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taydin
04-17-2010, 02:11 PM
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
04-17-2010, 02:25 PM
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
04-17-2010, 02:36 PM
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
04-17-2010, 03:28 PM
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
04-17-2010, 03:40 PM
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
04-17-2010, 03:55 PM
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
04-17-2010, 04:18 PM
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
04-18-2010, 11:25 AM
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
04-18-2010, 11:28 AM
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
04-18-2010, 11:42 AM
It's called a tool post grinder.

BadDog
04-18-2010, 01:46 PM
As for wearing gloves, google "degloved". And then don't. If you do manage to get caught, being "degloved" could actually be a good thing, considering the alternative. There was a pic circulating a while back that showed what was left after a guy was wrapped around a shaft in a lathe, not for the faint of heart...

Using sandpaper isn't the ideal way to handle this. By the time you get enough material off to make it fit, you are likely to have a shaft that is less round than before. A "tool post grinder" (commercial or dremel lash-up) would be far better. But watch the abrasive on your lathe slides! Once it gets in, the only way to get it out is disassembly and cleaning. And even that may not help. Ever heard of a "abrasive charged cast iron lap"?

JTiers had a rather in-depth thread recently on something (often) called a "shear bit". I think that might be worth reading.

If the application allows, another thing to consider is turning a guide/pilot on the shaft. This would be about 0.001" undersize to allow easy and quick alignment of the bearing before pressing into place. Not sure why the resistance to pressing it on. If you don't have a suitable press, it's often not hard to mock up something, or find one to use at a friend/neighbor's shop. Even your hammer application can work, just make sure you're driving safely on the inner race.

Boucher
04-18-2010, 02:09 PM
This is a good place for a lathe file then sanding paper on it for a polish.

taydin
04-18-2010, 03:46 PM
As for wearing gloves, google "degloved". And then don't. If you do manage to get caught, being "degloved" could actually be a good thing, considering the alternative. There was a pic circulating a while back that showed what was left after a guy was wrapped around a shaft in a lathe, not for the faint of heart...


Just found some academical links, but I get the point. If the glove somehow gets entangled in the chuck, the result wouldn't be very pleasant... No gloves near the lathe anymore!


Using sandpaper isn't the ideal way to handle this. By the time you get enough material off to make it fit, you are likely to have a shaft that is less round than before. A "tool post grinder" (commercial or dremel lash-up) would be far better. But watch the abrasive on your lathe slides! Once it gets in, the only way to get it out is disassembly and cleaning. And even that may not help. Ever heard of a "abrasive charged cast iron lap"?


I shifted the carriage away and laid a PVC sheet onto the ways before doing this sandpaper thing, so the ways should be OK. But the dust went a little bit into the chuck teeth. I vacuumed and brushed the chuck afterwards...


If the application allows, another thing to consider is turning a guide/pilot on the shaft. This would be about 0.001" undersize to allow easy and quick alignment of the bearing before pressing into place. Not sure why the resistance to pressing it on.

Not sure I understand this... Are you talking about turning a ring that is 0.001" undersize and then use this ring as a template when removing material from the shaft?

sansbury
04-18-2010, 04:00 PM
A fairly safe way to use sandpaper is to cut a strip off, throw it over the part, then hold the two ends to work it back and forth over the part while the lathe is running. This keeps your fingers farther from the chuck and if something catches, the paper should tear or pull out of your fingers before anything gets pulled in.

taydin
04-18-2010, 04:21 PM
A fairly safe way to use sandpaper is to cut a strip off, throw it over the part, then hold the two ends to work it back and forth over the part while the lathe is running. This keeps your fingers farther from the chuck and if something catches, the paper should tear or pull out of your fingers before anything gets pulled in.

Yup, I tried this. But the material removal rate is very little when I do this. Actually, in one instance, I did sand the shaft for a long time like this, and when I measured the diameter, it was more than before! I guess the shaft heated up too much and expanded more than the material removed...

In order to get some removal within a reasonable time, I had the sandpaper wrapped around the shaft and was pressing it down using both of my hands :eek:

taydin
04-18-2010, 04:26 PM
If the application allows, another thing to consider is turning a guide/pilot on the shaft. This would be about 0.001" undersize to allow easy and quick alignment of the bearing before pressing into place. Not sure why the resistance to pressing it on.

Ok, I thought about this more. I think you mean turning the end of the shaft 0.001" less and then press it onto the bearings...

Before doing the sanding operation, I tried carefully and slowly approaching the surface of the shaft. Once I saw that the cutter made contact, I engaged autofeed. But to my disappointment, after 10mm or so, the cutter wasn't touching the shaft anymore. It seems while the shaft diameter is very precise, it isn't very straight... Maybe I should have bought 22mm round stock and reduced it to the required size...

BadDog
04-18-2010, 06:59 PM
Ok, I thought about this more. I think you mean turning the end of the shaft 0.001" less and then press it onto the bearings...

Basically. When the application allows, I turn a small section that is a "hand fit" to get the bearing lined up. The tool nose radius provides a nice fillet shoulder that the bearing race squares against, and even if your press setup isn't perfect, the part of the shaft inside the bearing race keeps it square as it presses into the full size shaft.