What is MIM?
Here is an older post that I have copied here, what is MIM?
By popular request, here's the post from Mr. Herb Belin of S&W -----------------------
"I have read with much interest the many comments in this forum pertaining to MIM, MIM Parts and the use of same in a S&W product. So far I have come away with several impressions and they are "people in general don't like/trust MIM parts" and "no one has said why" I will take a stab at this issue and see where it goes.
As background to our decision to use MIM in some areas of our Mfg Process we took a long hard look at our "Life Time Service
Policy". It was clear to us that any change in any of our products such as the use of MIM components had to show equivalent or better performance and durability to those components that were being replaced or the "Lifetime Service" would haunt us forever. The second consideration was to determine if the change was too radical a departure from S&W mainstream design.
For the performance and durability issues we decided that if MIM could be used for the fabrication of revolver hammers and triggers successfully this would truly be an "Acid Test". There is nothing more important to a revolvers feel than the all-important Single Action Sear that is established between the hammer and the trigger. Mechanically few places in a revolver work harder than at the point where the hammer and trigger bear against each other. If these surfaces wear or loose there "edge" the "feel" is lost. Initial testing was on these two critical parts. Over time we arrived at a point where our best shooters could not tell the difference between a revolver with the old style hammer and trigger and the new MIM components. Special attention was given to their endurance when used in our very light Magnum J frames such as the early prototype 340 & 360 Sc's. None of our revolvers work their components harder than these small magnum revolvers. Throughout this testing MIM held strong and finally we determined that this change judged on the basis of durability and feel was a good one.
The second area of concern to S&W was our customer’s reaction to this departure from the traditional. Many heated, intense discussions resulted but in the end the decision was made to move ahead with MIM.
The issue of cost was only one of the considerations in making this decision. Equally as important was the issue of part-to-part uniformity and the result of this of course is Revolver-to-Revolver consistency. We found that revolvers that used MIM hammers and triggers required almost no Fitter intervention in those areas during final assembly and final inspection and Trigger Pull Monitor rejection rates dropped markedly on finished guns. From an internal process point of view it appeared a "Winner".
Lets shift gears for a moment and talk about the MIM process. It is unclear to me as to the reason for many of the negative feelings on the forum concerning MIM. Typically when people complain and aren't specific in the reason why, the problem is often created by a departure from the "Traditional". Perhaps that is indeed what is bothering some people when they view MIM.
The term MIM stands for Metal Injection Molding. It holds some similarities to Plastic Injection Molding and many differences as well. To start we would take a finally divided metal powder. This could be stainless or carbon steel. Today even Titanium is being used in some MIM fabrications. We would mix the metal powder and a thermoplastic binder (generally a Wax) forming slurry of sorts when heated and inject this mix into a precision mold and finally form what is known as a “Green Part". This part is roughly 30% larger than the finished part it will become at the end of the process. Interestingly enough the Green Part at this stage can be snapped in two with simple finger pressure. The Green Parts are then placed in a Sintering furnace filled with dry Hydrogen gas and the temperature is brought almost to the melting point of the metal being used. Over time the "Wax" in the Green Part is evaporated, the metal fuses and the part shrinks 30% to it's final correct dimensions. At this stage of the process the MIM part has developed 98 to 99%of the density of the older wrought materials and a metallurgy that is almost identical. Dimensionally it is finished and no machining is required. However the job is not yet done and the MIM parts are brought to our Heat Treat facility for hardening and in the case of Hammers and Triggers, Case Hardening. Depending on the particular metal alloy that was used at the start of the process we apply a heat treat process that is the same as would be used if the material were the older wrought style. Final hardness, Case thickness and core hardness are for the most part identical to parts manufactured the older way.
Lets look for a moment at how we achieve dimensional precision when comparing these 2 processes. The old parts were each machined from either bar stock or a forging. Each cut and every resulting dimension was subject to machine variations, Cutter wear, operator variations etc. If every operation was done exactly right each and every time and the cutter didn't let you down you would have produced a good part but sometimes this didn’t happen resulting in a rejected gun and rework or in the worst case an unhappy customer. With MIM parts you must still machine to very high tolerances and your cutters have to be perfect and your machinist has to be highly qualified but all of this only has to come together one time. That time is when the injection mold is made. Typically a mold for this process costs S&W between 30,000 and 50,000 dollars. Once it is perfect every part it makes mirrors this perfection and you have in my view a wonderful manufacturing process.
Hopefully this description will help us all better understand the MIM process.
Please forgive the spelling errors and misplaced punctuation. I have no spell checker on this and the phone continues to ring!
Have a Great Weekend,
Currently S&W is paying about $1.20/Lb for stainless steel bar stock. Raw MIM stainless steel inject able material costs $10.00/Lb."
Probable causes for a cylinder that doesn't open or close smoothly:
loose extractor rod
bent extractor rod
loose thumb piece nut
short or worn center pin
end shake cylinder or yoke
dirt under extractor
yoke out of adjustment
worn locking bolt
worn yoke button
worn yoke screw
tight yoke screw
ammunition rubs frame or primer flow
worn recoil plate firing pin bushing
Here is a picture of what flame cutting looks like. The gun is my 629 44 mag.
Flame or gas cutting is quite normal and is nothing to worry about. If a revolver has been fired it will have some degree of flame cutting. Flame cutting usually stops after a time and won't get any worse.
DCU or timing
DCU (doesn't carry up) or basic timing is easy to check.
How I check basic timing is pretty easy to do. Without touching the cylinder or holding back the cylinder just gently cock the hammer slowly in single action, (on all stations) and see if the cylinder stop snaps in place before the hammer falls. When the trigger is pulled to the rear (MIM triggers will rotate the cylinder further when the trigger is pulled) and the cylinder stop didn't snap into the cylinder notch, then you have a problem.
One thing I must insist, before swapping the parts make sure it's not a loose fitting ejector causing your dry fire and poor carry up problems. This is true with non-pinned cylinders. Drop empty cases into the chambers then check your carry up. Sometimes the gun is fixed when cases are inserted in the chambers.
As the extractor ratchets, hand, and hand window wear, they get to a point where the hand won't rotate the cylinder far enough to allow the cylinder stop to snap in before the hammer drops (carry up condition in DA mode). This is not a hand length issue, it’s a hand width problem. If you take the side plate off and watch carefully in the hand window while pulling the trigger, you will see the hand cams off of the ratchets and the left side of the hand tip positions against the right section of the ratchets to rotate the cylinder those last few degrees.
Unless a gun has been fired a zillion times, the hand and the window seldom show wear, except for the left tip of the hand. Most times when you see a hand loose in the window, it's because it was shipped that way.
Factory hands run from about .093 to .095" and wide hands run about .098 to .100" There shouldn’t be more than .001 side play between the hand and window.
If the hand has worn thinner, or the hand window has worn wider, or the edges of the ratchets have worn thinner, you will start loosing carry up. Without fail, the ratchets are the most usual cause.
There are several solutions. The absolute best one is to replace the extractor (ratchets). Unfortunately, that’s a factory fitted part and you can't buy one without sending the gun back to the factory.
The next best option is to fit a wider hand. This will have nearly the same effect as a new extractor because it will take up the slack from ratchet wear and make the cylinder rotate a few more degrees. When the wider hand is fitted, you must widen the left edge of the hand window so the hand will be positioned a few thousands to the left.
An undesirable option is to lay a fine bead of weld on the right edge of the hand window then dress some material from the left edge of the window. This will position the hand closer to the ratchet and make the cylinder rotate further. Welding that thin of metal is sure to do some damage to the finish and could ruin the frame.
You can bend the tip of the hand slightly to the left. This will also cause the cylinder to rotate a bit father. The hand is made of some very hard material and will fracture if you try to bend it without first softening it. You have to heat the tip of the hand until it is red hot then let it cool to room temperature. It will then be soft enough to bend without breaking. Once you get the hand tip bent, you must re-harden the hand. Herein lies the problem. Most of us don't have the proper metallurgy skills to re-harden the hand. If it is soft, it will soon wear out.
One of the very precision specs in an S&W revolver is the hand-to-ratchet clearance. A few thousandths makes the difference between good and bad carry up. You want the hand tip to be touching the ratchet but not binding on it when the trigger is pulled. Basically a zero clearance.
If you insert the tip of the hand into the window, it should move freely with minimal side play. The tighter the better, as long as it doesn't bind.
The left side of the window creates a reference point in respect to the ratchets. You must move the reference point to the left before the wide hand will make the cylinder rotate more. That means you must take metal off of the left side of the window. Use a "safe" window file for this. Taking metal off the hand or the right side of the window moves the reference point the wrong way.
Once your wide hand is installed, the tip of the hand may bind on some of the ratchets. This will cause a bad gag in DA trigger pull. With the side plate off, watch through the hand window as you cycle the gun in DA. You will see where the hand cams off of the ratchet and the point of contact between the hand tip and each ratchet. Dress the ratchets at the point of contact so the hand is touching but not binding.
Go slowly changing the hand. It's easy to do and will fix your timing (DCU) problems.
To correct push off
The hammer notch must be square, sharp and not rounded. It's only .005 inch tall so use a eye loupe to inspect it. Use a new stone to true it up, if it's not square. (I like ceramic stones for this) Then after the hammer is proper, stoning the trigger will correct the push off.
Remember use a 6 inch long stone. Changing the stone length will affect the trigger pull.
To test the trigger hook, you scrap it on your thumb nail. If nail is scraped off, you got it sharpened correctly.
If you want a lighter single action pull, you dull the angle by lightly stoning the angle parallel to the table top.
New style firing pin removal
When you pull the side plate off, the upper pin (that aligns the plate) is the firing pin retainer. Remove the pin, spring and firing pin will come out.
Find a perfect fitting screwdriver.
Remove the grip.
(The best way to remove grips is to unscrew the screw a few turns and tap the head of the screw with the screwdriver handle to push the right grip panel off the frame. Prying the grips with a screwdriver should be avoided).
Unscrew all the screws on the side plate, don't mix them up.
Roll gun over, push the thumb latch in and open the cylinder.
Slide yoke and cylinder off the frame, go easy not to scratch it.
Roll gun back and find a plastic hammer and tap the frame (near where the grips would be) and the side plate will come loose.
Remove slide plate.
(The proper way is to use the handle of a screw driver and strike firmly the frame where the stocks normally are. Keep your thumb over the side plate and it will pop off easily).
Lift out the hammer block (sheet metal part with a hole on one end, hook on the other).
Loosen strain screw and remove main spring. (J frame insert cotter pin into hole, remove from stirrup).
Pull thumb piece to rear. Press trigger to cock hammer, lift hammer up and off the stud.
Find a screwdriver, and pry the rebound slide off the stud, rebound spring and stem will fly out so keep a finger over opening.
Pull back the hand to clear window, then wiggle trigger off the stud.
You can remove the cylinder stop and bolt but the gains are not worth the trouble.
But if you must, then flip gun over and unscrew thumb piece.
Flip gun back, pull bolt to rear and lift out of frame, watch out for plunger and tiny spring.
Pry out spring for cylinder stop, let the stop drop down and remove stop.
Reverse the operation to put it back together. Find a special rebound slide tool to put the rebound spring back on. It looks like this:
Remember all the guns from J frame to X are nearly all the same internally.
The hammer block sits all the way up before the slide plate goes back on. One end sits on the stud of the rebound slide, the other end sits in front of the hammer. Picture of 500:
Here is a picture of a 686 without the hammer block:
CYLINDER DETAIL STRIP
Once the cylinder in on the table it can be totally taken apart.
Guns which are about 40 years and newer the ejector rod is counter-clockwise. Older than this the threads are clockwise.
Place empty cartridges into the cylinder.
Using a drill chuck, turn extractor rod and unscrew it. (Don't mar the knurl on the end)
There are 2 springs inside, a collar, center pin and the ratchet. Keep finger pressure on the ejector rod so the springs don't fly off the table.
Reassemble in the reverse order. You don't need lock tite on the threads.
You could use a tool like this instead of the drill chuck, if you prefer:
X FRAME CLYINDER:
Here is a picture of what the parts look like.
To disassemble: turn knurled end clockwise, remove nut and slide the yoke off. Be careful not to mar the knurl. I use a clothes pin under pliers.
Then using a drill chuck, turn extractor rod clockwise to remove the extractor assembly.
(all threads are counter-clockwise)
Here is a picture of all the J frame parts spread out for your pleasure.
The larger frames K,L & N have an additional part called the extractor rod collar. This part shows in the earlier diagram of the model 29 I posted earlier.
A note on working on extractor rods:
"Hand tight" is not good enough, and you don't need locktite.
Put the rod into a drill chuck and tighten it a little further.
A second way is to purchase a tool that will give you the needed leverage. The tool is sold on ebay or Brownells.
Remember don't grab around the knurl or you will mar it which will look unsightly.
Also, add fired cases into the cylinder BEFORE tightening it down to avoid damaging the pins (if equipped) or spline.