What Boring Bar

[Gary]

I have a QCTP (AXA) and need to get a boring bar that's 6" long to bore out my faceplate(s). I like the idea of the SHARS replaceable carbide tips and I'm inclined to go that route. Do I need a 5/8's or 3/4 bar; or is a 1/2 inch diameter bar OK?

 

[Brad]

I would go as big as you can get in there. Rigid is your friend.

 

[Gary]

Kinda what I suspected. However the largest drill bit I have is a 3/4" and I think I might need at least a 7/8" bit to be able to use the larger bar.

 

[Brad]

I find you can get by with a little less rigid but need to take lighter cuts.
How big will your tool holders handle? That may be the bigger consideration.

 

[Gary]

The AXA holder will take a 5/8 or 3/4 round bar.

 

[KeithB]

What specific carbide insert type were you planning to get? TPU types are positive rake, you can get them in several corner/tip radii, and they are relatively cheap and widely available, They also exert the least force and resulting deflection. The triangular and trigonal types with a negative rake geometry are also good but require more cutting force. The square/rectangular negative rake types require the most cutting force.

Negative rake cutters are tougher and stronger and less likely to chip; positive rake tools need less force to cut. If you're not planning to make it a production operation positive rake is fine.

It's always a balancing act between size/rigidity and use in small holes. The more you can "choke up" on a boring bar the better off you will be. Most vendor's catalogs will list the bar diameter and minimum starting hole diameter for most boring bars.

 

[Gary]

"Positive 80 degree CCGT inserts" is what the bar I'm looking at uses. 5/8's x 8" bar with a CCGT32.51 insert.

 

[Pistolero]

The bending stiffness is a cube function of the height of a beam.......so even a little
bigger is a lot stiffer.

What Brad said is dead on.

Bill

 

[smokeywolf]

Gary, If you don't have a 1/2" dia. bar, I'd consider getting the 5/8", since you can't bore a 3/4" hole with a 3/4" bar, but you can bore a 7/8" hole with a 5/8" dia. bar. I would also give serious consideration to forking over the extra money for a solid carbide boring bar. I've bought several on "that auction site". I have a preference for "Circle" brand solid carbide bars.

 

[KeithB]

Smokey makes a good point. Carbide is much stiffer than steel and can make up for being a smaller diameter. We only have one, it is a 3/8" diameter bar that uses a VERY tiny triangular insert. Figured the increased strength and rigidity made it worth the extra cost.

 

[Gary]

Ordered a 5/8" bar from Shars this morning. I only plan to bore out Aluminum for my projects.

 

[KeithB]

Bet you will be happy with that. Looking forward to pictures of what you do with it.

 

[Pistolero]

Interesting. I hadn't thought of carbide as a material for a insert type tool
holder, normally thinking of it as the cutting part. Stiffer is better, clearly.

Learned something new, again.

Bill

 

[Ian]

I've been looking at the small, carbide internal threading bars, but keep shying away because I can't get the "glass rod" thing out of my head. One little skip or eating a chip and SNAP there goes $50. Maybe I'm just too paranoid.

 

[Pistolero]

The same thing crossed my mind, too. Ian. This small ones would worry me the most.
I have seen 1/2" or bigger with inserts on the web, and while they are just as brittle,
the load to break is far higher.

Bill

 

[KeithB]

Carbide is brittle. But it is far more rigid and stronger than the steel most boring bars are made from. If something happens to break a carbide bar you would also have bent/broken a steel bar. (I won’t even tell you about my box of broken bars. Snot happens.)

 

[Ian]

Little toy lathes like mine have a lot of flex in everything (even with the gibs snug and evenly loaded), so the whole mess between tool and bed can load up and pop sometimes. I'm getting better at setting tool angles and understanding what I need to do to make my machine work smoothly, but I'm still an infant..