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An interesting way to make a multi tooth form cutter

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  • nickel-city-fab
    replied
    I remember seeing something like this mentioned in Ivan Law's book, but it was only a few sentences and then he got into the Eureka mechanism as a tool to do the same job. The first time I read it (last year) I kinda "got it" and then I tried to read it again... fuzzle... I decided that I actually have to *study* the book, because it's heavy reading. Might be the best 12 bucks I ever spent though, just for brain exercise.

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  • Ringo
    replied
    BCR, thanks for a better drawing,

    I looked at post #58 and thats on page 4 and the whole page is good pictures.
    shows how he offset the ellipse by taking a flat spot on the mandrel shank, and it offsets itself in the chuck jaws. cool

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  • alanganes
    replied
    Our old friend Sir John Stevenson wrote about this method here and on the old rec.crafts.metalworking usenet group years ago.

    The RCM article was sort of archived here:

    https://metalwebnews.com/howto/gear/gear1.html

    A bit of diligent searching on this forum should turn up his posts as well.

    Edit: OK, found it. Lots of related discussion in the thread, it's worth reading through. If you are in a hurry, the good part with photos starts at post #58

    https://bbs.homeshopmachinist.net/fo...involute-shaft
    Last edited by alanganes; 11-20-2020, 06:31 PM.

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  • BCRider
    replied
    Ringo, this might help. I did up this sketch to clarify it in my own mind for the future as much as hopefully help you.

    The center pivot bolt does not need to move. The cutter blank rotates around it and we only need indexing pin holes for each. What is needed though is to cut deeply enough that the lobe we cut away covers an angle that is at least the proper angle for the number of teeth. In this case 4 so we need to cut deep enough to cover at least 90°. Otherwise we leave a little land of the original dimeter. Not really a big deal as long as we cut it away later. But still untidy.

    With the 4 positions cut we get the shape in the first of the smaller examples. Then it builds from there.

    I drew myself into a corner with this one as you can see where the indexing holes end up partially left. Needless to say this would complicate the last cut. But I wasn't about to go back and re-do the drawing to offset the indexing holes. This just shows why it's a good reason though to do this up ahead of time on paper or CAD and avoid such a pitfall.

    But provided it would let us mill it this way we could still use the pins to locate the lobes and the front cuts and end up with a nice disc cutter. Just need a suitable slitting saw style arbor to use with the cutters that can be made in this manner....

    Click image for larger version

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  • J Tiers
    replied
    Originally posted by Ringo View Post
    I had to draw this with my morning coffee, I can see it, but, not the way clickspring did it.
    he never relocated the hold down bolt, how he did that?
    I understand my picture, but it moves the hold down bolt each facet
    how he maintain the bolt to lathe center?

    ....................
    The hold-down bolt is off-center on the arbor, by the amount needed to produce the larger radius for the relief.

    So it was producing the larger radius already, and all he needed to do was change which part of the tool that radius was produced on by rotating the tool.
    Last edited by J Tiers; 11-20-2020, 10:25 AM.

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  • TGTool
    replied
    IIRC John Stevenson showed that the difference between the two button method and the true involute form only amounted to tenths in the gear sizes we're working with.

    There are also several strategies for developing the cutting clearance. If you're making a tool with two hardened steel buttons the right diameter and the right spacing, you can provide the clearance by slanting the face where the buttons are installed. That compromises the circular form slightly but allows resharpening without losing any button diameter. The more theoretically correct tapered button installed on the flat means that it must be ground very carefully to the correct diameter and can't be resharpened without becoming a smaller circle.

    In one of his last iterations of making a fly-cutting tool with the gear form, he used a tapered end mill as used in the mold building industry. A washer with the target inside diameter was pushed onto the tapered end mill to get the right profile on his fly cutter with necessary clearance. A clever solution.

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  • Ringo
    replied
    I had to draw this with my morning coffee, I can see it, but, not the way clickspring did it.
    he never relocated the hold down bolt, how he did that?
    I understand my picture, but it moves the hold down bolt each facet
    how he maintain the bolt to lathe center?

    Click image for larger version

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  • Ringo
    replied
    I still don't get it.
    Cutting the one-button or two-button form ,,,,,,I get that part.
    I don't see the mandrel offsets of the elliptical portions.
    there is a threaded hold down bolt, and a corresponding pin locations, on the lathe, this cuts the ellipse
    but,
    how did the pin locations align with mill setup to cut on the apex of the ellipse?

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  • J Tiers
    replied
    Well, you can take a look at a gear cutter, or a hob sometime..... or look at one of the cutter grinding booklets put out by Norton and others years ago. The gear cutters have a curved relief, and can have a rather low angle of relief directly behind the edge.. Many cutters with ground relief have only 3 to 4 degrees relief, others may have 10, it just depends on the material being cut as to the optimum relief.

    You will find that is not a practical limitation.

    Yes, the amount of relief varies with the displacement of the circular relief center from the cutter center.

    They are not really visible angles. At the radial face of the cutter, the relief angle is really the difference between the tangent line of the cutter OD at the radial face and the tangent line of the relief circle at the same point. There is no flat surface as with a cutter where the relief is a flat ground with the wheel. These sorts of cutters are ground on the radial face.

    For instance, in my example cutter above, that difference is 7 1/2 degrees, even though there are 8 cutting faces. The relief is then 7.5 degrees, which is a medium amount of relief, as such things go.

    The connection to feed rate is obviously that you cannot feed faster than the relief "falls away". That limit is where you would get a rub. But that is only a theoretical limit, and theory falls down here.

    In actual practice, the feed rate is far less, and is expressed in thou per tooth, or "chip load". A rate of 5 thou per tooth is pretty large for many cutters, and takes a goodly amount of power. To illustrate the reason why the feed rate is low, and generally will not come close to "following" the relief, look at the example.

    The illustrated cutter has a relief that puts its surface at 0.031" below the OD at the point where the cutaway for the next tooth starts (about halfway between tooth faces). To get into trouble with that would require a chipload of 1/16" per tooth, which is way beyond the capability of the tool or the machine it would be used on. The corresponding rate of advance would be a half inch per turn, and at an SFM of 100ft/minute, feed rate would be away up at 80 inches per minute on the 2.5" diameter cutter. Not gonna happen.

    No, the true limit on these cutters is going to be the chipload and allowable SFM, you are not going to get in trouble at any reasonable, meaningful number of teeth And, of course, as the number of teeth goes up, even if the allowable chipload per tooth decreases somewhat, the allowable feed likely goes up as well.

    The method of making a multi-tooth formed cutter looks like it will remain practical up to at least 10 or 12 teeth. Above that, you do not have a formed cutter at this size, you are starting to have a saw.😁
    Last edited by J Tiers; 11-20-2020, 02:31 AM.

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  • Paul Alciatore
    replied
    J Tiers said,
    For a cutter, the relief need not be that large, it just needs to not rub. That may depend on the exact shape of the cut profile. As long as there is "draft" on the profile, then theoretically, "any" amount of relief is OK so long as there is definite relief.
    That is not completely true. The angle of the relief is connected to the rate that a cutting tool can be feed into the stock. Generally speaking, lathe tools with relief angles around 5 degrees or even more will have enough relief for almost any feed rate that it makes sense to use. So we do not worry about it so much with them. We just grind them and happily use them to make chips.

    But when using this method for making a multi-tooth cutter, as you increase the number of teeth, that relief angle decreases due to the geometry. With enough teeth, it could easily drop below one degree and the cutter may start to rub if it is fed into the work quickly enough. What happens is the angle of the ramp that the cutter is cutting into the stock becomes greater than the relief angle and the two will rub. The relief angle must be greater than the ramp angle of the cut. So, when the relief angle starts to become relatively small, you will have to slow down the feed rate.

    Sure you could make a 20 or 30 tooth cutter with this technique, but it would be difficult to use.

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  • Paul Alciatore
    replied
    Basically he was using the button (circular cutter) method. He just did it one side at a time in stead of both sides at once. So he only needed one round lathe tool. And that tool was at an angle, so it was converted to an eclipse. That may be more suited for cutting cycloidal gears. I have wondered about using a round button or cutter like his for making gears with involute teeth. There are errors with the two button method because the buttons are round and the gear teeth are not. I think a carefully calculated ellipse could be a closer fit to the involute curve.

    The other thing he was doing was to offset the cutter blank and then cutting circular arcs four times around it's outer edge. Then he cut a bit over half of each arc off with the milling cutter. That created the needed clearance with the offset arcs and the rake angle of the cutting edge with the milling cutter. This method has been used and explained before. It only works for a small number of teeth on the cutter you are making. If you tried to make a cutter with, say, 12 teeth, it would not work so well. Five or six teeth may be the practical limit. The four tooth cutter he was making is a good number for this technique.



    Originally posted by Dan Dubeau View Post
    I'm always impressed at his level of finishing. That's an area of mine that is severely lacking. The things I machine just never need that level of finishing, so I never bother. Not even sure I could if I wanted too.

    Using his fixturing would be pretty straight forward with the button cutter method no?

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  • J Tiers
    replied
    Actually, it can be done with any number, but the total relief gets smaller with the number. So he probably figured 4 was enough. For a cutter, the relief need not be that large, it just needs to not rub. That may depend on the exact shape of the cut profile. As long as there is "draft" on the profile, then theoretically, "any" amount of relief is OK so long as there is definite relief.

    There is probably an optimum radius of relief for any size cutter.

    Here is one with 8 edges (I do not show any profile, but it could be anything



    And the geometry of the relief. The size of the hole and its location is arbitrary as long as it is outside the center hole of the tool being made. Here it just indicates the actual center of rotation. An actual hole at that location could be used to index, I suppose, if the center hole of the cutter is offset on an arbor similar to clickspring's. Probably a different location would be better.
    The radius of the relief is not a number that is needed, the 0.625 is sufficient if 8 holes are drilled to use as index holes, and the offset is correct.



    Proof of actual relief

    Last edited by J Tiers; 11-20-2020, 12:28 AM.

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  • J Tiers
    replied
    He offsets the tool he is making such that the back relief is a circular arc, but the rest of the cutter does not intersect it, so he can just spin the whole thing. I don't think he could do it with 8 edges on the tool, which may be why he makes it with 4 only. I'd have to look at it more carefully to prove that, though, so he could just consider that 4 is enough.

    The usual "relieving system" just moves the cutting tool in and out with a cam, so that the tool being made spins on its center, and the cutting tool moves in and out to form the back relief. That can be done with as many cutting edges as are considered to be needed.

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  • Ringo
    replied
    I just browsed Law's book, I didnt see that cutter mandrel
    Can someone help me out?
    I understand Clickspring turning the eccentric, but, I cant see how the cutter back clearance reflects to the correct amount of eccentric.
    That looks like the single button method he used, just a different way to hold the 'button' or tool

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  • nickel-city-fab
    replied
    IIRC the method that he used is described briefly in Ivan Law's book, but it still takes quite a bit of skill and imagination to properly pull it off like Clickspring does. I seem to recall also that he teaches engineering, so that certainly helps.

    That video was beautiful, in that it describes things clearly with nothing extra. I am sure the fantastic finishes he gets are due to having a very rigid setup compared to the size of the parts, and also to spending some time with the honing stones on his tools.

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