Rotary Table setup question

I have to cut a 7.915 in radius arc, with a chord length of 2.042 in. The width of the arc is 5/16. I'm using a 10 in rotary table, so I will have some overhang. I'll be starting with a rectangular piece of steel. Is there an accepted method to get the long sides of the rough rectangular stock tangent to the true rotation of the rotary table?

I don't know if this helps, but it's my go to for all things circles , tangents and chords etc etc

http://www.1728.org/circsect.htm

If it's a onesie, just measure from the center of the table to each inside corner of the rough blank. Clamp to the sub-plate. Now, if the blank is too long & the arc segment is wide, there will be a possibility that the OD will not land on the blank. so rotate the rotab with the OD under the spindle where it is supposed to be, and see if everything ends up within the borders of your blank. If not, move the blank out until it does, but always keep the end corners the same distance from the center, more or less. For the typical oversize blank for a onesie, there will be enough waste area you can do these set up measurements with a tape measure.

If you are making multiples and want them all to come out to some feature of the part that you intend to fixture by bolting through, then it will be necessary to attach the sub-plate securely to the RT, and lay them out (the positions to drill/tap/ream for the holes you want to dowel/bolt through) accurately. This can be done by scribing the appropriate radius arcs with the table under a scribe in a collet in the spindle, and using trig to figure the amount of degrees to rotate the table between holes.

If you keep track of where the center of rotation of the rotab is on the CL of one of the mill axes, it is also practical to use mill X/Y to put the holes on a tangent without rotating the table. This would be easier with a DRO with canned bolt hole cycles; but you can look up offsets in places like Machinery's for that "jig bore" type of layout.

smt

Just set the table to zero-put couple bars in your R/T slots push job against them-So your table is set at zero,your slots will be in line with "x" axis-your job is pulled up against the two bar stops you have in your r/t slots-to get the side edges roughly central just measure equal/equal to each side of your r/t.Centralise your cutter to a spigot in your r/t center hole before starting-Then you can index the machine table to correct distance for your arc

onecut said:

Just set the table to zero-put couple bars in your R/T slots push job against them-So your table is set at zero,your slots will be in line with "x" axis-your job is pulled up against the two bar stops you have in your r/t slots-to get the side edges roughly central just measure equal/equal to each side of your r/t.Centralise your cutter to a spigot in your r/t center hole before starting-Then you can index the machine table to correct distance for your arc

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The OP's project sounds like homework.

Depending on the tolerances required and the sizes of the stock used, we used to cut oversize by enough to be able to clean up all 4 sides while the part was clamped on the table. Usually meant moving the clamps during the job.

If this is a multiples run long enough, make a sub plate the can be bolted down on the RT. If it's onesie-twosie work, a plate of aluminum thick enough to allow the end mill to clean the edges without mucking up the surface of the table.

We did it a LOT, and dialing in the RT to the spindle became second nature. As did keeping one's fingers away from the DRO once it was zeroed in.

Lotsa Trig!

Cheers
Trev

If this is a multiples run long enough, make a sub plate the can be bolted down on the RT.

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Needs a subplate regardless. He mentions a 10" rotab and a 16" OD (15.830) ....

On second thought, if the question is "hang the whole rough blank off the edge" with no subplate, then just make sure it extends far enough to cut a 8" radius from the center to the shortest point and call it good. It's not easy to get a clean inside edge that way unless the "rough blank" is thick and you grind the back of the part down after milling deep enough and bandsawing free. But it works.

smt

Send it out, sounds like a wire EDM job.

Seriously, if one edge of the part has to be tangent to the centerline, put the part on parallels and cut it. Know where centerline of the rotab is and using a new endmill, cut to that offset. Depending on how good your set-up is, you should be able to get within less than 0.001"
Easy Peasy, piece of cake
JR

put the part on parallels....

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Why?

It's either cut from a heavy overhung block out almost 3" past the rotab edge, or it's on a (presumably quick, consumable) subplate



smt

Why?
To cut the tangent edge without worrying about the endmill getting sucked into the top of the rotab. If I understand his dimensions, its 5/16 thick plate. Shouldn't be a problem with overhang if you don't try to take a huge cut.
JR

He's not cutting over the 10" rotab. He's cutting out in space roughly 2" & 3" beside it.

smt

Stephen, read my post again. They mentioned one edge tangent to CL.
JR

OP:

Is there an accepted method to get the long sides of the rough rectangular stock tangent to the true rotation of the rotary table

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Maybe we are both interpreting the Q differently than the OP expresses?

My interpretation is that he wants to set it up so that within the given rectangular blank, the machined arc falls within the borders of the blank, which at 7.915" OD radius/7.6025" ID radius will be well off the edge of a 10" (5" radius)rotab.

One way to do off-hung work is to use a blank that is thicker than necessary. I don't cut into this aux table because it gets use for various large arcs.

I also did not want the part to collapse or chatter while cutting the internal slot (which has a partial straight lead-in ramp if it looks slightly off) Besides keeping the cutter out of the aux table, the webs supported the thin sections of the work while machining. This method also simplified the fixturing, with screws & clamps outside the area of the finish part.





Bandsaw part well outside the finish lines. (10 x 24" chuck for scale):



grind to thickness to reveal the part:



Different part, larger arc, multiple pieces with dowel location to the rotab. I bolted on a subplate drilled/reamed the dowel and bolt holes and just milled into it (milled into the subplate). More stable than parallels& no chance of cutting into a hard parallel Since there were about a dozen of these football shaped parts symetrical about other holes, slots, and features, the fixture plate made sense.




Either way, no parallels necessary when off the edge of the rotab.
But we are getting into a pissing match at this point. I like you too much to take it further.

Perhaps the OP should show a picture?

smt