Tips for press fitting a pin?

JPMacG said:
I need to press fit two .250 pins into a 12L14 part. The pins are W1 drill rod, not heat treated.

I did a test piece using a scrap of 12L14 and a ".249" reamer. The hole seemed to be slightly undersized, about .2485 I think, based on my pin gauges. I was able to press in the pin but it took an enormous amount of force. My small arbor press did not work. I had to squeeze it between the jaws of a large vise. I'm afraid I will damage the 12L14 part if I use that much force.

Are there any tricks that would help? Should I put a lead-in on the pin? If so, what angle and how long? Should I lubricate the pin and hole prior to pressing? Should I polish the pin? This is a one-of-a-kind part, so I could turn down the pin diameter, but I'm afraid it would be difficult to get the diameter just right.
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This is a problem that us hobby guys run into and the issue is that the size of the hole the pin has to fit into must be of a precise size. What is not apparent is that the reamer must be sized to take into account the material the pin is being fitted to. What's more, you have to select the correct pre-reamer drill so that the correct reamer cuts a hole with the tolerances you require.

You can't just take a 0.250 pin and subtract the fit tolerance to choose a reamer. In the example you gave, you chose a 0.249" reamer. A better reamer size would have been a 0.2495" to 0.2496" reamer. You would need to drill a pre-reamer hole between 0.2455" to 0.2395" ID, which would require a drill size between 0.2414" to 0.2353" ... closest would be a B-drill. THIS would get you a 0.0005" press fit. I'm attaching the source for these calculations below.

I dunno about you but I can't afford a specialty reamer. Most undersized sets are about 0.001" under and they have tolerance ranges so they may not work; if you need a 0.2495" reamer then you are going to spring for a specialty reamer. If you do this for every press fit pin you do, it is going to get expensive. Yeah, I know you can make do but too tight a fit and you risk upsetting the material around the hole and your parts may not fit; too loose and the parts that fit together may have excessive movement.

Most of the time, I determine the fit I need and then I bore the hole to size rather than use a reamer. It is far easier and cheaper to do it this way but you have to be able to bore pretty accurately because you're working in tenths. It might surprise you but I try to bore precision holes on my Sherline mill. I get close to the size I need by adjusting the leadscrew on my Criterion head and once I'm within a thou or less, I use the variable speed motor to creep up on the final size. If you leave the boring head alone and increase speed in small increments, the boring bar will cut larger as speed goes up. Since my speed has infinite control, I can pretty precisely size the hole this way; I mean in the low tenths. Sherline machines are NOT toys.
 

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Tom Lipton did a video last week with a good explanation of achieving a press fit for a couple of locating pins:


BTW, even with his skills and experience, one of the holes ended up a slip fit rather than a press fit!

Craig
 
WCraig said:
BTW, even with his skills and experience, one of the holes ended up a slip fit rather than a press fit!
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And that's when Loctite saves the job.

I have used the "increase the RPM" trick with a boring head to shave another tenth or two.
 
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