2mm had been removed from the diameter, which amounts to an actual 0.1mm depth of cut of the truing tool. 

If I have done as many as ten truings in the 5-6 mos. I've had the stone, I'd be surprised.  That could account for 1-2mm of diameter lost, plus the just under 0.5mm today.  Since the stone is 5mm smaller than when new, about half the stone diameter loss may have been due to wear during grinding. 

Does that seem reasonable, from a general use perspective?  I do not have a clue.  Does it speak to a faster than expected wearing, as if it were defective in that manner?  Probably only support can answer that very well, thinking they would have studied this previously. 

Rick

It would be hard to answer this... without knowing how much it was actually used during this time.  (What does "general use" equal?)

This stone seems to be a bit of contradiction... the groove/flat spot seems to indicate a stone that would wear rapidly, while it glazing over or smoothing out and needing refreshing indicates a stone that doesn't wear fast enough.