Cutting Tools and Saws

A saw is a row of small chisels, and every question in this topic comes back to what those chisels are shaped like and how many of them there are.

Rip against crosscut

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The tooth shape follows from the job. Cutting along the grain means separating fibers that already run the way you are going, which a chisel edge does well. Cutting across the grain means cutting each fiber through, and a knife point scores both walls of the cut before the waste is removed - which is why crosscut teeth are beveled on both faces.

The figure keeps the blade length the same in both panels on purpose, so the difference in count is a fair comparison rather than a drawing accident.

Set and kerf

The kerf is the slot the saw cuts. It has to be wider than the blade, or the blade would be gripped by the wood the moment it got below the surface.

Set is how that is achieved: the teeth are bent alternately left and right, so the row of tips is wider than the plate behind them.

More set means less binding and a rougher cut. Less set means a smoother cut and more risk of the blade pinching.

A saw with no set at all binds immediately, and that is the question this part of the topic asks.

Hand saws

A coping saw and a keyhole saw both cut curves, and the distinction is practical: a coping saw's frame limits how far from an edge it can reach, while a keyhole saw has no frame and can be pushed through a bored hole into the middle of a sheet.

The hacksaw rule

Blade choice for a hacksaw is by teeth per inch, matched to the material's thickness. Common blades run from 14 to 32 teeth per inch.

At least three teeth must be in contact with the work at all times. Fewer than that and a tooth catches on the edge and snaps.

So: thin material takes a fine blade; thick, soft material takes a coarse one. Cutting thin sheet or tubing with a 14-tooth blade strips teeth, which is the symptom a question describes.

The blade is installed with the teeth pointing forward, away from the handle, because a hacksaw cuts on the push stroke.

Snips and shears

Aviation snips are color-coded and the code is asked. Red cuts left, green cuts right, yellow cuts straight. The two you are most likely to be asked for are red and green, and they are the ones that curve.

Bolt cutters are levers in series, which is why short jaws and long handles shear hardened steel. That is the mechanical advantage lesson with handles on it.

A pipe cutter leaves a clean square end and no chips, unlike a hacksaw, but it raises a burr on the inside that has to be reamed out.

Power saws

Blade direction differs and it matters for the finished face. A circular saw and a jigsaw cut on the upstroke, so the tear-out appears on the top surface - which is why you cut with the good face down.

Know it by sight

Aviation snips are color-coded: red handles cut left, green cut right and yellow cut straight.

What you can skip

Across the 58 questions on this topic in our bank:

• Japanese pull saws, scroll saws and dovetail saws never appear.
• Cutting metal with heat. Plasma and oxy-fuel cutting never appear.
• Blade tooth counts by number matter only for the hacksaw: know that thin metal needs a fine blade so at least three teeth touch the work.

Where people lose points

Swapping rip and crosscut. Rip goes with the grain.

Thinking more teeth cuts faster. More teeth cuts finer and slower.

Thinking set makes the cut smoother. It makes the kerf wider so the blade clears.

Using a coarse hacksaw blade on thin material. Three teeth must engage.

Installing a hacksaw blade backward. Teeth point away from the handle.

Using a wood saw on metal. A hacksaw's fine hardened teeth are the point.

Work one in under a minute

A technician is cutting thin-wall steel tubing with a hacksaw and keeps breaking teeth off the blade. What is wrong, and what should change?

The blade is too coarse for the material.

A hacksaw needs at least three teeth touching the work at once. On thin wall tubing the material is thinner than the space between the teeth of a coarse blade, so at moments only one tooth is engaged.

A single engaged tooth takes the whole cutting load and catches on the far edge of the cut, and it snaps off.

The fix is a finer blade - 24 or 32 teeth per inch rather than 14 - so several teeth are always in contact and the load is shared.

The general rule falls out of this: thinner material, finer blade. Thicker and softer material takes coarser teeth, which clear the chips better.

Where this leads

The saw tooth is a wedge and the bolt cutter is a lever, so this lesson keeps returning to simple machines, and the filing and planing that follow a rough cut are the next lesson.