Selecting correct bore size for gears (for motor shafts)

I have a motor with a 5mm shaft. I want to attach a gear to it, but should I pick one with 5mm bore, or does it have to be wider for some reason? I find lots of gears with a bore of 6mm and only few with a bore of 5 mm so it would be easier to get one with a 6mm bore.

Also I found this site which uses a “MOD” system to tell if one gear will fit into a different gear. But there are also other sites that use this MOD system. Is it “universal”? Will a MOD 1 gear from the site I mentioned fit a MOD 1 gear from eBay?

Are you talking about pinions? You should be looking for ones drilled for grub-screw perhaps? Its fairly easy to file a small flat on a shaft so a grub screw will work.

If you have the facilities to drill through (lathe or drill-press with drill-press vice) then you can get one with an undersized hole (or just pilot hole) and enlarge it yourself.

If you don't have digital vernier calipers you can now get really cheap digital ones, definitely worth getting, since a 5mm shaft might be 4.98mm or 5.00mm and that's the difference between a sliding fit and an interference fit (arbor press required).

Bore size should be the same as shaft size and any MOD1 gear will mate with any other MOD1 gear, irrespective of how many teeth there are.

With respect to gear wheel fitting, some are specified as having collars. These can be seen on the smaller gearwheels in your link. These often have small screws fitted to grip the shaft.

bestanamnetnogonsin: I have a motor with a 5mm shaft. I want to attach a gear to it, but should I pick one with 5mm bore, or does it have to be wider for some reason? I find lots of gears with a bore of 6mm and only few with a bore of 5 mm so it would be easier to get one with a 6mm bore.

Many hobby and model engineering shops sell brass tube. You may be able to get some with an ID of 5mm and an OD of 6mm so that you can use the 6mm gears on a 5mm shaft.

...R

Adapters used to be available for this purpose but from memory were v expensive.

Cheap plastic gears are normally available in certain standard sizes designed to be a friction fit on standard axles. Not really designed to carry torque

Are you sure its 6mm and not 1/4 imperial , its common still.

Normal practice with gears is to buy with pilot hole then bore to size.

For completeness 1/4 inch is exactly 6.35mm Caliper gauges are good to 0.01mm and invaluable for this kind of stuff.