hollow vs flat grinding thoughts

[Elden]

Jan,

   When I  stated my jury had not reached a verdict yet and was talking about measuring the angles, I was referring to how the Angle Master measures angles versus a machinists style protractor. I am wanting to verify that the AM measures the tip of the bevel not theoretically but experientially. Obviously the machinist protractor will contact two points, the tip and the heel of the bevel, bridging the the concave. Theoretically (in my thinking anyway) the AM measures the tip of the bevel? My jury is supposed to decide if that is true or false. I am in the process of analyzing that today and am planning on getting a sharp chisel out of the deal!

   While setting up the Tormek, I realized another possibility of error introduction. Wear of the corner of the AM where it contacts the grinding wheel will change the reading of degrees. Looking at mine, that corner is slightly rounded.

[Jan]

Thanks for your explanation Elden. 
Jan

[grepper]

Thanks for the extra prod Jan.  You (and Herman) explained it perfectly well... Uhh...Several times.  I had just skimmed and missed the point.  So I re-read the entire post.  Sharpening at a particular angle, the concavity would not exist below the sharpening angle.  What I had not known and even thought about was how a hollow grind bevel angle was measured.  Makes total sense. Very interesting and informative.  Learn something new every week or two!

With a big 250mm wheel, the effect near the edge is minimal, but more so for T4 owners.  But as you say, "The hollow shape of the bevel becomes much more important when grinding really thick tools".  I guess if you were after a wide, thin bevel, a flat grind would be the way to go. I wonder if I could tell the difference on an onion.

Cool info.  Thanks!

[Elden]

My jury has come into the court room before the judge to issue their verdict. Ok, forget the drama.

   After grinding the chisel,  the AM (Angle Master) showed that a 28 degree angle had been formed. The General machinist type protractor showed 30 degrees. As I stated, the corner of the AM pointer that contacts the grinding wheel is slightly rounded. If it was not, the AM  would probably read about 27.5 degrees or a little less. This is fairly thin Stanley chisel as it is 0.125 (1/8) inch or 3.175 mm thick. If the chisel were thicker, the machinist protractor would register a higher number of degrees.

   Thus I believe it can be said experientially, the AM does register degrees at the tip of the bevel. Also that as Jan stated, there is more metal behind the tip of the bevel on a hollow grind than a flat grind it is measured at the tip of the bevel.

   Good job Jan and thanks for pointing it out! I know mathematics and drawings are great, but sometimes the old Missouri mule in me says, "Show me, I am from Missouri!" You proved it on paper, I wanted to see it in steel and more than that, how does the AM register.

   So also, THANKS TORMEK for great products!

[Stickan]

Hi Guys,
Interesting subject to follow and good explanations.
I had prepared a long explanation but did not need to press "post"!

I wish you all a happy Easter!

Best Regards,
Stig

[Jan]

Stig, thank you for joining us. Please be so kind and post your explanation. Plurality of views may help to understand this interesting subject even thoroughly.

Happy Easter!    Jan

[Jan]

My jury has come into the court room before the judge to issue their verdict. Ok, forget the drama.

   After grinding the chisel,  the AM (Angle Master) showed that a 28 degree angle had been formed. The General machinist type protractor showed 30 degrees. As I stated, the corner of the AM pointer that contacts the grinding wheel is slightly rounded. If it was not, the AM  would probably read about 27.5 degrees or a little less. This is fairly thin Stanley chisel as it is 0.125 (1/8) inch or 3.175 mm thick. If the chisel were thicker, the machinist protractor would register a higher number of degrees.

   Thus I believe it can be said experientially, the AM does register degrees at the tip of the bevel. Also that as Jan stated, there is more metal behind the tip of the bevel on a hollow grind than a flat grind it is measured at the tip of the bevel.

   Good job Jan and thanks for pointing it out! I know mathematics and drawings are great, but sometimes the old Missouri mule in me says, "Show me, I am from Missouri!" You proved it on paper, I wanted to see it in steel and more than that, how does the AM register.

   So also, THANKS TORMEK for great products!

Thank you Elden for posting your results!

Your results are in agreement with the geometrical considerations presented in this thread.
The edge angle measured with Angle Master at the cutting should be smaller than the angle measured with machinist type protractor. This is because the protractor probably measures the mid angle of the bevel.

Keeping in mind the limited accuracy of your measurement,  the angle difference 2 degrees is even numerically OK. Really good job Elden, congrats! 

Jan

[Ken S]

Stig, would you please post your comments?

As I recall, Tormek holds a patent on the anglemaster. That would indicate the engineering department must have invested a considerable amount of development time. I believe we would all enjoy and benefit from learning about it.

Incidentally, I saw a photo of an earlier version of the andlemaster. (the 100). The current model seems moch more advanced,

Ken

[Ken S]

Yesterday I ground two Blue chip chisels to twenty five degree bevels. I ground one with the T7 and the other with the T4. I wanted to see the difference in the amount of hollow grind between the two Tormek grinding wheel sizes. The mathematical part of my brain tells me there is a difference.  If I examine them very closely I think I can see a difference, the T4 grind having more hollow grind. However, I am glad one chisel was ¾" and the other 5/8", because it would be very difficult to tell the difference if both were the same width.

I can easily spot a blade ground hollow with a six inch grinder. The two Tormek grinds hardly looked hollow ground to my eye. I don't pretend to have any expertise in this area. I would not let worries about the amount of hollow grind influence me in deciding between a T7 and a T4.

Incidentally, a Supergrind or T7 with a wheel worn down to eight inches is essentially a slower T4 with a wider wheel. (The T7 has a speed of 90 RPM; the T4 120 RPM.

Ken

[Herman Trivilino]

Suppose you measure the bevel angle on a chisel with a carpenter's tool, something like this:

http://www.amazon.com/Swanson-Sliding-T-Bevel-Stainless-Fittings/dp/B0008IUWDY/ref=sr_1_14?ie=UTF8&qid=1428028426&sr=8-14&keywords=carpenter%27s+protractor


You have two straight edges that meet and pivot so that the angle between them can be adjusted. One edge is lined up with the back of the chisel, the other with the surface that you grind away when you sharpen the chisel. That edge will indeed not meet that surface and there will be a hollow due to hollow grinding. Since that metal is missing, the chisel is indeed weaker than one that was ground flat.

But using this carpenter's tool in this way to measure the bevel angle is not the way we measure the bevel angle when we use the Angle Master!

What we do is measure the angle at the very tip. If you drew a straight line from that tip it would not meet the heel of the beveled edge in like the carpenter's tool does. Instead it would pass through the steel. There is steel on both sides of that line. Some is on the side closer to the back of the chisel, some is on the opposite side. It's this extra steel on the opposite side that makes the chisel a stronger tool for having been hollow ground. See my drawing in Reply #1 of this thread.

[Herman Trivilino]

   Thus I believe it can be said experientially, the AM does register degrees at the tip of the bevel.

As you also stated Elden, wear on the plastic tip can effect the accuracy of the measurement. As can many other things. My point was and is that the Angle Master is designed to measure the angle at the tip. I'm glad your measurements confirm that it's working as designed.

[Stickan]

My explanation of the material being behind the edge is already covered, Jan´s drawings and the other comments are what I had written so is would only be the same.

The angle master WM-200 is very good and are measuring the tip of the edge. It is compensated for the hollow grind. If you use a regular "angle setter" or a protractor you will fine that the degree will indicate that its not the same as the anglemaster. A protractor are not compensated for a hollow grind but for a flat surface.

Stig

[Ken S]

Thanks, all. That is valuable information.

Ken

[Elden]

   Very interesting everyone. Herman, your drawings and explanation makes sense now that it soaked through. Initially I thought your drawing looked like a skew chisel.

   When honing a hollow ground chisel  on a bench stone, the bevel angle is being increased if the bevel is contacting the stone at the tip and the heel of the bevel. So honing (which is working to form a flat bevel) a hollow grind causes a more blunt angle. Interestingly, there will be more metal left supporting the tip now than there was supporting the hollow grind you just honed. You have just formed a micro-bevel because the heel of the bevel is holding the angle of the stone at a greater number of degrees than that of the hollow grind. There is a nifty way to micro-bevel your chisel or plane blades if you are so inclined.

[Herman Trivilino]

   When honing a hollow ground chisel  on a bench stone, the bevel angle is being increased if the bevel is contacting the stone at the tip and the heel of the bevel. So honing (which is working to form a flat bevel) a hollow grind causes a more blunt angle. Interestingly, there will be more metal left supporting the tip now than there was supporting the hollow grind you just honed.

Yes, simply because the bevel angle is larger. We do this all the time. We make the bevel angle larger when we want the chisel to be stronger.

You have just formed a micro-bevel because the heel of the bevel is holding the angle of the stone at a greater number of degrees than that of the hollow grind. There is a nifty way to micro-bevel your chisel or plane blades if you are so inclined.

I've been doing it this way for years. It's a quick way to refresh the edge on a dull chisel.