# Boring between centers?



## TTD (Apr 15, 2018)

...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!


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## Rooster (Apr 15, 2018)

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.


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## TTD (Apr 15, 2018)

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



Rooster said:


> 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 .


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## British Steel (Apr 15, 2018)

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...


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## ThunderDog (Apr 15, 2018)

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.


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## Nogoingback (Apr 15, 2018)

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.


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## f350ca (Apr 15, 2018)

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


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## Janderso (Apr 15, 2018)

Pretty darn creative if you ask me


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## benmychree (Apr 15, 2018)

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.


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## f350ca (Apr 15, 2018)

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

Greg


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## brino (Apr 15, 2018)

Hi Todd,

I have never done it personally, but have seen it in many manuals....

The Southbend classic "How to Run a Lathe" shows it:



So does the Atlas "Manual of Lathe Operation" aka "MOLO":



As Greg mentions I'd think adjusting the cutter for the final pass to be kinda tricky.

-brino


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## pdentrem (Apr 15, 2018)

Place a bushing in the bore to support the bar. If you have the room, bore the end to near finished size, make a bushing that is sliding fit to the bar and place it in the counterbore after the cutting tip is in the end of your work. It will support the bar and reduce the deflection. 

Use a parabolic drill will help reduce the wandering of the drilled hole as it will clear the scarf quickly. Quite often drill deflection is due to the scarf making random movements to the drill as the chip pack up somewhere along the flutes. This why pecking is important. A gun drill would be a proper drill to use but pricy and not everybody has the equipment to use it. You can still use the techniques of bushings and reaming to size at the end.


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## benmychree (Apr 15, 2018)

f350ca said:


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


I doubt that it would be detectable.  Another thing is that even a small increase in diameter has a large effect on stiffness, for instance, given a 1" diameter bar, increased to 1-1/4" has 5 times the stiffness, and it matters not whether a hardened alloy steel is used for boring bars or arbors so far as stiffness is concerned, as the modulus of elasticity varies only a tiny amount for soft and hard steels or alloy steels and is not effected by heat treatment; this is why solid carbide is used for boring bars that have to bore deep holes; the ratio for HSS is about 5:1 of diameter to length for overhung boring bars made of HSS, or any steel for that matter.
How perfect does perfect have to be?


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## TTD (Apr 15, 2018)

benmychree said:


> 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.



So basically (or exactly) like a follow rest of sorts? Hmm…I like that idea!

Why is it that sometimes the most obvious answer is the one that eludes me the most??? 

Pretty sure I could fab something up quick & dirty for the occasion. 6.5” of material + cutter would leave me 1.75” - 2” of “relief space” at the front once rear of hex bar clears cutter. Enough room for a small follow rest to fit. Heck, if I’m really worried about flex in the cutter bar (as Greg stated…good point, btw) I could make two thin (1/2”+/- wide ) follow rests - one in front & one following behind. She’d be a tight fit, but I’m getting used to that by now with this lathe. I’m sure I can make it work.

Regardless of what route I may take regarding follow rest(s) and/or bushings etc, once the initial “rough” drilling is complete & everything is set up ready to use the line boring tool, I have no intentions of removing anything from anywhere at any point until I’m finished (or at least think I’m finished). I’ll measure the rough drilled diameters at both ends before mounting, then basically rely on math alone after that. Initial set up is undoubtedly going to be critical & finicky, but if cutter bar, starting & finished diameters is known, then it’s not hard to figure out how far to protrude cutting bit out from bar after each pass. That’s my plan of attack, anyways. That’s how I’ve been doing the bottom radius on my other breeches/scope rails with good success. This will be pretty much the same procedure. The few radius’ I have done with my 3/4” bar took 12 passes @ .006” DOC & a final pass @ .003” (.075” total DOC)

 For this project my final diameter of .469” - .470” isn’t carved in stone, either. I have a .020” leeway (+/- .010” of target i/d)  to play with so not worried there. Barrel isn’t machined yet, so it’s basically aim for .470”, hope for somewhere between .460” - .480” & ultimately machine the barrel to suit whatever I end up with within that range. Going to be conservative considering the size of this particular cutter bar, so going to make light passes of .002” - .003” DOC for its maiden voyage & see what transpires

Not asking for much…all I want is the hole centered at both ends & straight in the middle! 

Once I gain some experience & fine tune my methods with this set up, then, & only then, will I even consider trying to match an i/d to an already finished o/d.

Thanks to everyone for all the tips/advice so far…it is most definitely appreciated!


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## Chipper5783 (Apr 15, 2018)

When you make the bar, cross drill it at about a 45° angle (not straight across).  That way you end up with a longer bore, and there would be enough room to put a small set screw behind the tool (of course you still have the clamp screw as well).  Then you can advance the tool a predictable amount.  I agree with the other comments above, about using as large a bar as possible - but you can't go too tight.  You'll need some room for chips.  Try for something about 0.4" - which gives very little room if you go straight across.  It also gives you less that 0.02" of chip clearance on a 7/16" starter hole.  I suggest very light cuts on the first couple passes (taking just "angle hair").   I can't take credit for the idea - I think I saw it in one of Guy Lautard's Bedside reader books.


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