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How can I make a tool to punch shapes in foam sheets?
smart51 - 11/12/12 at 11:03 AM

I've worked out that my roll hoop takes 20% of the RW power in drag at top speed. By adding an aerodynamic profile, I can cut this to 2%, which gives me more top speed. I've been thinking how to make this profile, to fit strights and curves, and I've come up with the idea of cutting out a whole load of identical shapes from a sheet of polystyrene foam and sticking them together. To make the pieces all the same, I need to make some sort of "biscuit cutter". It has to be sharp enough to cut cleanly. How do I do this? Or is there a better way?

Slimy38 - 11/12/12 at 11:16 AM

Hot wires are normally the best option for foam? Use very stiff wire, bend it to the profile you want and then use that to cut the shapes?

designer - 11/12/12 at 11:45 AM

Hot wire cutter using formers:

http://www.ch601.org/tools/homemade/hot_wire_bow.htm

Bluemoon - 11/12/12 at 05:03 PM

^^ didto as per model aircraft foam wing hot wire cutting...

PSpirine - 11/12/12 at 07:41 PM

Cut rough shapes out of the foam (serrated knife works well depending on density).

Glue them all together to get one long stacked bit.

Profile with hot wire as mentioned.

nick205 - 11/12/12 at 08:55 PM

Could you make a mould in sections and fill with expanding foam then trim back to fit the roll bar?

blakep82 - 11/12/12 at 10:02 PM

quote:
Originally posted by nick205
Could you make a mould in sections and fill with expanding foam then trim back to fit the roll bar?


thats a god idea! certainly less wasteful than a block and cutting around half of it away

will be interesting to see if it works as well as you hope.

BobM - 12/12/12 at 06:51 AM

I can see your question has already been answered but ...

quote:
Originally posted by smart51I've worked out that my roll hoop takes 20% of the RW power in drag at top speed.

Are you sure about this?

And if it's true then I'd really question this:
quote:
Originally posted by smart51By adding an aerodynamic profile, I can cut this to 2%

smart51 - 12/12/12 at 06:59 AM

Thanks for the replies. I don't really want to make the pieces by hand as I want them all to be the same. I'll keep thinking about it. I might have a punt on shot wire though.

smart51 - 12/12/12 at 07:04 AM

quote:
Originally posted by BobM
I can see your question has already been answered but ...
quote:
Originally posted by smart51I've worked out that my roll hoop takes 20% of the RW power in drag at top speed.

Are you sure about this?

And if it's true then I'd really question this:
quote:
Originally posted by smart51By adding an aerodynamic profile, I can cut this to 2%


My 3 wheeler makes 22 BHP and a top speed of about 75. Round bars are terrible for drag. Even on a seven, the hoop's drags makes a noticeable impact on the power used. A perfect aerofoil has a drag about 27 times less than a round bar. My 10x reduction allows for a less than perfect aerofoil.

BobM - 12/12/12 at 07:05 AM

quote:
Originally posted by smart51I don't really want to make the pieces by hand as I want them all to be the same. I'll keep thinking about it.

Seems you've misunderstood the replies. The idea is that you make a pair of identical plywood formers in the profile you want. You then pin them on opposite ends of the block of foam you want to cut. You make a bow with the hot wire held tight and drag the hot wire around the formers. You can then make your foam pieces as long as you like, as has been stated model makers use this method to produce long accurately profiled wing sections that are then veneered.

02GF74 - 20/12/12 at 11:48 AM

it is worth trying the biscuit cutter route as it will not involve any fumes.

i would guess a thin piece of steel bent into shape an soldered/welded, sharpened at tthe buisness end and a lip on the other to help it keep shape.

the only problem I foresee is a the tool is pressed into the foam, the foam will compress so the sides may not be parallel - dunno if the will happen - you will have to find out yourself.

an example showing rolled top edge to help maintin shape.
[img]http://i.ebayimg.com/t/METAL-BISCUIT-PASTRY-COOKIE-CUTTER-Various-Sizes-Designs-/00/s/NjQ4WDU5NA==/$(KGrHqJHJCoE9!Npbh)hBPWJ-4QoDw~~60_3.JPG[/img ]

trextr7monkey - 20/12/12 at 02:22 PM

The hot wire cutter gives off poisonous fumes if used on polystyrene etc ,the proper blue styrofoam is very expensive, we cut the flane retardant stuff ona bandsaw but it is very dusty and unpleasant to work,
atb
Mike