Basic question: fitting rod to bearing

[taydin]

Hi, I have wheels with two 20mm ID bearings at each end. I also have round rods whose OD are 20mm. The rod seems to be ground, so my caliper shows the OD to be exactly 20mm. Without removing any material, the rod doesn't go in to the bearings. It might go in if I pound on it with a hammer. I want this rod to go into the bearings with a tight fit, just a few light strokes with a hammer should be all that's needed. I tried making the OD of the rod 19.9mm, but that got too loose.

Right now I am thinking of fastening the rod to my lathe and swipe a piece of sandpaper over the rod.

What is the recommended way of handling this?

[ptjw7uk]

Problem I have found is that the rod has a burr or lip at the end, so just a lick with some 400 grit paper.
If the ground rod is accurate it should be a sliding fit so you just need to ease the ends a bit, just take your time or it will be slack as you have found.

peter

[BadDog]

If both are perfect and the same size, you won't get it to go on easily. Depending on diameter, it will require some level of press to fit. You might also be dealing with a bar that is tri-lobed (or more). A common mic can't detect this, but it will drive you nuts trying to fit, and is very common with "centerless ground" stock. If so, the press difficulty jumps considerably. Often doing something like this includes freezing the shaft and/or heating (within limits!) the bearing.

[metalmagpie]

Hi, I have wheels with two 20mm ID bearings at each end. I also have round rods whose OD are 20mm. The rod seems to be ground, so my caliper shows the OD to be exactly 20mm. Without removing any material, the rod doesn't go in to the bearings. It might go in if I pound on it with a hammer. I want this rod to go into the bearings with a tight fit, just a few light strokes with a hammer should be all that's needed. I tried making the OD of the rod 19.9mm, but that got too loose.

Right now I am thinking of fastening the rod to my lathe and swipe a piece of sandpaper over the rod.

What is the recommended way of handling this?

Merhaba, Taydin! I suggest you measure the rod using a micrometer, as calipers really aren't all that accurate. You may find that your shaft is very slightly oversize. I'm not used to working or thinking in metric, so bear with me. I'm not talking about much, maybe 0.005mm (a ten-thousandth of an inch if I got it roughly right). If your shaft is *any* oversize then you will be dealing with a press fit. And you don't have much room for pressing the bearing onto a too-large shaft before it starts making the bearing not work correctly.

Last time I was in Turkey I didn't see many old-fashioned incandescent light bulbs - most people were using the fluorescent bulbs - but if you have an old table lamp try putting a bearing directly on top of the bulb for a few minutes, then try to pop it onto the shaft.

Good luck!

metalmagpie

[BadDog]

Good catch, I saw caliper, but apparently registered as "mic". I would never try to make such measurements with a caliper.

For shrink fitting, I've used a toaster oven on lowest setting (too hot for some bearings) and shaft in the freezer (if it will fit). Also just set the bearing in the sun for an hour or so when it was "close". But you have to be quick, you have only seconds before it seizes and you are in trouble if not fully seated.

[taydin]

Thanks for all the responses guys. I will try heating the bearing with a heat gun and then pushing the rod in. Unfortunately I don't have a freezer in the shop, so can't cool the rod...

Now I also have an excuse to get myself a micrometer. I have never needed one so far. It probably will be a mechanical one (not fond of all the digital crap that's out there, and the price is just right).

[Black_Moons]

Other things can be used for extreme cold.

'Freeze in a can' Aka, 'Air in a can' that was built upside down, can spot freeze things.. unfortualy id suspect you'd need a lot to cool down a shaft.
Cans of butane upside down also work.. but be careful about displaceing too much oxygen and affixating yourself... Or explodeing yourself with nearby ignition sources

Dry ice.. Ok a little harder to find for most but just throwing it out there.

Ice from your freezer.. a little wet but oh well. Mix with some salt and I think you can actualy get like -5c~ or something outta a really good freezer.

Oil! Just stick a bucket in the freezer for awhile when SWMBO isent looking....

Hmmm let me think what else is good for negative C tempature...
Oh, you could send the peice to evan come next winter! I hear his back yard is good for -20c!

[taydin]

Ok guys, I bought a micrometer and measured the rod. It was exactly 20.00mm. Didn't expect such an accuracy, hmm...

So I attached the rod to my lathe and make a few passes over the rod using 240 grit sandpaper. I used my welding gloves to hold the sandpaper, which nicely isolated the heat. Then I measured and there was no change in diameter. So I did many more passes, still almost no change.

I switched to 120 grit sandpaper and really pressed hard. This time the diameter went down to 19.995mm. Not a lot of progress ...

I took a grinding disc (about 3") and pressed its side against the surface of the rod. After a little grinding, the diameter was down to 19.99mm. Tried to fit the rod, didn't fit.

I brought down the diameter down to 19.98mm, tried again, and this time the rod seemed to be going in if hit lightly with a hammer.

So it seems, in order to obtain a tight fit, a clearance of 0.02mm (about 0.001") is necessary.

Then I heated the bearings with my heat gun, pushed the rods in. They went in about halfways before the top bearing seized the rod. I pushed the rest in with light hammer hits.

[taydin]

While doing the grinding and sanding work, I thought about a lathe insert which had a grinding stone or sandpaper in front of it and you used it just like any other cutter. But it would remove very little material. This would have helped me remove material more uniformly. With the sandpaper, I had to stop the lathe many times, measure the diameter at many spots along the rod and work on spots that are higher...

Is there such an attachment?

[Lotek]

It's called a tool post grinder.