CNC test piece: six machine accuracy tests sized to your stock
Enter your stock size and choose what to check on a CNC mill: backlash, squareness, Z repeatability, hole diameter, surface or circular accuracy. You get sizes, how to measure, what a result means, a G-code skeleton and a table to print.
Opens without an account 3 times a day. A free account removes the limit.
- 6 testsplus an optional dovetail
- 8 mmedge margin for clamping
- G-code skeletoncopied with one button
- 4 materialsaluminium, brass, mild steel, POM
What a CNC test piece is
A CNC test piece is a trial cut in cheap material that shows whether the machine moves exactly where the command says. This free online tool from EduFacturing builds one from your stock size and tells you how to measure it. You pick one of six checks and can add a dovetail.
CNC backlash test, squareness test, circle test: six checks
Sizes the test to your stock
From X, Y and Z it leaves an 8 mm edge margin and builds a feature from the free space: a 41.6 mm circle for 100×80×12 mm stock. The fields accept from 30×30×5 mm, but the tests need more: about 32 mm a side for the circle, about 83 mm along X for backlash.
Checks backlash, squareness and repeatability
Circular accuracy, backlash (circle, square and diamond in a row), squareness by diagonals, Z repeatability (3 to 8 steps), hole diameter (5 circular channels) and surface (3 zones). The dovetail is an add-on.
Says how to measure and what it means
For each test: what to measure and with what, and what the observation means, in an “observation, meaning” table. An oval can mean backlash on that axis (the tool’s reading), unequal diagonals mean axes that are not square.
Gives a G-code skeleton and a table to print
The skeleton (G21, G90, G54, passes of up to 1.5 mm) copies with a button. The table has columns for nominal, three measurements and deviation.
When to cut a CNC test piece
A test piece is cut in cheap material before the real job.
A new or moved mill
The backlash test shows which combination of axis moves is at fault.
A periodic check
The print table has room for the date and the machine, so the deviation can be compared over time.
A suspected serial error
Steps at a fixed pitch show a systematic error in Z, and five identical channels show the spread.
A teaching workshop
A student cuts a piece in scrap and measures diagonals and diameters.
How the test piece sizes are worked out
The sizes are calculated from the stock with a few fixed rules: workshop practice, not a standard.
Free space
8 mm is left from every edge for clamping. For 100×80 mm stock, 84×64 mm remains.
Feature size
The circle and the rectangle take 65% of the free space. With circle, square and diamond, the cells are 10 mm apart and each feature is 65% of its cell. Below a 10 mm feature size the circle, rectangle and circle-square-diamond tests are not built.
Depth
Typically 30% of the thickness, between 2 and 6 mm; the surface test, the holes and the dovetail use other rules.
G-code
Each feature is cut as a channel along the tool centre, in passes of up to 1.5 mm. Spindle speed and feed are sample values for the material.
| Applies to | Formula | Meaning |
|---|---|---|
| Free space | U = size − 2 · 8 | along X and Y, in mm; 84 and 64 mm |
| Circle | D = 0.65 · min(Ux, Uy) | 0.65 · 64 = 41.6 mm |
| Diagonal | d = √(a² + b²) | sides of 54.6 and 41.6 mm give 68.64 mm; equal diagonals mean right angles |
A CNC test cut sized to your stock: what is different
The result is read, not guessed
Each test comes with observations and what they mean: an oval, a rounded diamond, unequal diagonals, uneven steps.
It says what is not calibrated
Spindle speed and feed are marked as sample values, and the base width of the dovetail is not calculated because it depends on the cutter.
The skeleton cuts a channel along the tool centre, not a cleared pocket: real clearing needs CAM. The “holes” are circles along the tool centre, not drilled holes. Spindle speed and feed are sample values, not cutting data. In the surface test the three zones carry the same feed and only their outlines are cut, so the feed per zone is changed by hand in the G-code.
The 8 mm margin, the 65% fill and the depths are not a standard and not from the rules core, which has no such rules for a test piece. The circle and stepped tests follow the principle of ISO 230-4 and ISO 230-2 as a method. The thresholds (under 0.02-0.03 mm for the circle, under 0.05 mm for the diagonals) are a guide: what is acceptable depends on the part.
Questions about CNC backlash and accuracy
How to check CNC machine accuracy?
Cut a test piece in cheap material and measure it. The tool builds the piece from your stock and gives the measuring method for each of the six tests.
What is backlash in a CNC machine and which test do I start with?
Backlash is the dead travel of an axis when it reverses. A new machine starts with the backlash test: a circle, a square and a diamond in a row. The circle catches backlash on both axes, the square on one axis, and the diamond, which needs both to reverse at once, shows where the problem is.
How do I check squareness on a CNC mill with calipers?
By the diagonals: a rectangle with right angles has equal diagonals. A difference under 0.05 mm is good. One diagonal longer means the X and Y axes are not square; the cause is usually mechanical (levelling, worn guides) and it is not compensated by software alone.
Why are the steps cut in one setup?
All the channels are cut without moving the stock, so the steps are compared by Z alone. Equal steps mean repeatable Z, a systematic error points to the Z scale, and an irregular one to a mechanical cause.
How do I read the hole diameter test?
The skeleton cuts five circles along the tool centre with no radius compensation: the nominal is the diameter of the tool-centre path, and the measured size includes the cutter diameter. Compare the five channels with each other: a large spread means poor repeatability.
Is the CNC test piece tool free, and do I need an account?
Yes, it is free. Without an account it opens 3 times a day. A free account removes the limit.
The CNC test pieces is free. Pro opens the rest
Pro costs 15 EUR a month and opens 45 Pro tools.
Related tools
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CNC G-code viewerCNC G-code viewer online: paste a milling program and see the cutter path from above, with lengths and a time estimate. Part of Pro, 15 EUR a month.
Make a test piece for your stock
Opens without an account 3 times a day. A free account removes the limit.