Hi guys, I am starting off in a ne

[Bharath47]

Hi guys, I am starting off in a new role on CNC programming, I am not understanding how we could specific tolerance while programming or to control tolerance in the program, I am working on NX12 for programming. This question may sound silly but it's stuck in my head for a long time.

 

[BrianE22]

It's a function of the machine, not the g-code generated. If you tell the machine to go to 3.5 on the X axis and 2.1 on the Y axis the machine will go to exactly 3.5000, 2.1000 within the limits of the precision and accuracy of the machine. If your machine is off .005 then the 3.5000 could be 3.4950/3.5050

 

[Bharath47]

Hi Brian, thank you for responding to my message,

It makes sense now, so apart form machine resolution, what else influence the tolerance of a part being machined. I know it's going to be a long list of parameters, if you know any material or book that I can refer, please let me know, it would be a great help. Thank you

 

[BrianE22]

Machine accuracy, machine repeatability, operation (drilling, reaming, turning, grinding, honing, interpolation, boring), tool wear come to mind.

 

[looslib]

The NC program should be to the nominal dimensions of the model/drawing. The tolerances come into play as the parts are machined and are determined by the items listed in Brian's second note. The only time tolerances come into play for the programmer are when you have to deal with material characteristics and tooling materials. Inconel does not like thin cuts. If you do rough passes, be sure to leave enough material for the final pass to be within characteristics of what the material will actually cut and not end up rubbing and causing work hardening of the finish surface. It is a fine balance between depth of cut, RPM and feed rates.


"Wildfires are dangerous, hard to control, and economically catastrophic."

Ben Loosli

 

[BrianE22]

Bharath -

I thought the following might help:

https://mae.ufl.edu/designlab/Lab Assignments/EML2322L-Tolerances.pdf

 

[joycejo]

The tolerance achieved is also a function of quality of the cutting tool. A good tool will not cut well in a bad holder.
Tool deflection will affect the quality of the finish and dimensions.
The material will need to be considered. Are you cutting aluminum or titanium?
Very different cutting parameters and tool selection required to achieve accuracy.
The rigidity of the setup needs to be considered.


John Joyce
Manufacturing Engineer

NX 1984 Vericut 9.2.1

“Opportunity is missed by most people because it is dressed in overalls and looks like work.” - Thomas Edison

 

[dvd]

Some of your tolerance programming and understanding needs to anticipate wear of the machined part - for example, a sheet metal die, consisting of a punch and die, will wear with use. In addition to proper clearance between punch and die for the material type and thickness, it needs to be remembered that the punch will become smaller with use, and that the die opening will become larger with use. So, producing stamping dies at the middle of tolerance may result in very short tool life. Bottom line, you need to understand what the machined parts will be used for to decide the best machined part size to program.

 

[enginesrus]

And that is where a true machinist comes in. As well as the one running said CNC machine does the proper adjustments to maintain the tolerance. You as a programmer are just establishing the proper dimensions to cut to and the process. The programmer only needs to worry about the tolerances if some action in the program makes adjustment at the machine impossible to maintain the tolerance, like if the cutter is grabbing or pushing the part due to depth of cut or size of selected cutting tool etc. or one of many other variables.