Designing for Manufacture

General

• Objects in CAD often look bigger than they really are. Use scale drawings or measured bits of paper or card to get a sense of the actual size.

• Always measure your motors/sensors/servos/fasteners that will be interacting with the manufactured part. The CAD models provided are simply downloaded content and may not be identical to the physical part.

• Leave clearance wherever one part inserts into another. If they are designed with identical dimensions they will not fit together. The inserted part needs to be slightly smaller than the receiving part. It’s also a good idea to avoid sharp corners in your geometry when doing this, as these are more likely to cause fit problems.

• Where there are fasteners in your design, include them in your CAD assembly. This will help you make sure that you have chosen the right length, and identify if there may be collisions with protruding bolts

• Remember to consider how you will assemble your real design! For example, it must be possible to access every bolt with a screwdriver during assembly, so be careful of obstructions and create extra holes for access if necessary.

Laser Cutting

• The laser beam is about 0.2mm wide, so your designed dimensions in CAD will be different in the laser cut piece:

  • Inner dimensions will increase by 0.1mm all around (e.g. 0.2mm diameter increase for a hole)

  • Outer dimensions will decrease by 0.1mm all around (eg a 10mm wide tab will come out ~9.8mm)

  • For more information on applying this to holes and nut recesses, see our wiki page on hole tolerances.

  • We have done experiments with acrylic to find the optimum fit for the 3.96mm brass rod you have been provided with:

    • For a push fit, use 3.8mm holes.

    • For a hammer fit (really tight) use 3.75mm holes

• Acrylic is a brittle material and thin structures will easily break. Avoid long thin structures or tapered points in your design.

• Nominal plastic thickness may vary in reality; e.g. a 3mm sheet may be slightly thicker or thinner than 3mm. Allow for this if you are fitting a laser cut part in a narrow gap. If you know which plastic sheet you will be using, you can measure the thickness with calipers to find out for sure.

3D Printing

• Don’t try to 3D print everything! It is not always the best option, nor is it all-powerful. Always consider whether an already-available mechanical component or a laser cut part might be better. 

  • Many flat structures are much quicker and better to produce by laser cutting, in particular mounting plates or large panels. 

  • Do not attempt to print screw threads, long thin rods, worm gears, or other structures that require very fine resolution or low friction. 

• 3D printing is slow (2 day turnaround for most parts) and mistakes are costly! Always double-check your work before submitting it to be made. Where possible, get your design checked by a technician first.

• Holes will often come out slightly smaller than designed, especially small diameter holes, or cavities for holding nuts, and it is wise to add extra clearance, especially for holes that can’t be drilled out if they are too tight. For example: with a slot for a 5.4mm diameter M3 nut, I would add +0.3mm to the hexagon diameter in my design. For more information, see our wiki page on hole tolerances.

• FDM 3D printing builds a model from layers of plastic extruded in lines from a nozzle. This has several implications that you should always keep in mind:

  • In the Z-direction, the standard layer height is 0.254mm (can go as low as 0.127 for special parts if required). This means that 1mm distance in Z is only 4 layers, so what might look like a smooth curve or diagonal in CAD will in fact be a series of steps, and very small details can disappear entirely. It also means that vertical dimensions will always be rounded up or down to the nearest layer.

  • In the X,Y plane, the extrusion width is around 0.4-0.5mm. Any feature smaller than this in X and Y will not exist, and a single extrusion is very fragile anyway. You should have no feature below 1mm thickness, and aim for at least 2-3mm walls for strength.

  • A 3D printed part is much stronger in the X,Y plane than it is in Z, because the boundaries between the layers create weak points. Avoid tall, thin vertical structures that can flex, as they may just snap off. Using thicker structures or reinforcing with fillets at the base can improve this.

• Our Stratasys F170 printers use alkali-soluble support material which is dissolved in a bath after printing. This process is much slower for deep cavities and ‘blind’ holes (holes which are closed at one end). You can avoid a much longer production time by avoiding deep cavities and/or adding little extra holes for the alkali solution to get in.

Requesting Manufacture

All manufacturing requests should be emailed to roboticsjob-group@york.ac.uk (not directly to Alejandro Pascual) so that any available member of technical staff can handle the job.

3D Printing

• Make sure that your parts are within the volume limits given at the start of the project. Designs which exceed these limits will be rejected.

• We are able to accept Inventor part files directly, and this is preferred. If you are using software other than Inventor, please send us your design in STEP format.

• If you need multiples, include this in the name, e.g. “2 x gripperFinger.ipt”

• Email your CAD files to roboticsjob-group@york.ac.uk. If you have special requirements (part orientation or print configuration), make these clear in the email. Plastic colour cannot be specified, as jobs will be processed in batches with parts from multiple students.

• Your parts should be ready in 2 working days, unless there are delays caused by staff shortages, heavy printer usage, or manufacturing problems.

Laser Cutting

• Export your geometry to .DXF file(s) by right-clicking on the face and selecting “Export Face As”. Instructions on how to do this are in the lab script for 2D CAD (Spring Lab 1)

• Choose which material you are using. You must specify the thickness where there are options (e.g. 3mm or 5mm for acrylic), and if you have a preference for a particular colour that you know is available in the material store, you can specify this too.

• Submit your .DXF file(s) to roboticsjob-group@york.ac.uk with clear instructions about the material to use. If you have more complex requirements such as engraving, speak to a technician.

• Your parts should be ready by the next working day.

Fabrication Materials

Laser rubber Open	2.3mm thick	Gripping pads for fingers. Can be laser cut and engraved, or simply cut with knife or scissors.
Medium Reflex foam Open	~7mm thick	Very soft gripping pads (experimental!). Easily cut with scissors, can also be laser cut.
Acrylic (Perspex) for laser cutting	3mm and 5mm thickness Various colours available	Most flat parts like mounting plates and panels. Much faster than 3D printing.
Acetal (Delrin) for laser cutting	2mm, 3mm, 4mm, 5mm thicknesses	Gears and sliding surfaces. Smooth, wear-resistant finish, can thread screws directly into it, less brittle than acrylic, good for press-fits
ASA for 3D printing		Complex 3D structures that can’t be laser cut. Very versatile, but slow (2 day turnaround for parts with support material)

Mechanical Components

Item	Dimensions	Usage
Brass Rod   Open	3.96mm diameter	Axles for rotating parts. Use with bearings or fit into appropriately sized holes.   For push fit in laser cut acrylic, use 3.8mm holes. For hammer fit (really tight) use 3.75mm holes   There are vices, hacksaws and files provided in your tool drawers. You can use these to cut the rod to length and smooth any rough edges.
Bearings Open	13mm outer diameter 4mm inner diameter 5mm thickness	Rotating joints, particularly under high load. Fits onto 3.96mm brass rod. Press fit into 3D printed housing.
Springs Open	Various	Provide returning force, either in compression or extension. Compression springs can be contained at each end by cylindrical holes or inserts. Extension springs can be stretched between holes or posts.
Nylon Wire Open		Actuator cables for transmitting pulling force. Tied around parts through holes or around posts. May also be clamped between a bolt and washer.
Open Machine screw fasteners	M1.6 x 5mm M2 x  6mm,10mm M2.5 x 6mm,12mm,16mm,20mm M3 x 8mm,12mm,16mm,20mm, 40mm, 50mm M4 x  10mm, 16mm M8 x 20mm	Fixing things together. Use bolt and nut combination to produce clamping force. Do not rely on threading screws directly into printed plastic, and use fasteners rather than glue wherever possible.
Open Self-tapping screws	6.5mm and 9.5mm lengths	Attaching to servo horns only. Use clearance holes in the part to be attached.   Alternative approach is to use a machine screw in the centre of the servo, and a 3D printed part which engages with the horn.
Loctite 454 Cyanoacrylate glue Open		For when the use of fasteners or push-fits is not possible. Use only when necessary, as glued parts are permanent and not ideal for prototyping. 45 second cure time.