Rotary Broaching

Now I know the easiest way to do what I want is with a push broach and make them as two pieces, but I want to make some specialty hex drivers with a one
piece body. That leaves me with rotary broaching. The thing is I am not going to make a bunch of them so buying a rotary broach holder or spending a day making one isn't all that wise of use of my time or money. However once I was setup to do it rotary broach inserts are a lot cheaper than push broaches.
Recently I had need to do some cross drilling that would have been easiest with a preset cross drill on the lathe. Its slow and tedious to center up and drill the part on the mill, but of course I can do it. I don't have to do it often so again making a cross drill for the lathe isn't good use of my time.
Then I had an aha moment. Make it fairly robust and use it for rotary broaching as well. Centering would be a little tedious, but getting my 1 degree back angle from center would be dead easy to do after that. Then lock down all the gibs not in use and advance the carriage. I doubt it would ever work on the mini lathe or the small lathe, but maybe the PM1440 could handle it.
If I can use the same tool for both applications it almost makes it worht making one. I might still know where to find it by the next time I need it. Driven with hand held drill when cross drilling and of course driven by the stock when rotary broaching.
I don't think I've ever cross tapped stock in the lathe, but if I don't see why I could not add enough functionality to do that too if I need to.
So what's wrong with it? I know rotary broaching takes a lot of power. The lathe is big, but its only 3HP. Flex in the compound, cross slide, and carriage could be an issue. If a spacer is used internally in the cross dril that rides on a thrust bearing I think the drill itself could take it. I also considered bronze bushing with a 1/2 shaft collet chuck because the bronze can take so much force.
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wrote in message > I added the other groups to clutter them up the way they do to us.

STOP THAT! LOL. I've been posting metal working questions and projects to try and get people thinking about our craft again now that the political shit storm has subsided a little.
The flex you talk about to get an angle has the same affect as a 1 degree angle with a rotary broaching tool.
The idea with the tool post cross drill part is if its left adjusted in a tool holder I can just drop it in place and drill a cross hole with no setup. 10 seconds vs 2 minutes or more to take it off and transfer it to the mill. I've got three parts I have made that require cross drilling and might wind up being small scale production parts. One would benefit from a turning operation afterwards. I can mount a collet in the lathe, but I am leaning towards putting the 4 jaw on and leaving it in place for all the one off projects.
Actually the mini lathe CNC conversion may get a 3C collet closer and get used for some of those parts, but the conversion is only half done and customers just won't pay me unless I promise to make their parts. LOL. I am leaning towards also CNCing the small lathe with a 3C collet closer as well.
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wrote in message

Can you lock the spindle while you drill?
Mine locks by engaging the back gears but there's detectable play in the gears and bull pin.
I bought a router motor to mount on the tool post and then realized it wouldn't handle the range of operations I do to round parts on the mill, especially cutting large wrench flats, and the setback to allow for a drill chuck + bit would make the mount too wobbly.
--jsw
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wrote in message

Yes, I have a foot brake that cuts power to the motor and stops the spindle. I figured if standing on the foot bar was to much hassle I could add a bar or catch to it.

Yeah, mine too.

Somethings are just easier on the mill. I admit that. My big mill is actually pretty good inspite of being 35 years old. I do sometimes CNC mill round parts with it. They usually come out within .0015 of round if I go modestly slow. Often better Obviously external hexes are easy on the mill. I even thought about doing a modified internal hex where the flats of the driven object ride on internal CAM lobes like a lot of the multi size wrenches we have started seeing in the last couple decades. My issue with that was that its really really slow to mill out something like that for an 1/4" hex with the necessary 1/16" end mill.
My plan is to use a straight shank ER collet chuck. It goes in bearings or bushings in a chuck of steel mounted directly in the tool holder. That's a pretty solid setup. Its used for high speed spindles. I'm so into the idea now I plan to build a couple of them. (there goes my efficiency) Atleast one each with bushings and bearings. Any drive connection flex will be a near non issue as the chuck itself will be right up against its support bearing rather than suspended out from it. The big issue will always be the drill bit flex.
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    Good for you!
    Enjoy,         DoN.
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    Yes. Recent experience -- in MSC, a Dumont 1/4" square push broach is $101.??, and a 1/4" rotary broach is about $60.00.

    Hmmm ... for cross drilling -- have you ever seen the tailstock tool for the purpose? A Morse-taper mounted round pad with a V-groove across it. Just slap the workpiece (assuming cylindrical) into the V, and with a drill chuck in the headstock spindle, drill away. I've seen them in old _How to use a Lathe_ books. Either the Atlas/Craftsman one, or the South Bend -- if not both.
    I would actually like to have one of those myself.
    As for centering up on a mill -- the same trick as used for aligning tailstock and headstock centers can be used. A piece of thin material -- a 6" pocket scale, or a razor blade, and you can tell which side is getting the center of the bit by which way the thin piece tilts.
    Of course -- another trick which I have read about, but not yet tried, is to drill the center of the round stock in the lathe with the size bit desired, then part it off, and when dross-drilling time comes, put the round stock in a vise with the round parted off part on top. Use the hole in the parted off piece as a drill bushing to get the bit truly on center.

    Likely -- ideally with the compound removed and the tool bolted directly to the cross-slide. This eliminates several locations for play.

    For the drilling and threading, you would want the spindle locked as you are describing it. Or -- if using a variant of the V-pad in the tailstock, you want a releasing tap holder in the lathe chuck or collet so you don't tap too deep.

    I have no experience with the rotary broaching (though perhaps I should get that). However, I just did some broaching in A2 drill rod using a 1/4 square broach. After making a fixture to hold the drill rod vertical and setting it up in a 3-ton arbor press (and broaching a hole which was 3/4" long) I got about half way through before I could not put enough weight on the lever (my 3-ton does not have the ratcheting lever arm -- and one of my own arms is still recovering from being broken a couple of years ago), so I had to move it to the 20-Ton Harbor Freight Hydraulic press to finish. This with Molybdmium DiSulfide grease as a lube.
    So -- if the rotary broach takes more force, I don't want to play. :-)
    Enjoy,         DoN.
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I have one that I never used, so I just went through the motions without turning the lathe on. I don't like having to swap the hand holding the work against the vee to reach the power switch on the headstock while the spindle is running and the drill is in the hole. Backing out the tailstock could let the work spin. It seems as dangerous as hand-holding a small part on a drill press.
--jsw
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    Add a foot switch to stop the spindle. (It helps if you have a relay-switched spindle motor with buttons -- there you just need to add a NC switch in series with the stop switch.
    Enjoy,         DoN.
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Like the lathe milling attachment it seems to be a workaround for those who don't have other machine tools. It provides only hand layout and punch mark accuracy lengthwise.
Long ago I cast a white metal bolt-down vee block for my drill press that served the same function. The drill bit holds the block centered while tightening the bolts. Before the bit breaks through I raised the bit and work and turned the work onto the flat tops to give clearance underneath.
-jsw
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