How To Fix a CNC Router That Isn’t Cutting Square

There are few things more frustrating than running a job with your CNC router only to find that, for whatever reason, things are not lining up as they should. If it’s any consolation, you are not the first, and we are here to help you square things up.

To fix a CNC router that is not cutting square, you will need to calibrate each axis. This requires leveling the machine and setting each axis so that they are perpendicular to each other.

For the purposes of this article, it is assumed that you are using a typical 3-axis CNC router. Read on to learn how to level and calibrate the axes of your machine for a perfect cut each time.

1.  Level Your CNC Router

Leveling machines is important for a number of reasons. The larger and more industrial the machine, the more important leveling becomes. So, hobby-level machines can be leveled with a little more forgiveness than an industrial machine. The idea is to have the machine and its components perpendicular to the downward pull of gravity – you can think of this as your first point of reference.

Leveling Equipment

That old carpenter’s level you found kicking around in the shed might work for hobbyists, but to accurately level a CNC machine, you will need a machine level. Starret Machinist’s Level (available on Amazon.com) is a great option that allows you to simultaneously level in both directions. Leveling feet are always a better option than sticking shims under the table legs. Here are some machine leveling feet options for you to choose from (available on Amazon.com.)

Level your CNC machine as follows:

1. Remove your work holding and spoil boards from the machine.
2. Place the level on the machine diagonally – check both diagonals to get an idea of which side is higher.
3. Release the lock nuts on the feet and turn them so they are almost all the way in.
4. Work systematically by placing the level on each side in turn, adjusting the feet out so that the machine level is centered.
5. Once you are satisfied that each side is level, double-check the diagonals.
6. Lock the feet in place using the locknuts, being careful not to turn the feet.

Leveling the Router Table Surface

For smaller CNC router tables, it is less important to level the entire machine than it is to level the bed. This means skimming off some of the bed (usually a sacrificial surface called a waste board) so that the surface is parallel to the X-Y cutting plane of the router or spindle.

A few things that I have found to be important when leveling the bed are:

1. level the entire bed, not just the work area or the area that the router can travel to. This is because many times the workpiece will have to lay flat even outside of the work area and leaving a portion of the bed raised above the flat working area will prevent the workpiece from laying flat.
2. A smaller step-over will remove some of the tramming errors in the router setup. it may take longer to run the flattening program but the results will be worth it.
3. If you have clamping tracks in your bed, remove them before surfacing the spoil board. If you are sure that they will not interfere with the material removal – the choice is yours.
4. Lightly sand with 80-grit sandpaper mounted to a flat surface after machining to remove any ridges caused by tramming errors.

2. Square Your X and Y Axes

Even a slight deviation on either axis of your CNC router can have unwanted consequences. Your Y axis – that is, the axis your gantry moves along – is used as a baseline here.

Adjustments may be necessary on the gantry (the X axis) but before you do anything, make sure there are no loose bolts on your gantry, as this could cause your machine to cut incorrectly.

There are a couple of ways you can go about squaring your X and Y axis, depending on how accurate you want to be. You will need, at the very least, a good straightedge to draw a line running parallel to your Y-axis.

You will also need to draw a line perpendicular to this, then line your gantry up with the perpendicular line. If it is not square, you should be able to eyeball it and make a crude adjustment as necessary by loosening the bolts and using a rubber mallet to tap it into place.

Back To School – Using Pythagoras to Square Your X/Y Axis

For a more accurate squaring of your gantry, you will need to use the Pythagorean theorem. Depending on the size of your machine, you will have to determine how big your right-angled triangle will be. For this, you will need a ruler long enough to take the measurements, but the basic principle remains the same regardless of the size.

The Pythagorean formula is as follows: a² + b² = c². This is to say that the square of the opposite plus the square of the adjacent side of a right-angled triangle will equal the square of the hypotenuse – that is, the side of the triangle with 2 acute angles.

To make it easier, you can break the measurements down into 3, 4, and 5 units accordingly, with the size of the units determined by the size of your machine – for example, you can decide each unit is 10 cm (3.93 cm).

Square your X and Y axis using the Pythagorean Theorem as follows:

1. Put a V carve bit in your spindle and choose an arbitrary starting point near one corner of your table.
2. Mark the starting point using a piece of tape with an X drawn on it – it helps to slide the tape under the V-Carve bit rather than trying to center the bit over the tape.
3. Program your machine to move 4 units along the Y-axis from this point. Mark the point where it stops by sliding another piece of tape marked with an X under the point of the bit.
4. Command your machine to move the spindle 3 units across the X-axis from this point. Use another piece of tape marked with an X to mark where it stops.
5. You can now measure each side of the triangle formed by the spots you have marked.
6. Using the Pythagorean formula, check if your hypotenuse is indeed 5 units long – using the example from before, it should be 50 cm (19.68 cm) or 5 units.
7. Make any adjustments and run the test again to verify that everything is square. You may have to repeat this process a few times to get it right.

3. Tram Your Spindle

Aligning your spindle so that it is perpendicular to the X and Y axes is known as tramming. To do this, you will need a reasonably sized engineer’s square. This Groz 6-inch Graduated Square (available on Amazon.com) features a hardened steel blade and a 4-inch (10.16 cm) stock that should do the trick for most hobbyist machines.

Tram your spindle as follows:

1. Remove the bit from your spindle if one is installed.
2. Lower your spindle to just above the height of the short edge of your square.
3. Place the square flat against the spindle in line with the X-axis, holding it so that it is flush against the vertical surface.
4. Slowly slide it down until it contacts the spoil board.
5. If the long edge of your square is not flush with the spoil board, adjust the alignment of your spindle.

You can repeat this process along the Y-axis; however, most CNC routers don’t allow for much adjustment in this regard. All you can really do in this case is shim it at the top or bottom using aluminum foil. Once you are satisfied, make sure everything is tight.

A quick way to check the tram on the spindle is to insert the largest diameter bit you have into the router. Spray a small area on your table with spray paint and let it dry. Move the router bit close to the table over the painted area and turn the router on. With the CNC controller in single-step mode, step the router down until it barely contacts the table. This will remove a small portion of the paint and reveal how close you are to square and plumb.

This illustration, while not perfect, is close enough to call good for my woodworking hobby addiction. I seldom use bits larger than 1/2 inch in diameter so this accuracy will more than cover my work. However, I do use this large diameter bit to flatten my spoil board so I want my tram to be as close as possible as shown in the illustration.

NOTE: If you use small diameter bits in your CNC work, tramming is more forgiving that when using large diameter bits.

4. Cut a Test Piece

Now that you have done all that work to get your machine squared up, all that is left to do is test it out. No need to do anything too ambitious – ideally, you will want to use a piece of throwaway material and cut a simple square or rectangle just to check if everything is in order.

If you are still unhappy, it is recommended that you make use of a professional machine calibration service to level and align your machine.

Conclusion

It can be extremely satisfying when working with a machine that just cuts perfectly square. For most people, leveling the machine, aligning the X and Y axes, and tramming the z-axis will take a little time but the rewards will be lasting.

Tramming your spindle is another important step that should not be ignored – a misaligned Z-axis may not be as noticeable as the others, but if you are looking to cut as accurately as possible, it can have a noticeable impact on the quality of the work you produce.

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