PTC Creo tolerance allocation example

Tutorial for tolerance setting by an example in PTC Creo.

1. IN CREO –> OPEN A SKETCH CONTAINING THE INTERFACE BETWEEN TWO PARTS
   1. In this example, the sketch is a part of a skeleton model in Creo 6.0 and represents the mating between two parts – a “CROSS” and a “PLATE”
2. IDENTIFY THE FIT THAT SHOULD BE TOLERANCED
   • In this case, it is intended to make as close a fit as possible without overlap of the parts
3. ADD A REFERENCE DIMENSION TO THE SKETCH
4. READ THE GIVEN DIMENSION AND LOAD THE MEASURE INTO THE RD8 TOLERANCE STACK-UP CALCULATOR
   1. In this case, 40 mm
   2. The RD8 Tolerance Stack-up calculator can be found here
5. INSERT THE NUMBER OF ELEMENTS INTO THE CALCULATOR
   1. In this case, there are two elements – the height of the CROSS and the height of the hole in the PLATE part.
6. ENTER THE IT-GRADE OF THE GIVEN PRODUCTION PROCESS
   • In this case – IT11 for laser cutting
   • See RD8 documentation for IT-Grades and recommended IT-bands
7. READ THE OUTPUT OF THE CALCULATOR
   1. In this case: +/- 0.16 mm
8. ALLOCATE THE TOLERANCE TO THE FIT
   1. In this case, tolerance is added to the top and bottom of the sketch. The 0.16mm could also be allocated to one side only. 
9. EXAMPLE OF A FIT ON A SMALLER MEASURE (OF 5 MM)
10. ASSUMING THE SAME MANUFACTURING PROCESS
    • Still assuming that the parts are made by laser cutting processes (IT-Grade = IT11)
11. READ OUTPUT AND ALLOCATE TOLERANCES TO FIT!
    • It is seen that the tolerance stack-up is smaller for smaller dimensions
    • This reduces the variation of fits

TAKEAWAY
If a minimum variation is intended (a Robust Design), then the important fits should be placed on small dimensions!