Boring small diameter holes with end mill held in lathe tool holder?

That's right. Started doing this a little while ago on a lathe tool holder with a couple of allen screws (Hardinge tool holder). Just be sure the one of the four cutting teeth at the end is 90 degrees to cutting surface. Started doing this due to chatter using HSS boring bits. This chatter appeared when hand-cranking a 10 degree slope in a 1/4 internal diameter cylinder. The only thing is that the cutter contact is almost nothing and I just plow through very slow in. On the way out there is still some slight cutting, due to deflection?

(Here "small" means 3/32 or less)

Is this a question or just a statement? We do this on occasion if needed as well. In particular, we have one part that requires a .22" full radius so we use an indexable ball end mill as a bore bar. Works well.

There was a post a little while ago about boring and somebody stated that a boring bar can be ground out from a worn end mill. I'm using a brand new one with success and I have four positions it can be rotated before it's worn out.

Been doing it for years. I usually clear the flutes behind the tip so they don’t rub.

I do this when re-cutting the center in a shaft.
I'll use 1\8" to 5\16".
Never throw away those old end mills.

same here end mills can be handy like this. have to locate the cutting tip well and for me instead of grinding back the flutes i just angle the tool post to give the rest of the endmill clearance

Same, done it often since a friend told me to try it. A little angle in the cutter gives adage relief but the mill doesn't need it, right? Nice option for small holes.

I do this all the time for small holes, I also relieve the flutes so they don't rub, only a issue if you bore deeper than one flute revolution though. I've also ground a fair number of custom ID threading tools from snapped taps, typically smaller sizes.

Just did a hole in wood and stainless 316 today.

On a lathe I move the bit into the hole with the bottom of the tooth doing the cutting.
Moving the bit out of the hole causes the side flute of the tooth to do the cutting.
Which direction is preferred?

I've also taken a broken tap ( 5\16-18), and ground the end flat next to the first good tooth. Then grind off and relieve all the other teeth back to the shank.
This give me a small threading bar.
I try to leave the other flutes stock, and grind them when needed.
That way a 4 flute can be used 4 times.

When I first got my lathe I had a lantern tool post, some tool holders and carbide endmills. I had no square tool steel so I put 1/4" endmills into the tool holders and tried things out with that. I had to clock the endmill so a cutting edge was correct but it worked fairly well.
Bill D

Probably a little four flute end mill is a bargain considering you have four cutting points.

An option for boring non standard size holes is grind the end mill flutes to the correct OD. I use them in place of reamers on some parts. The other option is grind the flutes a couple thousand under size of your finished reamer size.

Guys use to save all their resharpened end mills that were now undersize after being sharpened, you can easily make custom sizes. Using a surface grinder and indexable spinner, they come in handy for so many operations.

Hi All:
It's a good technique for shallower holes as you all have pointed out.
Where it becomes a problem is in deep holes, and the reason is that so much of the carbide is cut away to make all the non-working flutes that the bit that's left is not as rigid as it needs to be for a deep hole.

I bore lots of skinny holes, and my favourite tool is still a custom ground bar with an oval cross section.
I start with a spun down carbide blank (usually a broken carbide endmill) and I make it so it will fit into my pre-drilled hole with just the minimum clearance I need to drop it into the hole and back it out again without touching the sidewall.
Then I offset the bar a bit and grind relief on the shank that will be closest to the cutting edge, and I grind it to the same diameter as the rest of the shank, leaving a bit at the tip untouched.
That gives me my oval cross section and provides a place for the chips to go.
Then I split the tip of the blank, and grind my tip profile on it, often just freehand by eye.
This is the strongest bar geometry I've found and lets me sneak into some pretty tiny bores with confidence.

So yeah, the endmill trick is a great one and I use it too when I'm too lazy to grind a proper bar, but it only works up to a point...typically an LD ratio of maybe 3:1 for sizes under 1/8" diameter.
Beyond that the endmills tend to scream when you only load one flute, and as others have pointed out, the flute will touch the sidewall in more than one place unless you cant the whole works, which reduces the depth you can go and increases the minimum diameter you can cut for any given endmill size.

Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining

rons said:

Probably a little four flute end mill is a bargain considering you have four cutting points.

Click to expand...

Especially considering that I bought them used on ebay for my woodworking router. I think depth of cut might have been an issue but all I remember doing is opening up the inner bore on some washers to make them into a coverplate over a bellows seal. Actually a silicone baby bottle nipple to seal the shift lever on the apron.
Bill D

Per post #2. Question or statement? Micro 100 makes boring bars that will easily do less than .100" ...? List price around $30-40. But I get if you are in a pinch and need something now