Press fit problems 2

[PrecisionT]

First off I am new here so HI all..... Now the problem. I am trying to press fit a .125/.1253 X .312 416 stainless pin at a 40HRC into a .124/.1245 hole in 154CM at 58HRC and it must pass threw the part and stick out .086 off each side. Both the pin and hole are chamfered but when I press the pin in it just peels off the material from the pin and it is then loose in the hole more like a slip fit. I have tried shrink fitting this with dry ice/acetone with the same problems. Any help would be greatly appreciated.

Thanks, Travis

 

[TVP]

Are you using a lubricant, such as a molybdenum disulfide-based paste? Here is a link to one suitable product:

http://www.dowcorning.com/applications/search/default.aspx?R=395EN&DCCSF=393EN

Have you inspected the chamfer on the hole? Many times the intersection of the chamfer with the ID is not concentric, leaving a step instead of a smooth and/or rounded transition.

 

[PrecisionT]

Hey TVP,

No I have not tried that paste so will give that a try. I have tried a lubricant but it was just kroil. I have inspected the chamfer and hole and it is fairly concentric but I do have .0001/.0002 runout in the hole and about the same in the pin. Can that cause this peeling affect? If the paste don't work I may also need to try and get more of a temperature change. I realized that I am only getting about .0001 shrinkage on the pin so I may need to get a much bigger temp change but I really wanted to stay away from heat because of discoloring, because of finish requirements and the finish work must be done before the pin is installed. Thanks for your reply I appreciate it.

Travis,

 

[dgallup]

How good is the surface finish on both parts? Any heat treating scale? I have found heat threat scale to be the equivalent of sandpaper making it practically impossible to press parts together.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[PrecisionT]

Hey dgallup,

No there is no scale. They both have a 32 or better finish.

Travis,

 

[TVP]

Travis,

I don't think run-out in that range (0.0001 to 0.0002 inches) is causing the problem. I think that it is the surface effects (roughness, stainless oxide layer, lubricant, etc.) together with the limited temperature differential. Don't use acetone, as this will remove organic lubricants. I think that the MoS[sub]2[/sub] paste will be a significant improvement.

 

[hydtools]

What is the shape/size of the chamfer on the hole?

Ted

 

[Screwman1]

Too much press for that diameter. Cut it in half and you should be OK. I don't think that you will ever be able to get a .125 dia part in with over .001 press. You also normally want your pin to be harder than the hole.

 

[btrueblood]

Generally press fit works better when the pin is harder than the hole, but you probably know that. Could you use a roll pin or spring pin instead of solid dowel? Try honing/lapping the hole entry chamfer to round the edge a bit? Liquid nitrogen for the chill? Do realize that a 1/4" pin will warm very rapidly, especially once it makes contact with the hole, you probably want to smack it home in one push.

 

[PrecisionT]

TVP, I am using the acetone with the dry ice for a poor mans liquid nitrogen. My Nitrogen container won't be in for another week. I have some of the molykote coming in tomorrow.

hydtools, It is .008/.01 on the hole and about .02 on the pin.

Screwman1, Yeah it is extreme the holes are all right at the high so we are pretty much about a .0007-.0008 press fit. I know this is still extreme at this diameter. We also got a change after this run on the pin material so on the next run we will be switching to 420 but still will be softer then the mating part. I just don't have an option to get the pin harder then the part.....well not a cost effective one anyway.

btrueblood, Yeah I would love for the pin to be harder but don't really have much option there. No I can't use a roll pin or spring pin in this application..... I wish. And yes the pin heats extremely quick I honestly think we are gaining nothing here because by the time it is taken out and put in place in is basically insignificant.

And I just want to say thanks again for all the help it truly is great to have a place to do this!

Travis,

 

[hydtools]

PrecisionT, what is the included angle of the chamfer?

Ted

 

[israelkk]

I believe your problem is galling. You are trying to press fit a stainless steel into a stainless steel. The main problem is the length of way the pin needs to travel through the hole where it accumulated the galled material during the long travel in the hole. The trick I use in such cases is to use stepped diameters hole and pin. For example: create three stepped diameters on the pin and three stepped diameters in the hole this way to press fit the pin you will need only one third of the relative travel between the pin and hole to press fit the pin the hole.

 

[mfgenggear]

precision
if my calcs are right.
based on linear expansion of .0000062 in/in

can you heat the part with the hole to a 300 deg. F this will expand to approx. .0014" on dia
use a conventional open air oven. 500 deg F would be even better.

the pin shrinks .0003" on dia @ -120 deg F (Dry Ice)
as you now well know that the pin must be as hard or better yet harder than the mating part.
use anti seize or lube as suggested by TVP.

You must press the pin very quickly or it will seize half way. then back to square one.
have a simple press tool to keep the pin inline.

is it possible to buy a std MS .125 dia pin the cut to the correct length.
these pins are very cheap.

HTH
Mfgenggear

 

[mfgenggear]

correction
the hole will expand .00017" @ 300 deg F
the pin will contract .00003875" @ -120 deg F

Mfgenggear

 

[PrecisionT]

hydtools, 90 deg.

israelkk, I don't see it as galling. It is peeling the material right off like peeling a potato. Unfortunately I can't use a step hole/pin as I need this pin sticking out both sides of the part at it's proper size. After pressing the pin in I can press it back out with hand pressure and the pin is smooth and the exact same size as the hole like I was broaching it.

mfgenggear, I actually tried that I can't go over 400 but did go to 375 with the part and dry iced the pin with the same results. I have a fixture for alining everything but the 10 seconds it takes from pulling everything to pressing it must be undoing to much of it.

Not sure what to do at this point I have some of the molykote showing up tomorrow if that don't work we may just have to make the pins from something I can get to a higher HRC.

Travis,

 

[mfgenggear]

Precision

it appears that if the hole & pin have more than metal to metal clearance . there will be a problem with the existing pin material.
with .0008" press it will not clear. even with the temp's posted on my first post.
I am afraid unless the lube works it may require a remake of the pins.

Mfgenggear

 

[PrecisionT]

mfgenggear, Yeah I am afraid that is going to be the solution. And I also think I will skip the 420 and go with 440C then I can get within a couple of points of HRC of the part.

Thanks again, I will let you know if the lube works or not.

Travis,

 

[hydtools]

90 degree included angle is still a sharp edge. It should be 30 or less.

Ted

 

[PrecisionT]

hydtools,

Do you think changing that to 30 would stop my peeling/broaching problem?

Thanks,Travis

 

[mfgenggear]

yes I agree with hydtools. it would very much help.
it would be easy to modify the pins and give it try.

mfgenggear