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Old 3rd July 2013, 03:21 PM   #1
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Default Machinist type question

Sorry for the vague title; didn't know how else to word this.

I have a .125" diameter hardened steel rod I need to secure in a piece of 306 brass. Will it be possible to press the rod into a .120" hole drilled in the brass piece?

I don't have much experience working with metals so I don't know if the brass will have enough "give" when trying to press the rod in.

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Old 3rd July 2013, 03:35 PM   #2
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Try googling "interference fit". There are calculators and stuff. I think .005" is going to be too much on such small parts.

Last edited by Andrew Eckhardt; 3rd July 2013 at 03:41 PM.
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Old 3rd July 2013, 03:35 PM   #3
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The rod should be checked first to find the exact size with a micrometer. A #31 index is
.120" dia. and it will drill about .002" oversize, a test hole should be tried first, but trying to push the rod in may not work for the size difference. Heating the brass first will help and grind a taper on the end of the rod and use some grease should help
PeterC.
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Old 3rd July 2013, 03:46 PM   #4
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Yeah I had a feeling .005" would be too much. Thanks for the "interference fit" tip, would never have thought to look for help with that term.

The rod is precision ground HSS, so the diameter is exact.

I have a lathe, but I just recently got it and don't have much experience with it yet. It has a 4-jaw chuck, so I'm thinking if I just move the piece a couple thou off center the #31 drill bit will make an enlarged hole. Either that or grind a super tiny boring bar, but since I've never used one nor ground my own tools yet, that would probably be a disaster.
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Old 3rd July 2013, 04:02 PM   #5
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Heat the brass and, if possible, put the steel rod in your freezer for a bit.
Should slot together easily with no force needed to press together.
Once both pieces are at room temperature it will be almost impossible to separate them again.

Try a test piece first to make sure nothing cracks.
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Old 3rd July 2013, 04:11 PM   #6
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Yeah I might try the heat differential thing. I only have a couple of the steel rods so I can only afford to make a mistake once or twice.

BTW, I just took a look at a press fit calculation web page and I felt a small part of my brain explode.

Press Fit Engineering and Design Calculator
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Old 3rd July 2013, 06:12 PM   #7
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I didn't take any notes in shop class and the first cutter I made 15 years later from angles that "looked good" out of a 3/16" blank was a sort of combo turning/boring tool. I first needed it and made it as a super tiny boring bar. I mostly make small parts and that thing has probably removed by volume almost as much material as all my carbide tools. I even make finishing face cuts with it. My advice is go for it as soon as you can find the time. Let us know how it goes with the press fit project..
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Old 6th July 2013, 09:04 PM   #8
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Your success at pressing them together for an interference fit is also going to depend on how straight and rigidly you can hold the pieces.
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Old 6th July 2013, 11:08 PM   #9
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Quote:
Originally Posted by Andersonix View Post
Your success at pressing them together for an interference fit is also going to depend on how straight and rigidly you can hold the pieces.
I plan on making a jig out of a larger brass rod that will keep both pieces aligned.

The brass rods came in the mail today, so I will probably get at this tomorrow. One more question though; should I drill a smaller relief hole for the air that would otherwise get compressed under the rod?

I'm starting to get the hang of this lathe stuff anyway. Ground my first tool; a tiny 1/8" right hand facing bit. I used it to cut an 11 degree target crown on a .22 cal pellet gun barrel....

Click the image to open in full size.
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Old 7th July 2013, 05:54 PM   #10
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Air will compress, but a liquid lube will not...
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