I am a “hobby” machinist. I’ve read some books, I’ve watched 1000s of YouTube videos. I’ve even been in the shop and made things. But I’m still a hobby grade machinist.

Occasionally, there are things that you just don’t know you don’t know.

The bible for machinists is the Machinery’s Handbook. It contains maths, it contains formulas, it contains specifications, it contains just about any knowledge you might need to machine something.

I’ve looked at the section on bearings, and it didn’t click.

Then one of you sent me some feedback on the Casinator. It was obvious from the outset that they knew what they were talking about. They rattled off specifications and bearing identifiers without issue. I had to spend some time looking things up.

One of the things that we deal with is attaching things to one another. You can glue things together with molten metal, but that has its issues. You can attach things with bolts and other fasteners. You can also shove something into a hole that is too small for it with enough force that it goes in and stays in place.

That last thing is called a “press fit”. Assume you have a 1/4in hole, and you want to press a 1/4in shaft into that hole and have it stick. If the hole is 0.2500 and the shaft is 0.2490, the shaft clears the hole. As long as the shaft doesn’t expand or the hole shrinks from heat, everything is fine.

But we want it to stick. We can either make the hole smaller or the shaft bigger. The Machinery’s Handbook will guide you to the correct values.

Bearings are designed to be held in place. One of the most common methods is a press fit. For the types of bearings I’m using, this means that when the bearing is pressed onto the shaft, it stays. When the bearing is pressed into the faceplate, it stays.

The amount of interference is based on many factors, some of those factors are known only to the manufacturer of the bearings.

Thus, “nominal”. With the R4ZZ bearings I am using, they are nominally 5/8 OD and 1/4 ID.

They are not 0.6250 and 0.2500. The OD is large, and the ID is smaller. The interference fit is built into the bearing design.

My job is to hit the proper sizes when I machine the pockets and shafts. That is to say, my pockets must be 0.6250 within the given tolerances, and the shaft must be 0.250 within the given tolerances.

No maths for me! Just hit my numbers.

Thank you!

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By awa

3 thoughts on “You Guys Are Amazing!”
  1. when I worked for a railroad I got to know a couple of the machinists there. you have these huge wheels 3 feet in diameter and 900 pounds and an axle 10 inches in diameter. wheels are “press fit onto axle with THOUSANDS of an inch difference… its amazing what we can do..

  2. Machinists…a dying profession, at least here in America an other Western nations. And when those skills are gone we’ll pay hell, literally, to get them back. We are losing almost all the skills that built western civilization and and very few people even realize that it’s happening.

  3. When dealing with interference fits don’t ignore Thermal Assistance. Work had hydraulic presses of varying capacity and lots of precision fixtures but all I had at home was a good 6 inch machinist’s vise and 1/4″ wall steel tubing in several diameters. More than once my bacon was saved by a 350 degree oven and a few dollars worth of dry ice.

Only one rule: Don't be a dick.

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