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Thread: Single-tooth broach geometry?

  1. #1
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    Default Single-tooth broach geometry?

    I'm part way through making a 4" aluminium drive wheel for a belt grinder - I'll get there eventually! I've got it bored and I'm just trying to get the keyway cut. It needs an 8mm wide key slot about 3 - 4mm deep and 45mm long....although I don't think it'll be an issue if it goes all the way through.
    I tried what I'd done with some delrin previously - essentially an 8mm square HSS bit sharpened like a chisel and held at an angle for clearance. With the scaling up of width and the harder material, the stick-out just makes it too flexible. I think I need to make a boring bar style of tool with a single tooth cutter in the end. I've seen youtube videos of similar efforts but nobody seems to agree on geometry. What sort of angles do I need on the mounting hole and the tool bit please?

    I'm thinking a 20mm bar - that's what's lying around shouting "Make me into a boring bar!" - cross-drilled to 11.5mm * and with a set screw to retain the HSS at the end. One of the videos suggested slanting the hole by 7 to get some built-in relief and then grinding side relief on the tool bit so that it doesn't rub. Is this the sort of geometry that's appropriate for this and will I get away with a slightly over-size hole for the tool bit?

    I guess I'm probably making this harder for myself by over-thinking it....but would appreciate some pointers to avoid going nuts!

    Many thanks,

    Gareth

    *The 8mm square HSS should have a diagonal of 11.31mm but 11.5mm is the closest drill I have without setting up a boring head....which I could do if you think it's really necessary.

  2. #2
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    A broaching bar will differ from a boring bar in that the hole or slot for the broach will be centered where on the boring bar you want the slot or hole lower so the top surface of the tool bit is roughly at center height. But otherwise there's no reason not to make your own broaching bar.

    The easiest way would be to drill a round cross hole the same size as the tool bit or just a hair smaller as you're suggesting and angled so the tool bit has about 5 of back rake. It'll also want to have some side rake on the sides of the face. But don't take that taper all the way back. Just cut away enough to make the depth you need and another 1mm that has the inevitable curve from the corner of the grinding wheel.

    If doing this myself, and I have, I'd dress out the hole with a square file that has one side ground smooth as a safety edge to make working the flats a little easier. And I'd mostly work the "back" side of the hole to give the tool bit more support area. So you would be left with what looks a bit like a long "D" shape in the end with the flat being away from the nose of the bar. That'll support the cutting tool well.

    A hint for aiding with fitting the tool bit is to grind or stone a slight short taper for about 1mm long and only tapering in a few degrees on the "back end" of the tool bit so you can see when it's starting to be the right size more easily. And from that point file carefully and make good use of the safe edge to aim your efforts at the spots needed. Marking the faces of the tool hole with a felt pen frequently so you can see where you are focusing your file work helps a lot too. And use the curved taper along the first portion to aid with controlling where you do the removal.

    You might mess up the first time. So start with a longer raw stock bar and cut off the mistake and try again. You'll have learned so much the first time that the second attempt will likely work. Or if not then for sure the third. Do the seating hole for the cutter first so you're not also cutting off and throwing out the rest of the time and effort if you mess up. THEN cut to length and drill and tap for the set screw.

    For the tool bit grind the side faces for end and side relief of around 3. And the sides of the cutting face can and should also angle in by about the same amount towards the bar so that only the end of the tool does any cutting. Again using some felt marker on the edges and a magnifying glass if needed can do a lot for getting the relief faces to just come up and kiss the actual leading corners and no more. If you're working with an 8mm cutter you want all the angles to run up to the two opposite corners of the cutting edge and leave no more than a small pencil point worth of felt marker that you then kiss off with some fine stone work so the width across the two outside corners is just barely affected when measured.

    Such a bar will happily take a couple of thou cut per pass using your lathe as a broaching tool. For aluminium it might well take more. But don't force it or the carriage will try to lift and pivot on the lathe of your size. Live with more cuts that are easily made. The work will go fast enough in any case.

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    Thanks BCRider. Something like this then, if I've understood you correctly?

    10mm hole drilled at 5 backward slant and then squared off at the back to 8mm:




    Then the tool bit ground to give relief at both sides and the bottom:



    Set screw hole would be at 5 off-axis aswell so that the end of the screw meets the face of the tool bit cleanly.....just didn't model that until well after I could be bothered to change it

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    Quote Originally Posted by Cenedd View Post
    I tried what I'd done with some delrin previously - essentially an 8mm square HSS bit sharpened like a chisel and held at an angle for clearance. With the scaling up of width and the harder material, the stick-out just makes it too flexible.
    The answer is in that quote - you're trying to take too big a bite in each pass for the size of the tool. Thin the tool so it's only a couple mm wide, and make multiple passes across the slot. It goes pretty quickly with no drama.

    The exact geometry doesn't matter much as long as there's enough back clearance; slight positive or neutral rake on the cutting edge is fine. You just don't want so much negative rake that the tool digs in deeper than intended.

    I do a lot of single point broaching on both the lathe and mill for certain parts I make, and have found most of what people online claim is essential is really just bias because they've only tried one way. The big key is just taking a narrow enough bite to avoid tool flex. Broaching aluminum is easy; what I've described above works even in hardened tool steel.
    Last edited by Yondering; 05-09-2019 at 04:05 PM.

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    One other thing to add that's not part of your question, but important for tool life, is to keep the chips clear of the work. Dragging the tool backwards over the cut is fine, as long as it doesn't get chips caught and rolled underneath; that can dull or chip the cutting edge of the tool, and messes up the broached finish as well.

    Most of the time I use an air blast to blow chips away; a mister works well for that, coolant on or off depending on the application.

    As long as you keep the chips clear, you can even use carbide inserts for this.

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    Yep, exactly like that 3D drawing but for one slight change. The sides of the front face should also taper inward slightly from the full width at the nose of the to slightly narrower at the end of the blank where it goes into the bar. As you show the front face of the cutter is square and all the same width. That will make for more friction than you want. It would also encourage a taper to the cut if the tool is not dead on in all respects. Making the tool taper in width up to the ends of the side relief grinds will make it more tolerant and able to cut a more consistent width.

    Also the angles don't need to be as extreme as you've shown but they do show the right idea.

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    Quote Originally Posted by Cenedd View Post
    I'm part way through making a 4" aluminium drive wheel for a belt grinder - I'll get there eventually! I've got it bored and I'm just trying to get the keyway cut. It needs an 8mm wide key slot about 3 - 4mm deep and 45mm long....although I don't think it'll be an issue if it goes all the way through.Many thanks,

    Gareth

    *The 8mm square HSS should have a diagonal of 11.31mm but 11.5mm is the closest drill I have without setting up a boring head....which I could do if you think it's really necessary.
    Is it keyed because you may be interchanging different diameter drive wheels?
    Could you accurately bore the drive wheel for the shaft and pin them together?

  8. #8
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    Quote Originally Posted by reggie_obe View Post
    Is it keyed because you may be interchanging different diameter drive wheels?
    Could you accurately bore the drive wheel for the shaft and pin them together?
    Or..... Reggie's idea suggests to me a fixed press fitted cross pin in the shaft that fits into a slot in the pulleys. The slot being similar to the drive dog notches seen on shell mill heads.

    That way the drive would be positive and the quick change drums would interchange quickly and very consistently.

    That is, if you're building your own machine and have the freedom to make such design choices....

  9. #9
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    I made one recently by cross-drilling some steel bar with a 6mm drill near the end and tapping a thread in the end for a clamp screw. Then I took an old 6mm carbide end mill, ground it flat on the end, with a bit of rake then ground the side down to the centre at that end (like grinding a D-bit). Locked the carbide cutter into the hole with the clamp screw and got broaching, using the carriage feed handwheel.
    Peter - novice home machinist, modern motorcycle enthusiast.

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  10. #10
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    Quote Originally Posted by Yondering View Post
    The answer is in that quote - you're trying to take too big a bite in each pass for the size of the tool. Thin the tool so it's only a couple mm wide, and make multiple passes across the slot. It goes pretty quickly with no drama.

    The exact geometry doesn't matter much as long as there's enough back clearance; slight negative or zero rake on the cutting edge is fine. You just don't want so much negative rake that the tool digs in deeper than intended.

    I do a lot of single point broaching on both the lathe and mill for certain parts I make, and have found most of what people online claim is essential is really just bias because they've only tried one way. The big key is just taking a narrow enough bite to avoid tool flex. Broaching aluminum is easy; what I've described above works even in hardened tool steel.
    Thanks Yondering. I suspect that my current tooling theory is flawed from the outset and I only got away with it previously because the delrin was so soft (relatively) and the cut so small. If you think of a square tool bit with the end ground to around a 30 nose sort of like a square chisel, you'll have a pretty good idea of what I've got currently....and the stick-out is necessary but too much.

    I hear what you're saying about taking it in multiple passes. I'm trying to take very light cuts and go progressively deeper with each stroke. Each cut is with the carriage wheel and the depth adjusted on the cross slide. If I were to make the tool thinner and effectively split the 8mm width into say 2mm widths, I'm not sure how I'd go about varying the height of the tool in that scenario. Just adjust up and down on the QCTP as you would for centre height?

    When you say "slight negative rake", have I got it correct in my model - like a chisel - or are you suggesting the other way like a scraper? Neutral is obviously purpendicular to the surface being cut, I'm just not sure which way then is positive....but I've a feeling you're suggesting I should be tipping it the other way to what I've modeled.

    Quote Originally Posted by BCRider View Post
    Yep, exactly like that 3D drawing but for one slight change. The sides of the front face should also taper inward slightly from the full width at the nose of the to slightly narrower at the end of the blank where it goes into the bar. As you show the front face of the cutter is square and all the same width. That will make for more friction than you want. It would also encourage a taper to the cut if the tool is not dead on in all respects. Making the tool taper in width up to the ends of the side relief grinds will make it more tolerant and able to cut a more consistent width.

    Also the angles don't need to be as extreme as you've shown but they do show the right idea.
    Cool, cheers. Not that great at thinking about all the relief angles in my head yet....I'm impressed I got two right!

    Quote Originally Posted by reggie_obe View Post
    Is it keyed because you may be interchanging different diameter drive wheels?
    Could you accurately bore the drive wheel for the shaft and pin them together?
    This is a wheel for direct drive so it is to fit on the 24mm diameter motor shaft which has an 8mm key. There's an M8 thread in the end of the motor shaft that should stop the wheel coming off. I was thinking I might make the keyway stop short of going all the way through as a means of stopping the wheel going to far on and hitting the flange....but I'm open to other ideas. I'd prefer not to drill through the wheel to put a set screw in if I can help it. The bore is a pretty good fit for the shaft - it may be a thou or two over as it was slightly under at first and a spring pass took it a little over.

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