How can I make this feature?

[DavidR8]

I have drawings for a knurler arm and I can't quite figure out how to make one of the features.
The feature in question is the semi-circular notch at the top.
I have a few approaches in my head:
1) a 7/8" annular cutter but I've never tried using one with only partial engagement in the stock
2) a boring head set to the correct diameter
3 a flycutter with an appropriately ground cutter set to the correct diameter

Workholding is also furrowing my brow...

 

[Janderso]

I'd use a boring bar in the mill. I recently had a similar need, the boring bar worked great. I just snuck up on the radius feeding in about .025" at a time.
This is the radius I am referred to.

 

[C-Bag]

I’d definitely go with the annular cutter. I have a 7/8” dia endmill I use a lot and use it a lot like an annular cutter for such cuts.

work holding I’d go with the usual clamp blocks on each end and a backing block/fence across from where the half moon feature is going to be cut. I’m sure there’s nothing surprising.

 

[pontiac428]

Depends on how precise it has to be. I'd vote for the boring head, because you can creep up on an exact dimension after roughing. The fly cutter is less precise but accessible. I don't know anything about the accuracy of annular cutters, the only ones I have are for drilling out spot welds in sheet metal, so I don't know whether they're good to hundredths or to thousandths.

 

[francist]

If you do opt for the annular cutter, clamp a sacrificial piece alongside the part you desire so you don't have partial engagement. Just toss the scrap piece afterwards.

 

[Janderso]

If you do opt for the annular cutter, clamp a sacrificial piece alongside the part you desire so you don't have partial engagement. Just toss the scrap piece afterwards.

I've never used an annular cutter. They don't like interrupted cuts?

 

[francist]

I've never used an annular cutter. They don't like interrupted cuts?

Nor have I, but my experience with anything like that (milling cutter, hole saw, etc) is that there is usually a tendancy to slide out of the cut and away from the workpiece if less than half of the cutter diameter is engaged. A sacrificial piece on the outside gets you back to full engagement so no drifting.

 

[GeneT45]

Second the boring head. You probably have one, and they're reasonably quick and accurate. I've done this to cut the front radius for a "doktor" sight on Glock pistol slides.

GsT

Edit: egregious typo...

 

[DavidR8]

Thanks gents, I have a decision to make as I actually don't own any of the options.
That said I think I'll opt for the boring head for a couple of reasons.
1) it's a multi-use tool
2) I like the potential to 'sneak' up on the dimension.

@francist I like the idea of scrap stock to avoid the sliding out of the cut tendency.

 

[rabler]

Relative neophyte, but my first thought would be using a 7/8" endmill to side cut it. That's a fairly large endmill and not one I would have on hand, so for a one-off I'd go with the boring head.