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finishing a plate

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  • finishing a plate

    I'm making a block-off plate that sits in the cupholder of a car. It's 3/8" Aluminum billet (well, it was...every time the head crashes, I have to take a little more off.) it's roughly 2.75" by 8", which is outside the envelope of my crappy vise on my Mini-Mill.

    So I grab the hold down kit and clamp it to the table. Only there's the clamping area where I can't mill, because the CLAMPS are there.

    I'm having reasonable luck with an end-mill, then running over the surface with a Random Orbital Sander for a matte finish, but man, ANY screwup and i've gotta hand crank over the whole surface again, taking another 0.010 off.

    My fly cutter maxes out at about 1.75-2.00 inches, and obviously can't get too close to the clamps.

    So how do you hold something like this firmly and get a good, homogenous, finished, surface on it?

  • #2
    You can hold something like that with low profile edge clamps so you can access the whole top surface. There are hold downs for a vise that apply down pressure to the workpiece to keep it flat, or a variety of device that bolt in T-slots and grab an edge such as toe clamps.
    .
    "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill

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    • #3
      Have you thought about super-gluing it to a block that's either threaded for your clamping studs or would fit in your vise? (You can knock the super-glued piece off when done).

      Also, since I forgot once time: does your vise allow you to move the jaws to be outside jaws?
      Last edited by Tait; 11-08-2011, 02:21 PM.
      Hemi-proprietor,
      Esoteric Garage

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      • #4
        Originally posted by Tait
        Have you thought about super-gluing it to a block that's either threaded for your clamping studs or would fit in your vise? (You can knock the super-glued piece off when done).

        Also, since I forgot once time: does your vise allow you to move the jaws to be outside jaws?
        No, it's actually not a machineist's vice, it's just a better than average drill press vice.

        The superglue is an interesting thought as it would minimize bowing of the part...and the back side can be ugly. :P

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        • #5
          simple. Make sure the table is clean and make sure the workpiece has no burrs on it. Set up one clamp at each end closer to one side so you can take a 1.5" pass with your flycutter and miss the clamps. After that pass. Stop...don't touch the Z axis or the quill. Use a 3rd clamp to temporarily hold it in position while you move the 2 primary clamps off center to the side you just cut..remove the third temporary clamp...then move the Y axis over and cut the other half.

          If I'm doing multiple flycut passes, I like to do them in the same direction...so I'll walk the table over on the Y to get the tool clear of the workpiece, run the X back to the other side of the workpiece, move the Y over to get ready for you next pass, and go for it. Then your surface finish will be the same after both passes.

          Use some aluminum foil underneith your clamps to protect the finished surface.

          What's the crashing of the head you speak of? What are you crashing it into?

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          • #6
            I use super glue often to hold parts for machining. When finished a little heat and the bond lets go.

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            • #7
              Originally posted by lbhsbz
              What's the crashing of the head you speak of? What are you crashing it into?
              Taking too deep a cut using an end mill that should be replaced. There's a percentage of my tooling that should be outright replaced as it was what I used to learn metalwork and is less than perfect as a result.

              lbhsbz, do you end up with a change in surface between passes? I'm using the low-rent, HSS, single bit fly-cutter and getting a perfectly homogenous finish is nigh on impossible.

              Won't matter if I hit it with the sander, but still, it'd be nice if I didn't have to.

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              • #8
                Passes?

                Originally posted by rocketsled
                lbhsbz, do you end up with a change in surface between passes?
                Some say multiple passes screw up the finish; whatever. If you take multiple passes on the same path eventually nothing more will come off, probably on about the third pass. Then see if adjacent lanes/passes match up. Otherwise you're not measuring your tram, your measuring how flexible and hard working your mill is on adjacent "lanes".

                If they don't match up by a measurable height, the head's outta tram left and right. Or outta tram in some other way. Imagine tilting your mill head to intentionally make a saw blade profile... its like that but maybe only a couple thousandths instead of 45 degrees to make a saw blade.

                If they don't match up in surface finish, welcome to machining aluminum. There will always be some difference.

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                • #9
                  Throw away your dull endmills...they'll do less damage in the trashcan. Shop ebay...you can buy a "lot" of random endmills for about $30...get 2 or 3 lots and you'll have enough for the rest of your life as a hobbiest.

                  The flycutters you have are fine. I've got a set too...although I rarely use them, they work good. Use a sharp cutting edge (flycutter body is angled, so use a cutter with a square tip on it) with very little or no radius. I've had good luck on aluminum simply using an unground HSS bit....you can dress it up a bit if you want, but the shape is already ideal. More radius will result in a smoother finish, but less radius on the point will work better with flimsey machines. Use less radius and use WD40 as a cutting oil. Slow your RPMS down. You shouldn't need but about 800-900 rpms on a 1.5" flycutter. Also, try taking passes at .005". .010" might be a bit much for your machine. The lighter the depth of cut, the better it will work.

                  Additionally, I imagine you dont' have power feeds...the mini mill will move around pretty good sometimes cranking the wheel by hand. Get yourself a battery drill or variable speed drill and fit a socket in the chuck that fits the handwheel nut, and use this as a poor mans powerfeed....slowly. You won't have forces moving the table around as you try to crank the handwheel, and the results will be much better.

                  I started off with a mini mill....the table would either flop around or was sticky...very difficult (if not impossible) to get the gibs adjusted to the point where you could easily turn the handwheel and the table didn't flop around....it was always either too tight or too lose.

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                  • #10
                    This is all awesome advice and gives me some things to consider. (I LOVE that powerfeed idea...my hands end up _killing_me_ after making a full pass. And holding and swapping the hold down points sounds like making it a two pass ordeal rather than what I've been doing thusfar.

                    The more I mess with this stuff, the more I realize how much farther I have to go.

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                    • #11
                      It just takes time. You'll soon realize that more often than not....the confusion is due to overthinking things. There's generally a fairly simple way....but since we've all got slick machines, at first it seems we look for the more complicated way. I'm work with a bunch of people who overcomplicate things, and I'm lazy, so I try to find the easy way.

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                      • #12
                        Got to get the basics right.

                        The tram needs to be right to achieve the best finish. A radiused HSS tool ground as shown will produce an excellent finish. Ignore how it is in the fly cutter that is just for holding it for the photo.
                        Byron Boucher
                        Burnet, TX

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                        • #13
                          Thanks for the help guys. Part turned out...well, as good as anything else on the car.

                          (pop quiz: See if you can tell the operating envelope of my mill!)


                          IMG_0832 by Matey-O, on Flickr

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                          • #14
                            Looks pretty good from here - are you happy with it?
                            Hemi-proprietor,
                            Esoteric Garage

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                            • #15
                              Why machine it? I'd hit it with a DA then buff it by hand with some scotch bright. Follow that up with something like Never Dull and you're finished.

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