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Drilling the straightest hole possible

flyfisherman246

Plastic
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Jun 28, 2021
I have searched all over for this answer and not quite satisfied with what I have read. I am looking to build a revolver cylinder where I will use a twist drill bit to drill all the way through front to back. Then going to use a chucking reamer to get exact size round hole for the bushing on the chamber reamer to follow. We all know drill bits tend to wonder and drill holes that aren't round, and we also know reamers follow the existing hole. So ideally you would want the straightest rounded hole possible for the chucking reamer to follow. We are talking about a roughly .5" hole going through hardened steel about 1.9" deep.

Here is the best way I can dream up and want your guys opinion on this. Use a twist drill bit (HSS or Carbide)to drill straight through one size under 1/2". Then follow this hole with a 1/2" square carbide end mill to help straighten the hole. Then finally follow this hole with a .504" chucking reamer which should follow the hole of the end mill, keep it round, and exact size to fit the bushing on the chamber reamer.

I would imagine using a boring head would be the best solution here, but not willing to spend that kind of money right now on this personal project. I don't have much experience cleaning up holes with an end mill, as I usually use a boring bar in a lathe for rifle barrels. Do you think following the twist drill hole with the end mill would be beneficial? Do end mills cut straight and round when chasing a hole? Or is the twist bit the best I'm going to get and should skip the end mill and go straight to the chucking reamer?

Thank you
 
You didn't mention the length of the hole. But if you're using HSS, then you definitely want to center drill/spot drill first.
 
He says 1.9 inches deep.

I assume you are using a mill to do this. You can take an old end mill and relieve the flutes so that you have a cutting edge at one tip. Use it as a fixed size boring bar after drilling, then ream to diameter.
 
You might drill it 31/64 or 15/32, follow with a .500 gun drill/reamer, and finish with a .5038 reamer.

Then put the part between centers and tickle the OD.

Oh, my bad..I misread the Op and thought he was making the barrel. sorry/ignore this post.
 
sounds like you are pretty new to this, gun manufacturers use a stub length drill to start the hole 1" or so, then gun drill. anything else will walk around or droop from gravity. but this will probably fall on deaf ears as id bet the op wont respond after the first post
 
Flyfisherman....I do not want to discourage you in any way but I see some issues here. One you say this is a hardened cylinder. That could definitely influence your tooling and technique. How hard? You should know this as it is a safety issue. The other thing is that you might want to make darn sure the cylinder holes line up with the barrel correctly in battery. I know this sounds like a no brainer but I have actually owned a few revolvers where there was a slight misalignment between one of the bores in the cylinder and the barrel. You do not want anyone standing off to the side when you use that bore. Shaved lead at the speed of sound is no fun for bystanders.
 
sounds like you are pretty new to this, gun manufacturers use a stub length drill to start the hole 1" or so, then gun drill. anything else will walk around or droop from gravity. but this will probably fall on deaf ears as id bet the op wont respond after the first post

This was your response 2 hours after I asked the question? Not sure how you came to the conclusion that I won't respond. This was my first post on this forum. If you have nothing productive to contribute to my question, or don't know the answer, please don't feel obligated to throw something out there.

Thank you for everyone else who has responded to my question, I sincerely appreciate it. I will look into the modified end mill technique, that sounds interesting. This will be done on a bridgeport in a 3-jaw chuck. And forgot to mention it, but yes holes will be center drilled before twist drilled.

Thanks
 
Sorry I didn't mean hardened, I meant tempered. Around 42 RC with 4140 or 17-4 PH. This cylinder will be fit to the frame with an oversize tight fitting bolt for solid lock up, and then line-indexed/line bored for proper alignment. Thank you everyone for your replies.
 
That's still hardened... Hardened doesn't just automatically mean it's as hard as it can get. Generally, you harden something to full hardness then temper it to achieve the desired level of hardness.

So, yeah, anyway, point being: saying it's hardened doesn't mean he can't cut it, contrary to what seemed to be implied by some respondents. Almost all gun parts that are in the business end of a gun are hardened... It is good to be concerned about how hard the cylinder is, which is why crossthread asked; you don't want to create shrapnel the first time you fire the pistol. Too hard and you might have that problem. Sounds like that is just about right though.

All that said, I'd use a boring bar or the end mill "bore" technique before reaming. There should be no real need to relieve the extraneous flutes for that either. If your mill is in good shape the end mill will cut a hole darn near as smooth and round as a reamer but straight like a bored hole. Spot it and check or cut a test hole first.
 
Are you saying using a standard 4 flute end mill should cut as straight and true as a boring bar? Meaning no need to relieve the flutes like was mentioned earlier? I guess long story short, that is my question. Will a square 4 flute carbide end mill cut a straight true hole (after a drilled hole) like a boring bar?
 
In a Mill,spot, then drill around .010 under your finish size, no moving the table.

Find an old carbide endmill just a bit larger than your finish sized hole. Then, in a spindex, indicate it in to round in a good collet, and grind the O.D. to around .001 smaller than the finish size.

Install the tool you've just made in a collet in your mill, and check for runout. then finish the hole.

From here, you should be able to hone it to an exact size.

If you don't have the ability to make your own tool, do a search for "Jig bore cutters". They come in all sizes and will finish a hole as straight as possible.
 
Are you saying using a standard 4 flute end mill should cut as straight and true as a boring bar? Meaning no need to relieve the flutes like was mentioned earlier? I guess long story short, that is my question. Will a square 4 flute carbide end mill cut a straight true hole (after a drilled hole) like a boring bar?

As long as your machine is tight and set square and you are taking a fuzz cut, yes, it absolutely will. Take the same precautions as when you are boring; in other words check tram, make sure all table clamps are tight, etc.

As I mentioned, you can cut a test hole on some scrap and check it for yourself.
 
I just want to reiterate that however you make your hole, load a collet for your endmill for the "boring" phase. Don't cheat and put it in your drill chuck for this job.
 
I just want to reiterate that however you make your hole, load a collet for your endmill for the "boring" phase. Don't cheat and put it in your drill chuck for this job.

What he said^^^^^^^^^^^^

Just like the lady said" No Capes!"

"No Chuckes!"
 
Is this for a Smith & Wesson 500?

Don't want to be a "Debbie Downer" but I hope you know what you're doing...



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