Boring between centers?

[brino]

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

 

[pdentrem]

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.

 

[benmychree]

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?

 

[TTD]

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!

 

[Chipper5783]

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.