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Measuring grinding angle question

Discussion in 'Woodturning Discussion Forum' started by Randy Anderson, Nov 18, 2020.

  1. Randy Anderson

    Randy Anderson

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    Getting my new Robo Rest grinding platform (which I really like) brought this to my mind again. I'm sure this is clear to many of you and makes you wonder how someone who has been at this for a while can still need clarification but - here I go. When folks say they grind their tools to a certain angle I've been confused over what angle they are referring to based on how they describe it. I'm not always sure they are talking about the same angle. I've included a crude pic to clarify my question. The confusion came to mind again because the Robo Rest does not measure the grinding angle the same as my little digital angle gauge. The digital angle gauge starts at 0 deg. The Robo Rest starts at 90. So, my little digital angle gauge is measure angle A while the Robo Rest is telling me angle B. If you use a digital angle gauge to set your platform it's easy to get confused between what it says and what your metal angle finder will say when put it in the flute and the arm put against the bottom bevel. I've seen videos of guys saying "set your platform to XX deg" using a digital gauge which is different than the way I think most think of the angle they want.

    So, when a turner says they grind to XX deg my assumption is they are talking about angle B in the picture. When they say the included angle like on a NR scraper is XX deg I assume they are talking about angle A, doubled of course.

    Pic - handle to the right, horizontal dash line is the bottom of the flue or center of a NR scraper.
     

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  2. Dennis J Gooding

    Dennis J Gooding

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    Assuming that in your sketch the flute is up, then the grinding angle is A.
     
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  3. Randy Anderson

    Randy Anderson

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    Yes, flute is up but, now I'm more confused. Angle A is what my digital gauge says. Angle B is what my Robo Rest says (yes, they add up to 90 deg). Articles I've read on how to sharpen a gauge refer to angle B as the one they are speaking of.
     
  4. hockenbery

    hockenbery AAW Advisor Staff Member

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    Angle A is the angle of the cutting edge.

    Anyone referring to a 30 degree bevel on a spindle gouge is talking about angle A.
    Same for an 80 degree bevel on a bottom of the bowl gouge.
     
    Last edited: Nov 19, 2020
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  5. Emiliano Achaval

    Emiliano Achaval Administrator Staff Member

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    I have never seen anybody measuring or talking about B. If you use a protractor, it becomes clear what you have to measure.
     
  6. Timothy Allen

    Timothy Allen

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    Reed has said that you cannot rely on his rest for accurate angles (he has discovered it is about 5 degrees out from what he had intended), BUT it *is* very precise (i.e. highly repeatable). Once you determine the setting for the angle you like, you can go back to that setting quite easily.
     
  7. Timothy Allen

    Timothy Allen

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    Another thought to keep in mind: With Reed's roborest, the pivot point for the rest is right at the face of your grinding wheel. IF (big if) the rest is installed at exactly the right height and distance from the wheel, the angle of the platform would (theoretically) always be relative to the vertical tangent of the face of the wheel. With the Wolverine rest (for example), the pivot point for the platform is located away from the wheel, so as you angle the platform, the front of the platform rises up on the face of the wheel, changing the tangent on the wheel that you are measuring your grind angle relative to. Thus there is a changing relationship between the angle of the platform and the angle of your grind over the range of adjustment.
     
    Last edited: Nov 19, 2020
  8. Roger Wiegand

    Roger Wiegand

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    Perhaps you're conflating the angle of the platform with the angle produced on the tool-- one should be the complement of the other, ie if you set the platform at 0 degrees (horizontal) your tool will be ground at 90 degrees.
     
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  9. john lucas

    john lucas AAW Forum Expert

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    most of us turners refer to the grinding angle as A. Machinists refer to B. I've had this discussion with my friend who is a turner and master machinist. As long as well agree on one or the other it's OK so we can have a discussion and understand what we each mean.
     
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  10. Randy Anderson

    Randy Anderson

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    John, your statement helps me think I'm maybe not an idiot. I've always thought angle between top (inside bottom of flute) and heel was it, set my digital gauge to that number and away I go. All works. If I wanted a 70 deg included angle on my NR scrapers I set it to 35, ground both sides and all was good. If I want 55 deg on my gouge I set and moved on. Then I set my robo rest in place and hang on - it considers level as 90 deg. I get complementary angles and can figure out how to get it done, just want to make sure I was speaking the same language as I read and hear. Just need to remember that robo rest gives me complementary angle, angle to face of wheel, etc vs platform angle to level. Not hard. That said, if you look online it's fairly easy to get a bit confused based on how folks draw their angle lines, speak about setting their platforms, etc. Even a few threads here on the topic. When I did a search here last night I didn't notice one that I see this morning - sorry for starting over on a covered topic.
     
    Last edited: Nov 19, 2020
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  11. William Rogers

    William Rogers

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    I use the Robo Rest, however I really don’t care what angle my tool measures. For my roughing gouges I set it to 45 degrees on the Robo Rest and I like that setting. Is it 45 degree, I really don’t know or care. Seems we get hung up on a protractor reading vs what we like. I have skews that I grind to different setting on the Robo Rest for a variety. If it is ground to the 25 degree setting I know it will cut nicely, but not stay sharp as long as one ground to the 35 degree setting. Are they really 25 and 35 degree? Haven’t a clue.
     
  12. Doug Freeman

    Doug Freeman

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    The actual tool bevel angle is only important if you wish to communicate to someone what angle you use.
     
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  13. hockenbery

    hockenbery AAW Advisor Staff Member

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    I both agree with you and disagree with you.

    a few degrees off is fine.
    However gouges and most turning tools will not do certain jobs unless the angels are close to what is needed for a particular use.

    Ellsworth grind nose is 60, leading edge of the wing 40-45, wing 25-30. Change by 5 degrees and the tool will not cut as effectively when doing the roughing cut, pull cut, flute up shear cut. Push cut..

    spindle gouge with a 30 degree bevel can do details that a spindle gouge with a 45 degree bevel cannot do and will leave a cleaner surface.
    Anywhere between 25-35 will work about as well as the 30

    a bottom of the bowl gouge with an 80 degree bevel can cut inside bowl shapes that a 60 degree bevel cannot do.
    A 60 degree bevel can cut inside shapes that a 40 degree bevel cannot do.
    75 will work about as well as 80. Get much about 80 and it won’t cut very well.
     
    Last edited: Nov 19, 2020
  14. Mark Jundanian

    Mark Jundanian

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    You're not an idiot, this is a complicated and confusing question. I agree with all the answers you've gotten so far, but I'll hazard another; if you already understand than feel free to ignore this one.

    I can't speak for machinists, but in your diagram the angle A is the "grind angle" and the angle that matters to woodworkers. For a single bevel tool (best to leave out NRS for a moment) it is the angle whose apex is at the cutting tip and is formed by the intersection of a line drawn along the bevel with the line drawn along the long axis of the tool (e.g. the floor of the flute). It represents the cutting edge as "seen" by the wood. But when any two lines intersect two angles are formed, one will be 90* or less (acute) and the other will be 90* or more (obtuse). These two angles always add up to 180*.

    So imagine for a moment that you have a Robo Rest grinding platform that is set up perfectly and you have the platform set for the 60* setting. A line drawn along the surface of the platform will intersect the surface of the wheel at a point. A tangent line can be drawn extending from that point of contact up toward the ceiling, and the two lines form an angle, but that angle is clearly not 60*. In fact it's 120*. But our tangent line also extends down from the point of contact towards the floor, as well, forming an acute angle. This angle is 60* and represents the grind angle. Thing is this angle is out of sight, harder to think about and as a practical matter inaccessible to measure, but it is the pertinent angle. I find it helpful to look at the grinder from the side and try to imagine seeing through the platform to the underside. So if you were making setup blocks for your grinder platform you would actually make your "60*" setup a 120* angle and your "35*" block a 145* angle.

    For a double bevel, negative rake scraper the included angle is the sum of the top and bottom grind angels so that could be 10 + 60, or 35 + 35.

    But what if what you can easily measure is angle B in your diagram. A + B = 90*. So you will have to subtract B from 90* to get the value of A. (Though I confess, off hand I'm not sure how one would measure B directly).

    Sorry if I've beaten the dead horse, here.
     
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  15. Randy Anderson

    Randy Anderson

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    Thanks to all. I feel better now. I'm back to my original understanding of how turners measure angles on tools. I don't intend to get crazy with gouge angle analysis and changes but part of the reason for getting robo rest was to try a few different ones and see if I noticed a difference. Until now I've stuck with one angle for consistency and changing the platform wasn't always consistent. I have more gouges now so maybe different ones for different tasks is something I could adapt to.
     
  16. robo hippy

    robo hippy

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    Some how, I don't think more coffee will help me get my mind around all this geometry.... I remember hearing a long time ago that machinists measure the bevel angles differently than we turners do. As near as I can tell, grinder platform angle setting B yields tool bevel angle A. At least I think.

    I think part of the reason the angles are not dead on are because of tool thickness. You will get a slightly different angle on a 1/4 inch thick tool than you will on a 1/2 inch thick tool. I put a piece of 1/2 inch thick plywood on top of the platform once to see what it would do. It made the grinding angle about 5 degrees more acute/pointy. I am guessing that if you set the platform about 1/4 inch below the center of the wheel you will have the angles match a bit closer as you get down to the 30 to 40 degree settings than you get if the platform is set to exact center height.

    I will never forget after learning the 40/40 grind how when sharpened on my platform at the 40 degree setting, it just felt too pointy, and I used the 45 degree setting. It felt/cut a lot better at that setting. I eventually compared it to another turner's 40/40 grind and they were almost identical.

    robo hippy
     
  17. Mark Jundanian

    Mark Jundanian

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    The geometry can be a bit much. Sometimes I have to resort to actually drawing on paper. Just to clarify, @robo hippy , what I'm saying is that to get a grind angle of A, rather than set the platform to angle B, you set your grinder platform to an angle equal to 180 minus A.

    I think this is absolutely true as is the set up. For me the #4 position is closer to 35*, but my platform is 1/8" low. How close the platform is to the grinding wheel will also have an effect.

    But at the end of the day if your setup is consistent your results will be, too.
     
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  18. hockenbery

    hockenbery AAW Advisor Staff Member

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    To repeat a grind angle I just put the bevel on the wheel and adjust the platform so that the tool resting on the platform makes full contact with the wheel.
    Bright light look from the side
    No light coming through it’s on the wheel.
    Light coming through top or bottom I adjust the platform.

    if I want to change the angle I adjust the platform.

    i have a handy little plastic gauge from Tormak the has small angled slots for the different angles if I ever need to check one.
     
  19. Mark Jundanian

    Mark Jundanian

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    The Robo Rest settings already take the math into account. When I spoke of the platform angle being 180° - A, I was refering to setting up a freely positioned platform using a setup block. For example, the Stuart Batty system. Indeed he labels his setuo blocks 30/150, 60/120, etc.

    @Randy Anderson , in your specific example I would double check the Robo Rest installation. With the platform at the #9 setting a ruller laid flat on the platform should intersect the very center of the wheel. Then with the platform in the #9 position the center of the platform should be 1/8" away from the wheel surface. Small variations in these two parameters will cause significant changes in the grind angle.

    For me, I'm off 1/8" on the height and I have figured out that's a 5° error when I'm in the #3.5 or #4 positions.
     
  20. Randy Anderson

    Randy Anderson

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    Thanks Mark, good to know it makes that much difference. I'll check. I just starting using it and since my grinder stations are set up I wasn't sure where it would be used most. On the most likely grinder I'm a good 3/8" high so need to raise the grinder up to match.
     
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  21. RichColvin

    RichColvin

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    Randy,

    The angle you have as A is known as the bevel angle, and 2x A is the included angle.

    DB61DA9C-0021-4D5E-B785-CA0CBC2B44E1.jpeg

    Kind regards,
    Rich
     
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  22. Doug Freeman

    Doug Freeman

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    you missed the point. In the context of the message, Mr Rogers is happy with how his tools work, therefore not knowing the angle is not important to him, yet it is if he wishes to communicate to someone else. How do you tell him he does or does not conform to your described angles for tools(I dont disagree with them)? He has to measure them in order to even have a discussion.
     
  23. William Rogers

    William Rogers

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    Doug, this is in context to using the Robo Rest as a guide. I use the Robo Rest and for instance I grind one skew at the 25 degree setting. I have no idea if it is really 25 degrees or maybe 23 1/2 degrees. I don’t care, it is how I like that tool. I just use the settings on the Robo Rest and don’t worry about the precise angle. With all the 40/40 talk I spent time grinding my bowl gouges to match the 40./40. I use the Hannes vector jig and feel it grinds the 40/40 truer than using the Various-grind IMO. But after doing this I didn’t like the 40/40. I did measure my previous grind and it was 45 degrees that I prefer. I determined this by trial to see what I liked best. I do know my angles within some degree of certainty, but my tools are the way I like them. I use stop block with the Hannes jig just as one would do with the Various-grind, so my grinds are consistent.
     
  24. Russ Braun

    Russ Braun

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    Whew!!
     
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  25. Dean Center

    Dean Center

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    Randy,
    It looks like the included angle on the tool you show is >90 degrees, probably 100 or 110. You have the platform set at 55 degrees, and it appears you are grinding angle A at 55 degrees. When you double that number you come up with 100 or so degees, which is consistent with the appearance of the tool.

    BTW, if that's a NRS, you have a total angle that is blunter than most of the forum members have recommended in the past (60-90 degrees seems to be the popular range).
     
  26. Randy Anderson

    Randy Anderson

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    Dean, it was less than 90 (barely) but not near as much as it should be - part of reason I got a new rest is so I could get some consistency on my edges. Turns out it pointed out an error in how I was going about it. You're right re my statement on using 55 to get 35 - I had been going round and round to make sure I was thinking correctly and stated it wrong. The 35 deg (90-55) would be from tool center up to level, NOT tool center back to vertical. Duh... Figured everyone else was correct and knew what they were doing, I was the dense and confused one and let it be. They probably wondered how I knew which end of a gouge to use. Odd how something this simple can get twisted in your head till you sit down and draw it out and look at it. I do angle work a lot in my flatwork builds and seldom get this far into the weeds on confusion. Once I locked my mind on a wrong reference point I was in the ditch - I'm ok now I think. Net, to get a 35 deg bevel on an edge or a 70 deg included angle on a NR scraper set the rest to 35 - done.
     
    Last edited: Nov 23, 2020
  27. Mark Jundanian

    Mark Jundanian

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    Keeping in mind that a setting labeled 35 might be a little more or less.
     
  28. Randy Anderson

    Randy Anderson

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    Correct, don't really care if a few deg one way or the other, just consistent when I move it for my scrapers or hollowing tool bits, chisels, etc.
     
  29. Brian Deakin

    Brian Deakin

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    If I remember correctly from my college days We were taught the following
    Arcuate is about reproducibility Example you step on the bathroom scales and the weight recorded on the scale is 62.5 kg You then step on and off the scale 5 times and each time the scale states 62.5 kg The scale is accurate
    Precision is about how close the answer is to the true value If you were to weigh yourself on a superior scale the weight may read 62.547 kg ,this figure is a greater level of precision
    Areas were it matters examples weighing active ingredients when preparing medicines, weighing valuable items eg. Gold Manufacturing engineering components on a space rocket
    Areas where Precision is not needed ,weighing vegetables ,weighing coal, measuring rough lumber
    You need to decide which is important depending on the circumstances
    So to summarize Reed,s rest is accurate but not precise that is you can reproduce the same angle every time (it is accurate) but is an approximate of the true value
     
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  30. Mark Jundanian

    Mark Jundanian

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    Your explanation is close, but below is a Wikipedia article. It's probably TMI so here is an excerpt:

    "In the fields of science and engineering, the accuracy of a measurement system is the degree of closeness of measurements of a quantity to that quantity's true value.[2] The precision of a measurement system, related to reproducibility and repeatability, is the degree to which repeated measurements under unchanged conditions show the same results.[2][3] Although the two words precision and accuracy can be synonymous in colloquial use, they are deliberately contrasted in the context of the scientific method."

    https://en.m.wikipedia.org/wiki/Accuracy_and_precision#:~:text=In measurement of a set,the measurements to each other.
     
  31. Doug Freeman

    Doug Freeman

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    Wikepedia is right for once! It agrees with my 35 years of mfg engineering knowledge.
     
  32. Brian Deakin

    Brian Deakin

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    Thank you Mark for providing the correct explanation
    It has been 40 years since I attended the lecture where the words were explained and I should of have verified my understanding before posting
     

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