What Does It Matter If The Stone Is Out-Of-Round A Little?

[RickKrung]

Reading some of the posts in the SJ wheel discussion made me wonder, what does it matter if the stone is out of round a little bit?  At least for hand held jigs/operations. 

Using Wootz's applet, I checked to see what the angle change is for a 1mm eccentricity in the diameter (that is 0.5mm at the stone/edge interface).  For a stone at 245mm, 139mm projection and 16º starting angle and USB height of 169.12mm.  Changing the stone diameter to 244 and adjusting the angle until the USB height was nearly identical (USB 169.13mm), the angle changed to 16.3º. 

Assuming lateral travel is slow enough to expose an edge to the full excursion of the diameter at least once along the entire length, what does that angle difference mean to the sharpened edge?  A slight flattening of the concavity.  Less sharpening action at the apex.  Since we sharpen until there is a burr, the latter would mean it would take a little longer to raise that burr.  For production sharpeners, I can see that mattering, but to the more casual sharpener, like me, what does it matter? 

Rick

[cbwx34]

Reading some of the posts in the SJ wheel discussion made me wonder, what does it matter if the stone is out of round a little bit?  At least for hand held jigs/operations. 

Using Wootz's applet, I checked to see what the angle change is for a 1mm eccentricity in the diameter (that is 0.5mm at the stone/edge interface).  For a stone at 245mm, 139mm projection and 16º starting angle and USB height of 169.12mm.  Changing the stone diameter to 244 and adjusting the angle until the USB height was nearly identical (USB 169.13mm), the angle changed to 16.3º. 

Assuming lateral travel is slow enough to expose an edge to the full excursion of the diameter at least once along the entire length, what does that angle difference mean to the sharpened edge?  A slight flattening of the concavity.  Less sharpening action at the apex.  Since we sharpen until there is a burr, the latter would mean it would take a little longer to raise that burr.  For production sharpeners, I can see that mattering, but to the more casual sharpener, like me, what does it matter? 

Rick

Actually, it's 16.295°

The reality... probably doesn't matter as much as we're led to believe.  I'm sure if you're trying to achieve a very high level of sharpness, you might see some difference, but for the average sharpening, I'd say very little.  And it may be more important for some tools, other than knives.

There is a claim with regular waterstones that if not kept relatively flat, the issue will "compound" over time... in other words, the knife can start "digging in" to the stone more as it goes into the curve... getting dished faster.  Not sure if it would hold true with the wheel though.

My guess, is there's a lot of unsubstantiated reasons for it... with little supporting evidence.  One of those... if you can't tell, it doesn't matter, if you can, it does?

[Ken S]

Is this our forum machinist asking is it matters if the wheel is out of round? Does it matter if I drop my precision Starrett square on a concrete  floor? 

On a practical level, I suspect not. I doubt that many of the old Swedish large grindstones which inspired Torgny Jansson were round. Some of us, myself included, like to cut down the chance of error by eliminating as many gremlins as possible.

Ken

[wootz]

Rick, you will see you can't apex the edge when you change to the 2nd wheel.
E.g. you've bevelled and got the burr on an SG wheel that is a little out of round, then move to an SJ wheel to hone where won't be able to reach the apex in the portions of the edge that "ducked" by 0.3 degree on the SG - and where you are not able to reach the apex you can't deburr it at the same rate as the "proper" portions of the edge, so you keep honing and overhoning, and when you finally get to those "ducked" portions and deburr them, the edge apex on the other "proper" portions of your edge get ground off by overhoning, i.e. "rounded". At his point in time you logon to this forum to post a thread "why I cannot get a shaving edge off the SJ?".

[jeffs55]

I have sharpened many knives on a wheel resembling an egg. A little out of round does not matter.

[Ken S]

Jeff,

At the risk of being the devil's advocate, if you had to sharpen a knife and could choose between two Tormeks, Which one would you choose, the one with the wheel with the egg shaped wheel or the one with the freshly trued wheel?

I have certainly sharpened my share of tools with out of round wheels and not flat stones. I have also had gremlins which went away when I trued my grinding wheel. I spent my working life as a troubleshooter for Ma Bell. Whenever I don't find a source of the trouble, I try to eliminate possible trouble sources. Truing up the grinding wheel is a quick fix for a possible gremlin.

Ken

[cbwx34]

Rick, you will see you can't apex the edge when you change to the 2nd wheel.
E.g. you've bevelled and got the burr on an SG wheel that is a little out of round, then move to an SJ wheel to hone where won't be able to reach the apex in the portions of the edge that "ducked" by 0.3 degree on the SG - and where you are not able to reach the apex you can't deburr it at the same rate as the "proper" portions of the edge, so you keep honing and overhoning, and when you finally get to those "ducked" portions and deburr them, the edge apex on the other "proper" portions of your edge get ground off by overhoning, i.e. "rounded". At his point in time you logon to this forum to post a thread "why I cannot get a shaving edge off the SJ?".

I'm not sure how being out of round would only affect portions of the edge?

Jeff,

At the risk of being the devil's advocate, if you had to sharpen a knife and could choose between two Tormeks, Which one would you choose, the one with the wheel with the egg shaped wheel or the one with the freshly trued wheel?
...

To me, it "feels" better sharpening on a trued wheel, and I'm sure is a more accurate and better edge, but maybe a better question would be, what difference would you see (especially in use)? 

[jeffs55]

Jeff,

At the risk of being the devil's advocate, if you had to sharpen a knife and could choose between two Tormeks, Which one would you choose, the one with the wheel with the egg shaped wheel or the one with the freshly trued wheel?

The freshly trued wheel every time. I do not remember how long it took me to notice the ever increasing ellipticity of my wheel. Is that even a word? It will work even when out of round but it is always better to use a tool in the correct way.

[wootz]

I have sharpened many knives on a wheel resembling an egg. A little out of round does not matter.

No doubt on your egg-shaped wheel you get a convex edge boasting fortified properties compared to the oversubtle edge off a trued wheel.

[jeffs55]

I have sharpened many knives on a wheel resembling an egg. A little out of round does not matter.

No doubt on your egg-shaped wheel you get a convex edge boasting fortified properties compared to the oversubtle edge off a trued wheel.

What the heck does that mean?

[Ken S]

Jeff,

My Australian is a little rusty. I think it means either "never needs sharpening" or "the shrimp's on the barbe". 

Ken

[Grizz]

i'll have the shrimp on the barbe while we wait !!!

[jeffs55]

My stepson just brought me his knife set for sharpening, Henckels or Wusthoff. Guess what? My wheel is egg shaped again. I have an aftermarket "truer" that has worked before, guess it will have to work again. I sure have not used it much for this to happen.

[Ken S]

Jeff,

I wonder if your shaft might have become bent. I remember Jeff Farris posting that he always removed the grinding wheel(s?) of his Tormek(s?) whenever he traveled. At one time he was traveling thirty four weeks a year. And, that was before the left hand thread EZYlock when you needed a hammer and a wrench to loosen or tighten the nut.

Machinery rebuilder Robert Vaughn wrote an article for Fine Woodworking where he routinely checked the straightness of drill press shafts with a dial indicator. He would hit the shaft with a hammer until it was straight.

A dial indicator would quickly check your shaft. If you do not have one, a local machinist, auto machine shop or motor repair could easily do it. Or, you could remove your grinding wheel. Set the usb in close. Place a piece of wood, plastic, or metal between the shaft and the usb. (This is just like using feeler gages to set distributor points in the old days.) Slide your "feeler gage" in and out setting the usb until you feel a slight drag. Use your other hand to rotate the shaft by turning the leather honing wheel. If the shaft is straight, you should continue to feel the same drag. It the drag changes, your shaft is bent. Straightening a shaft with clamping pressure is no big deal. (This is another example of the value of knowing a good machinist.)

Plan B would be to just replace the shaft. It you have a pre EZYlock SuperGrind or T7and have been thinking of upgrading to a stainless steel EZYlock, this might be a goid time to do it. The EZYlock also includes a new set of bushings. I converted my first T7. It is not difficult. The hardest part iwas reprogramming  my brain  to think left hand thread.

Another Jeff Farris tip was to true the wheel frequently. As a Tormek demonstrator, Jeff wanted to avoid problems. Keeping his wheel trued was a good way to lessen the number of problems.

Do keep us posted. I think your solving the egg shaped wheel problem will benefit many readers.

Ken

ps your package is being mailed tomorrow..

[Jan]

In my understanding an oval grindstone is today rather a curiosity. In ancient times it was probably used to grind a convex grind on swords. 

It is not so easy to imagine how an oval stone works, and so sometimes ago I have prepared a model. I have assumed that the oval is an ellipse with axes ratio 4:5. The blade is guided by a jig which fixes the blade protrusion from a pivot.

Assume that when the vertex of the oval stone meets the edge than the wheel grinds the desired bevel angle.

When the stone rotates towards the co-vertex, than the contact point with the blade moves away from the edge and the bevel angle changes its size.

When the co-vertex of the oval stone meets the edge than we have reached spot where the change of the bevel angle is maximal.

Jan