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Username Post: Edge Pro, angle calculations        (Topic#794815)
Journeyman KnifeNut!
05-06-07 13:22.33 - Post#1221872    


1. measuring set-up (5579)
2. sharpening "triangle" (5577)
3. setting pivot post height, 66.3 mm = 14° (5580)
4. 9 mm thick riser block, 6° (5583)

I. Measuring set-up

The first Edge Pro tip I learned from this forum was to record the distance from the front edge of the blade table back to the knife guide, so as to always use the same setting when sharpening a given knife. That is an easy measurement to make, but what about sharpening angle? The colored marks on the pivot shaft indicate 2° increments, and are spaced almost 9 mm apart, hardly enough for your average obsessive! In addition, ever since I began buying and using Japanese knives (some of which are asymmetrically bevelled), I have wanted to be able to set and sharpen much lower angles than the lowest indicated setting of 13°. In order to make the necessary measurements and calculations, I made several temporary modifications:

(see photo 1: 5579)

1. Knife table. The black Edge Pro knife table is slightly chamfered on its edges, making it difficult to take measurements along the 30 cm (12") Staedtler rule. So I held an engineer's rule flat against the end of the table, and taped a white 4x6 card to the bed, carefully aligning the edge of the card to precisely coincide with the end of the bed as though it had never been chamfered. Both the sharp edge of the card, and its whiteness, made taking subsequent measurements much easier.

2. Tape blank. Since all water stones wear, I standardized my measurements by using a new tape blank supplied by Ben Dale as the hypotenuse of the triangle I was creating and measuring. To facilitate supporting the 30 cm ruler, I attached a few spare riser blocks of 3 mm plexiglas to the undersize of the blank with 3M double-sided tape. This ensured that the line I was measuring was effectively the same one as the surface of a stone or polishing tape.

3. Ruler. Especially when calculations are involved, I find that I prefer to work in the metric system, and so purchased a precision 150 mm metric rule from Mitutoyo, model 182-111, to use with the Edge Pro. For quick work, millimeters are pleasantly precise, and for very careful work (with either close-up glasses or a loupe), I can read and/or estimate within half millimeter markings fairly easily. On the other hand, I find 1/32" intervals a bit coarse, and 1/64" intervals very hard to read. The Mitutoyo satin chrome metric rigid rule can be found here for about $15 USD:

Rigid satin chrome rules like the Mitutoyo (or Starrett equivalent) are ideal for the sorts of measurements I was making, as they are both wide enough and thick enough not to rock, even when being stood on end. Once again, a strip of 3M double-sided tape made it easy to find the correct position for the rule, and then lock it into place each time.

II. Measurements and calculations

To begin, I wanted to check the accuracy of the marked angles on the Edge Pro, and arbitrarily selected 13° as a starting point. I made sure that the 15 cm metric rule was perpendicular to the bed, and used the double-sided tape to stick it to the square, black plastic block at the top of the pivot shaft (above the colored angle markings). This placement automatically formed a right angle with the blade table.

(see photo 2: 5577)

Next I needed to take two measurements:

1. Opposite. The height on the pivot post where it intercepted the line formed by the polish tape blank.

2. Hypotenuse. The distance from the front edge of the knife table (where a blade is sharpened), to the edge of the metric rule on the pivot post.

The sharp corner of the 30 cm metric rule made it easy to read out the height of the triangle on the 15 cm rule, and the white card made it easy to read out the length of the line thus formed. Using simple trigonometry (sin = opposite/hypotenuse), I then calculated the sharpening angle formed by these lines. In this case, my height (opposite) was 50.5 mm, and my hypotenuse was 224.5 mm, for a sin of .2249. Using the calculator's inverse sin (arc-sin) function, we find that this is exactly 13.0°, so Ben Dale's marking is very precise (as we would expect!). Here is an image of the Edge Pro set to 14°, with a post height of 66.3 mm. Again, I am taking my measurement from the top of the block in which the pivot post is seated, to the top of the block at the top of the pivot post.

(see photo 3: 5580)

In order to calculate in advance the amount that the pivot post needed to be raised or lowered to change it by one degree, I measured the distance between the most extreme positions marked, 52 mm, between the 13° and 25° marks, and divided 52 mm by the 12° difference to get 4.33 mm/degree of change. I proceeded to lower the post by roughly that amount, take measurements and calculate to verify that these values would hold for all the angles between 20° and 7° (with the pivot post all the way down, I measured 7.16° as the lowest position the stock Edge Pro Pro can assume).

III. Angle settings

These are the calculated, and verified, pivot post heights for each angle, in millimeters:

7° 36.0 mm
8° 40.3
9° 44.7
10° 49.0
11° 53.3
12° 58.7
13° 62.0
14° 66.3
15° 70.7
16° 75.0
17° 79.3
18° 83.7
19° 88.0
20° 92.3

After cutting a riser block out of 9 mm thick plexiglas (48 x 74 x 9 mm), I was able to determine additional post heights for 5° and 6°, which is as low as I need to go for any of my current knives.

5° 35.0 (with 9 mm thick riser block on blade table)
6° 39.5 (with 9 mm thick riser block on blade table)

(see photo 4: 5583)

Note on marked angles. As has been discussed before, while the "marked" or calculated angles listed here are quite accurate, they don't take into account the fact that almost all blades taper in thickness even before the primary and secondary bevels are reached. If a blade, in cross-section, was a rectangular block sharpened only on one edge, then the marked angles would truly describe the actual included cutting edge angle. However, as the bodies of most blades are also wedge-shaped, the actual included angle will be several degrees more acute than the values listed above. I wonder, for example, just what the actual angle of my Takeda bunkabocho is, since it tapers radically, in addition to being sharpened at the 9° angle setting. I thought it was sharp when I received it, but it's now the one knife that my fingers won't slide on when doing the Murray Carter three-finger test!

IV. Adjustments and stone/blank thickness

I have still not used the Edge Pro enough to feel truly expert with a whole variety of knives, but after doing these calculations and studying its construction carefully, I came to a few additional conclusions that may be useful to other users.

1. Fine angle adjustments. Using the marker technique to color the bevel, we all know that small adjustments in raising or lowering the pivot post can make the difference between polishing the bevel or adding a micro-bevel to the cutting edge. I now know that changing the post height by 4.3 mm means changing the sharpening angle by 1°, which means that a 1 mm change equals about 1/4°, a very fine increment indeed.

2. Stone/blank thickness. Using a dial caliper, I measured the thickness of my current set of Edge Pro stones and polish tape blanks, and was surprised at what I discovered. I lap my stones frequently on DMT Dia-Sharp plates when I am sharpening, and so they're quite flat (I also check them periodically against an accurate engineer's square). The thickness, however, varies between stones by as much as 1.5 millimeters, with the thickest at 7.3 mm. The polish blanks are substantially thinner, as low as 5.8 mm, and ranging up to 6.4 mm. Seeing how different the tape blanks are one from the other, I am going to drop them off at a machine shop next week and have them all milled to the same thickness so that once I'm polishing I won't have to make any further angle adjustments unless it's for a specific effect (the difference in the thickness of the PSA tapes themselves ought to be negligible). That will mean lapping them again, but the consistency will be worth it.

As for my stones, I now know that as they stand — and they don't change thickness very fast — I'll need to adjust my pivot post height up and down with each stone change so that I'm hitting the bevel at an equivalent angle at each grit. The rule here is that as stone thickness decreases, the effective sharpening angle increases, which is why micro-bevels are formed by raising the pivot post and stone arm slightly (raising the post increases the angle).

This leads to a simple rule for stone thickness: if you are moving from a thicker to a thinner stone, lower the pivot post by the amount of difference between the stones. Example: my MF (medium fine, 220 grit) stone is 6.9 mm thick. My EF (extra fine, 320 grit) stone is 6.4 mm thick. When I'm ready to change between these stones, I need to lower the pivot post by .5 mm to compensate.

I still have some questions about changing guide clip bolts, guide clip lock bar clearance and raising the guide clip when using thick riser blocks on the "Pro" Edge Pro, but I'll save those for another post.


--- Christopher


Journeyman KnifeNut!
Edge Pro, angle calculations
05-06-07 15:49.05 - Post#1221969    

    In response to C_Campbell

Nice work. I learned a few things in a quick read. I'm gonna read it again later.

Edited by matt321 on 05-06-07 15:49.37. Reason for edit: No reason given.
Member KnifeNut!
Edge Pro, angle calculations
05-07-07 10:00.28 - Post#1222390    

    In response to matt321

Whoa nelly, thats more then I wanted to know before breakfast<^((((>=&l t;

Edited by Lockguy on 05-07-07 10:01.26. Reason for edit: No reason given.
Master Member KnifeNut!
05-07-07 19:09.13 - Post#1222863    

    In response to Lockguy

Christopher, that is one excellent post. I'm just starting to work with the EP and this is an excellent review of your use of this device.

Nubatama Stones, Diamond/CBN Sprays
Natural Stones

Ken's Corner

Master Member KnifeNut!
05-08-07 04:43.10 - Post#1223127    

    In response to ken123

Great post Christopher.
One of the really nice things about the EdgePro is it's ability to remove the absolute minimum of metal every time it is used to re sharpen and keeping accurate measurements on each blade certainly helps.

Journeyman KnifeNut!
Edge Pro, angle calculations
05-08-07 05:33.14 - Post#1223163    

    In response to Nosmo

Thanks all, I'm glad the information is proving useful. For a relatively simple device, the implications of various changes in how one sets up or uses the Edge Pro are surprisingly complex, which helps explain why there are already so many helpful threads on its use!

I do like the fact that with just two measurements, the Edge Pro gives us a means of establishing an entirely repeatable sharpening set-up for each knife. It is interesting, however, that a number of people have remarked on the fact that a blade doesn't seem to reach its highest sharpness until it's been through a few use and sharpening cycles (The Tourist mentioned this phenomenon again with his SnG just last night). I've hardly begun to experience this because I have seldom worked a blade up through all the polishing tapes to anything like an ultimate edge, but I'm looking forward to seeing it for myself. With good records and patient technique, I expect that I won't be needlessly grinding away unnecessarily, just refining a geometry that's already established.
--- Christopher

Journeyman KnifeNut!
05-08-07 14:51.33 - Post#1223771    

    In response to C_Campbell

"I now know that changing the post height by 4.3 mm means changing the sharpening angle by 1°"

Being curious, I did some quick post height measurements on the Apex for comparison. Based only on measurements of the post height to the marked angles (without confirming the actual angles) the Apex varies from about 3mm per degree to about 5mm per degree. It isn't linear.

Chico Buller
Master Member KnifeNut!
Edge Pro, angle calculations
05-08-07 19:27.05 - Post#1223967    

    In response to matt321

  • matt321 Said:
It isn't linear.

And that's not the only issue.

"As for my stones, I now know that as they stand — and they don't change thickness very fast — I'll need to adjust my pivot post height up and down with each stone change so that I'm hitting the bevel at an equivalent angle at each grit. The rule here is that as stone thickness decreases, the effective sharpening angle increases, which is why micro-bevels are formed by raising the pivot post and stone arm slightly (raising the post increases the angle)."

Due to manufacturing tolerances, and the wear factors of the stones and mounts, the thickness of the abrasive on the arm meets the bevel with slight differences. But I find the change is very different and sometimes radical.

I notice this most often as I polish. For example, I can be using stones to actually remove some degree of metal without much of a problem.

(That is, unless I'm using a very worn 180, and a very new 600.)

However, when I switch to the polishing tapes, I notice that I hit the bevel 'high,' and I must adjust the arm 'up' on a Pro model.

While I have adjusted the relative angle of the fixture (changing the degree of attack), I am hitting the bevel at the correct and accurate angle of engagement.

I consider this like "adjustable shock absorbers." I want the car to ride level, so when weight is added, I add spring tension, oil or air to bring the vehicle level once again.

In other words, my setting must be changed to return adjustments to a relative zero. You do the same thing when adjusting a rifle scope when you change reloads.

I consider this a very key issue in terms of repeatability. If I like the performance of a knife at 20* when sharpened with new stones, I might have to re-adjust the arm to 21* as the stones wear and thin.

Changing one element changes the equation. We say that water boils at 212* as a uniform point of reference. Not so in the higher altitudes of Denver.

"Dwade is a good sharpener--for a Canadian."

See there, with the change of info, you knew exactly what I meant...

Edited by The Tourist on 05-08-07 19:29.35. Reason for edit: I needed to tease Dwade.
Chico Buller
Master Member KnifeNut!
Re: Edge Pro, angle calculations
05-08-07 19:49.34 - Post#1223973    

    In response to Chico Buller

In re-reading my last post I want to make it clear that I do not disagree with C_Campbell. I simply use this 'angle of attack' as a daily recognition of wear. Wear to the stones, and wear to the knife.

And I consider 'wear' and Q/C to be major factors here--I do lots of very hard knives. That is a personal comment, and not meant as an insult to Campbell's fine treatise.

If I was going to pack my Harley with heavy luggage and plot a course down mostly Interstate highway, I would add several pounds of air to each tire. The level of the bike would change, and the heated tires would continue to change over time.

My point is that the initial angle is seemingly less important than the relative constant angle of working performance.

In figuring springs and dampening to motor vehicles, they often refer to 'unsprung weight.'

But I agree with C_Campbell on his premise, and I compliment Ben on his Q/C in keeping the relative working tolerances as close as they are.

Consider the tens of thousands of stones, individual Edge Pro fixtures and the ham-fisted amateur blade grinders who are making a concerted effort to destroy his machines. It's a wonder anything ever gets sharp.

Having said that, I wear out a lot of stones--and black magic markers.

Edited by The Tourist on 05-08-07 19:53.15. Reason for edit: No reason given.
Journeyman KnifeNut!
Re: Edge Pro, angle calculations
05-08-07 23:49.27 - Post#1224031    

    In response to Chico Buller

Tourist, I think that the way you are thinking about the fine adjustment of the "angle of attack," i.e. as a means of keeping a constant angle between stone and steel in a system with lots of variables, is truly clear and useful.

I had an interesting experience recently in teaching my father how to use his Edge Pro Apex. I had given him my unit when I decided to upgrade to the Pro model, but he hadn't gotten around to using it much, and wasn't clear on how to use it well. The process of teaching him how the various settings interact, and trying to help him refine the pressure he was putting on the arm (too much!), the degree to which his stroke stayed within the left/right confines of the blade table (not!), the importance of the gentle stability of the grip on the knife to keep it settled during sharpening, made it clear why we see occasional posts from users who are having trouble getting good results from the Edge Pro.

The Edge Pro is beautifully designed, and fabricated to very fine tolerances, but like any good tool it really is only a beginning, and the journey to its mastery is rather longer than one might imagine. Even sorting through the various posts on the subject on this forum is quite a job, and that's before we take into account the ways in which the most experienced sharpeners have personalized the process: "sneaking up on the edge," etc.

In the end, the point I tried hardest to impress on my father was the degree to which he needed to think about the Edge Pro as a mechanical system in which he needed to keep his input as consistent as possible. The machine holds its settings, but he needed to develop his: to practice slowly and build a knowledge into his muscles so that the left hand and the right hand effectively mirrored each other, to keep his touch delicate and controlled, to record his angle and guide settings so that he could return to them.

Matt321's point about the linearity of the Apex is an interesting one. I still have my father's Apex here, so I'll make a few measurements later this week and see if I can come up with a some equivalent numbers for pivot post height vs. sharpening angle.

I laughed at the reference to wearing out a lot of black magic markers. There is a certain irony to the fact that one can work out the mathematics of degree changes to several decimal points, but all that counts in the end is seeing what happens to that sliver of black marker on the edge!
--- Christopher


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