Solid modelling

 19 November 2021

I think the main difficulty with solid expanded foam modelling is the sanding required for proper shape and texture. However, if gloss over this seemingly inherent difficulty, expanded foam is about the lightest modelling material easily available for flying models.

The least work is profile planes. Where the wing and tails are to the planform without regard to the cross-sections and the fuselage is to side profile. In the case of light RC models, a profile fuselage is sufficiently strong and 'stiff', especially if wrapped with tissue or paper which takes care of the tensile and torsional strengths.

Next 'level-up' in aesthetic is to make the wings and tails airfoiled and to make the fuselage conform to the planform as well. The performance of the wings and tails are almost maxed out and the fuselage has lesser drag. 

The ultimate is to have the wings and tails tapered from root to tip in thickness and to have the fuselage conforming to cross-section.

Carving the wings with HWC is relatively easy but compound curved fuselage is difficult. Not all aeroplanes' fuselages have compound curves and some are easier to achieve by building up the fuselage cross-sections.

Instead of cutting fuselage to side profiles, some can be cut to the planform. In these cases, the planform fuselages can be stacked together to the depth required and cut to the side profile. Turtledecks and front deckings can be curled paper with some formers.

I wonder if it is easier to sand expanded foam (polystryene) in water. 

Don't forget that XPS can be sanded. However, if used in slab construction, it is probably twice as heavy as EPS. Depends on the density of EPS comparison.

 

Foam Cutter

3 November 2021

Making angle templates.

1. Draw baseline and angle on paper.
2. place straight template over angle.
3. tape wooden block over baseline.
4. snip away excess template material that protrudes from base of wooden block.

A pair will guide straight cuts to:

• wing roots at dihedral angle
• wing seat of nacelles to dihedral angle
• chamfered edges 

1 November 2021

Metal coat hanger, wooden stool, screws.

Why must making a vertical or inclined cutter involves drilling a hole on the worktop? It would be simpler if a metal frame is screwed to the underside of the wooden stool (or worktop), with the hotwire outside the wooden stool. The "C" frame may be held perpendicular or even alongside the wooden stool. To adjust the hotwire, slide it along the metal frame. Short circuit? That's what electrical tape prevents.

The same wooden stool functions as a worktop with feet. Being made of wood, it is relatively easy to screw in small screws to hold the metal frame. Since it is a worktop with feet, it can also be converted to a sheet cutter. Alternatively, the seating of the stool can be a worktop for temporary fixing templates to be used in conjunction with a separate bow cutter.

Main task is finding a wooden stool, an item that was easy to spot in the past. Now, plenty of plastic stools but difficult to find wooden stool. An alternative to a wooden stool is to find a wooden chopping board and use some rubber feet.

15 October 2021

Horizontal is vertical on its side. Cylinders and cones can also be cut with horizontal HWC.

Drop a bow cutter on 2 supporting columns from a workboard. Thin or thick sheets can be sliced to thickness, it became a sheet cutter. If a block is held between a C-Channel with pivoting grips/pins, rotating the foam block under the bow cutter can result in cylindrical or conical cuts.  

24 September 2021

Vertical or Inclined Cutter: Frame and Worktop

Construct a square frame with 4" x 4" clearance from 3/8" square wood. The 2 column pieces are drilled at both ends, The 2 beam pieces have additional drilling in the centre. The beams are bolted over the columns so the stretched hot wire is 3/8" away from the columns. To stretch the hot wire, pass the ends through the holes in the centre of the beam pieces, make loops and tension with rubber bands and a simple hook. Holding the square frame by hand, it is a hand-held cutter. If a floor is packed up across the columns to reduce the distance to the hot wire, it can be used as a sheet cutter. If a bed is attached with holes drilled to clear the frame and the frame is perpendicular, it can be used as a vertical cutter. Similarly, if the frame can be secured at an angle, it can be used as an inclined cutter. With a bed, circular objects can be cut if the bed is drilled to receive pivots, it can be used as a circle cutter.

Maybe this cutter and a bow cutter covers all my needs/wants.

23 September 2021

Bow Cutter: Beam and Levers

A simple 18" hot wire cutter for my small things.

A piece of 3/8" square can be the beam. Holes are drilled along the beam to attach the levers. The many holes means that the hot wire can be shorter.

Ice cream sticks can be the levers. Drill 3 holes in each ice cream stick, the central holes are for attaching to the beam, one end is for the hot wire and the other end is for the tensioner. Each lever is made from bolting 2 pieces of ice cream stick. A total of 4 ice cream sticks.

Assemble the beam and levers with 6 bolts and nuts. Wrap and coil the hotwire ends to the bolted levers. Tensioner can be rubber bands on the other end of the levers.

Wire the assembly and the hot wire cutter is ready for some low voltage current.

Hand-held Cutter

This is to cut foam with very short length of hot wire, maybe only 2"-3" and operable by 1 hand. It can be constructed like the bow cutter, just with a shorter beam of 3"-4". Useful for specific applications and since it uses the same levers like the bow cutter, the only additional work is making the short beam and a short hot wire.

Sheet Cutter: just a work top

This is to make thin foam sheets from thicker foam sheet. The basic idea is to have a tensioned hotwire separated by spacers to maintain the height off the work top.

A short plank or anything that is workable, rigid, and have a smooth and flat surface will do as the work top. I think 12" cutting length should be enough and since I wanted thin sheets, the surface does not have to be wide, maybe 3" is enough, anything wider is just more at ease.

If the worktop is thick enough to accept screws, then 1 can be screwed to grip the hot wire. At the other side, a short nail as a radius before tensioned by rubber bands to another anchor point.

Incline Cutter: Raking wire

The idea is is to hold the hot wire at an angle. And preferably adjustable angle at that, so maybe a work top is important. No idea how to do that easily yet.