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Low Stress Drilling

glbreil

Cast Iron
Joined
Feb 10, 2008
Location
McLeansboro, IL
I know questions like this are very common but while searching for answers I couldn't find anything with same goal in mind.

I want to remove some metal with a twist drill before boring but I want to do it gently without inducing anymore stress or pressure on the fixturing setup than absolutely necessary.

This is part of the process of chambering a rifle barrel in a Hardinge CHNC II lathe.

The material (416 Stainless 28RC) will be set up and dialed in running as true to the center of the barrel bore as I can possibly get it and the goal is for it to remain that way through out the entire process without disturbing it.

The barrel has a .250 pilot hole and I will be opening that up with a 7/16" cobalt twist drill going 2" deep. I will have about 80 psi oil coolant meeting the drill to help evacuate the chips.

After drilling it will be bored to the same taper as the cartridge.

So I am looking for recommendations on speed and feeds as well as peck increments.

Again the goal is to keep the stress low. It's not production and time isn't a real factor.

Any thoughts on this would be appreciated.

Thanks Gary
 
The problem Glbreil, is that twist drills have been a very effective tool for at least the last 3000 years. But not as effective without proper pressure or "feedrate". Drills in general are the first most effective way to remove material, turning second. I understand your dilemma, so the least pressure you can apply there would be a high performance drill or drill$. Hopefully indexable that way you can run faster RPM and lower feed and no pecking. Second I would try just a regular twist drill HSS run 25 SFM (guessing 2000 is your max RPM on the CNHC) and a feed of .005"-.006" Per rev, again no pecks. The reason for no pecks is that you have a pilot and so the chip isn't going to break unless you stop the tool, and you don't want to stop in 416 just ram that bitch through. The chip is going to be nasty but just go for it you are boring it to size any way, and it's only about 4x the diameter of the tool.

Robert my ±2
 
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The problem Glbreil, is that twist drills have been a very effective tool for at least the last 3000 years. But not as effective without proper pressure or "feedrate". Drills in general are the first most effective way to remove material, turning second. I understand your dilemma, so the least pressure you can apply there would be a high performance drill or drill$. Hopefully indexable that way you can run faster RPM and lower feed and no pecking. Second I would try just a regular twist drill HSS run 250 SFM (guessing 2000 is your max RPM on the CNHC) and a feed of .005"-.006" Per rev, again no pecks. The reason for no pecks is that you have a pilot and so the chip isn't going to break unless you stop the tool, and you don't want to stop in 416 just ram that bitch through. The chip is going to be nasty but just go for it you are boring it to size any way, and it's only about 4x the diameter of the tool.

Robert my ±2
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hss drill at 250 sfpm ?? on 28RC hard steel ?? got to be a typo
 
If you want to reduce thrust needs, sharper drill, lower feed and im becomeing a ever bigger beliver in TIN coated in steel drilling, just helps the stuff slide up the flutes.

On a pre piloted hole going only what 4 diamiters deep i would not peck, every peck blunts the drill + increases the chances of jamming swarft bettwen hole bottom and flute requiring a big feed force increase to cut through.

Assuming your gripping it hard enough to not move with a tapered reamer cutting over 2" of length its kinda hard to see how its even going to notice the drill thrust. Compared to the torque your chucking methods going to have to deliver to do the ream. You want to reduce force, bore it not ream it!! The reaming is the high force imprecise bit!
 
He mentioned he was going to bore it (twice) using his CNC lathe and I see no mention of a reamer anywhere in his post.


Thanks for the replies so far. Actually I am going to finish ream the chamber but did not mention it because I didn't consider it an issue at all as far as pressure on the setup goes.

I have done all this many times on the manual lathe where you can feel the amount of pressure that you are applying, just not on the CNC where it is going to keep pushing regardless.

Just for information a chamber reamer does cut very easily with very little force at all. Most people who use them on manual equipment push them with the tail stock and actually hold the reamer with a very short handle so that they can feel exactly how much force it is taking to cut as they feed the reamer in.

Again this is not the case on a CNC and I will have to determine the speed and feed for the reamer as well but I already have some pretty good information on that.

Unless someone else has more thoughts I will probably give the 65SFM and .003 per rev a try. I guess the question is still to peck or not to peck.

As I mentioned before I will have cutting oil at about 80psi coming down the pilot hole to push the chips out. When I said peck I was actually thinking of backing all the way out each time so that any chips could clear completely.

Thanks Gary
 
Yeah if you have a supply of oil coming from the pilot bore, it should keep things real well flushed any how even at just 1-2psi, 80 sounds positively messy!
 








 
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