A gunsmith tells me MIM parts can't be polished. Can that be true?

I don't buy it. While I'm not certain that they are MIM, I suspect that the thumb piece for the cylinder release on the new stainless revolvers are MIM. It's a small part that could easily be produced with the MIM process at what would be a fairly easy cost savings.

I've polished these parts on my 610 and 620 and the result is they look chrome plated. However, I did find that the surface would take on a "marbled" look when I tried to refine the finish by simply using a polishing compound. What I did to get a good and level high polish was to wet sand from 800-1500 grit, then further refining with a Diamond Lapping Paste, and finally do the final polish with a high grade polishing compound. Point is, IMO it can be polished, however the approach used has to follow the traditional steps for polishing without any shortcuts.

I'll also note that I wouldn't think about or suggest polishing internal MIM parts. I don't know the hardness or material specifics for the rebound slides in my MIM revolvers but do know they are very hard. In addition, fully polishing a "working" surface is NOT good practice, lubricant's don't "wet out" well on fully polished surfaces and you actually want some scratches to retain lubricants. I've been designing and building industrial machinery for a lot of years and if you have gibs and ways that have become fully polished by use they will then start to gall rather quickly unless you change to a lubricant with a high degree of "wetting" agents and that's only a short term "crutch". The best long term fix is to grind or "shape" the running surfaces to provide those essential scratches to retain and spread your lube.

From my experience doing trigger jobs on newer S&W revolvers most of the MIM parts have a pretty good to excellent finish except for some occasional mold parting lines or molding flash. It is easily polished off with a stone with no detrimental effects that I have noticed.

Look at the hammer and trigger guys, it's dark gray metal. If you polish it, it will still just be somewhat shiny dark gray metal. What's to gain? Am I missing the point on what parts you want to polish?

__________________
Dick

They can be polished, but since they're not as dense as forged parts they won't polish AS WELL, or with the same result.
This is assuming you're talking about engagement surfaces on hammers & triggers, not trying to make shiny jewelry out of a MIM part.
Denis

"This is assuming you're talking about engagement surfaces on hammers & triggers, not trying to make shiny jewelry out of a MIM part."

You assume correctly, Denis. I am asking about engagement surfaces that come into play for action/trigger jobs.

Thanks, guys. All of you in one way or another have confirmed what the gunsmith told me.

"I've been designing and building industrial machinery for a lot of years and if you have gibs and ways that have become fully polished by use they will then start to gall rather quickly..."

scooter, you must mean a type of steel other than pre-MIM traditional steel for internal engagement surfaces, right?

I've been told by the Performance Center that the MIMs don't polish up as well, and by my local S&W certified longtime gunsmith that the results are not the same.

As far as polishing engagement surfaces goes, honing & stoning have been done on those in Smiths & other guns for decades.
It's part of what constitutes a true "action job", as opposed to merely swapping into lighter springs.

Keep 'em at least very lightly oiled & there should be no problems with galling, especially in non-stainless surfaces.
Deliberately introducing lube-retaining scratches on mated working surfaces is a new one on me.
It was never brought up at the brief S&W armorer's school I went to years ago.
Denis

I have a 66-5 3.25" and the hammer and trigger were both jeweled, like a rifle bolt, and have a mirror-bright polish. Were these items not MIM?

No idea.
Look at the back of the trigger, is it hollow?
Polishing, to the naked eye, is not a good way to judge the degree of polish involved.
Under magnification the surface that looks bright & shiny to your eyeball will look markedly less smooth.
Denis

Yes, back of trigger is 'hollow.' The sides of both trigger/hammer are 'burled' as in jeweling.

(Brown tint is reflection)

MIM.
Denis

I figured such.

The ability to be polished is what I questioned re: other posts re: same.

(Looks like someone did a nice job anyway )

Those have been machine turned.
You CAN polish MIMs, just not to the same degree, as mentioned earlier.
Denis

Thanks for the clarification. (Should hold a bit of lube, I'd think.)

Take care.

Good luck.
Denis

Look at this hammer, it is MIM and you can polish it as just as bright as the gun if you want.

Plus on the trigger job, There has been alot of bull hockey said about it, the forged parts only has a .004 -.006 thickest of hardness (case harden) where the MIM is just as hard in the center as the out side is. I can take MIM and polish the contact points to where you can see yourself in them and have no problems with it. Light oil is all I ever use except on a new guns first range vist I use grease (White) to help the surfaces out some. I did polish one of my guns to a mirrow finish and put over 5000 rounds down the tube with no sign of wear to the hammer or trigger, some of the mirrow finish was a little dull but not bad at all. That gun is in the safe for retirement .

Again- VISUAL brightness is not the indicator. Not talking about polishing to match a frame, or to look at your gorgeous face in.
Yes, you can polish MIM parts BRIGHTLY. You can polish through the external colors.
What your eye sees isn't the same as what the mating surfaces feel though.

MIMs are more microscopically porous than forged parts.
MIMs don't polish down as slick as forged parts, despite what your eyes see.

You spend an equal amount of time & effort in polishing the engagement surfaces on a MIM and a forged part, stick them under magnification, and you'll see a difference.
Whether you feel it in the trigger pull or not is another matter.
Denis

I only have my eye and feel along with the end results, a MIM system can give a smother and slicker action than you can get with the forged any day of the week. For a action job the MIM is "KING", Looks and collection the forged system is "KING". Two different systems to pick from to your likeness.

"... a MIM system can give a smother and slicker action than you can get with the forged any day of the week. For a action job the MIM is 'KING"' Per Bullseye

That is exactly the issue in this thread.

"... a MIM system can give a smother and slicker action than you can get with the forged any day of the week. For a action job the MIM is 'KING"' Per Bullseye

That is exactly the issue in this thread. And I thought it was about MIM not taking more than a 'grey' polish

I am not told by either the PC or any gunsmith I've discussed it with that MIM takes a better and smoother action job.
Denis

But, to avoid sinking into another MIM vs forged argument, I'll leave it here.
Denis

So to drag this one a bit further,
MIM can absolutely be polished.
My preferrence:
Stainless steel finish then bead blasted, Polish trigger and hammer for contrasting against flatter bead blast finish.
I have done it on 2 pistols. a 620 and 686-6. I also had it done at S/W on a 60-10 when they did a complete action job on it. I guess it depends on what you wanted it for aesthetics? I used to have a few picks but deleted them. My current 60-10 and 686 have highly polished hammer and triggers.

Guys, please let me try to make the question clear. I am asking whether internal MIM engagement surfaces can be polished for the purpose of action/trigger jobs.

A gunsmith at a gun store I frequent tells me "no", and the consensus here seems to be more or less in agreement with the gunsmith, i.e., it can be done, but not nearly as well as with the old forged parts. I don't recall the reason for the gunsmith's negative opinion, but I think he said the MIM surface might be damaged--don't hold me to that.

My question is in no way a slam of MIM parts or of polishing external MIM parts for aesthetic purposes.

Everyone here agrees that MIM parts can be polished to make them shiny, and Bullseye takes it a lot further.

The S&W-provided MIM FAQ (Sticky) on this board (and even on S&W's own web site FAQ) does not address the question. So, yesterday I sent an email to S&W. I'll post S&W's reply when I get it.

<climbing on soapbox>

Synthesizing the results of the discussion here, I think the
answer may be clear. I am not an expert but putting together
all the information from those who profess to be such:

MIM is hard all the way thru but slightly more porous. This may
be an advantage to hold the slight amount of lubricant necessary.
No one doubts the hardness or function of the forged parts.

Hence I call BS on the guy who claims MIM can't be effectively
polished for action surfaces. It doesn't have to be spacecraft
worthy, only smooth enough to work properly and hold a very little
oil.

<falling off soapbox>

---
Nemo

I've cleaned up engagement surfaces but there wasn't much to do on them as they were already pretty smooth to begin with. The rebound slides I've seen on new guns were black which appeared to be Teflon coated(?) the wear was even and minimal on them. However the rough action on some guns was due to metal flakes/residue and dry crude. I've changed a few parts that apparently left the factory in that shape. If it's a new or new to me gun I clean and inspect them for any uneven wear. After a few range sessions the insides get another cleaning and then it gets decided if they need anything. Polishing the suffices smooth will not improve much if anything there is more damage done to guns from people that like to polish the insides or play gunsmith.

UPDATE

Last weekend I sent a specific question to S&W asking whether or not MIM parts can be polished for action jobs:
"A true action job, I'm told, involves polishing internal parts, and maybe new springs. But, some say MIM parts can't be polished for that purpose because it tends to ruin the natural surface smoothness of the parts.
"So, can MIM parts be polished for an action job?
NOTE: This is not a slam on MIM parts. I have no problem with them."
"Thanks,"

Got a nice reply (as usual) from S&W today, but they said only that they do many action jobs on "guns that have MIM parts". I replied today as follows:
"Yes, I know you do action jobs on revolvers with MIM parts, but my question was, do you polish the MIM parts for the action jobs?
Thanks"

I'm beginning to think that the answer to the question is pretty close to what srgvaz and others have posted above. In my own words:
Yes, MIM parts can be polished by skilled people for an action job, but generally a full polishing job is unnecessary with MIM parts, and the time, effort and risk are not worth the small reward which can be better obtained by other means.

I'll let you know what S&W says in its reply.

Like I said, you can polish them till you can see yourself, "BUT" a good action job doesn't require it. There is way more to a action job than polishing the parts.A burr doesn't care where you can find them, making sure parts aren't rubbing the sides. Make sure pins aren't causing drag, burrs on the inside of the frame, if it can cause drag it has to be fixed. I can take a trigger to 3 to 3.5 lbs, but afer 250 rounds because of the drag the crud causes bring the trigger up to 5+ lbs. Smooth is the word for a action job like Keystone is in Beer .

I'd be surprised if S&W gives you much more of an answer than that.
Denis

My gunsmith did an action job on my 625. Said it wouldn't be as good as non -MIM trigger but he'd do is best. I am extremely happy. It was much improved over the factory. I think people might be splitting hairs on how good an action job you can get but I'll say you can certainly improve the factory action.

UPDATE

Received a reply from S&W today. The answer to the question is that S&W gunsmiths do polish the MIM components when doing an action job.

So that settles that.

That was no surprise, of course they do.
Denis

Hello,

I have recently bought a magnetic tumbler after reading about how good and efficient a polishing machine it is. The first item I polished was a SS small part. After 30mins of tumbling, the ring was dull and had a frosted finish or what others call an orange-peel effect. I then noticed on the product page of the website that says: "Not a Final Polish May leave behind an orange peel effect on smooth broad surfaces.", so I tried to polish smaller items like a few pairs of gold and silver earrings. They too came out looking dull and frosty. Other items such as bracelet and necklace came out okay but I find the finishing quality inferior to that of a rotary tumbler.

I've tried using liquid and powdered burnishing compounds, experimenting on different amount of water used and extending the tumbling time but to no success. I have seen videos in youtube and read online articles showing how bright and shiny a seemingly plain ring (among other jewelry) can be after it has been tumbled in the magnetic tumbler.

The good thing about it is that it doesn't remove delicate designs on the jewelry during the tumbling process and produces a very even finish.

So what did I do wrong? Why am I struggling to get a bright and shiny finish as advertised? How should a magnetic tumbler be used in conjunction with other polishing tools? Also, should I polish my now frosty-looking ring with Tripoli then rouge or can I just skip it and apply rouge instead?

Any help is greatly appreciated!

I will upload photos my work and comparisons upon request. Thanks!