How Long Does a Boring Head Shank Have to Be?

Steve H. Graham · Jun 3, 2017

I have a Flynn boring head about 3" in diameter. It's a model 43. It came with a crazy arbor. The arbor is about 2" of 5/8" shank, internally threaded at the top end, with a taper resembling an R8 at the bottom. The tapered bit is maybe 7/8" long.

Problem: I can't get this stupid arbor out no matter what I do. I managed to put some decent torque on it by cutting a keyway in the tapered part and appying a hook spanner, but the boring head turned in the vise when I really cranked down on it.

People are suggesting I cut the tapered part down to 3/4" in diameter and use the boring head with a 3/4" R8 collet. My question: will a stub that short be sufficient to anchor the boring head in a collet?

I can make the stub an inch long by cutting into the arbor flange when I turn it down, but I can't do much better than that. I could conceivably build it up with MIG wire and make it longer.

The other alternative is to bore the arbor out of the boring head and remove the threads with pliers.

The boring head has a screw hole that opens onto the arbor threads. I didn't see anything in there that looked like Loctite. I filled the boring head with a combination of acetone and ATF and left it for a couple of days, and it didn't loosen up.

I'm afraid to screw around too much with the mill vise, because I don't want to bend anything on the boring head, and I don't want to use the dovetail or tool hole to apply leverage because I don't want to screw them up.

apt403 · Jun 3, 2017

My big shop made fly cutters (4" and 5") have 3/4" shanks. I can take a 3/4" DOC in 6061 with a radius form tool (that's a fun job) no problem with the power feed. The BP runs out of rigidity before the tool. As long as you're choked up tight in the collet I think you'll be more than okay with a boring head.

The shear strength on a 3/4" shank tool made of alloy steel is quite high.

Red James · Jun 3, 2017

Your 'stub' shank will hold just fine, turn it straight and go.

Steve H. Graham · Jun 3, 2017

I really appreciate the replies, but I want to make sure before I kill myself with a flying boring head: a shank 7/8" long and 3/4" in diameter held in an R8 collet will work with a 2-3/4" Flynn Model 43?

moonlight machine · Jun 3, 2017

Steve H. Graham said:
I really appreciate the replies, but I want to make sure before I kill myself with a flying boring head: a shank 7/8" long and 3/4" in diameter held in an R8 collet will work with a 2-3/4" Flynn Model 43?

Look in your R8 collet, it probably only has a 3/4 or 7/8 wide grip area anyway. It will be fine.

apt403 · Jun 3, 2017

moonlight machine said:
Look in your R8 collet, it probably only has a 3/4 or 7/8 wide grip area anyway. It will be fine.

Plus, the worse you'd be looking at is wrecking your part and the gripping surface on the collet - unless you decided to retract the tool when it started spinning in the collet, it's not gonna go flying anywhere. Snapping a carbide endmill is much more dangerous.

Steve H. Graham · Jun 3, 2017

Thanks again. This will make my life a lot easier.

Bill D · Jun 3, 2017

You may need a drawbar extension to grab it.
Bill D

moonlight machine · Jun 3, 2017

Bill D said:
You may need a drawbar extension to grab it.
Bill D

The drawbar screws into the collet on an R8 setup. The drawbar will be fine

Davis In SC · Jun 3, 2017

I have a big Criterion Boring Head that had a 3/4 shank.. I put it in a 3/4 Cat 40 holder in my Hurco, and got a lot of chatter and vibration.. Went to Travers and picked up a Parlec Cat 40 1- 1/2" threaded holder.. Huge difference, smooth finish, no chatter.

Steve H. Graham · Jun 5, 2017

I am stuck with R8, and it looks like I won't even get the full benefit of that unless I bore this arbor out and put in a new one.

Wild West · Jun 5, 2017

Steve H. Graham said:
I really appreciate the replies, but I want to make sure before I kill myself with a flying boring head: a shank 7/8" long and 3/4" in diameter held in an R8 collet will work with a 2-3/4" Flynn Model 43?

Post a pic of it so we can get a better idea. Put a steel rule next to it so we can get an idea of the actual size.

mo841 · Jun 5, 2017

The shank in there right now is most likely a left handed thread so make sure you are trying to turn it the right way when trying to remove it from the head

Monarchist · Jun 5, 2017

Steve H. Graham said:
I am stuck with R8, and it looks like I won't even get the full benefit of that unless I bore this arbor out and put in a new one.

A Wallbanger, Gamet, Narex, Chandler-Duplex, or Tree could justify fabbing a new mount.
DeVlieg wants having the whole 'infrastructure' to go with it.

Flynn (and Criterion) boring heads OTOH, are among the simplest, least-costly, and most common in the used market and have inexpensive Chinese knock-off copies that 'mostly just work' as well.

Might be time and money ahead to just go and find one that was a drop-in fit R8 hard-tail, not collet, and didn't need any f**k-with at all.

awander · Jun 5, 2017

mo841 said:
The shank in there right now is most likely a left handed thread so make sure you are trying to turn it the right way when trying to remove it from the head

What possible reason would they have for using a left-hand thread on the arbor?

Monarchist · Jun 5, 2017

awander said:
What possible reason would they have for using a left-hand thread on the arbor?

Reason? Why did MOPAR once put LH wheel lugs one side of a motorcar, RH the other, when a five-bolt wheel shouldn't care? And did not.

Same reason a dog licks his anatomy.

He is able to. No one really cares to interfere.

gbent · Jun 5, 2017

I have never seen a left hand thread on a boring head shank, even on a Flynn. If you take a large depth of cut the head will unscrew from the shank. I have seen a lot of right hand threaded boring heads with welded in shanks so they could be used left hand rotation without unscrewing.

Steve H. Graham · Jun 6, 2017

I learned something new about the boring head. The arbor flange, the boring head body, and the tapered bit at the bottom of the arbor are not aligned. I was trying to put the body in a 4-jaw chuck, and I found I could indicate the flange, the taper, or the body, but only one at a time. I stuck it in a mill collet and measured the runout, and I found that the body and the taper run out quite a bit.

I was going crazy trying to get it indicated, and then I found out it was impossible. I just assumed Flynn would put everything on the same axis. I know it doesn't matter, but it seems odd to make a tool this way. I'm used to things lining up whether they need to or not.

Now I figure I should just chuck the body, indicate the face and outside as well as I can, turn the taper down to a stub and not worry about it.

Steve H. Graham · Jun 7, 2017

I appreciate the help. I turned the stub down to 3/4", and it looks like it will be fine. The shank turned out to be a little over an inch long, so my rough measurement of 7/8" was off.

The outside of the boring head was hard to indicate because of all the holes and things, so I decided to indicate the face of it. I figured that way, even if the body and shank were not concentric, they would not be at an angle to each other. I indicated the face as well as I could, and I was satisfied with getting the body within a few thousandths.

I wish I could take it down to 1/2" in diameter so I could swap it in and out without removing the collet I use most of the time. I don't trust a 1/2" shank on a boring head.

06 07 17 Flynn boring head with arbor trimmed to stub small.jpg

Lakeside53 · Jun 7, 2017

If you simply cut that flange about 20 thou from the head down to the internal thread diameter, it will probably unscrew with you fingers. I've had two Criterion heads that no reasonable force could get the old shanks out, but by releasing the flange pressure they unscrewed by hand.

How Long Does a Boring Head Shank Have to Be?

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