Why settings drift
Fences get bumped. Stops loosen with vibration. Hinged and sliding parts wear. A scale painted on a tool was set at the factory, and it may not match reality once the tool has been moved, dropped or used for a while. Regular checks catch drift before it shows up as gaps in joints or parts that are the wrong size.
The checks themselves are not complicated. They rely on a reference that you trust more than the tool being checked.
Choose and protect your reference
A reference is only useful if it is true, so treat it differently from your everyday tools.
- A good square. An engineer’s or machinist-style square is typically more precise than a stamped framing square. A combination square is handy but has more moving parts.
- A straightedge. This can be a purpose-made steel straightedge or a precision-ground steel rule. A board edge from the hardware store is not a reliable reference.
- Keep it separate. Store the reference where it will not get knocked about, and do not use it for rough layout, scraping or prying.
Even a reference needs testing. Verify it before relying on it, using the methods below.
Two self-checks
Flipping a square. Hold the square against a straight edge, draw a line along the blade, flip the square over to the other side, and draw a second line from the same point. If the lines coincide, the square is likely true. If they diverge, the angle between them is double the square’s error. As a hypothetical example, suppose the lines are 1/32 inch apart at the end of a 12-inch blade. The actual error is half of that, about 1/64 inch over 12 inches. That is small for rough carpentry but can matter in tight joinery.
Flipping a straightedge. Draw a line along a straightedge on a flat surface, flip it end for end, and draw along the same edge again against the same marks. Any gap between the lines indicates the edge is not straight. This works best with a sharp pencil or a fine knife line, because thick marks hide small errors.
Checking fences, stops and gauges
With the machine unplugged, clean the surface first, because debris causes false readings. Then:
- Fences. Hold the square against the fence face and the table. Look for light between the blade of the square and the fence. A fence that leans will tilt every cut. On a table saw or router table, also check that the fence is parallel to the miter slot or to the bit’s direction of travel, according to the manual.
- Stops and miter detents. Set the angle to the factory stop, such as 90 or 45 degrees, and test against a reference square. Detents on a miter saw can drift, and a quick comparison catches that. Sometimes the best verification is a test cut: cut a scrap, then check it with the square, and cut a mating piece at the opposite angle. If a corner of two 45-degree cuts does not close to 90 degrees, adjust and retest.
- Drill press tables. Put a reference square against a chuck-held rod and the table to see whether the table is square to the spindle. A tilting table can be knocked off by a heavy workpiece.
- Gauges. Marking gauges and sliding bevels should hold their setting. Set one, tap it lightly, and check that the setting has not moved. A sliding bevel can be compared to a known angle block, or a reference square, to see whether it locks without shifting.
- Cut width. Saw kerf and depth settings can also be verified with scrap, since the scale on the tool may be off by a small amount.
A related sign of trouble is blade runout, a wobble in a spinning blade. A wobble found during a check may point to dirt on the arbor, a damaged blade or a worn bearing, not just a misaligned fence.
If something is out of tolerance
Make a small adjustment according to the manual, then retest. Avoid chasing the error with big changes. If you cannot bring a setting into line, look for a mechanical cause: a worn cam, a loose bolt, a bent rail, or debris. If the cause is not clear or involves safety-related parts, consult the manufacturer or a repair professional. Squares that fail the flip test and cannot be adjusted should usually be replaced or demoted to rough work.
A reasonable rhythm
A quick square check before any critical cut costs seconds. A fuller check of fences and stops is often done at the start of a project or each season, and after any move, drop or major adjustment. Machines that get heavy use deserve more frequent attention than occasional ones. The best schedule is the one you will repeat, and noting the dates in a small notebook helps you see patterns.
Safety
Unplug machines before reaching near blades or bits, keep fingers clear of edges while holding squares against blades, and wear eye protection when running test cuts. This article is general information, and your tool’s manual describes its approved adjustment points.