pin & hole slip tolerance

[jjbiot]

Hi all,

I need to put a turned pin into a hole. The pin is 0.157 inches in nominal diameter. I need to tolerance both the hole and pin so that the latter slips into the former under a force of 3 to 4 ounces (which is the force created by the moving stem of a Mahr-Federal dial indicator per spec). The pin being machined will thread on to the bottom of the dial indicator stem which has a 4-48 female thread. The gauge is a uMaxum II XL (

http://www.mahrfederal.com/index.php?NodeID=11586).

Question: if the intent is to have an easy fit with a sliding action that is solely a function of the force of the stem, how tightly can I tolerance the diameters of hole and pin yet still achieve an easy sliding action with only 3-4 ounces of gaging force? Basically, when the pin moves over the hole it has to fall into place with any encouragement by the user. The depth of the hole is about 0.5 inches.The pin a slightly longer, and functions as a probe.

I am familiar with the various classes of fit - for example

http://www.engineersedge.com/tolerance_chart.htm,

but it's the slip fit relative to applied force part of the problem that I am struggling with.

Any help would be much appreciated.

 

[MechNorth]

Have you read the relevant section(s) in Machinery's Handbook? I believe it will give you what you're after.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

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[ak762]

I would start from open catalog of Mold components to find something similar. At least fit H7/g6 with Ra0.4 is reasonable choice with your particular application.

 

[drawoh]

jjbiot,

This is a mechanics of materials question. You need to work out the interference needed to generate your forces.

I have worked out low-force interference fits in the distant past. For a pair of completely circular features, you cannot do what you want, with metal. For my design, I used a nylon tube. It was rarely taken apart, so I did not have the problems with wear that you are going to have.

Can you slit something to make it flexible?

--
JHG

 

[jjbiot]

Thank's much for comments.

When the 0.157 (call it 4mm) pin moves over the hole it has to easily and smoothly fall into place. The force pushing on the pin is 3 to 4 ounces from the gauge, and that's the only force available to get the pin in the hole. I'm not clear on the reasoning behind the comment that this cannot be done with metal, or on why this would be characterized as an interference fit.

I'm looking for a slip fit, and the question was how tightly I can tolerance the pin and hole and still get an easy and smooth slip under 3-4 oz of force (no lubricants). H7/g6 is a good candidate, as has been kindly suggested. I can see other possibilities though.

Using H7/g6 in the 3mm to 6mm range, that would mean a bore of 4.000mm to 4.012mm, and a pin tolerance of 3.988mm to 3.996mm.

Q1: If I machine the parts out of 303SS, with an Ra0.4 (16Ra) surface, does that get me a pin that slides easily and smoothly into the hole under 3-4 oz of force with no lube? Using anything other than 303SS or equiv had not occurred to me. Due to the nature of the use this application requires metal. Plastics are out.

Q2: Other candidates for a slip fit are H8/f7 and H7/f7, especially if the surface requirement is relaxed. If I use (H8/f7) for a hole toleranced at 4.000mm-4.019mm and a pin toleranced at 3.978-3.990mm, with a 63Ra surface, does that get me a pin that falls smoothly and easily into a hole under 3-4 oz of force with no lube?

 

[drawoh]

jjbiot,

Sorry about that. I read your insertion force as an interference fit. Read up on the limits and fits in your Machinery's Handbook. The RC series fits are running fits allowing movement of your parts.

There is a table in the Handbook, showing how machinable the dimensions are. Search through the section on Limits and Fits.

Don't forget to chat with your fabricator.

--
JHG

 

[cjccmc]

"...with only 3-4 ounces of gaging force?"

Sounds like you have to make full insertion with 3 oz max of straight in push? Lots of variables may enter in, surface finish, alignment, maybe compressing some air if it's a blind hole, etc. Sounds like you could benefit from experimenting with size and finish combinations if you want to optimize this. If I had to pick the tightest fit that would easily insert with 3 oz, I'd put it at .001" of clearance with a 32 finish.

 

[ak762]

totally agree with cjccmc
My suggestion was as input idea of precision threshold.
With lapping pin or hole you can slightly lose precision but achieve result.