When to stop grinding?

[Herbert Flower]

Good Morning All,

A couple of weeks ago I switched from "white wheels" to CBN wheels (on the grinder). Cool running, effective grind, no wheel dressing, and no shrinking wheel diameter. All good things, right? However, my experience has been somewhat mixed, and often frustrating. Wheras I seem to be doing well enough with gouges (using jig), my results with skews have been quite inconsistant (use platform). I'm one of those who has been depending on the "spark edge" as a signal to stop grinding. Recalling Alan Lacer's comments on the lack of such a defined spark when using CBN wheels, it has occurred to me that I"m not really sure what to look for, in terms of edge readyness if you will, when using these wheels. Yes, my skews are getting long and "pointy". It may well be that the age related MD has brought me to a new place. In any event, I was wondering what your experience has been. Is there a learning curve? Are there specific things I can look for? (Other than the skew simply does not perform well.) I'll very much look forward to your thoughts and comments.

Best,
Herb Flower

[David M Peters]

I look for the back edge of the tool to become flush with the wheel. There's a black shadow line between the two surfaces which disappears as the two bevels meet. That's how I freehand sharpen, anyway.

When using a gouge jig I'll just make a few passes and then inspect the edge, looking for the absence of reflected light. When refreshing a gouge edge I'm removing so little metal that the "shadow gap" trick no longer works. Same goes for a skew.

[Dave Fritz]

You can darken the surface with a marker, then see how much has been sharpened.

[Reed Gray]

Having a platform that is easy to adjust for angle and distance from the wheel, so it is easy to repeat is a big help. If nothing else, I take the tool off the rest and look to see if the bevel is clean or maybe 'fresh'. After that, I hone the grinder burr off. I didn't used to do that. I have found that the diamond hones, even the 1000 grit ones do an okay job, but they still leave a tiny burr. I finally found a use for my old Tormek and use the honing wheel. If nothing else, get a leather scrap and load it with oil and honing paste. I picked up a black stick, made by Norton I think, at the big box store locally. I asked for a honing compound and they didn't know what that was, but they said 'we have a polishing compound'. The honing makes a big difference. I have a 1000 grit CBN wheel, and even with the lightest touch, it left a burr. It would cut pretty good in one direction, but not the other, and that was because of the burr. I do use the skew more for peeling cuts rather than for bevel rubbing slicing cuts. I am far better with gouges and scrapers than I am with a skew...

robo hippy

[Herbert Flower]

Thankyou for your suggestions. I'll have a chance to try them later today. As a follow-up, I did discover that the face of my grinding wheel was slightly cupped, and that wheel is being replaced by the vendor.
Thanks again,
Herb Flower

[roger wiegand]

If you hold the edge up in a strong light and sight down the edge you should see no "glints" (bright spots) on the edge. At that point it is ready to hone and make really sharp.

If you look closely and have a good light you can see a gray trail of ground metal particles coming off the CBN wheel and coming over the top of the knife just like the sparks do on a traditional wheel. It's not as obvious, but it works just the same.

I like the sharpie trick for adjusting the platform. I hone my skews many, many times between grinder sharpenings, so I typically have a fairly wide band of bright polished metal at both edge and heel of the bevel. That's almost as obvious as the sharpie mark to watch disappear as you grind.

[John K Jordan]

I am not sure what you mean when you say "getting long and pointy." Perhaps provide photos, drawing, or a better description.

What is the measured included angle of the bevels, measured perpendicular to the cutting edge (not along the tool shaft axis)?
What is the angle of the edge measured from the straight side of the shaft?

There is definitely a learning curve.

There are various variables, some arbitrary, some a matter if preference:
The skew angle.
The included angle, measuring both bevels at once
Straight edge vs curved edge
The amount of curve


My method.

• I make setting templates to set the wheel to the angle I want for the skew. This is quick and accurate. The first one is for my negative rake scrapers but those for my skews are similar. The second one is to set the platform to 90-deg for hand scrapers. I don't have a picture of those I use for skews but they use the same idea.
  
  NRS_IMG_7907.jpg template_angle_IMG_7898.jpg
  
  BTW, use a sturdy platform. A wimpy platform made from sheet metal may flex and make sharping correctly difficult. The Wolverine platform is good.
  
  
• To match an existing bevel, I mark the existing bevel with blue sharpie, hold the skew with the straight edge horizontal and against the wheel. Adjust the platform until it fits the bevel, sighting from the side with a light on the other side. Rotate the wheel a tiny bit by hand to see by the sharpie scraped off if the platform is set correctly, adjust if needed.
  
  
• Sharpen by holding the edge perfectly horizontal, press the shaft of the skew tightly against the rest so it doesn't tilt even a tiny bit, slide the tool up until the bevel touches the wheel, then holding horizontal slide the skew CAREFULLY sideways a little bit. Be careful not to angle the tool even a bit. Examine the bevel and repeat. The goal is a completely smooth bevel from one side to the other with no odd spots from angling the shaft.
  
  
• If an older tool is carbon steel and not HSS, sharpen gently and cool often with water. If the edge turns blue even for an instant the hardness of the edge is destroyed.
  
  
• If one bevel is shorter than the other, start grinding on the shorter bevel.
  
  
• With CBN you will usually not see sparks over the edge but you can sill see when the wheel reaches the edge. If that bevel is shorter than the other, repeat sharpening until they are close to the same width.
  
  
• Repeat on the other bevel. You can feel for the grinding bevel on the top side.
  
  
• Make sure the bevel is sharp and straight all the way from short point to long point. If the skew was abused or poorly sharpened there may be extra work to repair the points or edge.
  
  
• Examine the bevels for approximately equal width and fix with additional sharpening if desired.
  
  
• The same method is used for a skew with curved edge but the difference is the entire tool must be rotated smoothly while grinding the entire edge while pushing forward enough to apply an even force against the wheel. The pivot point will be at the approximate radial center of the curve. If the curve is not an arc of a circle, which is usually the case, some coordination is needed to adjust the sliding motion and rotation over the grind. This may take some practice.
  
  
• The "oval" skews with curved shafts are difficult to hold straight, as are round skews. I sharpened several of those this week.
  
  
• After grinding, remove the grinder burr with an extra fine diamond hone or a stop. I strop hones on a hard piece of MDF resawn on the bandsaw to give one rough side. I rub some kind of honing compound into the rough surface. I hold the skew with the bevel flat against the surface, raise the handle a fraction of a degree, and slide the tool towards me while pressing down significantly. Repeat on other side. This leaves the edge shaving-sharp.

I sharpened about a dozen skews in the last few days, some for myself, for others, some for a demo in a class. I use a 600 grit CBN wheel on a 1/2-speed grinder. My left arm is almost bald from testing skews.

The easiest way to learn is to spend a little time with someone with experience.

JKJ

Good Morning All,

A couple of weeks ago I switched from "white wheels" to CBN wheels (on the grinder). Cool running, effective grind, no wheel dressing, and no shrinking wheel diameter. All good things, right? However, my experience has been somewhat mixed, and often frustrating. Wheras I seem to be doing well enough with gouges (using jig), my results with skews have been quite inconsistant (use platform). I'm one of those who has been depending on the "spark edge" as a signal to stop grinding. Recalling Alan Lacer's comments on the lack of such a defined spark when using CBN wheels, it has occurred to me that I"m not really sure what to look for, in terms of edge readyness if you will, when using these wheels. Yes, my skews are getting long and "pointy". It may well be that the age related MD has brought me to a new place. In any event, I was wondering what your experience has been. Is there a learning curve? Are there specific things I can look for? (Other than the skew simply does not perform well.) I'll very much look forward to your thoughts and comments.

Best,
Herb Flower

[Richard Coers]

On a skew, I will hit it with a hand hone about 4-5 times before I go back to the grinder. But really there isn't much to look at or wait for sparks. A slight touch on each side should be all that is needed. Now if you are reforming a new skew different story. But on a formed skew, less than 5 seconds on each side and it's ready to go.

[allen thunem]

when to stop grinding????
when i run out of steel
sorry I just couldnt resist.
been locked up too many days

[Herbert Flower]

Thanks again to all.
After I discovered the CBN wheel was concave on the grinding edge, I contacted the vendor and the wheel was replaced. All good now.
Best,
Herb Flower

[Don Stephan]

When the skew bevel has been refreshed up to the cutting edge, a slight burr will be felt on the opposite side if one CAREFULLY rubs a finger or thumb from the heel of the bevel to the cutting edge. Flip and repeat on the other side of the cutting edge. Some hone off this burr after grinding, I simply return to the lathe and the burr is quickly removed by the wood.