4140 is comparatively easy to machine but not so easy that it is possible to accurately bore a complete chamber and garantee an acceptable finish. Boring is simply not an easy task most due to tool deflection. The CNC ability to optimize speeds and feed can provide a fairly consistent finsh and even a reasoably good one but tool wear and chip clearance, especially where boring tool length has to be over 10 times the diameter, will result in a high number of reject pieces. There is a good reason precision holes are finished by reaming and/or honing. These are the best ways to achieve the finish and precision required.
Boring a 30/06 chamber, for instance, would be the eqivalent of boring a 4 3/4 inch hole 20 inches deep with a tapered section connecting to a 3 1/2 inch hole about another 3 1/2 inches deep and doing this with a 2 1/2 inch diameter bar. CNC or not, it ain't gonna happen. Not with any degree of precision and an acceptable finish. Boring as a means of roughing a chamber is an excellent technique but not for finishing. There are simply limitations to what a boring bar can do. Boring bars can do fairly well in holes with a depth of up to five times the diameter of the bar provide there is sufficient room for chip removal. Beyond that, performance becomes iffy. Regards, Bill.

You are correct Bill. I taper bore and follow with my chambering reamer. Can you imagine what the long thin boring bar does cutting the freebore and lead? It is doing an interrupted cut on the rifling.
Butch

Bill and Butch

I am not as expert in machining as either of you guys, but I certainly support what you are saying about boring tool flex. Years ago when I took machine shop classes one of the things that was emphasized was the difficulty of making dependably accurate cuts of more than 3 times the depth of the boring tool diameter.

The primary problem?...flex! It was emphasized, rightly or wrongly, that lathe tools cut the same way a metal-chisel does and that lateral pressure on the cutting tool shaft will make pretty much ANY cutting tool shaft bend easier and easier as the hole is deeper and the required shaft is longer.

AC,
That is why I use a solid carbide boring bar.
Butch

quote:

Originally posted by Bill Leeper:
4140 is comparatively easy to machine but not so easy that it is possible to accurately bore a complete chamber and garantee an acceptable finish.

4140 with the correct tooling can have a great surface finish. There have been very significant advances in tooling geometry and coating that can produce exceptional results. but it requires a significant investment in tooling and inserts with expensive coatings

quote:

Boring is simply not an easy task most due to tool deflection. The CNC ability to optimize speeds and feed can provide a fairly consistent finish and even a reasoably good one but tool wear and chip clearance, especially where boring tool length has to be over 10 times the diameter, will result in a high number of reject pieces.

The problems associated with tool deflection can be solved with the proper machining techniques applied. Heavy steel boring bars solid carbide boring bars All with insertable cutters have helped the machining industry greatly. Granted it doesn't eliminate tool deflection but it will reduce it to a level were it is no longer an issue. Spring passes also help. High pressure coolant has just about eliminated chip clearance issues along with peck cycles.

quote:

There is a good reason precision holes are finished by reaming and/or honing. These are the best ways to achieve the finish and precision required.
Boring a 30/06 chamber, for instance, would be the eqivalent of boring a 4 3/4 inch hole 20 inches deep with a tapered section connecting to a 3 1/2 inch hole about another 3 1/2 inches deep and doing this with a 2 1/2 inch diameter bar. CNC or not, it ain't gonna happen. Not with any degree of precision and an acceptable finish. Boring as a means of roughing a chamber is an excellent technique but not for finishing. There are simply limitations to what a boring bar can do. Boring bars can do fairly well in holes with a depth of up to five times the diameter of the bar provide there is sufficient room for chip removal. Beyond that, performance becomes iffy. Regards, Bill.

Here again I agree but for another reason.
Reamers are used simply because they are cheep, accurate tooling. It's that simple. Economics is the reason why reamers are used.
If you had a readily available source of precision cutting tools to allow you to produce a hole with an accuracy of greater than +/- .0005" For less then $100 each you'd be crazy not to use them. But if you had a $210,000.00 CNC lathe sitting idle With the proper programing experience it can produce a reamer grade chamber.

Don't get me wrong I'm not trying to argue or prove a point or anything like that. I actually agree with you on many points. But to say that a CNC can't produce a reamer grade chamber is just flat wrong. A good CNC screw machine with proper tooling can produce a reamer quality chamber. It just requires a substantial investment in tooling and programing time. When you compare the cost of reamers to the cost of the machine and tooling It only makes perfect sense to use a reamer. The only way it would pay to use a CNC is if you made a large number of a single chambering therefore setup time is reduced and scrap can be minimized. But in the world of rifles there are many hundreds of chamberings available and to go through setup and proving out a program on every single chambering would be a waste of time and money especially when a reamer is $100 to $200 retail. Not to mention they can be reground giving a bigger return on investment over the long run.
I'm simply saying it can be done I'm not saying it's better.

quote:

Originally posted by butchlambert:
AC,
That is why I use a solid carbide boring bar.
Butch

Butch- Did you ever snap one?

For a lot of chambers...especially the neck portions...I'd think the bar would have to be awfully small in diameter, and at least 12 times as long as its diameter, maybe 15 times or even more in some chambers.

I'm not saying they would snap on occasion, just asking, as I've never used anything that long and slim made of solid carbide. Thought I'd ask you because you have the experience to give me some good info for future use.

But I have seen a lot of carbide inserts broken in the shop...not mine as I prefer to make and sharpen my own tools from cobalt steel, so I've never felt the urge to try a solid carbide bar (or insert) of a size fit for working inside a chamber.


Another question....in the old days steel wasn't necessarily exactly the same in all parts of the length of a bar (not perfectly homogeneous), nor of the same hardness in every part either. Then one was faced with parts of the steel which would cut easier than other portions. That tended to contribute to crooked deep holes. I assume that is no longer a problem in industry? Or is it one of the reasons barrel bores are still often not perfectly straight?

Anyway, if is still something one encounters, I'd think that would be yet another reason to use a reamer...being full hole diameter, it would cut more evenly and be much stiffer than a boring bar made of the same material, wouldn't it?

AC, I taper bore to within .020 of OD and .050 short of the shoulder. I want the reamer to follow the bored hole. I do this on my short fat BR cartridges only. I don't believe there are any advantages to taking the time on a hunting cartridge. I still bore them, but not as painstaking as my BR chambers.
Butch