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Tech Info - Steel

Bright Steel ShaftingStandard matches can usually be found between stock bearing sizes and the diameter of shaft steel that can be bought. In the UK close tolerance, smooth surface finish steel round bar for uses such as shafting is usually differentiated from rougher unfinished steel bar by the use of the terms "bright" and "black". For shafting look for bright bar - a, say, 12 mm diameter bright bar will usually fit a 12 mm diameter bearing because the 12 mm bright bar is usually about 11.95 mm actual diameter and the bearing will be 12.00 mm (+ a wee bit) hole diameter.

There are other specialist shafting steels that can be bought usually described as "ground" or "precision ground" bar with even tighter tolerances (+0,-0.015 mm) and better surface finish (0.6 microns) than standard bright bar. One of the most commonly available of these in the UK is "Silver Steel" or more precisely high carbon bright steel to BS1407. This is commonly available in short lengths and is suitable for hardening (it contains about 1.1% carbon) but is also considerably stronger than mild steel in its unhardened supplied state and can often be used as bought. (I understand UK Silver Steel is approximately equivalent to US ground tool steel SAE 0-1.)

shaft machined from "black" steel barBright steel bar comes in many steel specs with, in increasing strengths, mild steel, medium carbon and low alloy steels probably of most interest to DIY-ers. For your more heavily loaded shafts and axles look at through hardened, low alloy steel bright bar (UK EN24(T) , US 4130 and 4340 specs or similar), these are common and are just about hacksaw-able when hardened moderately although getting the supplier to cut to length in diameters of 1/2" upwards helps a lot. In common heat treated forms they have strengths about 3 x that of mild steel and are drill & tap-able - but they are more expensive.

There are intermediate strength steels available too; in the UK EN8 (or BS970 080M40) medium carbon is quite common with stockholders and in the US SAE1040/1043 shafting may be found for duties where the stresses (eg bending) or surface hardness aren't quite so onerous.

The image on the right shows a shaft machined from "black" steel bar. The mounting areas for the bearings, sprocket and pulley are the only parts of the shaft machined to close tolerance.


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