What lathe tool to turn conical cavity?

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HandyNotHandsome

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I'm a new owner of an SB 9A, and one of the first projects I need to do is turning what amounts to a funnel - an internal cone - in mild steel and/or stainless. The small end (throat) could be as small as 0.150", and the taper is about 15° half-angle (30° total), with a length of maybe an inch. What tooling do I use for this? Boring tools that will go to 0.150" are very thin along their entire length, since the assumption is that they need to do a hole, not a cone. They are also not very long, so 1 inch might be a stretch. I predict that they would not last long with my skills :).

My tool-making skills are also not great, so if I grind one myself, it needs to be simple. I was thinking I could start with a 3/8" HSS rod and grind the end down into a cone that's a bit shy of 30° - perhaps 25°. The tip needs to be a bit smaller than 0.150". Then grind off the top half (top, as seen when the rod is mounted in the tool holder) of the end for a distance about 3/8" back from the tip. That's it. Although this will have 'some' clearance for the cutting edge, I suppose I should also grind the underside of the cutting edge back to a 7° angle. And a flat or two on the shank to make sure it gets mounted at the proper rotation and doesn't twist, in the QC tool holder.

Is there an easier/better way to do this? Is there an off-the-shelf product that would work?
 
Yeah, but a 0.150" boring bar looks like this:

1660089639274.png

The thinnest part of that shaft is probably less than 0.100" in diameter, and a bit less than an inch long (drawing is not to scale). It doesn't seem as durable as what I'm suggesting, though it is "off the shelf".
 
The boring bar does not know you are cutting a taper. It does not care. that is handled by machine setup.
If your small end finished size is .150 then you need a boring bar that is smaller than .150.
Is this a thru taper or a blind hole?
On the small boring bar shown it will only be the cutting point touching the part. the taper will allow the bigger shank to fit in with no issue.
Yes you will need to take a lot of light cuts to avoid breaking the bar.
Do some math and use a number of drill sizes to various depths to remove most of the material. start with the biggest one first and work down to the smallest. Then the bar just has to clean up the steps.
 
You could use a boring bar intended for a larger minimum hole and grind a 15° 30°conical clearance on the back side of the head.
 
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Flyinfool said:
The boring bar does not know you are cutting a taper. It does not care. that is handled by machine setup.
If your small end finished size is .150 then you need a boring bar that is smaller than .150.
Is this a thru taper or a blind hole?
Click to expand...
The narrow end of the taper terminates in an axial through hole - not blind. So I have some room to work.
I understand the tool needs and the operation (I've actually done this before, but on a larger part). My description of the tool I planned to make was just an attempt to get something significantly beefier than an off-the-shelf tool capable of getting into 0.150". I think something made from 3/8" rod will last longer - in my hands - than the tiny shaft on the commercial item.
 
I guess I might use a 2 flute 1/8'' endmill. I use endmills quite often as a boring bar.

Or maybe grind a tap to any shape you need. I make special tools out of worn and broken taps all the time.
 
When I started out as a newbie, all my bores were tapered.
Unfortunately, that's not what I was aiming for!

But seriously, you should use the top (compound) slide setting to give you the required taper.
A few test cuts in (aluminum) scrap should help you set it up at the proper angle.
Alternately find a male taper that is what you need, mount it between centres and dial in the top-slide angle until you see zero movement in a dial indicator running against the taper.

Brian

EDIT: to clarify the indicator would be mounted on the top slide.
 
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