This is the fifth in our series of posts where we are sharing drawings for the White Horse CAD Wobbler Engine. This week we will look at the Wobbler Engine Base, which is the first part of the second sub assembly..
Next week we will look at the main support, which is the backbone that all of the main parts attach to.
Lets look at one of the Base’s features in a little more detail. If you would like to refer to the PDF engineering drawing it can be found by clicking here… P9999-13101_Rev1
Most engineers will be familiar with geometric tolerating, but the casual maker may not understand them.
The 200mm width of the base had a datum symbol “A” attached to it. The 2 holes are at 75mm centres with a tolerance of ±0.3mm. This tolerance is not declared on the drawing, but if you look at the title block you will notice the “unspecified dimensions” section. As the holes centre dimension has 1 decimal place the tolerance is ±0.3mm.
Now, back to that geometric tolerance… The geometric tolerance frame is split into 3 sections…
- The type of tolerance (Symmetry)
- The allowed error (0.2mm)
- The Datum from which the tolerance relates (A)
This means that the centres can be between 74.7mm and 75.3mm AND they must be symmetric about the width within 0.2mm.
But why have we used geometric tolerances? The main reason is that for the engine to fit together properly the holes need to be in the middle of the base. This means that regardless of the finished width of the base (199.7mm to 200.3mm) the 2 holes must be symmetric about the centreline. Had we dimension the holes from one edge, they may not be symmetric.