chip contol when pre-boring

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fourspear

New member
I do not have any experience with boring small diameter holes such as would be common to pre boring for a chamber reamer. On the larger diameters that I am familiar with, chip control is an issue that can cause pretty and slick to go south in a hurry.
I realize that depth of cut would necessarily be small, 3-8, maybe 10 thousandths and feed rate accordingly slow.
Is this an application for coolant flush from the muzzle?
Is high speed or carbide the best choice for this and would anyone have any recommendations.
As I am thinking about this maybe the depth of cut is so small that the fine curls coming off don't pose that much of a problem. At any rate, I would appreciate and thank any for sound advice and experiences on this. Thanks, Red
 
I use carbide boring bars for small hole cuts. You can go over 1" deep on a .315" diameter hole. Can't make heavy cuts because the chips will build up inside the hole.

Micro100 is a common brand that comes to mind. Check MSC or any other machine tool catalog.
 
Boring a small hole can be a bit of a pain sometimes. If using insert carbide tooling one thing to consider is the depth of cut. Less is not synonymous with better chip control. In fact it can be the opposite.

A typical boring bar insert usually has a chip breaker. Meaning the insert geometry is designed as such so that the chip rolls into itself until it breaks off in a little spiral.

Pressurized flood coolant is obviously needed as well. Both for chip evacuation and to mitigate insert erosion. The best solution would be as you mentioned, pressurized through barrel coolant.

Sandvik makes a micro boring bar that is absolutely PERFECT for this stuff. It's a ground tool and uses a holder so if a guy has an accidental "crunch" it can be quickly replaced. The inserts run about 35.00 a piece and last for a long time. Very, very, very nice surface finishes are easily obtained. The only draw back is the reach. They are limited to about .600".

Last trick. Ever try just using a two/three flute endmill? Index a flute and kick the tool off at an angle so it doesn't gall the bore. Works surprisingly well. Take a diamond lap and roll a small radius on that flute and it almost acts like a "wiper" insert. The chip gullet on the cutter will push an almost continuous ribbon out of the hole if your feed rate is consistent which greatly aids in keeping the hole clean so that the tool doesn't load up and break.

Hope this helped.

C
 
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chip control when pre-boring

When pre-boring a chamber, I never considered surface finish to be the big issue, the reamer removes all the material from the holeyou have bored anyway. The critical issue was to get the hole concentric, properly sized and to the correct depth.

My big issue with chips has been to keep the work hardened swarf out of the bore where they could do damage. I always push a cleaning patch up the bore to just forward of the finished chamber when doing any pre-drilling, pre-boring or counterboring for the bolt nose.
 
Thanks to all for replies. I am sure experience will be the best teacher on this.
There are a lot of variables that come into play here, not the least of which is the depth that you are wanting to bore to.
I certainly do appreciate the wealth of knowledge and experience on this forum, it is a privilege to read and learn.
 
Reamers are affected by tool pressure, as is a boring bar....Don't expect the reamer to ream a perfect chamber if the rough bore is not true.

My point is that roughing and finishing are dependent on each other no matter what you are working on. So try to make the rough bore as true as possible.
 
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As was mentioned in a previous reply, the Micro 100 miniature solid carbide boring bars are ideal for truing & enlarging the drill-roughed chamber. They are made with different dia. shanks & lengths - the shorter ones are suitable for PPC/BR length chambers, while the longer ones are necessary for longer cartridges.

I've made a practice of taking light cuts - usually only .0025" per pass, which on my lathe shows as .005" on the crossfeed dial - and have yet to have a problem with chips.
 
Does anyone have the part number or specs of the Micro 100 miniature solid carbide boring bar used for 6ppc ?
Thanks
Adam
 
IMO, the safest and easiest way to prebore is to crank in, using a DRO or dial indicator, set the cut, then feed out. You don't need to take but a couple of cuts of about 0.005"-0.010" each to end up with a true hole that the reamer will follow if you predriled to about 0.020"-0;030" under chamber shoulder diameter.
 
Boring out +1. clean after each pass.light cuts= nice holes.

Chuck
 
Pre-Boring

I think you are making too much of this. The object of pre-boring is to establish a third point dead true with two pre-determined points. Finish, if anywhere with in reason, is a non factor.

I just took a small high speed steel boring tool, the ones that have a 1/2 inch round shank, and ground a tool with a 30 degree shoulder. If you rough-drill a 6PPC chamber with a .400 drill, that gives you about enough stock to bore so that the reamer will clean up and establish the correct dimensions and the correct finish.

I pre-bore with the compound, set off a tad over 1/2 degree on my Pratt & Whitney Lathe. That is pretty close to the taper of a typical PPC Case. (I actually have a mark that I determined long time ago). I pre-bore to where the reamer goes in about 1/3 way, then, with a pilot, I hand ream the chamber. Going in about 1/3 way gives you about .002-.003 to the side to ream. All the reamer is used for is establishing the final finish, and the final dimensions of the chamber.

There is a thread of me chambering a barrel, Gary Walters took allof the pictures. Do a search, "jackie chambering a barrel", the little boring bar I ground is shown.......jackie
 
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