CNC doesn't provide the requested measurements

Hey folks, can anyone help me out?

I'm almost done assembling a CNC machine, but I've run into a single issue: the machining is offset by approximately +-0.5 mm.

When I mill a pocket, it leaves it 0.5 mm higher, and when I profile, it's 0.5 mm lower. Sometimes it gives me more or less measurement, but generally it's around +- 0.5 mm. Occasionally, it gives me the exact measuremen

I've meticulously calibrated the motors (using Mach3) and even manually input exact coordinates, as I'm using 1204 ball screws.

I'm starting to feel desperate here. I'm programming with Vectric Aspire, and it's strange because I don't have any issues with another machine using the same software. It feels like there's something I'm configuring incorrectly.

Any ideas, guys? Could it be that the board is defective? I´m using the red bitsensor board

If the numbers in the code are correct then you need to take a look at the physical aspects of the machine.It might be a coupling slipping slightly or a motor mount flexing.In the worst case it might even be the structure of the machine flexing if it is one of those made of extrusions held together by brackets,but we can't see enough information to give more precise pointers.

Thank you for the response.

I also thought it might be a flexing issue, so I reinforced the Z-axis, which was the weakest. However, it's still the same. The entire machine is made of 3/8" thick 6061 aluminum plates. It has 16 mm round rail guides. It has Nema 23 motors with 4.5 amps (something like that).

So I don't know, as I mentioned, I have another machine with less capacity, and it works without problems. I'm starting to suspect it's the board. I tried setting it to 0 and moving it to rough coordinates, and it does give them and returns to point 0, so I don't think it's steps lost either.

If the numbers in the code are correct,it has to be a machine related problem.Do you have the means of measuring Z axis travel?It would seem to be a logical step to move the axis over a large part of it's range and to physically verify the distance travelled.A height gauge would be ideal and the result might highlight calibration issues which could allow you to introduce a factor that would correct things.If the travel is correct things might get a bit more "interesting".

To me it sounds like there is some mechanical movement on the Z Axis. If we look at what you've discovered pockets are 0.5mm in the positive then the forces on the cutter are upward and a little mechanical play here would cause what you are seeing. The bottom of the endmill is being pushed into the work piece hence more resistance.
Now the opposite is when profiling the endmill is trying to pull itself into the workpiece hence giving you lower cuts which is again indicative of mechanical movement in the Z Axis. This could be backlash in the ball screw or flex in the gantry, I too had similar symptoms till I stiffened the gantry with extra support.

That would be the next thing to check after verifying that the machine is actually travelling the correct distance in response to the Gcode.Either could be at the root of the problem and persistence will determine what needs to be done.

Whatkind of motors are You using, are You using ballscrews?

Guys, it turns out the screws were loose. They're made of aluminum flat bars and 4080 profile; they weren't properly aligned and were twisting the bars where the motors are attached, causing the ball screw to misalign and, I suppose, lose steps. I'm using a Makita router and a 1/8" bit, apparently it wasn't working very well, I replaced it, and now it works much better. Thanks a lot for your help.

It is always nice to know that a problem has been eradicated.All too often we have a post from somebody with a problem and try to respond with suggestions.If we don't receive a progress report it may be that the problem has ben overcome or they might have taken the machine to the dump out of frustration.I think we enjoy the triumphs and learning of them may help later arrivals who use the search function,with their own challenges.