In April of 2004 I purchased a Eagle America 196-9645 45 piece bearing
survival kit. I have not had a reason to use the 1/4 inch pattern
bearings until now. Today I attempted to use the 1/4 inch bearings and
they appear to have an interference fit on nearly all the shafts of my
router bits with the exception of only 2 or 3. The bearing ID measures
0.2490 to 0.2495. Most of my router shafts measure 0.249 as close as
my calipers can measure. The diameter of the router bits that work to
the ones that don’t vary by such a small amount that my calipers (which
have a precision to only 0.0005) are unable to show the difference
between the shafts that work and those that don’t. This tells me that
the ones that work vs. the ones that don’t vary by only a few 10ths of
a thousanth. The same problem seems to be true with the 1/2 inch
router bit shafts. I measured the 8 mm bearings and they appear to be
7.98 mm. (Actually, I have no use for the 8mm bearings, but they came
with the kit). I have no problems with the 3/16 inch bearings that go
on the top of the router bits.
I wonder if these 1/4, 1/2 & 8mm bearings are designed for an
interference fit? Has anyone gotten one of these kits or a similar
kit? What are your experiences?
Replies
Calipers may read to .0005 but they are at best only accurate to .001 of an inch. You really need micrometers that read in tenths to read with any accuracy for those tolerances. There probably is a slight interference to prevent the inner race from spinning on the shaft.
You could try to use heat and cold to change the dimensions to put them together. Put the router bit in to a small plastic bag (the baggy keeps the frost off) and put it into the bottom of your deep freeze or back of the freezer in the kitchen and give it lots of time to get as cold as the rest of the freezer stuff. Put the bearing into another baggy and hang it in you hot cup of "Starbucks" until it's hot. Can also use the oven (without the baggy) on warm, if your wife will let you. When the one is cold and the other is hot. Rip them out of the bags and get them together as fast as you can! If your lucky they will slide together nicely and you only have one or two burnt fingers. If not, you are going to have to force them the rest of the way. Get a small tube or pipe that will slide over the shaft of the bit (a small deep socket will work) that only touches the inner race of the bearing (if it touches the outer race or seal you will wreck it). Gently tap it into position or squeeze it with your vice.
You can do it without the hot and cold but it's not as much fun or as effective. At work we use liquid nitrogen to freeze bushings before installing, and they have an interference fit of .0015 to.0035.
Your fit is so close that just minor roughness or burrs are probably what is preventing the bearing from fitting. Just take a fine file and go around the stub on the end of the bit a couple of times just to clean it up, not change its diameter, and then bevel the top edge of the stub. Also clean up and check that the hole in the bearing is burr free.
Being that the bearings fit properly on some of your bits, the problem probably isn't with the bearings, which are usually held to close tolerances. If the problem isn't caused by burrs on the bit's stub, then the bit is probably slightly oversized and some careful additional filing will correct the problem without creating vibration.
These bearings shouldn't be an interference fit, they should slip on and off. If they were tight you would need a miniature bearing puller to remove them if you wanted to switch to a different diameter bearing.
John W.
Edited 1/31/2006 10:08 am ET by JohnWW
I understand that I could probably make it work by the heating/cooling approach, but I did not want to have to use a bearing puller or fool around with heating/cooling tricks. Heck fire, I have stop collars and I want to use this bearing kit on all my bits when I need certain features without resorting to tricks or reserving their use only to some of my bits that might work with them.
I finally made the one router bit work, but I used sandpaper to take it down a little. As far as I am concerned, that is not the way I want to treat my router bits. I am not a bearing expert, but I have got to believe that the bearing should be just over 0.250 in order to provide a clearance fit on the router bit. I will check with Eagle America and see what they know about this.
"A 45 Piece Bearing Survival Kit"
Pheewww, now I REALLY know why hand tools appeal to me so much.
This forum post is now archived. Commenting has been disabled