Drilling or machining vertical hole/pocket in edge of cylinder?

coherent

coherent

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Ok, I'm sure the smart way to drill the 4 through holes and then drill or machine the countersink pockets would be to do so before it was machined. But this is a precast aluminum spindle mount for a cnc router and I want to do so to fit my existing Z fixture's threaded mounting holes. I can drill the through holes from the bottom. But is there a recommended way to machine the pockets other than just use a center cutting end mill and vertically plunge them carefully? I have a long enough end mill, but none small enough diameter that long to allow helical pocketing or ramping on my cnc mill. Hopefully the photo is large enough to see the lines in the drawing where the holes/pockets go.
spindle mount.jpg
 
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You can also make a plug to fit the big hole and start with an end mill and then drill through. The plug needs to be a fairly tight slip fit and have a shoulder so you can clamp it against the vise jaw to keep it from moving.
 
+1 on the plug. Keeps the drill from walking when going through the opening. Also a piloted counter-bore can be used for the pocket.
 
Thanks for the suggestions. There are no holes at all in it now, so I'll need to drill the through holes (1/4) and then drill or mill the top four to 3/8 (they just provide access to the machine screws with an allen wrench) and then the countersinks/pockets in the lower 4 holes for the actual mounting screws. The piloted reamer makes sense... but of course another tool I don't have
 
4ssss said:
You can also make a plug to fit the big hole and start with an end mill and then drill through. The plug needs to be a fairly tight slip fit and have a shoulder so you can clamp it against the vise jaw to keep it from moving.
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+1, specially since you got to drill them other holes anyway
 
coherent said:
Thanks for the suggestions. There are no holes at all in it now, so I'll need to drill the through holes (1/4) and then drill or mill the top four to 3/8 (they just provide access to the machine screws with an allen wrench) and then the countersinks/pockets in the lower 4 holes for the actual mounting screws. The piloted reamer makes sense... but of course another tool I don't have
Click to expand...

The piloted reamers won't work unless the center hole is plugged. I'm assuming you want the 2 holes to meet. By the time the reamer hits the big hole, your pilot hole is gone and the reamer will walk. You won't be able to pick up the other side of the hole because the reamer wont side cut like an end mill. Piloted reamers are just that, a reamer that follows the original hole, and needs the original hole to follow like a counterbore. The way I'd do it is as I wrote above, make a shoulder plug, clamp it in the vise against the plug so it won't move, put the casting on the flat end like your drawing, and end mill the top hole just until you get a big enough flat bottomed hole to put a center drill in and to drill it, and drill thru the plug and the casting. Unless the center hole is plugged the drill will walk as soon as it breaks thru the center hole because the drill is not made as an end mill and also has no side cutting ability. You can use a piloted reamer, after you make a plug, as then there is a pilot hole to follow, but why waste your money when a drill will do the same thing, follow your original hole.
 
A key point is that the plug is as close to the same material as the part as possible. Don't use a steel plug in an aluminum part, for example.
 
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