SE-76 Squareness revisited

[stevebot]

I sharpened 5 plane irons for a wood worker yesterday, including two deemed too wide to sharpen by the local tool store.  SE-76 handled them all, but all were out of square by a couple of degrees. Squareness was this guys hot button (he brought his own square), so I had to carefully skew each iron in the jig to match his already square edges.  What is the point of a Square jig that is not square? More important, how do I fix it? Do I need to tune my jig to fit my machine?  Could the truing tool be the culprit?

For the record, I trued the wheel immediately before sharpening. I started with the irons squarely  up against the stops. Bottom plate was parallel to top plate, and since these are plane irons over 2" wide there was no question of being tilted in the jig like a chisel can be.

Any suggestions welcome.

[Herman Trivilino]

If the grindstone is not true, I can never get them square no matter how much I skew the mounting in the jig.

If the grindstone is true, then the only thing I know that will work is application of more force on one side of the tool than the other. I've never been able to get a square end by skewing the mount in the jig.

Perhaps another issue is alignment of the horizontal section of the US bar. If that section of bar is not parallel to the axis of rotation of the grindstone, nothing I know of can remedy the situation.

[Jan]

Steve, my suggestion is to try the recommendation from one Tormek demo. The assumption is, you are starting with square plane irons.

Lower the universal support and draw a line on the grindstone parallel with the bar of the universal support. Then align the edge of the plane iron with the line on the stone.



This should replicate the original iron squarness.* 

Jan

*P.S.: If the plane iron is not square, you will replicate the skew.

[Ken S]

Good suggestions.

My oldest metal plane, a very usable Stanley #4 jack plane from 1891, has a lateral adjustment lever, as do planes today. Torgny invented the Tormek. It seems the squareness issue is much older.

Ken

[Stickan]

Steve,
http://tormek.com/international/en/grinding-jigs/se-76-square-edge-jig/
Watch this video, mount the tool as explained. Use pressure where you need to remove material. If the edge is 90 degree when you start to sharpen, press in the middle, id you see that it starts to move, use pressure on the side where you need to remove more material.
Also, you freehand sharpen, check that the stone is flat. If not, true it before.

SE-76 is a very good and precise jig. Complaints is almost every-time a operator issue :-)
Be sure that the stone is flat and use pressure correct on the tool and be sure the tool is mounted correctly in the jig.

Sincerely,
Stig

[RobinW]

May I suggest that you search the Forum using 'se-76' and you will see that there have been plenty of posts about issues involved in trying to attain squareness. My own analysis considered in particular the knock on effects of small angular errors in two planes and had some 3D modelling to prove the point. Also the manner in how the SE-76 was manufactured. Some good analysis was supplied by by Mike Fairleigh.

I seem to post items at the moment where it may appear that I am an argumentative so and so, (it's not intended), but "SE-76 is a very good and precise jig" is subjective as I am not aware of any error or cumulative tolerances regarding accuracy for this jig or indeed the Tormek machines. Similarly Tormek advertising for the  T4 claiming "Precision improved by 300%". What precision? How measured? Compared to what?

Staying with the SE-76, after truing up the wheel, I got into the method of continually re-applying marker pen to the surface being ground and checking the grind with a square. If it was not providing the desired solution, I would loosen/retighten the blade in the jig, tweak as I think fit, change finger pressures etc until I get the right answer. The idealistic impression as shown in the referenced video is an infrequent visitor when I try sharpening!

[Jan]

Robin, I have read your thread "Square Edge Issues with SE-76"(http://forum.tormek.com/index.php?topic=1366.msg4173#msg4173 ) and have to say that I fully resonate with your experience and opinions. It is still very topical issue.

I would like to ask you, if you are ready to share with us your 3D drawings dealing with the effects of small angular misalignment and blade twist? If you have difficulties to display the images, let me know the address where they are stored and I will attach them. 

Jan

[RobinW]

Sorry Jan - I deleted them some time ago.

[Jan]

It's a pity, Robin.

I have simple 3D CAD software and I am trying to design a model, you had already several years ago. For me it is not easy, I have never worked with 3D solids.

Nevertheless my preliminary results show, that even 1 degree angular misalignment can has significant impact on the bevel squarness. In my thinking, small misalignments we are able to correct by the pressure of fingers during sharpening.

Jan

[Jan]

In the following simple drawings you can see the impact of 1 degree misalignment between the stone axis and the tool in the SE-76 jig on the resultant edge squarness. I have assumed a plane iron 3 mm thick and 50 mm wide, ground with a 25 degrees edge angle. 





For simplicity I have ignored the hollow shape of the bevel. The bevel was created by a SW milling tool, which is not entirely correct. Maybe this affected the size of the skew.

Jan

P.S.: Please take the figures above only as my first attempt to get acquainted with 3D CAD modelling. In the future I hopefully will be able to prepare more realistic drawings, including grinding stone and hollow shape of the bevel.

[RobinW]

I am having a day with many senior moments involved, one of which is that I can't see where I posted the values I used in the 3D modelling!

Thanks Jan - your diagram illustrates the point I had been trying to demonstrate.

Jan's demonstration above shows clearly how seeming deceptively small errors are magnified at the end or the blade.

If Jan's example then introduced an angular error round the axis running along the length of the tool, then a compound error of more annoying output results. This later effect would show different effects dependent on whether this second error is clockwise or anti-clockwise so 'adding' or 'subtracting' to the initial problem.

[Jan]

Thank you Robin for commenting my figure. 

You are correct, the 3D modelling reveals all the annoying output results, which are consequence of a compound error in positioning the tool against the grinding stone.

Jan

[Jan]

I have used 3D CAD modelling to get an answer to a question: "Can we get a square grind if the blade is twisted?"   

Imagine we are grinding with a 25 degrees edge angle a plane iron which is 3 mm thick, 40 mm wide and twisted around the red axis by 2 degrees.



This 2 degrees twist will cause, that we get trapezoidal bevel skewed by 4 degrees in the horizontal plane!



To compensate for the effect of the twisted blade we have to rotate the blade clockwise 4 degrees in the horizontal plane (around the blue axis in the first figure).



This fully compensates for the skew. What remains is a small change in the bevel width along the edge.

Jan

P.S.: As mentioned by RobinW, when compensating for twist, we have to select the proper orientation to receive the subtracting effect to the initial twist.   

[Herman Trivilino]

Jan, I suggest giving names to the angles to facilitate discussion of them. I suggest b for the bevel angle, so in your example b = 25°. I can't tell from your figure what you mean by the "red axis" but let's say a = 2°. Is the red axis in the plane of the blade? If so then a is the angle between the line formed by the edge, and a line perpendicular to the side of the blade. In other words, if a = 0 then the edge is square to the side of the tool, which is the goal.

If the axis of the grindstone is not parallel to the horizontal portion of the US bar, and the grindstone is trued using the truing tool, then a will not be zero when the tool is mounted squarely in the SE-76 jig. I believe this is the source of some of the problems people have with getting square ends.
 

[Jan]

Yes Herman, you are correct. 
The red axis is in the plane of the blade.

If the axis of the grindstone is not parallel to the horizontal portion of the US bar, then they can belong either to intersecting lines, or they can form skew lines which do not intersect each other.

In my understanding the 3D modelling has shown, that some types of misalignment can be almost compensated by simple means. In my modelling I have assumed that the grindstone was square=true with respect to its axis.

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.

Jan