Fixturing to cut an arc slot

[jwmelvin]

I'd like to use my BS-0 dividing head and PM-30MV mill to cut a slot in a piece of 1/4" thick aluminum. It's the front piece in this drawing (dimensions are actually not that critical and odd because designed in mm but drawn with in.; slot width not dimensioned but 3/8" endmill will work well):


I have seen this thread, and I think that's basically where I'm headed but I appreciate any guidance.

I have a large round plate (5/8" thick x ~11" diameter) that I figured I would bore the approximate center of and press in a short shaft to hold in the 3-jaw on the BS-0. Then I can mount the work piece on that plate (I guess with clamping screws on the straight sides. I'm considering tapping the center of the shaft in the tooling plate 1/4-20 so that I could clamp with that too, but it'd be nice to leave the center open to indicate off the hole in the workpiece. I suppose I'd like a nice way of centering something on the tooling plate, even if that means a short stepped shaft with one side that fits a recess on the tooling plate (the same recess I'd indicate on to align the spindle with the rotary table) and another side that fits a feature in whatever part I'm working on (here, the 0.315" hole).

Thanks,
Jason

 

[Lo-Fi]

You're basically making a mini pallet to put in the dividing head chuck so it's like a small rotary table? Sounds pretty good to me. I use the centering spigot method all the time on my rotary table, it works well. However. Be very wary of holding large diameter work by a small diameter. Very easy for the cutting forces to shift it with such large mechanical advantage.
A few thoughts:
Bore three blind holes in the back of the large plate big enough to take the ends of the jaws. Easy to accurately mark out with dividers on a circle and punch mark for drilling and gives you the exact centre hole position too. You'll want to make sure they're the same depth unless you can get them deep enough for the plate to rest against the next step in the jaws. This gives a vastly more secure register between the pallet and chuck. Even better if you have a set of reverse jaws for the chuck. Either way, set the holes on as large a diameter as possible.

Pics of the final setup please

 

[jwmelvin]

Bore three blind holes in the back of the large plate big enough to take the ends of the jaws.

What a good idea, thank you.

I had already started the bored center hole (my first time using a boring head), which complicates marking the three holes for the 3-jaw. I decided to continue boring for a slip fit plug that is center drilled, giving me a place to locate the center of the compass. That let me get a feel for hitting a target with the boring head; it was fun.

I will now mark out and create the pockets for the 3-jaw. My tooling plate isn’t that thick so I may make them holes not pockets.

 

[Lo-Fi]

Setup problems are the most interesting part of this hobby (or profession) in my humble opinion. Thanks for posting for it to be discussed

Through holes sound like an easy way to go.

It'll be a great tool for future use as you can always put drilled and tapped holes in to suit what you're working on. Easy when it's on the dividing head, as is milling concentric marks to aid positioning the work of you want to get fancy. I like it so much I think I might actually make one up myself!

 

[jwmelvin]

The quick-indexing feature of the BS-0 made drilling and boring holes for the 3-jaw pretty painless. Because I sized the center bore for a slip-fit insert, I super glued that insert to hold with the 3-jaw, and ran through each hole each time I expanded the boring head.

Now I’m ready to mount my workpiece and get started on the part itself.

 

[Lo-Fi]

Fantastic!

Some mini projects are worth the little extra time

 

[jwmelvin]

I can call this project a success! I finished the tooling plate with some tapped holes:

And used it to make the arc slot:

It was great having the center bore to first align the rotation axis with the spindle and then use an insert (visible in the top picture) to align my parts.