Predrilling chambers

[45flaco]

Hey guys. What's everyone's process for predrilling a rifle chamber?
I normally don't predrill, but I'm about to do six chambers, and the customer wants them all done with the same finish reamer. To keep the reamer as sharp as possible, for uniform chambers, I was considering doing a predrill.
Never done it before.
The cartridge is 375 H&H, it's a PTG HSS solid pilot reamer, it's cut one chamber alone, and finished a few short chambers.
The metal I'm reaming is 4140.
Any advice is greatly appreciated.

[Scorch]

Get a roughing reamer and a finishing reamer. Rough it to close to size, finish with the finishing reamer. Doing it this way, you could cut 10-12 chambers and have them turn out perfectly. Drilling a chamber is a recipe for disaster. Yeah, yeah, I know, your uncle's cousin's neighbor who was a gunsmith just chucked up a 7/16"" drill and had at it, and maybe it worked for them, but if you are doing it for a customer do it right.

[4V50 Gary]

What Scorch suggested. Rough reamer first and then finishing reamer. Finally a polishing stick.

[Unclenick]

Dave Manson told me a finishing reamer can do up to fifty chambers with good lubrication and before needing regrinding. I don't anticipate six will change it much.

[T. O'Heir]

"...all done with the same finish reamer..." That's how and why every chamber is slightly different from all other chambers. Pre-drilling isn't going to change that.

[45flaco]

Alright, looks like the consensus is to get a roughing reamer. I assume it doesn't really matter what brand, since this is a standardized cartridge? If not, I'll just grab one from PTG.

[Unclenick]

Shouldn't matter, but for a solid pilot type, you could save $35 by getting one from Dave Manson Precision Reamers, whose tools are top notch.

[Scorch]

I always use floating pilot reamers. It does make a difference.

[old roper]

Gunsmiths that chamber my rifles all use floating pilot reamers. This may help understand difference.

https://mansonreamers.com/2016/01/04...movable-pilot/

[Nathan]

If you predrill or rough ream, your finish Reamer pilot is touching air, right? How does that affect chamber accuracy/alignment?

[Scorch]

If you predrill or rough ream, your finish Reamer pilot is touching air, right?

No, that is not correct.

[Nathan]

Thanks, Scorch, can you explain?

[Unclenick]

The pilot is a centering guide. It is usually within a thousandth or two of the land diameter in the barrel blank and rides the bore to keep the front of the reamer centered in the bore by bearing against the lands whenever it tries to wander off. A solid pilot is part of the reamer so it doesn't turn independently of the rest of the reamer and thus there is rubbing against the lands when it has to touch them to keep the front of the reamer aligned with the bore. Good lubrication is required to prevent that from marking the bore.

A removable or live pilot reamer has a narrower post at the tip that is threaded for a retaining screw. The pilot is then a precision sleeve that slips over that post and the screw is put in to block it from slipping off the post, but leaves it free to turn on the post like a bearing journal. This means you can have a set of sleeves in slightly different outside diameters to get a closer fit to the bore diameter and, because it turns on the post, it can touch the bore without rubbing.

Either type of pilot can have grooves ground in to pass through-bore pumped lubricating fluid if you are set up for that. In the solid pilot the grooves are usually angled in the direction opposite the direction of the rifling it is expected to be used to cut a chamber in.

At the back end of the reamer, a floating reamer holder is used that allows the back end to self-center into alignment with the piloted end. Without that, a rigid hold on the reamer will allow any tiny misalignment of the bore and reamer axial centerlines to bias the back end of the reamer to one side, causing a wide chamber at the back end if the barrel is being spun and the reamer is anchored, or an off-bore-axis chamber if the barrel is stationary and the reamer is being spun.

[Nathan]

So, if predrilled or rough reamed, what is the pilot touching?

[tango1niner]

the bore...

[Unclenick]

Right. The pilot is out in front of the cutting edges so it sits in the bore beyond where the cutting is taking place. The roughing reamer leaves material for the finishing reamer to cut the final throat and chamber dimensions. It's just there to reduce the total amount of metal the finishing reamer removes to preserve its edge. That way, instead of lasting for 25 to 50 chambers before needing to be set back and reground, the finishing reamer can last through making a couple hundred chambers.

[Scorch]

Really simple. The chamber is only about 2.5" long (30-06 size cartridges), the reamer is about 8" long. The back s about 3" for holding the reamer, the reamer body (chamber and neck plus throat) is about 3", and the pilot is about 2"-3" longer. If you start out with a drilled or reamed chamber, the pilot often engages the bore when the shoulder of the reamer contacts the chamber mouth. That's so you can cut a chamber that's centered even if you are opening a chamber up. It's a pilot, it guides the reamer.

[HiBC]

And,FWIW,drill bits typically do not have pilots. They will pretty much follow another drilled hole,usually,but you can get some runout. Especially on an older.worn,light duty hobby gunsmith lathe. A #2 Morse tailstock is not quite the same rigidity as a #4 Morse tailstock.
If you were to predrill and introduced runout to the hole the reamer body would try to follow,the wobbly hole would try to introduce side loads to the pilot.

So now you have one of three situations. A floating reamer holder (good tool!) will allow the reamer body to wobble with your drilled hole,perhaps breaking off the pilot.

Or,if controlled by a perfectly true and rigid tailstock (A great blessing,but not every gunsmith lathe has one) you can just about bet the cutting flute contact will not perfectly agree with what the tailstock wants to do. If one or two straight flutes dig in,the cutter pressure goes way up.Things get ugly.Watch how the chips fill the flute . If one or two or three flutes fill with chips faster,one side of the reamer is cutting heavy...and you are cutting oversize.


There is one decent alternative to using a roughing reamer...but you'd have to melt a few brain cells and have the skills. And it will take some time,and maybe some risk.
Given you indicated both ends of the barrel true to a best fit pin gauge ,
If you have some nice,rigid little solid carbide boring bars,you could rough bore an undersize chamber,leaving steel for the finish reamer.

I've run a Monarch EE that was machine enough,and Ive done some small hole tapered bores to a shoulder that were the equivalent to boring a chamber. (plastic injection mold work) But manually boring a chamber versus using a reamer does not make a lot of sense. Time is money.

A good CNC lathe might make rough boring multiple chambers practical,but unless Ruger or Savage or another mfgr has found it more profitable..
Probably best to stick to tried and true.

A free,unsolicited tip. Chip control is critical. Pack enough chips in a reamer flute,the chips will gall and gouge. Pack few more,your reamer will break.

Thats reasonably obvious. But this next part is critical. Meticulously clean all,I mean ALL traces of chips from the chamber and reamer flutes before you run the reamer back into the chamber,every time. If you get even one sliver of a chip over the cutting edge of a flute,it will gouge a ring in your chamber,and maybe even chip the cutter.

I'm not racing the clock,but each time I clear chips,I wash ,brush,and blow off the reamer in a can of solvent. I blow out and run a patch or two through the bore,and visual the chamber area before the reamer goes back in.

[Unclenick]

...And that's yet another reason to get one of the inexpensive endoscope-type borescopes off Amazon or another site. Makes that visual a lot easier to do.