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    Default Broaching force

    Hi all,I have a small batch of internal splines to broach and will make up some guiding equipment to maintain accuracy (the pieces operate up to 12000rpm) the splines are 25mm module 1 24T with 30 PA, the material is a case hardening steel EN 39B.How many tons would i need to pull a spline broach through the job? The spline is around 40mm long.I'm looking at a 20T hollow hydraulic ram thinking surely it will be powerful enough but I'd like to here from someone who knows for sure. Thanks

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    I'm not an engineer, but I do pull similar parts. Seat of the pants estimate from me is that you need about 5 tons to pull that part.

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    Surely a 20 ton press will be enough. That said when dealing with specialty metals or very odd broaching procedures I contact
    Dumont 413-773-3674. We have tons of Dumont tooling, I believe there the best. Very helpful people.

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    Should be able to figure it out pretty easily. What's the shear strength of the material you are pulling ? How many square inches of material removed in 2" of broach length ?

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    Thanks for the replies guys . I haven't got a broach yet but if it fully forms the spline over say 300mm and taking the shear strength as 60% of tensile then with 40mm of the broach "working" the theoretical value would be less than a ton...this would surprise me if it was the case hence i would be keen to hear from guys who have done similar work and have a fair idea of the force needed.I'll contact the suggested company just to be sure i'm not going to over or under engineer the pulling jig.

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    Quote Originally Posted by jimmy s View Post
    Thanks for the replies guys . I haven't got a broach yet but if it fully forms the spline over say 300mm and taking the shear strength as 60% of tensile then with 40mm of the broach "working" the theoretical value would be less than a ton...
    Doesn't seem too surprising to me ... I think your 60% figure is low, should be more like 80% unless this is aluminum but 1 mod is very small teeth, you can do those teeth individually by hand in a Bridgeport (don't tell the boss}. Put 24 of them together and it's still no big deal.

    But you do want it oversize because the last thing you want is for the broach to get stuck in the part.

    Are you going to do this vertically ? Easier to oil up the broach and also get the splines straight because there's less sag. You're about to find out how messy this operation is, besides washing the broach and the operator in sulfur oil, we'd brush on some gloop with a lot of anti-seize additives every few parts. Handling the broach every part and cleaning the chips out of the gullets, even if you have an enclosure ... it's like being in a dinghy on a sea of cutting oil.

    One thing you do have to do - clean the chips out every single stroke. Otherwise you can strip a row of teeth right out of the broach. That's not pretty.

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    Thanks Emanuel - I envisage a vertical jig with a guide bush of some type on the nose of the broach and seat made to accurately align the job on the jig. The original plan was to do them on the slotting unit of a Beaver mill but then the ease and repeatability of broaching appealed.The parts are crankshaft gears of an engine i'm building - a twin crank two stroke with crankshafts pressing into the primary gears from each end hence the need for good accuracy.

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    Quote Originally Posted by jimmy s View Post
    Thanks Emanuel - I envisage a vertical jig with a guide bush of some type on the nose of the broach and seat made to accurately align the job on the jig. The original plan was to do them on the slotting unit of a Beaver mill but then the ease and repeatability of broaching appealed.The parts are crankshaft gears of an engine i'm building - a twin crank two stroke with crankshafts pressing into the primary gears from each end hence the need for good accuracy.
    If you have the option, leave a little material on all the surfaces but the bore (sized for broaching), cut the splines, then somehow put a mating male spline "mimic" accurately trued in a lathe, then mount the new broached flange on the male spline and finish turn all accessible diameters and faces. Done right, should give the best perpendicularity, concentricity, and balance when you're finished.

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    Quote Originally Posted by Milland View Post
    If you have the option, leave a little material on all the surfaces but the bore (sized for broaching), cut the splines, then somehow put a mating male spline "mimic" accurately trued in a lathe, then mount the new broached flange on the male spline and finish turn all accessible diameters and faces. Done right, should give the best perpendicularity, concentricity, and balance when you're finished.
    Have done some this way but the expanding splined arbor can be $$$$ ... did quite a bit of vertical broaching when I worked at Websterville, and generally we didn't need to go to that extreme.

    First thing with ALL gears is to do the bore and one face at the same time. KEEP TRACK ! This is the biggest problem with cutting teeth-only for people, they usually aren't real careful about this but it's like Rule Number One in cutting gears - bore truly square to one face. Then that face is your reference for all tooth cutting.

    The second is, if they are flat gears and going to be stacked for tooth cutting ? PARALLELISM ! You can put four or five blanks on a mandrel you think is strong - inch, inch anda half, even 2" - tighten the nut and bend the mandrel .025" or .030" easy. Hard to believe but put an indicator on it ...

    Then on a vertical broach, there's a bunch of play, we didn't try to lock everything down tight because the tension on the broach will pull everything straight. There's a groove on the bottom end of the broach that is gripped loosely by a collar, you'd ease in on the hydraulic cylinder to take a little tension first then let 'er rip.

    The grip on the broach was fairly loose so it could self-center and pull straight. And the part was free to float around on the plate for the same reason. It was just a flat piece of steel with a hole in it, nothing special. We had different drop-in anvils for different size holes.

    It worked pretty good, probaly not as good as finish turning off the spline but it was good enough for Hewland gears. Most important was cleaning the plate the part sat on so chips didn't make the part sit crooked. And the gullets under the teeth. One of my co-workers pulled a row of teeth off and collected his check that afternoon, bye bye. Made an impression, can you tell ?

    There was a hose with like a garden nozzle for slooshing oil all over the broach before pulling it. Also got all over yourself. As the broach pulls through, every row of teeth makes a little corona of oil, sploosh sploosh sploosh. And you can't really stand far away or you couldn't reach the parts and broach and handles to operate the machine ... wear rubber underwear, after a few hours of this one is a sulfurated mess.

    Horizontals take up a lot more floor space (air is free - at least so far) and the broach has a tendency to sag, so you need a follower. We'd frequently pull about 3' long broaches, the operator platform was maybe three steps up, made for a tall machine but small footprint. For the price they get for broaching machines I'd probably build my own too

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    I'm new to this site and i've got to say it's great to have guys that know their stuff chiming in ... The gear blanks have a 35mm journal either side of the gear and the plan is to finish grind these the last four or five thou after case hardening and this can be done on a light press fit splined mandrel.The teeth (mod 2.5 42T)I'm hoping won't move much during heat treatment. Previously i've had the teeth also finish ground but in recent times that has become very expensive so for the first pair of gears we'll see how the cut teeth work. I really appreciate the help and advice offered

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    Oh, one other hint (brain is slow these days, must be the cold weather.)

    All our broaches had a cylindrical lead-in before the teeth. The closer the fit was to the bore, the better the concentricity and straightness. So we generally honed all the bores on the blanks to a nice sliding but still snug fit. That gets your broach started straight. Honing is fast, was worth the extra step.

    About your grind stock, on this size four to five thou may not be enouogh. Seems like you don't even get a full circle around until you go thru three or four thou. I'd probably go ten (on the diameter) to be safe but that's easy to change as you get the process dialed in.

    I can get you a nice gear grinder if you decide you need ground teeth ... but it's not usually necessary for automotive. Aftermarket automotive even less so.

    But Harley now grinds the teeth on their cams so they will be quieter. That way they can make the exhaust louder

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    Broaching is one of the simplest metalcutting operations performed in a machine shop. It's still very easy to screw it up. I went through several vendors trying to get it done properly when I started making products that needed broached, finally brought it in house. I pull one part that is similar in size to your spec, it's 18T 20/40 30PA which very close to 1.25 mod, major dia is just under 25mm. we do it almost the opposite of what EG described, that shows there's more than one way to skin that cat.
    My machine is a rebuilt MiniBroach, was purchased as a used straddle broach and reworked into a hole broach. if you look at pictures of MiniBroach machines, they are really simple mechanically, mine is larger than most but still uses Thompson rod and ceramic linear bearings for guides, steel plate for the frame, no cast iron in the whole deal except for maybe the pump - would be easy to shop build an equivalent machine.
    Vertical is supposed to be a plus, but some of the worst parts I had done were done on a vertical pull down machine, there are even vertical table up machines that lift the part up over a stationary broach tool. I think the big help of a vertical is when the broach is very large and very heavy, it makes handling the tool easier and avoids the bend of the broach itself from gravity, plus it simplifies automation, removing the broach and changing parts is easier to mechanize.

    We do it horizontal, but our tools are less than 4 ft. long, and I don't see the need to dig a pit for a machine. We turn the parts in house so we can hold the OD,ID and face closely. We hold the part in 3 1/2" dowels, concentric with the pull head and square to the cylinder and guides, parts are tight fit inside the 3 dowels. A full pot is OK, but the dowels make the fixture easier to keep clean - there are LOTS of chips and oil to deal with as EG described. We bore the parts to fit the guide on the broach within .0005", and the knob on the broach fits the pull head to less than .001", the fixture, pull head and part are all managed and aligned carefully. If you start the broach in the center, it will stay in the center unless it's sharpened incorrectly and wants to veer in one direction as it cuts.
    That particular part turns about 6500 rpm as part of an assembly that's 18 lbs and 7.5" diameter, so we need it pretty concentric and square. We've pulled thousands, typically we see well under .001" runout as they come off the broach. Having good tools to measure the finished part is harder than broaching it correctly.

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    Thanks for the detailed info guys. If i get within .001" and finish on a splined mandrel i'll be pretty sure i've got a good part at the end of it

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    Quote Originally Posted by Mud View Post
    we do it almost the opposite of what EG described, that shows there's more than one way to skin that cat.
    Kind of funny But I'd agree, if you pull horizontal then the "let it self-align" method is going to be a failure. The fit on the broach seems to be the only thing we do the same ... I do like the honing step, the parts seemed to fit nicer that way. And it is quick.

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    A thought if you really want to grind after broaching - You can put a few round sizing flutes on the broach after (or before) the spline flutes, to cut the ID of the part to size. That way the ID is cut in the same operation as the splines and should be concentric and parallel to the splines, then all you need to hold it to grind is a round mandrel.
    At least in theory

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