3D Printing in Schools
Designing for 3D printing – opportunities and limitations
Clearance between moving parts – typically printed parts, from powder, have a bleed factor. This is dependant on the machine but typically it is good to leave an expansion, or bleed, gap of about 1.25mm between parts. This allows for excess to be sanded away and parts to join accurately without needing to file material away.
‘Angles’ to avoid support material
If you are using a powder machine then all internal angles are possible to print as the material is self-supporting. If using an extrusion 3D printer then consideration of how parts can be supported, support material can be removed and built is essential. Overhangs and internal structure can be built however some detail can be lost due to the ‘step’ of the extrusion layers.
Bosses, ribs and inserts for threads
3D printing allows for internal detailing to be considered as part of the design. Joining methods should therefore be considered and lugs, bosses and ribs are all possible using 
3D printing. Typically, for working models, it is useful to use brass inserts that means that you do not need a self tapping screw as the 3D printed material can lose some accuracy when cutting a thread. A simple hole in a larger cylinder allows for a threaded brass insert and a machine screw and provides a suitable, re-useable joining method for 3D printed parts. Due to torsional forces it is necessary to strengthen the lug with ribs, which is a simple extrusion and D1 x D2 chamfer.
When not to 3D print
With all tools there will be occasions when it’s more appropriate to use different techniques. Here are just a few examples.
You need lots of the same parts – 3D printing is not very
fast so check other methods such as CNC moulds and vacuum forming or injection moulding. Slot car guides – Mark Harmsworth for Boxford and CR Clarke
Secondary manufacture – 3D printed parts can be used to create
CNC machined hard tooling for large quantities or soft, silicon tooling for short production runs. Coin moulds – PTC
You need lots of different parts – You may not have the capacity to 3D print class sets of individual designs. Hand techniques or hand operated machines may be the only way you can do this.
Flat shapes – It might be better to laser cut or profile flat
shapes. Tripod leg – PTC, modified by Dave White, Clevedon School
Material is not the best – The materials for 3D printing are limited so what manufacturing processes do you have for the ‘ideal’ material? Nylon, PTFE, acrylic, solid timber, plywood,
metals
Boxes – Would it be better to laser cut and assemble a
box or create one using folded sheet techniques. Bike alarm – Staffordshire STEM Centre
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