Thread: Pattern bit tolerances?

If the bearing is a true sized bearing (most are) there is no way that they can be machined to match an undersized bit. The inner and outer races of a bearing are ground to very fine tolerances using specialist machinery and then the bearing is assembled and supplied to the bit manufacturer for installation on a bit.

Some router bearings are not true sized and use a ring of some sort or a square or triangle shaped fitting pressed over a standard sized bearing to achieve a custom size with the economy of mass produced standard sized bearings. The rings are sometimes used with rebate bits where the bearing can be swapped to change the rebate width, and the bit comes as a set with a number of bearings and a chart relating the bearing diameter to rebate width. Bits with the shaped fitting tend to be for use with synthetic materials like corian.

For the most part, resharpening bits is not really economical, as as you have discovered the bits end up undersized, or the profile is altered and doesn't match the profile produced before sharpening. For intensive industrial use, you can get bits with indexable/replaceable carbide inserts where you can renew the bit performance by releasing the carbide fixing screws and either installing new carbides or swapping to a fresh edge of the existing ones. Bits set up this way tend to more than double the cost of non indexable bits from the same supplier, and 4-10 times the cost of consumer grade bits. Carbides can be $10-20 each and there may be 2 or 3 on a bit. The value of these bits is situations where you are working high wear materials constantly and can change the carbide as needed while retaining the arbor for an extended period, instead of destroying conventional buts on a daily basis.

Thanks malb

I guess I'll buy a bit for box joints and not use it for general template stuff

Get yourself a new bit to use just for boxmaking and treat it as kindly as you can to get maximum life.

Keep the couple that you have had sharpened and use them for anything where you you are using a template for shaping, but absolute dimensional accuracy is not essential.

At work we have a TiN coated, solid carbide spiral bit from Carbitool that is kept exclusively for box joins. Wasn't cheap, but it'll last.