Squaring and tramming a CNC router
Squaring makes the X and Y axes perpendicular to each other, so a drawn square comes out square. Tramming makes the spindle perpendicular to the table, so a flat bit cuts a flat surface instead of a shallow arc. Different problems, different symptoms, different fixes.
The two adjustments
Squaring
Symptoms
- A drawn rectangle comes out with unequal diagonals.
- Parts that should be square are slightly rhomboid.
- Two parts that should mate do not, and the gap is a wedge rather than uniform.
- Circles come out slightly elliptical, tilted. Note that a circle that is oval along an axis is a different problem.
Checking it
- Cut a large rectangle, as big as your machine allows, into scrap. Bigger is better because it magnifies the error.
- Measure both diagonals with a tape or a long caliper.
- If they differ, the axes are not perpendicular, and the difference tells you which way.
The five cut method, borrowed from table saw sled squaring, is more sensitive still: make five cuts, rotating the piece between each, and the width difference across the fifth strip is four times the real error, which makes a tiny misalignment measurable.
Fixing it
This is machine specific. On most gantry machines it means loosening the gantry sides, squaring the gantry to the rails with a large square or by measuring diagonals, and retightening evenly. Some machines have adjustment provision, some require shimming. Consult your machine's documentation, because the wrong bolts loosened on some designs means a gantry falling.
Tramming
Symptoms
- Regular ridges after surfacing, in a corduroy pattern.
- Lines in a pocket floor that a smaller stepover reduces but never removes.
- One side of every surfacing pass deeper than the other.
- A flat bottomed cut that is measurably deeper on one side.
Checking it
Two approaches, one crude and one precise:
- The surfacing test. Take one pass with a wide surfacing bit and look at it closely. If one edge of the cut is deeper than the other, the spindle is tilted in that direction. Cheap, and good enough for most hobby work.
- A tramming gauge. An arm held in the collet with a dial indicator at the end, swept around to compare readings at the front, back, left and right. Far more sensitive, and it tells you the magnitude as well as the direction.
Fixing it
Tramming is adjusted at the router or spindle mount, usually by shimming between the mount and the Z plate, or with dedicated adjustment screws if your machine has them. It is fiddly, it is iterative, and it is worth doing properly once rather than repeatedly badly.
Z perpendicularity
A third, less discussed alignment: the Z axis should be perpendicular to the table in both directions. If it is not, a deep pocket will be wider at the top than the bottom, or vice versa, even with no tool deflection. Check by cutting a deep straight wall and measuring its thickness at the top and bottom. Deflection also causes this, so rule that out first with a light finishing pass.