How to Make a Tapered Slot?

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Active User
Sep 28, 2010
I need to make a tapered slot in the side of aluminum or steel round stock (work piece). See the pics. Ten of these are needed. They will fit to another piece which is not relevant to the problem at hand.

The round stock shown is 1" long if aluminum or 1.25" long if steel. The diameter is 1/2". The tapered slot is 1/8" wide and 1/4" tall on one side of the work piece. The slot tapers down to a 1/8" diameter hole on the other side of the work piece. The long sides of the slot are straight. The "floor" of the slot is round with a 1/16" radius. So is the "roof" inside the angled slot. I calculate the angle of the taper (top of slot) at 14 degrees, from horizontal, with the pin standing vertically as shown, but I could be wrong on that. The bottom of the slot is at 0 degrees from horizontal, with pin standing vertically as shown.

I can easily cross drill a 1/8" hole to make the bottom part of the slot. But making the tapered part stumps me. I could make a 14 degree wood triangle, angle the pin in the vise, and go at with a 1/8" endmill, but it would have to go in slightly further than 1/2" (due to slope of slot and resultant increased distance), and the endmill cutting surface is only 1/2" long. Even if it could reach as far as needed, I am nervous about breaking the endmill, even if in aluminum, much less steel.

My tools are a 1946 Southbend 9" lathe and a 1951 horizontal/vertical knee mill (Rotex, w/5 x 20 table).

Any ideas are welcome.

Thank you.



Isometric - 1.png

T Bredehoft

Active User
H-M Supporter - Gold Member ($25)
Dec 27, 2014
Your isometric view doesn't show us the taper....perhaps a cross section or two would help.

P. Waller

H-M Supporter - Sustaining Member
H-M Platinum Supporter ($50)
Mar 10, 2018
This is difficult geometry for conventional machining and would require a a 5 axis mill or WEDM.
If you have high surface finish and dimensional limits this will prove difficult to do manually.
The drawing shows a slot on one side and a round hole on the far side TB. It is also not symetrical.
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Mar 12, 2018
I would go about it on the mill. Reduce the shank on your 1/2” mill so you can go deeper without rubbing. I would do the angled hole first. Exactly how you said angle block under part in vise. Then put your piece horizontally and do the straight through hole with endmills. Then you can go side to side and get rid the f ridge between cuts. Make sure you pull up on depth to not cut tapered side. That’s what endmills are for. Very doable.


Global Moderator
Staff member
H-M Platinum Supporter ($50)
Feb 8, 2014
I've cut 1 inch deep with a 0.093 endmill, in tool steel without breaking it. So don't fret that too much. Light cuts and a steady hand. Normally something like this I would plunge cut with about a 20% stepover.

If you have a horiz/vert rotary table it would make life easier, but not required. I would make a fixture that holds the part and allows it to rotate around the hole centerline. Then after drilling the pilot hole, rotate a bit, plunge, rotate, plunge, rinse/repeat until you hit the stop at 14.0362°.

Could easily be done on a 3 axis CNC using a 1/8 ball end mill, but this also can easily be done on a hand cranked machine as I described above.
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