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You can't polish MIM parts

Discussion in 'Handguns: Revolvers' started by Bubba613, Jun 18, 2013.

  1. Bubba613

    Bubba613 member

    Another myth dies. Here are pics of the Smith PC637 Gunsmoke.

    Attached Files:

  2. Mauser lover

    Mauser lover Well-Known Member

    Oops, I didn't realize that you couldn't polish MIM parts. Glad to know you can though!

    Just as a side note, why can't you polish MIM parts? (in the myth)
  3. WC145

    WC145 Well-Known Member

    MIM parts polish up nice if done right -

    Bobbed and polished 360J hammer

    I don't have any good pics of the trigger but this one will give you an idea
  4. rswartsell

    rswartsell Well-Known Member

    At one time "sintered" parts were not good candidates for polishing because they had a hardened "crust" encasing steel of a less hardened character which formed the "core". Polishing could penetrate the "crust" and expose the softer "core" which would not perform to the standards of the hardened "crust". If you accept this as true, it once was not a good idea to fool with these parts. As I only dimly understand it, the current technologies used in producing modern "MIM" parts alleviate the vast majority of these concerns.

    I make no claim to being a metallurgist, or even a production engineer.
  5. Old Fuff

    Old Fuff Well-Known Member

    Yes you can polish MIM parts, but under high magnanification you will see tiny pits in the surface that don't go away. This is because the material is not truly homogeneous, but made up of metal particles fused together with the remains of a binder.

    You can get similar results by simply dry-firing the gun and let the parts burnish where they come into contact under pressure. Other then cosmetic affect, polishing elsewhere doesn't make any meaningful difference.

    Want to see the difference? Look at a hammer taken from a pre-war S&W revolver that was polished on the sides before it was case hardened, vs. what you see today. :barf:

    I won't bring up the fact that S&W cannot color-caseharden MIM hammers and triggers, and unlike Taurus doesn't even flash-chrome them for better appearence. All this of course comes under the heading of cosmetics, which is not critical for function but does relate to quality which is now lost. :banghead:
  6. PabloJ

    PabloJ Well-Known Member

    Don't let them BS you there is nothing wrong with the MIM parts.
  7. rswartsell

    rswartsell Well-Known Member

    Sounds like "don't think too hard and just trust me religiously", not a big subscriber to that. Current MIM production seems to work well, but telling the Old Fuff and I we are wrong "just 'cause" ain't feeding any bulldogs.

    Even without being a metallurgist, I understand that "natural" steel has a crystalline form that has the advantage of "chemical bonding" that takes advantage of the positive and negative charges of the outside shells of the sub-atomic orbits to "weld" molecular structures in a far more homogenous way than powder and binder under heat and pressure will ever achieve. There isn't anything that mysterious about that. I encourage resistance to "because I said so".
    Last edited: Jun 18, 2013
  8. Jaymo

    Jaymo Well-Known Member

    I still prefer forged.
  9. glocking26

    glocking26 Well-Known Member

    Smith + Wesson went to MIM parts for one reson to cut cost. They still use forged parts on some there P.C. Guns why.... Because they know there better. But you'll pay for them. MIM parts may hold up as good as forged parts but they look like parts off a toy gun. Oh yea I carry a 340pd and it shoots great but I still hate looking at the trigger...but that's just me.
  10. Jaymo

    Jaymo Well-Known Member

    I was going to say, "Lookit that. You CAN polish a turd. But, you still can't pick it up by the clean end."
    But, I decided to be nice.
    And, yes, I do have a few pieces that use MIM parts.
    Funny thing, my Judge hammer and trigger are either CCH or fake CCH.
    Go figure.

    That said, your hammer and trigger came out looking far better than I would have expected, being MIM parts.
    I would polish mine, but I'm just too darned lazy.
  11. DPris

    DPris Well-Known Member

    Fuff has it.
  12. fletcher

    fletcher Well-Known Member

    Steel in MIM parts also has a crystalline structure and relies on the same bonding method ("sea of electrons" around a lattice of cations) to join the atoms. The big disadvantage of MIM is porosity. Because of the use of binders, it is more difficult to achieve theoretical density. Internal defects really weaken a structure's static and fatigue properties.

    To the original post, and as noted earlier, MIM parts can certainly be polished, but the appearance will be dependent on the amount of porosity present.
  13. PRM

    PRM Well-Known Member

    Really - must be why S&W are replacing MIM Hammer Blocks with the older style stamped ones. The block in my Model 60-9 has snapped into (twice) with less than 200 rounds fired.

    Attached Files:

  14. evan price

    evan price Well-Known Member

    Polishing a MIM part is akin to polishing a slab of granite. Yes, the end product looks good, but even in highly polished granite countertops you still find pits and voids where there are gaps in the material.
  15. MrBorland

    MrBorland Moderator

    As far as I can tell, this thread's about putting enough shine on an MIM part to be pleasing to the eye. Whether it's too porous to be a true "polish" is minutiae, IMO. Looks good to me.

    And how well MIM parts perform, why they're used, their strengths & weaknesses, personal preference/bias, etc. have all been discussed ad nauseum in plenty of other threads.
  16. mope540

    mope540 Well-Known Member

    looks great.

    everything, including a fine diamond, looks pitted under high magnification
  17. fletcher

    fletcher Well-Known Member

    Its an accurate comment.

    Under enough magnification any surface will likely show some defect, but it won't necessarily look pitted. A properly polished surface on wrought material will show no pitted look well up to 1000x - we do this routinely with metallography. Keep going to many thousands, such as in a SEM, and one may see something.
  18. 460Shooter

    460Shooter Well-Known Member

    Not high road material.

    No offense meant, and no, I realize I'm not a moderator, but I'd keep comments like that to yourself. You can disagree without being rude. The mods don't take well to people who yell inflamatory things like that.

    Also, it is an accurate statement Fuff made. Old Fuff has been sharing great tips, info, and wisdom on THR for a long time. I have yet to read something he has written that is incorrect regarding handguns, or "DUMB", and he is well respected here. Please treat him, and all forum members with respect.

    Looks great OP. I've polished MIM, and it certainly does improove the appearance.
  19. Walkalong

    Walkalong Moderator

    Exactly. ;)
  20. CraigC

    CraigC Well-Known Member

    Ha! :D

    Proof that there are no dumb questions, only dumb answers.

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