Choosing a bandsaw blade in three questions

A bandsaw blade catalogue runs to hundreds of variants: carbon, bimetal and carbide bodies, a dozen widths, twenty odd pitches, three tooth sets. Faced with that wall, most buyers reorder whatever is on the machine, which is how a blade specified years ago for one job ends up cutting everything badly. The choice is actually three questions, and only one of them needs any maths.

What are you cutting? The pitch rule

The rule that decides tooth pitch is old, published and worth knowing by heart: count the teeth in the cut. Divide the engaged width of the workpiece in millimetres by 25.4 and multiply by the TPI, and the answer should land between 6 and 24 for metal, 3 to 6 for wood. Fewer than the minimum and the teeth straddle the work, snag and break. More than the maximum and the gullets pack with chips before they clear the cut, the blade rubs instead of cutting and the feed rate collapses.

A pitch written as 4/6 is variable: the tooth spacing cycles between 4 and 6 TPI along the band, which breaks up the resonance that makes a constant pitch sing and lets one blade cover a range of sizes. The arithmetic still applies, run at the coarse value for the minimum teeth check and the fine value for the gullet check. Tube and structural sections have their own table because the engaged width is the wall, not the outside dimension, and it changes as the cut moves round the section.

What machine is it running on? Length and width

The machine fixes two of the blade's dimensions outright. Loop length is pi times the wheel diameter plus twice the centre distance, with unequal wheels taken at their mean diameter, though the machine plate or the old blade settles it faster than the tape measure. Width is the widest band the guides and wheels accept, because a wider band is a stiffer band and a stiffer band cuts straighter. Run the widest blade the machine takes for every straight cut and only give up width when the work curves.

How tight is the tightest curve? The width chart

Contour work inverts the width rule: the blade must be narrow enough that its back clears the kerf as the cut turns. A 13mm blade will not turn a 40mm radius however carefully it is fed, it will bind, heat and wander. The honest reading of the chart runs conservative in both directions: pick the widest width whose published radius is inside the tightest curve on the drawing, and if the drawing asks for tighter than the narrowest published blade turns, the answer is a different process, not a hopeful attempt.

Band speed is the last variable and the one most often left wherever the last operator put it. The starting ranges are published per material family: free machining steels around 90 to 105 m/min, alloy steels 50 to 70, austenitic stainless 30 to 45 with constant feed and flood coolant because 304 work hardens the moment a tooth dwells, titanium down at 15 to 25. The chip check above is the feedback loop; the chart is only the starting point.