Boring between centers?

[TTD]

...at least I think that's what you would call it?

Hi guys, I need some advice from any/all more experienced than me on a setup I’m thinking of trying. You say there is no such thing as a stupid question…let’s see if you still think that after this episode.

I only have a 7x12 mini lathe w/milling attachment so keep that in mind, too (i.e- very limited work space).

Problem:
I need to bore a .469” diameter thru-hole x 6.5” long in some 1” aluminum hex bar (6061) for an airgun breech. I’ve made a couple breeches before with 1”x 1.25” 6061 flat bar with 1/2” bore, but always seem to struggle getting a hole bored from one end to the other & have it perfectly concentric. Usually chuck the bar stock in my 4-jaw, opposite end in a cat’s head & drill halfway or better with a jobber length drill. Then flip piece end for end & repeat…& hope the 2 holes line up perfectly (hasn’t yet). Reaming after makes for a smooth bore, but doesn’t really help concentricity much if the 2 holes are out of whack to begin with.

Proposed solution:
I made an adjustable radius-cutting tool a while back to cut the profile on the bottom of a breech to match whatever size tube it will be mated to (mostly .875” o/d ) . The “tool” is simply just an 18” long piece of 3/4” o/d CR1018 bar supported in steady rest (15” total workspace) with a chunk of 1/4” drill bit shank ground for the cutter at mid-span, held in place with a 8-32 set screw. Works awesome & much cheaper/quicker than buying a boatload of ball-nose end mills.

Cutter orientation for breech:


Cutting .968” diameter radius with same tool for a dovetail scope rail:



I was thinking that this time around I might try making another tool like above, but this time using O1 drill rod & only .430” o/d with 5/32 drill shank for cutter held in place with a 6-32 set screw. I would still initially drill (under size) halfway from each end with .453” (29/64”) stub/jobber drills, then slide the hex onto cutter bar, mount assembly in chuck/steady rest with hex held in milling attachment (indicated to ensure it’s centered, of course). Then I would be able to make my first pass @ say .455” to clean up bore & ensure things are squared up & running true, then re-adjust cutter to make a second pass on size @ .469” (.007” DOC). Could even split that into 2 passes if need be to reduce flex (.004” then .003” DOC). Heck, I’ll even make 7 passes @ .001” DOC if that’s what it takes.

Do you think this is even feasible, or am I a fool for even thinking about it?

My gut feeling is that .430” o/d @ 14-15” clear span between chuck/steady rest might be like using a rope or piece of spaghetti. Then again, maybe it will work. Regardless, unless I’m told it absolutely won’t work (quite possible), I may give it a whirl just to settle my curiosity….wouldn’t be the first (or last) time I made a bad decision. Just thought I would ask here first before marching on.

Thanks in advance for any insight you can offer!

 

[Rooster]

Greetings, that's some nice looking work. You might want too check out some Kieth Fenner video's, he has done some on line boring. I saw one where he had bushings on each end.
Also am curious why your using the steady rest and not the tail-stock with live center.

 

[TTD]

Thanks, Rooster & I will check out Keith's videos...thanks for the tip.

Also am curious why your using the steady rest and not the tail-stock with live center.

That's how I would prefer to do it, but there's just not enough room on my 7x12. Tailstock w/live center only leaves about 10 3/8 - 10 1/2" of work space....I need a minimum of 13.25" (6.5" material x 2 plus at least 1/4 - 3/8" worth of clearance).

I soooo badly wish I could afford a bigger lathe. Even a 10x22 would work perfectly for what I do 99.9% of the time. Can't see that happening anytime soon, though .

 

[British Steel]

If you have length enough for the work, the cut and the boring bar plus drive dog/whatever, it should work (light cuts, fine feed!) - I'd suggest that the breech blank should be *very* rigidly supported for its full length from the carriage, the between-centres bar is going to have a constant load and hence deflection from the full length of the cut, but should the workpiece be able to deflect you'd likely get a tapered smaller (but concentric) bore at the "free" end where it could deflect, or if it could only deflect in one plane an ovalised bore where it wasn't properly supported.

I'd suggest using the headstock and tailstock centres instead of the chuck and steady rest if you can (you may not have enough bed length?), you'll have to remove the work (hard to be sure you return it to the *exact* same position relative to the carriage) or the boring bar to measure the bore as you progress and you're more likely to get a shift in position if a) you have to back off the steady rest then re-position it and b) remove the boring bar from the chuck - dead-centres will pretty much guarantee that the bar goes back exactly where it was and each cut is concentric with the previous.

Dave H. (the other one)

Edit - saw the limitation between centres after I'd posted - lots of attention to everything going back in exact same places is going to be required...

 

[ThunderDog]

I did some line boring on my Myford "M" series. My cutter was close to the tailstock because I was low on stock material and it was serving as a dual-purpose tool. Long story short, it was close to the tailstock. It flexed with small cuts, but the accuracy was amazing for the age of the machine. Plus, it was so fun to use the lathe in a different way. I say go for it and realize that small cuts are just what needs to happen.

Here's the video of my first ever attempt at line boring. Jump to about 2:43 to see the process.

 

[Nogoingback]

I've never tried anything like that myself, but I have read a description of doing what you suggested and it sounds entirely feasible.
The author uses a boring bar similar to yours and showed it mounted between centers and driven with a dog. The work was mounted to the
carriage. He noted that the setup is rigid and accurate.

 

[f350ca]

I'd give it a go Todd, as Dave said the flex it the boring bar is constant so you should get a cylindrical bore. Only concern is having to remove the bar to measure. Getting it back in the same position and adjusting the cutter will be tricky.
If we ever get together for a coffee I'll try and collect some broken end mills for you, they have a hardened shank, drills don't, they'd make beter cutters. Broken carbide drill bits work great.

Greg

 

[Janderso]

Pretty darn creative if you ask me

 

[benmychree]

Chucking the boring bar would make it much more ridgid, and I'd use a bushing carried by the lathe's carriage just past the far end of the bore, this reduces the excessive overhang of the bar, all the way out to the steady rest. I note that it is always the case that parts that are turned between centers are much more liable to experience chatter and deflection than parts that are chucked on the driven end and supported by a center on the other end.

 

[f350ca]

A bushing would defiantly stiffen the bar but wouldn't that change the flex in the bar as you travel trough the bore?

Greg