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  • From: Lawrence London <lfljvenaura@gmail.com>
  • To: machinist@lists.ibiblio.org
  • Subject: [machinist] radius form tool opinions
  • Date: Fri, 7 Feb 2014 01:47:06 -0500

http://www.practicalmachinist.com/vb/general/radius-form-tool-opinions-279717/

  1. 02-06-2014, 08:16 PM #1
    DrWerm is offline Plastic
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    Default radius form tool opinions

    I work on manual machines only. I recently had to make three parts with a 1/8" radius grooved in them for a cam and groove type set up. I know this would normally be done with a form tool on the lathe. However, i just threw them on the rotary table and used a 1/4" ball end mill. This obviously took longer than using a form tool. The part needs to be mounted on the rotary table regardless due to three threaded holes running into the side of it. This is partly why I just used the ball end mill. I sent them off to cnc shops because eventually we will need about seventy of them. However, the circumstance could surely arise where I may need to make a bunch of them myself. Therefore, it may be worth it for me to spend a little time on this. My question is, how do i make a form tool that has an accurate radius? I have access to a tool and cutter grinder. I considered mounting a swivel base vise on the table, but figured setting that up for an 1/8" radius would be damn near impossible. Any suggestions or pictures of jigs that you guys have made for a similar situation? I was thinking about the swivel base vise and I suppose I would have to make a jig to hold the part fairly high above the vise to clear the grinding wheel for such a small radius. I only looked briefly but couldn't find a swivel base vise smaller than 4". Am I over thinking this?? Are there any tricks for making an accurate radius form tool by hand that I don't know of?? Stick with the ball end mill? Thanks.
  2. 02-06-2014, 08:34 PM #2
    travis.taylor is offline Plastic
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    Well I'd say it depends on just how accurate the profile needs to be. I've gotten to be quite good
    at free handing form tools and I've not come into a situation (yet) that requires dead nuts accuracy.
    What material are you working? If it's softer stuff, you might be able to use something like O1
    tool steel and use a rotary table or corner rounding mill to establish the radius, then harden it.
    Honestly though, if it requires any more accuracy than that and you value the job, I'd say have
    an edm/grinding shop whip you up a few HSS bits. Shouldn't cost too much and used properly
    only a few should last you quite a while...
  3. 02-06-2014, 08:35 PM #3
    johnoder's Avatar
    johnoder is offline Diamond
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    Get a good angle/radius dresser like this (assumes a surface grinder or other precision grinder is available)
    Attached Thumbnails Attached Thumbnails p1000326.jpg  
  4. 02-06-2014, 08:35 PM #4
    sfriedberg is offline Titanium
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    The trick with grinding a radius on a T&C grinder is that a conventional swivel base vise isn't the right shape. The problem is not that it's too big, but that the jaws aren't in the right place. If your T&C grinder has a radius attachment, you should make a simple bushing that fits it and holds your form tool blank.

    If it doesn't have a radius attachment, you can probably improvise. Start with any available swivel base. Make a plate that swivels on it; put the swivel hole toward one end and have lots of room off toward the other end. Mount a toolmaker's vise on the plate with the jaws parallel to the long axis of the plate. (This is 90 degrees turned from the usual swivel vise arrangement.) You want this vise to be adjustable in both axes on the surface of the plate: lengthwise to control radius, and crosswise to control alignment of the tool blank axis with the centerline of the radius. A radius attachment usually has a lengthwise slide for radius control, but if you're improvising, you take what you can afford to build. When properly adjusted, the centerline of the tool blank will intersect the swivel axis, and the end of the tool blank will extend one radius past the swivel axis (use a hard stop on your T&C grinder table motion). So the vise needs to be back from the swivel axis slightly, and adjusted crossways depending on the tool blank width.
  5. 02-06-2014, 08:36 PM #5
    sfriedberg is offline Titanium
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    John, I use a Fluidmotion like the one you show for dressing wheels. Do you also use it for forming tools?
  6. 02-06-2014, 08:44 PM #6
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    Actually I have never used it for anything, though bought it when thinking of making my own gear cutting milling cutters years ago.

    I intended to modify it to hold a workpiece instead of a diamond. Sort of a smaller version of my Engis(sp?) (P&W) DiaForm template dresser - also intended to be modified to hold a workpiece instead of a diamond
  7. 02-06-2014, 09:26 PM #7
    mark costello is offline Hot Rolled
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    My experience has been that the normal corner wear in a 8" bench grinder wheel is close to a 1/8" radius. Might make a "close enough" setup rather quick.
  8. 02-06-2014, 09:45 PM #8
    CarbideBob is offline Titanium
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    Do you really want to build a fixture for one form tool?
    It might be cheaper to find a local house.
    A .125 radius is easy in a swing fixture, wait until you try to do .002 corner rads.

    You need a decent swing axis on the bottom. A small rotary table with the handle unlocked works nicely but you need some kind of stops so that you don't "overswing".
    You then need some way to hold the tool on top and align it to the rotary center.
    A simple vise clamped to the table with an end stop works and you shim the part in/out and side to side as needed to get the rotation center where you need it for one offs.
    Normally you will need some height above the vise so you make a toolholder for the part to stick it up there 3 inches or so and you can put the relief angles into the holder so that you don't have to tilt the rotary table on the chuck.
    Bob
  9. 02-06-2014, 10:21 PM #9
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    shars.com - 13212quot Brazed Radius Tool Set
    Or you can buy these dirt cheap at a variety of places.
  10. 02-07-2014, 12:35 AM #10
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    I just made a metal sculpture with a 3/8 ball inside a 5/8 ball inside a 7/8 ball in a 1 inch cube. And I had to make 2 form tools for each ball size. I made them free hand on my bench grinder then lapped them on my lathe. The reason I needed 2 form tools for each size was the 3/8 ball was formed thru a 5/16 bore the 5/8 ball thru a 1/2 bore the 7/8 thru a 3/4 bore.




  11. 02-07-2014, 01:33 AM #11
    stephen thomas is offline Diamond
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    I've used my R & A dresser with a rat tail diamond to dress the radius in a wheel, to do form tools about that size. You do have to use neutral rake, with a true radius on the tool or the profile changes a bit. But a very small pos rake probably would not affect things much, depending on the app.

    I believe it might be possible to dress the radius a calculated distance past center on the wheel, to grind a form that would closely approach a true round when ground with a specific positive rake, but have not experimented.

    Or am I misunderstanding the question?

    smt



  • [machinist] radius form tool opinions, Lawrence London, 02/07/2014

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