Squaring the Gantry

I had a lot of trouble squaring my gantry with adequate precision on my CNC router. This meant that small parts were pretty good, but larger parts were noticeably out of square. Here is an outline of the more or less unsuccessful techniques I tried:

(1) I knew my two side rails were very close to parallel, and it seemed that they spanned the same distance. So I tried making sure that each side of the gantry was the same distance from the end. This does not work very well at all. You can make square-looking pieces, but nothing like a precise fit.

(2) Use a carpenter's square or speed square. The problem here is that the gantry rail and the side rails are not on the same plane. So it is somewhat hopeless to try to eyeball it.

(3) Two strings can be used to square a frame. You tape or tie two strings to the corners of the frame and hang them diagonally across the square of the frame. If the two strings are tightened to the same tension and the distance is the same, then the two strings will just barely touch. Unfortunately, that is a lot of 'if' to have in a precision squaring. I didn't have a way to ensure that they were tensioned the same and adjusting the gantry position changes both the distances and tensions at the same time. It ended up being very fiddly and the end result was not very square.

(4) Measure the diagonals of a frame. If they are the same distance, then the frame is not a parallelogram. If all the edges are also the same length, then the frame must be square. This was less fiddly than the two strings, and I got about half the skew by using this method. But it was difficult to measure the two distances with any kind of accuracy. I was using a tape measure and getting consistent results across the frame was very hard. It may be that a less flexible measurement tool would have been precise enough.

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These techniques may be good enough to lay out furniture, but I was still getting noticeable skew across my 1-meter CNC bed. Fortunately, I found one more technique that really worked:

Use a laser square. Laser squares are typically used to lay out tile in a room. I used this one Tiling/Flooring Laser | 40-6616 | Johnson Level & Tool Mfg Company and it worked great, but I presume any other brand would work equally effectively. Laser squares have two big advantages.

First, the laser lines extend all the way along your rail even for large machines. Speed squares and even carpenters squares are usually shorter than the rail for a large machine.

Second, The laser lines project downward at an angle. So the fact that your gantry and base rails are in different planes no longer matters. Put the square on the gantry and one line will project downward along the side rail.

The only issue with the laser square I used is that there is not really a mounting or clamping surface. If I could have clamped it in place and made fine adjustments. Instead, I had to hold it by hand and re-align it each time I moved to adjust the gantry. Perhaps a different model would be easier to clamp.

-Jonathon Duerig

 

I don't have a photo of using the laser square on my gantry. Partly because it is necessary to do it in the dark. See this video about somebody using it to lay out a floor:



Now, take a look at my machine.

Contrariwise -- A C-Beam Router

I put the laser square on one side of the x-axis gantry. The whole gantry is at one end (near y-max). The z gantry is in the middle somewhere out of the way. I sight one laser line down a line on the extrusion, often an edge. Then I see how close the laser line gets to a line down the y-axis rails. Then I tweak one side's position accordingly. Repeat until it is dead square. I found having an assistant is super-helpful since my laser square doesn't really have an easy way to clamp it.

-Jonathon Duerig

 

For this thread, I was mostly concerned with squaring the Y-axis with respect to the X-axis. My Z-axis is probably not 100% aligned, but it is good enough for the cutting that I do (3mm plastic). Giarc's advice is probably the way to go if you need a perfectly aligned Z-axis.

-D