Machining 4140 pre hard

Folks, I'm looking at boring a 5/8 inch diameter by 5 inch long hole
through 4140 PH, so I can make a barrel stub for my TC Contender.
From initial comments, I'll need carbide to do this... BUT... pure
carbide needs rigid setup, which I don't believe my Grizzly G9972Z lathe
has. Once I punch through a 1/2" hole, then I can whip out my CCMT
boring bar and do it.
Will HSS or cobalt drills get er done?
Heat treating would add more cost to this than it's worth. Really, if I
could find a clapped-out Contender barrel, I'd saw the barrel off, bore
it, and stub it. But barrels are not cheap on ebuy...
Reply to
Louis Ohland
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Pre-hardened 4140 maxes out at Rc 32 if it was treated properly. You should be able to do that with M42 ("cobalt") with no trouble. Probably M4, for that matter.
Reply to
Ed Huntress
Agree. I made some parts with it, and found it tougher than mild steel but still very manageable with HSS cutters.
Reply to
Jordan
I'm curious about 4140HT too, so I just faced some with HSS, no cutting oil, and it cut as easily as drill rod or a Grade 5 bolt.
jsw
Reply to
Jim Wilkins
Greetings Louis, Cobalt drill would be better but HSS will work too. As a bonus to the hardness you will find that you can get an excellent finish on the stuff using carbide. I would rather machine 4140 and 4340 in the heat treated condition just because it looks so much better. And the nicer finish also makes it easier to hold tighter tolerances. Eric
Reply to
etpm
Did some measurements, stub length is about 2 1/4 ".
Looking for 7/8 thick x 1 5/8 wide by 2.25" long 4140 HT (or thereabouts). Speedy metals has 7/8 x 2.5 flats, but I'd rather have stock that is fairly square lengthwise, I can always face to length.
Reply to
Louis Ohland
Some advise I got on this sort of thing matches well with my own experience:
You can work hardened 4140 just fine with HSS or cobalt, because 4140 can't get all that hard (it's only 0.4% carbon, after all). But it dulls the tools quickly.
For a _production_ environment, where floor space is money, and time is money, and skilled help is money, and everything else is money, it makes sense to own a superior machine and to use carbide, because you're not spending money changing tools, or discarding parts that were made with dull tools, or sharpening tools, or having a superior production monkey working the machine who can exercise the right touch to make a superior job with inferior tools.
But for a one-off in a hobby environment, just sharpen up a HSS or cobalt drill just for this one time, use plenty of lubricant, and go for it. You can take ten times as long to do the job and still be happy, and you should be able to get a satisfactory result.
Then when you're done, sharpen the drill again.
Reply to
Tim Wescott
A couple of points to avoid confusion: First, 4140 can be hardened up to around Rc 57 - 59 -- quite hard. Second, that's because manganese (or chromium) increases the hardenability of a low- or medium-alloy steel with less carbon that would be required for a plain-carbon steel.
While I'm at it, I realize that the pre-hardened form of 4140 frequently is called 4140 PH. But appending "PH" has meant "precipitation-hardening" for many years, and using it that way for "pre-hardened" can cause some confusion. 4140 does not precipitation-harden.
I don't think there's an official standard for pre-hardened 4140, but it's conventionally hardened to around Rc 28 - 32 when it's sold as "pre-hardened."
Reply to
Ed Huntress
Few years ago a friend gave me about 100 lbs of 1.375 OD X 2.5 inch 4140 round stock. It was left over from a guy who didn't honor a contract.
Machines realy nice with carbide and HSS lathe tooling and HSS drill bits.
Reply to
toolbreaker

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