How would you fix this workpiece and locate the cutting point ?

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compact8

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Up to this stage the workpiece is basically a brass cylinder with three 2.5mm-wide, 2mm deep keyways cut on the inner side. Now I need to cut out the fourth one at the marked position shown in the photo. This keyway is going to be 3 mm wide, a lot deeper ( 8 mm ) and slanted at an angle of 37 degrees from the longitudinal axis of the cylinder. The four keyways are to be evenly distributed along the circumference of the cylinder.

I will appreciate some suggestions on how to fix the workpiece securely in a slanted position so that it won't move when broaching is done ( with my hobby mill ) and also the method to bring the tool to the wanted position accurately. Thanks in advance.
 

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It would help to know what tools and machines you have available.
The easiest way would be using a tilting rotary table with a 3 jaw chuck which you probably do not own.
 
You could bore a hole or mill some V's on the angle you need in a clamping fixture. Line up the part with a DTI referencing the 2 keyways that oppose each other, locate the center and broach away!
 
The slanted part is what makes this difficult. Assuming you are using a mill. Fixing the part would mist securely be done with a chuck. If you have a mill with a tilting head the part fixing problem will be simpler as the head can be tilted at your slant angle to broach the keyway. If mot, you will have to use a tilting table, angle vise or sine vise.

In order to orient the fourth keyway with respect first three, you will want to align the previous keyways along the x and y axes. To do so,keys can be fitted to two opposing keyways and the part orientation adjusted so an edge finder or test indicator shows equal x position. A test indicator in the spindle or edge finder prior to tilting the head will allow you to find the center of part. This won't change in the y direction when the the head is tilted and a line along the x axis will run through the center of your new keyway. After tilting the head, you can find the edge of the bore with and edge finder and proceed with your cut.

Using an angle vise or tilting table is more difficult as your setup will have to be done at an angle. I would first align the vise or tilt table along one of the axes. Then the centerline of your cut and efge of the bore can be determined with an edge finder.
 
Hold the part in a V block set in the vise. Tilt the V block to get the correct angle. Use a sine bar or protractor or angle blocks to set the tilt. If you have a V block with a large enough clamp you can clock the existing slots at the surface plate. Then transfer the V block to the vise and set the angle.
 
Set them up the same way I did when I cut my tapered keyways with my shaper:

www.hobby-machinist.com

Cutting tapered keyways with my Rhodes shaper

Man I love my little Rhodes shaper! It's a very handy item to have in a shop and the size is perfect for the bulk of the work I do. I only got one right hand grinding wheel adaptor with my Cincinnati #1 tool and cutter grinder so I decided to make 6 of them; 3 right handed and 3 left. They are...
www.hobby-machinist.com www.hobby-machinist.com

I used a angle gage to set the angle for clamping in the vise.

Ted
 
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