Mechanical Details

With the mechanical design of the machine we have decided to make most of the enclosure out of Acetal copolymer as is FDA/Medical approved.
The mechanical design of the Automated BVM is being kept very simple with a set of plates and a linear stage that is driven by ballscrews.
All fasteners will be made of stainless steel.

Plates

With the mechanical design of the machine we have decided to make most of the enclosure out of Acetal copolymer as is FDA/Medical approved.

The copolymer grade offers excellent performance at a slightly lower cost than Delrin.
Acetal copolymer may exhibit center line porosity, but porosity free material is available.
Certain grades are FDA, USDA, NSF, Canada AG and 3-A Dairy compliant.

Low stress levels and high strength assure flatness and dimensional stability up to a maximum continuous service temperature of 180°F (80°C).

We will be releasing the files in DXF format for people wanting to cutout and pocket on their CNC routing machine and also as STL files for anyone preferring to 3D print the plates.

The down side with Acetal when designing is that the tolerance of the sheet thickness can be 0.2mm-0.8mm thicker and so you will need to skim one side with a fly cutter first, so the plates align nicely.

Other plates will be made of aluminium to give strength, such as the base plate, stage plate and brackets for the motor pulley setup.

Bearings and shafts.

We have now tried three different kinds of bearings.

Option1 Igus

The Igus bearings and shafts are FDA/Medical approved and for the prototype Igus UK has kindly donated them to the project.
We will be using the FJZM-01-08 flanged bearings on stainless steel shafts.

Testing them out, we found that they had enough backlash that the design of our paddles where the pressure gets applied made them dig into the shafts and not really smooth enough.

Igus FJZM-01-08 flanged bearings

Option2 LMK8UU

These worked really well and supported the paddle design well, but a little bit noisey.

Option3 Acetal bearings

We decided to cutout our own bearings out of some Acetal 30mm plates we had this end and these work really well.
We have included the CAD files in the BOM page for anyone wanting to cut their own out.

Ballscrews

We are using twin ballscrews configured as the picture below.