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HI I could use some advice with my new cnc machine.
check your tool diameter-it has to be perfect
It is unlikely that the tool diameter/radius is off since smaller parts are the correct sizes. There are a few other things it could be could you give some more information.
Is it a new/used machine?
If you cut one part #1 299mm and part #2 301mm will #1 turn out to be exactly 299mm and #2 300.7mm? In other words is the error exactly 0.3mm at exactly 300mm length part? And does it matter where on the table the part is located it is always off this much, either in X or Y?
0.3mm error seems almost negligible in the woodworking industry especially for a part 300mm or greater.
Thanks for the replies.
Can you rotate the part so it cuts the 700mmlong the other axis and see if the problem follows or if the 200 mm side is smaller
Rotating the part is a good test. Also, have you reviewed the actual code at the machine to compare if the numerical code values are accurate? If you are relying on a CAM package this could be a rounding error within the post processor which could grow proportionate to size of the feature.
Are you common line cutting or actually cutting around the part ? That has hit us a few times.
I cut round the pieces.
Since it worsens as the part gets longer its very likely due to rounding in the post processor as Mark T. suggested.
Are you able to program at the machine directly using software that came with the machine? If not maybe have your cnc guy write/modify a program manually to cut a part say 700mm and see what happens. This way if it cuts correctly then you have your answer.
Your machine needs to be calibrated. The calibration process starts by moving your machine a specific amount and then check the movement against a standard. Start by finding the most precise and longest steel scale (ruler) available, 1000 mm or longer. Carefully set the edge of a router at zero, then move the machine 1000mm under program control. Check the result of the movement against the scale. If it's not perfect, the machine needs to be calibrated. Contact the manufactures for instructions how to do it. There may also be local service companies that can do laser calibration, but it may be expensive.