Taper on helical interpolated bored holes

I like using a spiral boring approach on deep accurate holes. But still sometimes get taper (from deflection). I'm not really sure why though since I would expect even a dulled cutter would have such little loading a dulled corner wouldn't matter. Replacing with a sharp tool does the trick though.

Tonight I'm putting a 2" diam hole, 3" deep in 1018 using a 1" 4-fl HSS cutter 4" lg. Using a toolroom style bedmill, not very rigid, its a slow process but only 4 parts. After roughing in the hole and leaving .02 on the wall I spiral down in .02" steps. Disappointed to find a .017" taper from top to bottom. I know the cutter is dullish, but all the corners are still there. surface finish is good, and with such little loading I expected a straight hole even with a dull tool. Wouldn't the tool deflection be exactly the same at the top of the hole as at the bottom since the length of engagement is just the .02 from the top to the bottom?

The question is whether spiral boring is indeed the best chance of success when hole size matters? Or some other CNC strategy?

Looks like ill be boring these

I suspect a few things

1) HSS flexes much more than carbide
2) Yes, even though you are only cutting with the .02" bottom of tool, as it goes deeper, the flutes get more pressure on them
3) You said it was dull'ish "nuff said"

vmipacman said:

The question is whether spiral boring is indeed the best chance of success when hole size matters? Or some other CNC strategy?

Looks like ill be boring these

Click to expand...

Answered your own question there.

Do you want to keep that 1" end mill forever? Go to a bench grinder and back off the flutes so only about .250" at the end is sharp. Or chuck a boring bar (HSS type bit) in the spindle and use it as a single flute endmill.
The flutes of the 1" are not cutting because there is so little material left, and they are rubbing at that point.

dodgin said:

Answered your own question there.

Click to expand...

That was not my question though.

Flexy machine, bendy (cheap, badly ground?) cutter, concentrating wear on tiny bit of cutter, poor machining practices, etc.

Bore the hole. No chance of a round, straight hole with current methods.

I'm sure someone will disagree but I save HSS for the manual machines so chips aren't flying everywhere. As was mentioned carbide is much stiffer.

I have a pile of endmills and inserts sitting on my desk that "look" sharp. Finish, sound and spindle load say otherwise.

Not to be hard headed (too much), but I know how to fix it, and I know what caused it.
I want to understand the physics of question 2 more.

Phrased a different way my questions are:
1. Is helical interp the best CNC strategy for round deep holes, or is plain circ interp best? (given all other things equal)
2. What causes deflection to be greater at bottom of hole given that cutting pressure is constant (and low) top to bottom (with helical interp). In my mind it was like using a 4 pointed boring head and in my experience boring bar deflection is generally constant once in the cut a little ways regardless of depth.

Unless you are using a relieved tool more flute contact = more deflection. As the tip of the tool dulls this become more pronounced. I interpolate press fit bores both ways and can't say one is necessarily better than the other.