PartDesign Bearingholder Tutorial II

From FreeCAD Documentation
File:HolderTop2-1.jpg
Bearing Holder Tutorial - Finished bearing holder (top)

This is an introductory tutorial to modeling with the PartDesign workbench in FreeCAD. The purposes of the tutorial are to introduce you to two different workflows for creating a cast part with drafts and fillets. Depending on what other CAD programs you have been using, one or the other might be familiar to you. As a working example we will be modeling a simple bearing holder.

This is the second part of the tutorial. It will use what might be called the 'multiple body' workflow, using the (simpler) top part of the holder as an example.

Obviously, to follow through this tutorial you must activate the PartDesign workbench.

Design data

The holder should be able to hold a diameter 90mm bearing with a width of up to 33mm (e.g. DIN 630 type 2308). The bearing requires a shoulder height of at least 4.5mm in the holder (and on the shaft). The top part of the holder will be bolted to the bottom with two 12mm bolts. There should be a groove on both sides of the bearing able to hold a standard shaft sealing ring DIN 3760: 38x55x7 or 40x55x7 on one side, 50x68x8 on the other side.

The holder will be a sand cast with a minimum wall thickness of 5mm, a draft angle of 2 degrees, and a minimum fillet radius of 3mm.


Setting up the skeleton geometry

File:HolderTop2-2.jpg
Sketch of the skeleton geometry

Create a new part in the PartDesign workbench. Rename the Body that is created by default to Skeleton. This Body is probably activated already, which you can see by the blue background colour in the feature tree. Create a new sketch on the YZ plane containing the outline of the shaft, bearing and sealing rings. After finishing the sketch, make a revolution feature from it. This skeleton feature will later be used to reference the real geometry to it. This means that if you want to change any dimensions, all you need to do is adjust the skeleton feature's dimensions and the rest of the part will update accordingly.


The first body

Sketch of the first pad

Create a new body and make it active. The sketch for the first pad is shown on the right. It is placed on a datum plane with an offset of 5mm (wall thickness) from the skeleton face marking the side of one of the bearing sealing rings. Because all the important dimensions are taken from the skeleton, there are just three dimensions: The machining allowance (3mm) at the base as an offset to the XY-plane, the 5mm wall thickness from the outer diameter of the skeleton, and the two degrees draft angle. Two create the 5mm dimension, you first need to select the outer circle of the skeleton geometry as external geometry in the sketcher.

You are probably wondering why there is this small straight segment at the bottom of each arc. This segment ensures that there will be a draft angle of 2 degrees on the arcs. This might look like a lot of work for a very small benefit, but many CAD programs (and maybe FreeCAD one day) have tools that highlight a solid model in different colours and immediately show you all faces where the draft angle is not correct. You don't want that to happen to your model, especially after putting on a lot of fillets!

The first pad

When you have done the sketch (which is a bit tricky because of the 2 degree tangential lines), create a Pad from it extending up to the other side of the skeleton geometry, again with a 5mm offset to the side face. You don't need to create a datum plane this time, the "up to face" mode of the Pad dialog offers to input an offset.