SE-76 Squareness revisited

[Herman Trivilino]

At this moment it is not quite clear to me, what we get when we true the grindstone using the truing tool, while the axis of the grindstone is not parallel to the horizontal bar of the US. The truing will probably result in forming some cone-shaped grindstone body.

If the US and the grindstone axis lie in a plane then you will get a symmetrical cone of the type used for ice cream. If not (skew), then the cone is of a different shape. Picture a round funnel with the spout off center. The problem is that as you move the chisel from side-to-side while sharpening you change the bevel angle. You can't compensate for this by adjusting the angle a, that is, by mounting the chisel in the jig out of square.

There was a thread a while back about a shop teacher with a new T7 who never could get his chisels square. I suspect, but do not know for sure, that this was his problem. The T4 design, again I suspect but do not know for sure, was introduced to overcome this problem in the T3.

If the housing became twisted in the manufacturing process it would result in this problem. I recently bought a new dishwasher with this problem. You could get one side or other of the door jamb plumb (front to back) but not both. It meant that the door couldn't ever close tightly, resulting in water leaks. Had a heck of a battle trying to get the manufacturer to believe me. Fortunately, I prevailed.

[Jan]

Thank you for your explanation, Herman. 

The effect of a grind stone in the form of a right circular cone could be modelled with 3D CAD SW.

Jan

[Jan]

In the drawings below you can see the situation, when the grindstone has the shape of truncated right circular cone. Its edge is not square, 90 degrees, but only 88 degrees. One side of the grindstone has a diameter 200 mm, while the other only 196.5 mm.



Picture we are grinding with a 25 degrees edge angle a plane iron which is 3 mm thick, 40 mm wide.



My interpretation of the result is following, when the tool is mounted squarely in the SE-76 jig, we will get almost square end, despite the fact that the grindstone is not square, but conical!
(We have to use the same sleeves and the same US, which were used for "truing" the stone.)

Jan

[Jan]

If the US and the grindstone axis lie in a plane then you will get a symmetrical cone of the type used for ice cream. If not (skew), then the cone is of a different shape. Picture a round funnel with the spout off center. The problem is that as you move the chisel from side-to-side while sharpening you change the bevel angle. You can't compensate for this by adjusting the angle a, that is, by mounting the chisel in the jig out of square.

Herman, in principle you are correct, but the change in the bevel angle is negligible. For the situation shown in the drawings (25 degrees bevel angle) it is less than 0.2 degrees for the width of the blade.

Jan

[SharpenADullWitt]

Some days I come here, and see these posts, and I think I am in a classroom, being served a test (solve for *)

[Jan]

Sorry Randal, if it reminds you some unpleasant school experiences, it is not so intended. 

Jan

[Ken S]

Randal,

Your comment gave me a good laugh and made me think. My high school geometry class was fifty years ago, literally in the prior millenium. To put it politely, my math skills most days are less than well honed. After sharpening so many old dull pocket knives in Hartville, I have come to believe there may be a sharp edge in need of honing in this old gray matter. Do not lose heart.

At first glance, my simple single piece of wood, the kenjig, seems to have all the technical sophistication of a kindergarten wooden building block. However, the more I use it, the more I appreciate its solid trig foundation. I just reread Dutchman's angle grinding booklet (posted by him on this forum). I confess some of the fine points are still beyond my immediate comprehension, however, I am gaining a grasp of the basics, enough to find it a very useful tool.

I am very grateful for the math expertise Dutchman, Herman and Jan have brought to this forum. I do not see injecting more math as adding complication; I see it as making more things possible. I like Jan's quote, "Iron sharpens iron and one man sharpens another".

Welcome to Tormek 202, Randal. Your wit may be sharper than you realize!

Ken

[Herman Trivilino]

Herman, in principle you are correct, but the change in the bevel angle is negligible. For the situation shown in the drawings (25 degrees bevel angle) it is less than 0.2 degrees for the width of the blade.

Nice work, Jan. I cannot verify as I find it hard to think in three dimensions. But this is the case for the US bar and the grindstone axis being in the same plane, intersecting at an angle of 2 degrees. What of the more general case of these lines not meeting at all, being askew? That is when, I suspect, the real problems arise.

[Jan]

Yes Herman, I agree with you. I am gathering forces to attack the more general case. Please stay tuned to rethink the potential results. It is really easy to make a mistake when thinking in three dimensions.

Jan

[Ken S]

I believe the importance of regular use of the truing tool is under appreciated. I have been surprised twice recently by an out of true grinding wheel. The first time I had not trued the stone in a while, but "it looked OK" to me, a bad assumption. The second time happened shortly after that. I was thinking I had only recently retried the stone, however, this was during the woodworking show when I was using the Tormek extensively.

Not only was my grinding wheel not parallel with the universal support bar, it was also slightly out of round. This is not a product defect; it is a natural product from use. I have since become a twice believer in very regular retruing with very light passes. What normally happens is the diamond tool first removes small amounts from only the high spots, just like rounding a billet on the lathe. Gradually the cutting covers more of the circumference of the wheel and also the distance across. Once a very light cut covers the entire wheel, we are done.

I believe this should be the first variable to be eliminated from the problem equation.

Ken

[Jan]

I have simulated truing the grindstone when the US bar and the grindstone axis are askew. In the picture below it means, that the US bar was rotated by two degrees around the red axis and additionally by 2 degrees around the blue axis. The grindstone trued in this way is conical, its edge is not square but only 88 degrees. The rotation (misalignment) around the red axis has no effect on the conicity of the grindstone.



When the tool is mounted squarely in the SE-76 jig, we should get trapezoidal bevel skewed to 91,6 degrees. Surely an annoying output result



This skew can be almost corrected by mounting the tool not squarely in the SE-76 jig, but parallel with the side of the grindstone. In this case the bevel skew is reduced to acceptable 89.6 degrees.



Jan

[RobinW]

I love it!

Great effort Jan.

[Herman Trivilino]

This skew can be almost corrected by mounting the tool not squarely in the SE-76 jig, but parallel with the side of the grindstone. In this case the bevel skew is reduced to acceptable 89.6 degrees.

How can someone suspecting that they have this problem check for it?

[Jan]

Your response pleases me, Robin.  I'm following in your footsteps.

Jan

[Ken S]

I generally view sharpening problems from a basis of my telephone troubleshooting background. Things like careful and frequent truing of the grinding wheel eliminate some of the variables in the trouble equation. As we learn more about what the problem is not, we can focus more on what it may be.

My other factor in viewing troubleshooting Tormek trouble comes from my long hobby interest in machine shop technology and measurement. In simpler, pre digital days, machinists routinely used calipers to measure. A skilled machinist could measure within about .001" with a pair of simple calipers. Using a set of inside calipers to measure the distance between the grinding wheel and the universal support bar would give a very accurate reading of how parallel the bar was from the wheel. By making several measurements along the bar, the flatness of the wheel is also checked.

The beauty of calipers is that there is no actual measurement interpretation, just feeling the drag. I suggest placing a thin piece of paper between the grinding wheel and the caliper leg to preserve the caliper.

Ken