Dowel Pins/Interference Fit/Hardened Plates?
So.... I'm designing a bender. Think "tiny Diacro". I want to insert the dowel pins permanently into the tool steel plate with an interference fit. I've done this alot with 300 series stainless, but never tool steel.
I'd like to end up with hard pins in ground, milled, and hardened steel.
My questions:
Can I press or hammer the pins in if the steel is hardened, or do I have to insert them and then harden the whole thing?
Amy I nuts bothering to harden the plates? I guess the majority of the wear will be on the pins, but I don't want the pins to get loose in the plate.
I'm planning on putting .1885 pins in .1875 holes in .25" thick steel.
As for what type of steel, I'm thinking on using A2, M2 or O1, but am open to suggestions.
Thanks!
-Jeff
(PS: I searched CNCZone and Google groups before asking this question here.)
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Sounds good to me. If you use A-2 then you will have something that wears well and tends to hold size during heat treat. Ream the hole .1865. Buy an undersize ream and use standard dowel. You should not have any trouble. I would suggest pushing the pins in instead of hammering. .1875 pins will deform if you hammer on them. .001 press is no trouble on .1875 pin.
this is a box & pan brake? trying to picture it - are these the fingers you're talking about? . if it is a box & pan I’d really like to learn more about your design because I’ve always wanted to make one.
Definitely use a press, hammering would make a mess. Machinery's handbook says ream .001 under in soft and grind/lap 2-3 tenths under in hard. as there is possibility that the hole is not round after heat treating they specifically say to lap or grind. That’s a small pin, at you may have a problem with .001 interference, .0005 would be the max imo. get the undersized reamer, as ctate suggested.
you won't be able to grind the surface with the pin there, but it sounds a lot easier than holding tenths tolerances on with a lap
I’d do it with mild steel and then case hardening. with tool steel, you have to let it down (anneal) or it will be too brittle. Case on the other hand can be left dead hard because of the soft core. a heat treat shop can get a dept of up to 50 thou. you could cut the profile you want, get it carburized (carbon soaked into the outer layer), left soft, drill and tap all your holes etc and then send it out for hardening. you end up with a glass hard exterior and all of the threads soft (the way you want them). its also a lot cheaper than working in tool steel and easier to work. Although going this route would require proximity to a heat treating place – its too much hassle to get into that at home imo (although can done)
I'm making a micro bender, not a brake. Something sorta like a Diacro #1... see
http://cgi.ebay.com/ws/eBayISAPI.dll...tem=7559557460
But *way* smaller. My bender will have a max stock diameter of 1/8".
I've got a couple of presses, so no problem there.
I can see having .0005 interference on something so small. The good news is I can get pins in .001 increments around .1875, so if things go wonky the worst fit I will get is .0005.
Case hardening sounds like an option, but if A2 can be handled in the same shop as the milling, it seems it would be easier? Apparently A2 has low sulpher content, making it harder to machine?
As far as the distortion goes.... just how much should I expect from hardening? If the plates starts out as +/- .001 in flatness, how bad will it come out of the oven?
Should I maybe start with somewhat hard material and accept more machining time/trouble?
Why would you want the threads soft?
Thanks for all your help! While a small project, this is a big learning step for me!
-Jeff
If you use A2 it stays very stable and the surface comes back to you as was machined, meaning it can be 100% finish machined befrore heat treat (for your application).
If you do case hardened CRS, your plate will be an ugly scaley mess and need to be remachined after HT.
www.integratedmechanical.ca
i was thinking a sheet metal brake and was envisioning you using a lot of material making fingers, hence thought case hardening would be a lot cheaper. I've never used the air hardening steels (perhaps I should) so am used to thinking in terms of less stability, ie grinding after hardening. an oil or water quench can be enough move things around.
DaraBee's point is true (on the ugly mess), but if you have them run it through the pickle after quenching, it comes back clean and grey.
threads represent a narrow cross section - because things get more brittle when hardened, threads are better left soft. this might be more applicable to case hardening where its left glass hard and not tempered, but i've been told by tool and die makers a thread is better soft than hard.
What you say about the brittleness and small areas is correct Mcgyver, if you have a glass hard thread it will most likely shear when you do up the nut or bolt. If you manage to get it done up, then the first shock load will strip the thread. Think how easy it is to break a small drill (which is tempered) with a light tap, and then try tapping a piece of unhardened steel the same size. I normally wind bolts into tapped holes and nuts onto external threads before sending things for case hardening. This is so that the threads do not absorb carbon during the carbonising process and as a result remain soft. If the threads are hardened there is a strong probability of them cracking during quenching unless they are protected from the quenching medium, i.e. with nuts bolts etc. Tempering of the threads is likely to be hit and miss due to the small and changing area and the very small thermal mass relative to the rest of the part resulting.
The simplest things confuse the cleverest minds
I realized I can't use case hardening because I will need to have some interior features be precise.
If I go with the A2 steel, do I need to request that the tapped holes get screws put in them? Also, I assume I can request a lower level of hardness as this is not a cutting tool.
Thanks everyone!
-Jeff
Jeff, you'd want to temper something made out tool steel after hardening, right? once it's tempered its much less brittle. Tool steel already has the carbon in it so plugging with a bolt doesn't do anything - i.e. it doesn't stop the heat from reaching that location, although it may slow down the quench. (that's a bit simplified, but I think its accurate for parts this size)
Case hardening can be a two step process: 1) cooking the piece in a solution at temperature such that the carbon actually migrates into the steel, the longer the soak, the deeper it goes. 2) quenching it after bring to heat. While this often done in one sequence, you can take the piece from step one, do a bunch of work to it then do step 2. basically any surface before step one gets carburized, any surface created after step 1 is not. when its quenched, only the carburized surfaces are hardened. this way you can control which surfaces are hard or soft.
A case hardened piece gives you the best of both worlds - it's left glass hard (not tempered) yet is not fragile/brittle because of the underlying ductile steel (that hasn't receive any extra carbon). That you usually want to leave the case glass hard and not tempered spurred the concern with threads - I wouldn't worry too much if you are tempering the A2
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