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Making a 7/8" square hole in 6 bushings, photo heavy

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  • Making a 7/8" square hole in 6 bushings, photo heavy

    Recently I took on a project to put a square drive in 6 bushings. I didn't have a 7/8" square broach or a press that could handle the job but I did have a K&T slotting head for my 2H universal mill. The only problem was I'd never used or even cleaned the slotting head. I had 3 weeks to get the job done. The bushings blanks I received were 2 3/4" in diameter and 1 3/4" high and made from an unknown steel. Using the bouncing steel ball method they proved to be harder than cold roll but how hard I couldn't determine.

    The first thing was to see if I had all the parts to drive the slotting head and then get it working. This took several days because I had to order socket head screws for the drive hub then machine the heads of the screws to clear the body of the slotting head. Four days later the slotting head is on and working.






  • #2
    Next was to prepare the bushings for the 7/8" square. So all 6 bushings were bored to 0.937". Then I prepared a couple practice bushings since I'd never used the slotting head. Next was the tool geometry. The slotting head has a max 2 1/2" stroke and the bushings are 1 3/4" tall plus the max tool diameter I was able to use was 1/2". So I went to grinding the slotting tool. I'm not sure it's the best geometry but it's all I could come up with without ordering special material. I had 1/2" square HSS with Cobalt so I made it work. Both cutting edges were honed with a diamond hone.




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    • #3
      So the idea was to use the spacer to rotate the bushing 90 degrees after the depth of cut was reached. This should have work right out of the box but as you will see from the photos 90 degree rotation on the spacer isn't always 90 degrees. The photos show one corner that isn't 180 degrees from the opposite corner. This problem took me a while to solve. What I found was a burr in the detent slot and a sticky detent locating pin. Cleaning, deburring and lubricating did the trick but I still lightly taped the engage lever with a rubber mallet to make sure it engaged the detent fully.



      Check out the left corner


      Once I solved the 90 degree problem the slotting head did better

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      • #4
        The first bushing I machined looked really nice but measured poorly. I had a 0.010" taper in the square (0.005" per side) and I just couldn't get it out. I finally resigned myself to deal with the taper. After all cleaning up .005" with a file shouldn't take too long. Well I'm here to tell you it takes 40 minutes per bushing to cleanup the taper and that is using new files. I think the square 1/2" tool and probably the geometry wasn't the best choice but the biggest mistake I believe was machining both sides of each corner at the same time. Even though I was only taking .002" to .005" per stroke and always allowed a spring pass I think it was too much for the tool cross section and cutting edge profile. I ran the slotting head at 35 strokes per minute which was the slowest it would go. Using thread cutting oils probably wasn't the best choice either but it's all I had.




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        • #5
          Good job. Surprised you're getting a taper cut as that looks like a really stout slotter head. Presumably there is a little wear in the slide?
          West Sussex UK

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          • #6
            Maybe the bushings could have been shimmed to compensate for the taper?
            http://pauleschoen.com/pix/PM08_P76_P54.png
            Paul , P S Technology, Inc. and MrTibbs
            USA Maryland 21030

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            • #7
              Really nice work!

              I'm also surprised at the taper, as you obviously sharpened the cutter well. If it was tool flex, then just letting it run should have sorted that out. Maybe cutting the opposite corner would have given you a taper in the opposite direction? Almost sounds as if something wasn't square.

              Ian
              All of the gear, no idea...

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              • #8
                Must be square hole week up here.

                Last week a customer needed two threaded bushings for some bicycle sprocket/sprag thing that he was apparently putting on some generator setup. It needed a square hole so he could use some bike crank pedal "cartridge" thing- I don't pretend to understand his full reasoning.

                Anyway, I did a similar setup- I bolted a semisacrificial chunk of 1/2" to the top of my old Gorton rotary table, then milled a shallow pocket in it, to retain the bushing I'd made.



                I don't have a slotting head, so I first marked out and drilled the corners out...



                Then used a long 3/16" endmill to carefully shave away some of the remaining corner.



                After that, I used a homemade "broach" tool I got with my lathe many years ago- it's a 5/8" shank with a piece of 1/2" HSS pressed or maybe brazed in place. You fit in in the quill with a 5/8" collet, lock the spindle (in my case using the spindle brake and two bungee cords ) and use the quill as an arbor type press.



                The trick on the corners- in my case- was to start in the middle, broach to the scribe line, then traverse the table slightly to then cut along the scribe 'til you hit the corner. The DRO helped keep my center so I didn't get too far out of whack, but yeah, it was slow going, and I had to take small bites and lots of "spring passes".

                But, I got the job done and the guy was happy with it.

                Doc.
                Doc's Machine. (Probably not what you expect.)

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                • #9
                  Could the head have been trammed to compensate for the taper?

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                  • #10
                    Originally posted by Alan Smith View Post
                    Good job. Surprised you're getting a taper cut as that looks like a really stout slotter head. Presumably there is a little wear in the slide?
                    I was quite surprised at the taper too. I think most slotting heads are rarely used and this one was no exception. I tore it down to parade rest and there was little to no wear in the slide, body and gib. Slotting heads in general rotate 360 degrees on the X axis but offer no adjustments in the Y axis. Prior to machining, the slotting ram was set vertical to the spacer and the slotting tool was then checked to see if it was vertical, and it was.

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                    • #11
                      Originally posted by old mart View Post
                      Could the head have been trammed to compensate for the taper?
                      Limited tramming options on slotting heads but I did the best I could

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                      • #12
                        Originally posted by Ian B View Post
                        Really nice work!

                        I'm also surprised at the taper, as you obviously sharpened the cutter well. If it was tool flex, then just letting it run should have sorted that out. Maybe cutting the opposite corner would have given you a taper in the opposite direction? Almost sounds as if something wasn't square.

                        Ian
                        I totally agree squareness was likely to be an issue so I addressed it in the beginning. And I also agree spring passes should have cleaned up any tool flex that was happening but it didn't. That's why I'm leaning more toward poor tool geometry 1st and not enough tool cross section 2nd. If I had the time I'd get to the bottom of this before the next slotting job comes around.

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                        • #13
                          Curious how much time you estimated for this job, including making the tooling? Did you consider using the slotting head as a big die filer to clean up after the broaching?

                          1/2" square tool bit is more flexible than you think. Not wanting to buy a broach, you could have bought some 3/4" tool steel.
                          Had you mentioned this job two weeks ago, I could have mailed you a 3/4" lathe bit for the cost of postage.

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                          • #14
                            After looking at Doc's approach I should have drilled the corners to lessen the amount of material the tool had to remove. Well done Doc

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                            • #15
                              The taper in the hole might be due to insufficient relief on the cutting tool. Is it possible to tilt the slotting head to compensate for the taper? That would require feeding in only the X direction.

                              Nice Work.

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