Sharpening rotary broaches
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  1. #1
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    Default Sharpening rotary broaches

    Is there a simple way to sharpen the concave tip of internal rotary broach? The tip is a concave spherical surface (attached an exaggerated picture) and the radius is different for different broach sizes. I assume this can be ground using a small ball grinding wheel (or a small small wheel with circularly dressed edge) while the broach is rotated against the side of the wheel, on centre. I have a Cincinnati No. 2 Tool & Cutter Grinder and a setup like this does not seem to be easy. I have as well a Deckel SO, but again, I see no way to do it on it.
    I realize that excessive grinding of the tip will reduce the broach size, but just touching it up will still keep it withing tolerance.

    square-broach.jpg

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    I would use an indexer/grind-all at an angle to a surface grinder wheel and rotate the broach while feeding towards the wheel to generate the surface, much as you have suggested.

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    You could spin the tool in a lathe and use a Dremel tool with a very small
    stone to sharpen it.

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    Does the inside corner really go to the mid section? I would expect the inside cornet to go towards the outside corner.

    Does the supplier offer sharpening?

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    Quote Originally Posted by michiganbuck View Post
    Does the inside corner really go to the mid section? I would expect the inside cornet to go towards the outside corner.

    Does the supplier offer sharpening?
    In all the internal broaches I have seen the front is a spherical concave form. Suppliers do offer sharpening service, but I do all may cutter sharpener myself as I find it much more cost effective - especially since I do have all the tools and accessories to sharpen ends mills, drills, slitting saws, etc. This is the first tool I am having some difficulties.

    Quote Originally Posted by Mr_CNC_guy View Post
    You could spin the tool in a lathe and use a Dremel tool with a very small
    stone to sharpen it.
    I am looking for a more permanent setup. Besides, on the very small broaches this will not work.



    Quote Originally Posted by Mark Rand View Post
    I would use an indexer/grind-all at an angle to a surface grinder wheel and rotate the broach while feeding towards the wheel to generate the surface, much as you have suggested.
    This could work for the big broaches but the small need a much smaller wheel than I cannot mount on my surface grinder (or on my Cincinnati where I do have all that is needed for this setup).

    I wonder if anyone knows how this is done in production?
    Last edited by Wlodek; 04-26-2019 at 12:16 AM.

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    QT: [In all the internal broaches I have seen the front is a spherical concave form. Suppliers do offer sharpening service, but I do all may cutter sharpener myself as I find it much more cost effective - especially since I do have all the tools and accessories to sharpen ends mills, drills, slitting saws, etc. This is the first tool I am having some difficulties.]

    I have never sharpened one and have not even held one.
    But did have an odd special inside grind that I had to turn the wheel head on an angle so it would match the intersect angle of one side..then grind only half way across the grind..do all edges that way with only half way,,

    then I changed the angle of the head for the other side intersect angle and grind that side for the other half with a grease pencil rub match..

    Yes standing the part near vertical but at angle that was a little short of the original grind angle. using a smaller wheel OD for the grind..

    Might try to mount a die grinder on the Cincy on a double tool finger bar so easy to change wheel angle/position.. use a small wheels diameter.. then mount the cutter on an angle plate with a bar so it might change angle easy also.

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    Hi Wlodek:
    The concave form can be ground with an internal grinding spindle using a smallish diameter cylindrical shank mounted point...exactly the same as you'd use doing normal internal cylindrical or jig grinding.
    The unique thing is that you need to mount the internal spindle at an angle to the axis of the workhead so the corner of the wheel can drop into the concavity on the face of the broach.
    It's a problematic grind in that you have a large contact area of wheel to workpiece, so you can't push it very hard, but that's how it's done.
    The principle is the same as the method used to hollow grind the faces of a slitting saw...that's why you can see the swirly grind marks on the hollow ground faces; the corner of the wheel is doing the grind rather than the sides or periphery of the wheel.

    So all you need to do is find a mounted point of a small enough diameter that you can fit it into the hollow when it's canted over at a suitable angle to touch both the rim and the center of the existing concavity at the same time.
    Line up the corner of the wheel with the axis of the workhead (or broach), and you will make the concave surface you want when you advance the wheel gently into the face of the broach while the broach is spinning in the workhead.

    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining

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    I believe this concave form can be accomplished using your Deckel SO grinder with the appropriately-sized diameter wheel and the broach approaching the edge from an angle.

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    Quote Originally Posted by implmex View Post
    Hi Wlodek:
    The concave form can be ground with an internal grinding spindle using a smallish diameter cylindrical shank mounted point...exactly the same as you'd use doing normal internal cylindrical or jig grinding.
    The unique thing is that you need to mount the internal spindle at an angle to the axis of the workhead so the corner of the wheel can drop into the concavity on the face of the broach.
    It's a problematic grind in that you have a large contact area of wheel to workpiece, so you can't push it very hard, but that's how it's done.
    The principle is the same as the method used to hollow grind the faces of a slitting saw...that's why you can see the swirly grind marks on the hollow ground faces; the corner of the wheel is doing the grind rather than the sides or periphery of the wheel.

    So all you need to do is find a mounted point of a small enough diameter that you can fit it into the hollow when it's canted over at a suitable angle to touch both the rim and the center of the existing concavity at the same time.
    Line up the corner of the wheel with the axis of the workhead (or broach), and you will make the concave surface you want when you advance the wheel gently into the face of the broach while the broach is spinning in the workhead.

    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining

    Yes, this is exactly what I do have in mind now. I am looking into making this setup on my Cincinnati No2. I do not have the internal grinding attachment, but can make a holder for a micro die grinder that should do the trick. A small mounted point like this will require quite a high RPM.

    Quote Originally Posted by Dumpster_diving View Post
    I believe this concave form can be accomplished using your Deckel SO grinder with the appropriately-sized diameter wheel and the broach approaching the edge from an angle.
    I did try this with a smaller wheel (although not small enough for the small broaches) and the problem is that geometry; when the tool head is tilted forward, the work ends up quite below the the axis of the grinding spindle. This setup will work for large broaches with larger wheel.


    Thank you all for your suggestions.

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    Hi again Wlodek:
    Not to derail the thread or anything, but whereabouts in BC are you located?
    I'm in North Vancouver, right near the Seabus terminal.
    If you're interested in meeting some time, my door is always open.
    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining

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    Pardon my ignorance. But why would you need to reshape the most difficult concave portion of that cutter? I would attack the outer flanks that would be much easier to regrind. Then refactor the size change during use. Please let me know and thanks for the edification.

    Best Regards,
    Bob

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    Quote Originally Posted by rjs44032 View Post
    But why would you need to reshape the most difficult concave portion of that cutter? I would attack the outer flanks that would be much easier to regrind. Then refactor the size change during use.
    Your Allen wrench may no longer fit the resulting hex with either end or flank grinding, but the size loss will be worse with flank grinding.

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    Hi rjs44032:
    It's the best way to limit the dimension reduction across the flats.
    The grind is not difficult; in fact it's the easiest and fastest to re-work provided you know how it was done and have the toys.
    Wlodek sounds pretty well equipped with both a Cincinnatti and a Deckel cutter grinder, so it sounds like he has all he needs.

    In addition, a rotary broach cannot be "refactored" for size so far as I know, assuming of course that I'm understanding the term as I believe you are using it.
    They will cut the size of their big end, and cannot be adjusted or compensated for, so the less you can take off the form, the better.

    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    www.vancouverwireedm.com

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    Ok, I think I've got it. Thanks guys. It looks like either way you are going to lose geometry. Perhaps chrome plating then regrinding the center would be the ticket. Cool stuff.

    Best Regards,
    Bob

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    Quote Originally Posted by rjs44032 View Post
    Ok, I think I've got it. Thanks guys. It looks like either way you are going to lose geometry. Perhaps chrome plating then regrinding the center would be the ticket. Cool stuff.

    Best Regards,
    Bob
    One can actually sharpen the face quite a number of times without loosing much. Here is a typical 3 degree 0.25" square broach as new and sharpened by removing 10 microns (about 0.0004") - typical touch up to get it sharp, assuming it is not broken or damaged. As you see it will reduce the size by only 0.6 micron (0.00002"), an insignificant amount and still much better than the initial tolerances. Even ten times as much will still not make the broach out of tolerance.

    square-broach.jpg

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    Quote Originally Posted by implmex View Post
    Hi again Wlodek:
    Not to derail the thread or anything, but whereabouts in BC are you located?
    I'm in North Vancouver, right near the Seabus terminal.
    If you're interested in meeting some time, my door is always open.
    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining
    Thank you, Marcus, for the generous offer. I shall certainly would like to take you on it when on the mainland (I am near Courtenay) and maybe will send you a PM when planning this. I would like to offer to host you as well.

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    I have made several rotary broaches using a tool post grinder to get the necessary relief on the end of the broach. Putting the limitations of the material I used aside, they worked OK. Any reason not to use the same technique to sharpen a commercially made one?

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    Not pro and not something to be proud of, but the little 1/8" ish wide 3/4" ish dia grind stones and a dremel after dressing the wheels has worked for me on several broaches in the 1/4" size range both square and hex. cover the lathe ways and just have at it. Sure smaller tools are going to be serious fun, but the setups much the same with just a smaller wheel.

    Seam to recall when this came up before you only get a few sharpenings out of a tool any rate, not so much just the loss through grinding, but the simple OD loss through wear mounts up fast. Coated broaches are out there and are probaly a worth while cost on the more complex forms just to gain the life span.

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    Good morning Wlodek:
    You wrote:
    "Thank you, Marcus, for the generous offer. I shall certainly would like to take you on it when on the mainland (I am near Courtenay) and maybe will send you a PM when planning this. I would like to offer to host you as well."

    When the fates allow, I will greatly enjoy meeting you face to face.
    My Lovely Wife takes us over to the island from time to time to stock up on knitting and spinning supplies...We can connect then if that happens before you get a chance to visit here.

    In any event, we can take this separate conversation offline so we don't bore the rest of the contributors.
    Best to contact me by email; not on the PM private messaging service because I never think to look there anyway.
    Click on the link to my Implant-Mechanix website to get the email address...if I just write it into this post it'll get bot-grabbed and it'll take forever again to settle down (at least that's what happened last time I foolishly posted it to a site like this one).
    You could also just call me...604 899-8977

    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining


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    Now after you published your number, don't go far from your phone, Marcus.


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