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Broaching 360 Brass?

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  • Broaching 360 Brass?

    If I can't find a certain auto part, I may need to make it. It will entail broaching a 1/4 x 3/8" hole in 360 brass for about 1.2" (through). I'll end up making the broach so I need to know how deep of a cut I should make with each step and how long should each step be? I'll have a 5/16" guide hole in the piece so the broach will basically be cutting the corners.

    I've got some Starrett oil harding stock that is .250 x .375 (dead-on) that I can use. Any ideas for DOC and steps?

  • #2
    Hi Ken,

    Why don't you talk to the folks at DuMont and see what they have to offer. They probably have a stock item, or can make one for you at much lower cost and higher quality that a home-made part. A broach is not a simple exercise.

    And how are you going to get a 1/4" x 3/8" finished size with a 5/16" starting hole???
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    • #3
      Machinery's Handbook has information on broaches and tooth form, etc.

      Being that it is brass, it should not take too sophisticated of a broach to accomplish.
      Jim H.


      • #4
        This is part of a headlamp assembly off, I think, a 1952-56 Bentley. The guy says he can get the entire assembly for about $1300 (used) but not this connector by itself--Or at least not yet. The owner brought in both bucket assemblies and wants one re-made. I can see why! This piece is part of a bulb terminal that's pressed into the bucket. (missing on one)

        This is a one-off deal and looking at other Dumont broaches, I can't see this being cheaper than making one. I've got the stock. All I need is the cut info. If nothing else, I'll take it to the die filer or jig saw.


        • #5
          I don't have any info on depth of cut for you Ken but it should be doable in brass if you have the time. One other suggestion that might not be to expensive is make the blank with a round start hole and take it to a shop with a wire edm. It should be able to be cut in under an hour setup included.
          Jonathan P.


          • #6
            Ken, I've posted it here before, but here's a sample of some home made broaches. 1.5" inches deep is a challenge, it would be a bear to file but equally problematic in that your broach is going to likely have many teeth at once engaged and therefor have a lot of cutting for on it. the rectangular shape also complicates it compared to the simpler lathe work in making a hex or square broach.

            Have you thought about fabricating it? I've done odd shaped holes where you make up the profile via fabrication with silver solder the, turn the od of the fabrication then drop the whole thing into a round hole (comparatively easy to make ) while not rectangular, photo attached to show the general idea

            file, broach, fabricate, or if handy, as suggest by many, the edm might be the way to go



            • #7
              In 360 brass this is not rocket science.......

              Use your 1/4 x 3/8 bar stock unhardened.

              Put it in the four jaw chuck, dial it in running true. Turn a 5/16" diameter pilot on the end, maybe an inch long. Where the pilot meets the square relieve the pilot down to around 1/4" diameter or less. The relief allows the broached material a place to go, without a relief it can't go through.

              Push the broach in about half way through. Push it back out to clear the accumulated chip material. re-insert and push all the way through. Caution....Testing in some scrap material will give you an idea if you can actually go half way without jamming up with chip material, it might take three presses to go all the way through.

              With a homemade broach in 360 you'll have some breakout on the back side so it won't look pretty. Either use a sacrificial thin backing plate or make the part extra thick and mill the breakout off.

              I've done this kind of broaching a number of times. You don't need a stepped broach unless you have to make the part in a single pressing.

              If you didn't already have the rectangular stock I would have suggested using a hardened dowel pin as broach material. A little tough to machine, but it holds up for broaching.


              • #8
                Thanks for the tips guys. I won't hear back from the owner until maybe Wednesday so I've got a little time to play and test. After doing a little number crunching, I think I can broach it with a three or four step broach.

                I may have a trick up my sleeve.


                • #9
                  Ken, a couple of years ago there was a rather involved thread here about making broaches.
                  I can't remember who all was involved but they had all the numbers needed for a broach IE, depth and angle of cut, etc.
                  Someone may remember the post,( seems to me Thrud was involved in that one).
                  I have tools I don't even know I own...


                  • #10
                    Broaching 360 Brass?


                    If you have a surface grinder and a spin jig you could easily make a 4 step broach out of a standard high speed tool bit. The spin jig uses 5C collets so get a square one and grind your round starter section. Then back the wheel off about .020 thousanths and grind the next step and so on. You will have to come in behind each tooth and grind the relief and primary angle but it should not be hard to do.

                    Jim )KB4IVH)
                    Jim (KB4IVH)

                    Only fools abuse their tools.


                    • #11
                      It's only one part. I would file it. Brass files easier than anything else. Start with a round hole and make it rectangular. File close to finish dimension and then broach it through with a simple bar hollow ground on the tip to clean up the hole.
                      Free software for calculating bolt circles and similar: Click Here


                      • #12
                        This time..I agree with Evan. Doesn't look like it's that hard; but like everything else, sometimes too many opinions can screw with a man's way of reasoning to get to a simple solution.


                        • #13
                          A file is a good idea. Although I'd try the lathe as a shaper trick. The tool is much simpler than a broach to make.



                          • #14
                            Originally posted by Evan
                            It's only one part. I would file it. Brass files easier than anything else. Start with a round hole and make it rectangular. File close to finish dimension and then broach it through with a simple bar hollow ground on the tip to clean up the hole.
                            You're probably right Evan but I may make a couple of these as part of an assembly. I might be able to get $150-$200 for a connector assembly. Hard to tell because there's not much market for post-war Bentley parts (Rolls Royce). There's so few around but when you do find them, they're like gold. I was going to use my little Whitney-Jensen punch press to broach it but it looks like there won't be much to broach with a 5/16" pilot hole. I like the lathe idea too. Might give that a try since the piece will be drilled there anyway.


                            • #15
                              yeah my first thought was also a file, but then noticed the 1.2" through distance, at .250 not an easy task for most of us and still a pita for the best.