Measuring axis squareness

[kb58]

I built a 24"x36" gantry-style CNC router that uses paired servos to move the gantry in the Y axis. I'm only just starting to test things and to educate myself on all things CNC router. How do you guys check if gantry motion is square to the Y axis? The context here is that I'm just a retired guy in the garage who likes building things, so I don't have the $$$$ for high end measuring equipment. While a granite 30-60-90 reference triangle would be perfect, I just can't go there. The best (combination of cost and results) seems to be to cut some material into a large square, remove it from the router, then measure it for squareness with my mill's DRO. Is there another way that works better yet doesn't cost a fortune? The advantage here is that measuring it in the above method is "where the rubber meets the road", meaning that everything is taken into account and the resulting alignment is the bottom line.

So, what low-cost material do you guys recommend for test cuts, something that holds it's shape and edge long enough that it can be measured with an edge finder? Pretty sure wood is out due to spring-back and splintered edges, though Melamine might work. Obviously aluminum will be much better (the router has been designed to handle it), but it probably needs to be at least 3 mm to not flop around, so more than a couple pieces start adding up. Hmm, as I type this, I just realized that one sheet of aluminum could be used for many tests, cutting smaller and smaller test patterns.

Thanks.

[IPGregory]

There are a number of ways to check Gantry is square to Y, but a quick simple method I use on mine is the 3/4/5 Triangle method.

Mark the start point, then run a commanded distance along either the X or Y Axis, whichever is shorter that is divisible by 3. Say 300mm or 30".
Then run along the opposite axis another distance equal to the previous but by 4. Say 400mm or 40". Mark the end point.
Finally measure between the start point and the end point. The result should be a factor of 5 of the first 2. So 500mm or 50" for example.

If it is you are very close to square, if it's not, you are off.

Measurements are depending on your machine size of course but make them as large as you have room for.

[suntravel]

You can use a good quality flat angle at least 200x300mm size and a dial indicator. Also handy to test the z axis in both directions.

Uwe

[EastMemphis]

Another easy way to check for square is to measure the diagonals. Measure corner to opposite corner of the machine and that measurement should be the same. If it isn't you're out of square.

An easy way to check the plumbness of the Z is to make a long "Z" indicator. Take a coat hanger or some other piece of metal that will chuck up and be bendable and put a 90 degree bend with just enough wire so that it will fit in the chuck. Put another 90 degree bend at the other end of the wire with about an inch or two aiming down. One end of the Z goes in the chuck, the other end, the indicator, points down to the table. The longer the horizontal bit of the wire, the better the measurement.

Move the Z axis to the center of the table and lower the Z until the indicator end just about touches the table, then turn the chuck by hand 90% and check how much distance there is between the wire and the table at the 90 degree location. It should be the same. Do the same for the 180 and 270 positions. Any difference in the distance between the pointer end of the wire and the table indicates an off plumb condition. Adjust your Z axis and try again.

The benefit of this type of system is that it exaggerates the error greatly. It is easy to tell if you have a square and plumb Z or not.

[kb58]

Thanks for the suggestions. As a first pass, I put a transfer punch into the spindle, placed three targets on the table, and wrote a quick Intercon program to mark the corners of a 450 x 600 mm right triangle. First, X and Y were measured and Y was right on - at least visually with an engineering scale. X was off a bit, probably because it had been messed with and recalibration was overlooked. That was corrected and the test run again. The diagonal should have measured 75 mm but was 0.5 mm too long. With the vertical side of the triangle already there, I realized that instead of running the test over and over, to just leave the measuring scale in place, move the spindle over the problem point, and shift the right end of the gantry to place it exactly above. Much better now. The pic shows it at 76 mm because I don't trust the zero end of any scale, so offset it by one.

I'm fully aware that this is just a gross adjustment, that it's very likely no where near exact, but it's a start. As said above, I really want a large square, either steel or better yet, granite, and indicate off of it to a dial indicator. Problem is that I wouldn't use it enough to justify the expense. Ebay has them but even used ones hold there value. Eventually I'll machine a piece of aluminum, cutting my own "metrology square" and then measure the error off that. Thanks again.