CNC foam cutter Fannar Andrason Kristján Orri Magnússon Sveinn Haukur Albertsson

Functional Requirements 0 Cut airfoils out of foam for ornithopter design research(Leifur) 0 Wing profile must be able to vary with span 0 Twist, variable camber, sweep etc. 0 Specified tolerances +/- 0,5 mm 0 The requested dimensions are 50 cm wide, 25 cm high and 1 m long. 0 Objective: 0 Make the machine cheaper than if imported 0 Tailormake to the costumers needs.

Design Parameters 0 Hot resistance wire (Ni-Chr) 0 Each end driven by 2 stepper motors(X-Y) 0 Wire attach point on each end move independently in respective moving plane 0 Wire able to elongate/shorten without loosing tension -> spring mechanism

Design considerations 0 Main challenge: Prevention of jamming and bending 0 Center of action considerations 0 Saint Venants principle for bearings (3-5 rule...) 0 For best precision the machine has to be as stiff as possible 0 While keeping material cost at minimum 0 Axiomatic design 0 All parts replaceable without complicity 0 Parts that demand high precision built separately 0 Electrical and mechanichal parts decoupled

DP - Software 0 User input: Define airfoil profiles for each end + define a reference point (i.e. location of max camber) 0 A software interprets sketches and creates input signals for steppers, subject to: 0 Movement of each end starts at the reference point 0 Cutting time the same for each end to ensure correct lofting 0 Bigger profile on one end -> more speed of movement

DP – user input 0 Sketch 1: 0 Sketch 2: Ref pts. 0 Result: NACA6410 (30° twist)NACA9440

Frame structure 0 Standard L – profiles to maximize stiffness/weight and affordability. 0 Relatively easy to assemble/weld with good precision 0 Mounting platform for other parts Foamlinks hotwire cutter

X - Slider 0 Standard box and L-profiles 0 Plug-bearing made out of POM 0 Low friction on steel rod 0 Y – slider bolted to L-profile 0 A very demanding part to build! 0 Precision, precision…

Y - Slider 0 A steel rod on each side of lead screw to constrain the part to correct degrees of freedom 0 POM plastic 0 Electrical insulation 0 Low friction 0 Easy to press-fit rods 0 Lead nut bolted to inside of slider

Spring mechanism 0 Wire changes direction 90° 0 Rounded guide (half-wheel 60mm dia) 0 Adjustable spring tension 0 About 150N estimated sufficient 0 Rounded leader to ensure correct position of wire end

Motor mounts 0 Sheet metal 1 mm thickness 0 Stiff for constrained degrees of freedom but flexible to allow for imperfect fabrication

Lead screws and nuts 0 Ordered from Master Carr 0 Trapezoidal threads 0 10 mm dia, same as CNC mill 0 Pitch 2 mm 0 Very hard to find in Iceland which means they are the most critical components of the machine!

Hot-wire 0 Preliminary tests indicate that about 3A of current is sufficient to allow for slow movement(about 4 mm/s maximum) without kerf being inconsistent and/or too thick 0 Tension of about 150N sufficient to keep wire straight and prevent unwanted bending of wire

Steppers 0 After „feel“-testing steppers the following conclusions were made: 0 Precision(1,8° per step) sufficient to allow for no gearing -> Direct coupling using stop screws 0 24V input voltage gives sufficient torque 0 Maximum step input frequency of about 700 Hz means that maximum speed of lead nut can be about 6 mm/s

A closer look

Current status 0 Final design ready 0 Workplans ready 0 Preliminary hardware tests complete 0 Stepper motors and lead screws on order 0 Next step: Start building!

Thank you Da Vinci’s ornithopter design (1488)