How should I cut this?

[Mr Ron]

I'm cutting the following from 3/8 thick 6061 aluminum plate, qty 2. I will use my rotary table set up on a 6x26 mill. I will be using a 3 flute, 1/4" HSS end mill, specially for aluminum. How should I set it up on the RT? I think I should start at the left end and get the 2" long edge lined up with the Y-axis travel; mill that edge; then mill the 2 surfaces along the X- axis that are 90° to the first surface; repeat the same cuts on the right end then reset the RT to the mill's arbor center and cut the circular elements using the RT rotation. This will require me to move clamping devices out of the way of the tools path. Does this sound like the correct schedule of cuts? Would you stack cut the 2 plates or do them separately? I already made 2 of them, but I started by cutting the circular elements first. I stack cut them, but they didn't turn out like I hoped.

[gangel99]

In my relatively limited experience with RT work you need at least 3 clamps, 4 are preferred. Given that you are milling almost a complete circle you will have to reposition clamps. I try to have 4 so three of them can remain clamped while I move the 4th.

You might be able to have one on the L side (2" flat) that you do not need to move if your head does not interfere with the clamp. I think you also will need to mill the pieces separately because there will be too much chatter (or a lot more than 4 clamps) to get a decent milled edge.

[easymike29]

Can you put two more #18 holes symmetrically from the others and then use 8-32 hold down screws into a sacrificial plate?

Gene

[earlgo]

A suggestion from the stress world would be to add the .125R to the inside corners next to the clamp. This will prevent stress concentration failure at a future time.

--earlgo

[Dave_C]

Mr. Ron,

I almost hate to weigh in as there are so many ways to got a a project.

If I were going to make that item:

I would make a sacrificial plate that attached to my RT with Tee Nuts. Then I would make a pilot of 1/2" in the middle of the plate and put a dowel to center my workpiece on.

Now drill a 1/2" in the middle of your circle so that you can mount the workpiece on center.

Clamp it or use the two holes you show (more if you can) to fasten it down.

You can start the outside cuts easily and stop short of the square parts.

Leave the inside to the last cut as you will have to hold the outside with clamps unless you have adequate through bolts.

To cut the square parts you will need to turn the square parts so that they line up with the X and Y axis. The just cut up to your previous round parts.

Making the two meet is not an easy task and may leave slight tool marks depending on your skill level.

I would cut in layers of maybe .030 to .050" and not to the finished diameter. Leave about .010" for the final cut at full depth and it will clean up real nice with a climb cut.

Dave C.

[Mr Ron]

Thanks all. I realize there is more than one way to skin a cat. I just need to know which is the best way.
P.S. I like the sacrificial plate idea. Do you think if I add another #18 hole at the centerline on the left end, that will be adequate? I probably could add more hold down holes in the scrap area.

[SteveR]

I would leave the left and right sides to last. Mount to RT with extra on left and right, drill and bore the center, move the RT and mill the outside radii. Drill the #18's. Then bandsaw off the excess, L&R and clean up.

You call out your dimensions to .001, this implies (to me) tolerances of half a thou. For a router?

SteveR

[Harold_V]

You call out your dimensions to .001, this implies (to me) tolerances of half a thou.

It is quite common to see fractions displayed as decimals.Past experience (for me) indicates that there is a tolerance block specifying the amount permissible. Typically it is either ±.005" or ± 010". Based on that, to assume that tolerance would be only a thou would not be reasonable.

Greater or lesser tolerance was often indicated by using two place decimals, or four place decimals. Fractions typically allowed either ±1/64" or ± 1/32".

Since my time in the commercial shop, a whole different set of rules have replaced those with which I was familiar, so I may to off base in today's rules.

Harold

[warmstrong1955]

You call out your dimensions to .001, this implies (to me) tolerances of half a thou. For a router?
SteveR

Last place I worked, fractions meant a tol of +/- 1/32", two place decimal was +/- .03", and three place decimal was +/- .01". For more precision work, we used either limits, or +/- with tol(s) as required, or, it was stated on the drawing. I still use these 'rules' with my own drawings.
Job before that, was +/- 1/32", +/- .01", and +/- .001" respectively. I remember other standards from previous employers.

Just depends on who is doing the drawing & what their standards are. In cases where I get a drawing from a client, and I'm unsure...I always ask.

Bill

[Mr Ron]

It is just my personal preference to express eyerything in the same units of measure. In one place I worked at, fasteners were called out in decimal format; for example, 0.250-20UNC-2A. I usually would prefer 1/4-20, but this was mandated. As an amateur machinist, I don't assign or work to any set tolerances. To me, it either fits or it don't. I know this would rattle a lot of purist's, but none of my projects requires a whole lot of precision. I usually try to keep within ±.001 and make adjustments as necessary.