Tolerancing

Before dimension tolerances can be displayed check;

File > Options > Configuration Editor > tol_display > set to yes

and;

File > Prepare > Drawing Properties > Detail Options > Change

In the dimension tolerances section;

Option: tol_display   Value: yes

These setting should be OK in the department labs but may be required on your home installation if you haven’t set up the config files

 

Tolerances or Clearances?

How accurately a part is manufactured will decide whether an assembly fits together or not – that is the manufacturing tolerance.  But before we consider that we will use the nominal dimension to define a clearance – the gap between the two parts (the tolerance limits will also define a clearance!)

Look at any assembly around you and you will notice there is a gap between the parts which is dependent on the manufacturing processes, materials and product end use.

The size of the gap is defined in the nominal dimensions then the accuracy of the gap is decided by the tolerance limits – if the cavity ends up small and the insert big within the tolerance limits then we still want the parts to fit together.

 

Process

Create your dimensions

Dbl click to edit its Properties

Or select multiple dimensions – drag box or Ctrl select – and RMB > Properties

 

Tolerances > Tolerance mode

 

Tolerance Mode

 

Nominal – displays the dimension without the tolerance

Set your tolerance upper and lower limit in this mode

Limits and Plus-Minus – displays the range after the nominal dimension

Symmetric – show an equal upper and lower limit, the previous setting are asymmetric

 

Example

An alignment pin hole in your mould tool is manufactured to a high tolerance but it must be asymmetric. It could be minus zero, plus 0.05mm

The pin would be plus zero, minus 0.05mm. In this case the gap between the two surfaces would be 0.00 to 0.10mm

 

Standard parts

You do not specify clearances and tolerances for standard parts such as screws, bolts, nuts, washers, etc. – these are manufactured to a industry standard such as ISO.

 

Holes and Pins/Shafts

There are specific Tolerance tables for fits between holes and the cylinders which fit into them, which fits you select will depend on the mechanical operation of the assembly.  Look at the fit description table at the bottom of this page.

For location clearance tolerances in our mould tool design we shall (theoretically) use the tolerance bands H7 for the holes and g6 for the pins.

Therefore a 5mm pin would be 4.988 to 4.996 and the hole would be 5.000 to 5.012 – at the maximum, 0.024 clearance.  Actual machining in our workshops will be to much lower tolerances, more like H8/h8 hole/pin.