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Tilted keyways?

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  • Tilted keyways?

    Just cut two keyways with a 1/4 end mill in some 6061. Both keyways are tilted. That is, the sides of the keyway are not perpendicular to the surface of the work. It's as if the mill was slightly out of tram. The work consists of two flat pieces that were to be a sandwich with the key to keep them oriented to each other. The consequence is that they won't sit flat to each other when the key is inserted between them.

    One keyway was cut on the x axis and one on the y axis, and the mill is dead square and in tram. I pre-cut the slots with a 3/16 2-flute end mill and came back with the 1/4 2-flute to open them up. Both slots slant in the same direction relative to the direction of the cut. The only thing I can figure is that the end mill flexed. But there is quite a slant, so much that you'd think I was using a rubber end mill. Of course, I can go back and open up the slots and mill out each side square, but that means an oversize key and more fitting. An oversize key and fiddling with the fit negates the whole point of doing this, which was to be an easy solution to keeping these parts in line.

    Has anyone else had this problem?

  • #2
    What kind of milling machine are you using?
    Jonathan P.


    • #3
      Sounds like the end mill wasn't on center..Check the verticals. They should be equal.


      • #4
        Is your vice wore out where the movable jay may have lifted the work piece when tightened?


        • #5
          I think the problem is you thought endmills where not made of rubber.

          You should of opened it up by using the 3/16" and just taking two finishing passes with the same sized endmill
          Using a 1/4" in a precut slot will result in two flutes trying to dig in at the same time, one in climb, one in conventional. Im betting the result is the conventional cut skimed along as the climb cutting flute dug deeper.

          Never use an endmill to cut a slot to the endmills size and expect accuracy. Allways rough it out with a smaller endmill, then use.. well, any endmill smaller then the finish size. Maybe not the same one as you roughed it for verious reasons, but anyway.

          Allways expect heavy endmill flex in any roughing operation, thats one of the many reasons you take roughing *and* finishing passes
          Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.


          • #6
            Black Moons,
            While what you say is good advice and true, I do not see how flex is going to create any off perpendicular slot condition such as the OP mentioned.


            • #7
              Moons is dead on. Reread his post he explained it well.


              • #8
                Moons is right, when climb milling the cutter deflects, just like a spinning baseball will curve, a full size endmill will deflect.
                The proper cutter for a keyway is a keyway mill, not an endmill.

                When you rough with a 3/16 endmill. and then follow with a 1/4 endmil , EVEN THOUGH YOU ARE ON THE SAME CENTERLINE, the cutting forces are different. one side is cutting in conventional mode (deflects cutter away) and the other side is climb milling (pulls cutter)
                Such a set up can result with the 1/4" cutter pulling more to the climb side . Now add to that that the 3/16" cutter did the same thing, and you can envison such a keyway may be off .020" easily.
                The faster the feed, the higher the effect
                Last edited by Rich Carlstedt; 06-14-2010, 05:53 PM.
                Green Bay, WI


                • #9
                  THANKS, guys. Makes perfect sense. I'll remember this for next time.

                  In the end, I solved the problem with my old standby, brute force. Since the parts were 6061 and the key was steel, I just put it all together and smooshed (technical term) them together in the vise. I had nothing to lose since the alternative would have taken the rest of the afternoon. The parts fit together tight and square which was the goal. A couple of cap screws to seal the deal and we're in business.

                  The part, by the way, is a fixed 90-degree miter gauge for the table saw. The ones that come with the saws suck and the aftermarket ones are too expensive for what I need. A few bits of 6061 from the scrapyard and the problem is solved.

                  Thanks again.


                  • #10
                    I agree with Black Moons assessment. This is a common problem discussed in most machine shop operations manuals. The intended square cut keyway becomes a trapezoid due to flex and deflection of the end mill. It's best to use a a 4 flute mill for the finish passes to minimize flex and deflection.

                    Rich also hit the nail on the head when mentioning feed rate. Most machine shop manuals recommend reducing the feed rate by 25%-50% to minimize the deflection toward the climb cutting flute.


                    • #11
                      Agree with the assessment,
                      Greg has more going wrong here then is answered in these suggested reasons and replies. There was just too much missing information for such an absolute reply.
                      We did not see his setup, machine, speeds, type of cutter (other then 2 flute 3/16 and 1/4), condition of cutters, feeds, or depth of cut. There have been many successful machining operations that were not done by the book, simply by taking it slow and easy. His improper methods is one case where I believe this would fully apply, it could just as easily been successful had he done things differently even though not by the books.


                      • #12

                        Well, no pix of the setup but below is a shot of the finished part. The machine was a Clausing 8530 which was trammed square. Cut #1 was in a piece of 3/4 x 3/8 6061 held in a Kurt vise, stock sitting on matched parallels. The cut was on the x axis. It is possible but unlikely that the stock was not square and tilted in the vise although a tilt to the degree that the slot was off would have been noticed.

                        Cut #2 was to the bottom surface of a fabricated angle plate (two pieces bolted and pinned together) and held in the Kurt vise. The cut was on the y axis. I doubt the stock was tilted in the vise as the vertical piece of the angle plate, which was in the vise, sat down firmly on the base of the vise. In this case, Black Moons answer makes the most sense.

                        As I said, the error was in the same direction of cut for both pieces. If you were to sight down the cut like you were looking down a bowling alley, and moving the cutter away from you, the tilt was toward the right. This would be the side that was getting the climb cut. Thus the Black Moons answer adds up.

                        While there may have been better ways to fit these two parts together, it seemed that the easiest way to get them square and keep them that way was with a keyway. I know my mill cuts square and straight so there would be no alignment issues like I might have had if I had tried to clamp the two together, get them square and keep them that way while fitting the whole clamped-up gob into a drill press vise to drill for roll pins or dowel pins.

                        In the end, all is well as it is dead true and I've already made some cuts on the table saw that are dead square.

                        THANKS AGAIN guys. I appreciate all the help.


                        • #13
                          Ken, with that attitude, let me guess. Do you work for BP?


                          • #14
                            "In the end, all is well as it is dead true and I've already made some cuts on the table saw that are dead square."
                            In the end, that's pretty much all that counts

                            Good looking part there.!


                            • #15
                              Originally posted by tdmidget
                              Ken, with that attitude, let me guess. Do you work for BP?
                              No TD I don't, nor do I have any attitude. What I said is reasonable and very true. You seem to be the one with an attitude, and you can keep it. I am also reasonably experienced, just not a snob about it, if you get my drift.