How to Make Round Corners on Manual Mill

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Vern2
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Re: how to make round corners on manual mill

Post by Vern2 » Tue Jun 15, 2010 9:03 am

I deal with much smaller material. A 12" roter-table is more then I can handle. But I did for the first time use a 4 jaw chuck to drill an off center hole on lathe. It was so easy to do my rotery table will come with a 4 jaw chuck not 3.

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doorknob
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Re: How to Make Round Corners on Manual Mill

Post by doorknob » Sat Jun 26, 2010 9:18 pm

I have never used a rotary table on a mill - and I'm having some difficulty visualizing how you would set it up and use it to round off the corners of an aluminum bar.

Can someone describe the setup and operation(s) in more detail?

PixMan
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Re: How to Make Round Corners on Manual Mill

Post by PixMan » Sat Jun 26, 2010 9:56 pm

doorknob wrote:I have never used a rotary table on a mill - and I'm having some difficulty visualizing how you would set it up and use it to round off the corners of an aluminum bar.

Can someone describe the setup and operation(s) in more detail?
What I'd probably do is make a little aluminum sub-plate using a piece of 1/2" thick 6061, something with at least 2" of space all the way around the workpiece. For the workpiece shown in the first photo (assuming it's still just the 1/4" thick bar stock, before the round is attached), I'd put locating pin holes in the bottom of the sub-plate at what would be the centers of the desired rounded corners.

Now make a matching pin of steel to put in the center of the rotary table. One end of the pin to fit into the rotary table, one end to fit the holes in the workpiece. I've never seen a rotary table without a hole to use, BTW.

On the top of the sub-plate, make 2 pin holes to match up with the two bolt holes shown in the part. The pair of holes would be off-center in one axis so that when the pre-drilled workpiece is placed over the pins, the center of the corner radius to be milled would be over the center of the table's rotation. Move the machine slide off-center to the rotary table by the amount of the desired radius plus 1/2 the cutter diameter.

Cut 90º of radius, return to start position, flip the part over and cut the next one. Flip the part end-for-end, cut the third one, flip it over one last time, cut the last corner.

That's how I'd do it for short runs. If it were bigger batches I'd probably spring for a big corner-rounding cutter, stack up the workpieces and hold them upright in a vise. Or run them on a CNC or Prototrak. ;)

doorknob
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Re: How to Make Round Corners on Manual Mill

Post by doorknob » Sat Jun 26, 2010 10:41 pm

Thanks... I guess that the corner-rounding cutter approach sounds pretty attractive by comparison...

PixMan
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Re: How to Make Round Corners on Manual Mill

Post by PixMan » Sun Jun 27, 2010 5:47 am

While the corner rounding cutter is a LOT faster and very simple, many folks already own a rotary table but are resistant to putting out perhaps $100 (or more) for a large CR cutter they may use once. One could get lucky and find a used one somewhere, but they're usually too dull to work well in aluminum.

Moreover, the rotary table can make any radius at all (in the right hands.) To use a large CR cutter for this application you might have to sandwich the workpiece in between two thicker pieces of sacrificial material. Otherwise, the 1/4" aluminum may catch and become a part with a left turn in it.

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Vern2
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Re: How to Make Round Corners on Manual Mill

Post by Vern2 » Sun Jun 27, 2010 10:19 pm

What's a CR Cutter?

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PixMan
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Re: How to Make Round Corners on Manual Mill

Post by PixMan » Sun Jun 27, 2010 10:28 pm

Corner Rounding cutter.

BobWarfield
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Re: How to Make Round Corners on Manual Mill

Post by BobWarfield » Sat Jul 03, 2010 10:46 am

If the rounding is decorative, and not precision, I use my 12" disc sander. It goes real fast, almost too fast. Most rounded corners are decorative and don't need the precision of a rotab. Takes some practice to get a nice symmetrical blended round though. Don't try it for the first time on a part you labored hours over!

Cheers,

BW
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doorknob
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Re: How to Make Round Corners on Manual Mill

Post by doorknob » Sun Jul 25, 2010 10:05 pm

Maybe this should be obvious, but I'll ask it anyway...

How are you supposed to set up the corner rounding cutter so that it precisely rounds the corner without removing material from the part of the edge that is supposed to remain straight?

In fact, don't you need to align two material edges with the cutter? (The top edge of the material and the left edge, for example)

Do you locate the edge with an edge finder, then use the DRO to position the exact center of the spindle along the material edge, and then back it away from the edge by the distance between the center of the cutter and the centermost part of the cutting edge?

But that only works for the 'left' edge of the material, but not for the top edge, right? But maybe you can set the spindle Z height correctly by positioning the cutter against the material (with the spindle not running).

Or, do you just "dial it in" as it's spinning, and hope that it doesn't go too far into the top or left edges of the material?

Am I missing something simple here?

randyc
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Re: How to Make Round Corners on Manual Mill

Post by randyc » Sun Jul 25, 2010 11:51 pm

Hi doorknob,

I don't know if your question has an obvious answer, I think it's sort of an individual preference. With careful measurement of the cutter, the edge-finding technique can work. Most operations on a small vertical mill don't require a great deal of precision so there are other expediencies to simplify the setup.

Image

Strips of paper are helpful for tolerances around +/- .005. In the above sketch, the setup process includes raising the knee so that the workpiece and the cutting tool are close to one another. Placing a strip of paper on the workpiece, the quill is lowered GENTLY and locked into position when the paper strip is contacted. Backing off the knee crank by about .001, measure the thickness of the paper and zero the knee elevation scale by adjusting for the paper thickness less a mil. The paper will obviously be deformed by the cutter - one mil is just an estimate of the deformation.

Perform the same approximation on the "X" (or "Y" depending on the edge to be rounded) but with the knee lowered so that the radius of the cutter doesn't touch the workpiece as shown in the sketch - zero the scale, correcting for the paper thickness as described above. (EDIT: There's actually a simpler way to zero this axis to the cutter if the corner to be rounded is the "Y" axis: loosen the vise, zero the "X" axis scale and gently move the workpiece until it is in contact with the pilot diameter of the cutter. Correct the zero setting by about 1/2 mil, secure the vise and you're ready to cut along the "Y" axis.)

At this point, several options are available but my preference is to set the knee elevation to about 1 mil above the reference surface and feed the cutter incrementally with the "X" or "Y" handwheel within a mil or so of the cutter radius dimension. There will always be a "line" that delineates the transition from the right angle of the workpiece to the radius of the cutter. It's best NOT to cut a full radius - stay just a bit proud of that and hand-sand the transition to as near perfection as your eyes are capable of measuring, LOL. If you undercut, the transition will not be attractive and difficult to correct !

Long time ago, I made aluminum demos (for customer presentations) that were intended to represent semi-steel castings in the final design. Draft angles, fillets, chamfers and radii that the patternmaker would include had to be included in the model. The above technique worked pretty good and the surface was further "enhanced" (to look like a casting) by tumble-deburring with driveway gravel as a medium for 10 or 15 minutes.

Cheers,
Randy C

doorknob
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Re: How to Make Round Corners on Manual Mill

Post by doorknob » Mon Jul 26, 2010 12:34 am

Thanks for the detailed explanation and diagram, Randy...

The trick with the strip of paper makes sense - guess that I will need to actually try it out and see how well I can do it in the real world...

PixMan
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Re: How to Make Round Corners on Manual Mill

Post by PixMan » Mon Jul 26, 2010 6:30 am

Most quality-made corner rounding cutters also have a slight angle beyond the radius (where the paper shims would be touching) to give a little more "forgiveness factor" should it not be set correctly. The angle helps make smooth-looking blends if there's a little taper in the workpiece or if there's another alignment problem with vise not straight, parallels not parallel, etc.

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