lots of progress on the blades

[jacquesm]

I never thought making those foam blanks would be this hard. Anyway, it's getting there.

After 3 days of pretty hard work I have something that looks a little bit like a windmill blade in front of me. It's not yet fit for consumption but it has recognizeable twist and taper, as well as a decent airfoil shape. Hopefully tomorrow I'll have it perfect.

One of the more nasty little headaches in 3D cam is that you don't really have an infinitely small bit that  traces infinitely small paths. Real world router/milling bits are anywhere from .125 to .750 " in diameter for this kind of work. The best ones are 'Ball' shaped, uand have a long shank.

In order to compensate for the cutter diameter you have to perform an operation called radius compensation. The cutter software I wrote already does that, but only in the 'X/Y plane'. That's where the plasma cutter operates, Z isn't an issue at all, except for warp control and obstacle avoidance.

In order to do true 3D sculpting you have to compensate for the cutter radius in the 'Z' axis as well. This took quite a bit of fiddling but I think I have that licked.

The Z axis problem I had yesterday had nothing to do with the weight of the router, I messed up in my 3d adaptation of Bresenhams Line algorithm. That's fixed now and the cutter head now keeps perfect Z position. No need for springs to compensate for step loss because there wasn't any.

btw I *hate* routers... what an unbelieveable mess and noise. I got a tip to reduce the RPM on the router to cut down on the 'snow' (thanks again, Rob) that comes of the foam, I'll definitely look into that tomorrow, this is getting out of hand.

[TomW]

acquesm;

Ever consider a nice, quiet, hot wire cutter for the foam?

T

[jacquesm]

Hi TOm,

Yes, but I could not get it to work good enough.

If there is a 'proper' way to use a hotwire to cut foam blades with then I'd really like to know how to do it.

I did a few tests some time ago (a month or so) with a hotwire made out of the wire that's in toasters. It worked to a point, I could cut 'simple' shapes, but anything more elaborate (such as a 'hollow' back surface, and the the leading edge of the blades) did not work for me. Possibly I have the wire mounted completely wrong !

The way I did it was in a little plywood jig with the wire being about 3" long and cutting length wise across the blade. That limits surface features quite drastically. Even for a simple profile that would need another stepper to 'rotate' the wire. So at a minimum I would need a much better way to mount the wire.

Cutting with a 'fixed' profile shaped cutter wire would not work because like that you can not do taper (and twist is pretty hard like that too), even if that would be lightning fast for straight, no twist blades.

[windstuffnow]

  I use guitar string for foam cutting, an "E" string I believe it is.   Works quite nice, it seems to have just about the right resistance to maintain heat yet only draw a couple amps from the battery.  You have to go at a steady even pace, and make sure your follower is smooth otherwise you'll get vibration from the string.

  I use a ball end mill on my machine and a ball end follower matching the size.  It would be nice to have a CNC machine but I don't know enough about them to convert...

  My modest but functional blade duplicator...

Its done me well for quite a few years although you still have to hand carve the first blade.  Some of them I've kept the pattern for future use.

Can't wait to see a couple blades from your "sit back and watch the blades be made" machine...

Have Fun

Windstuff Ed

[windstuffnow]

  I use guitar string for foam cutting, an "E" string I believe it is.   Works quite nice, it seems to have just about the right resistance to maintain heat yet only draw a couple amps from the battery.  You have to go at a steady even pace, and make sure your follower is smooth otherwise you'll get vibration from the string.

  I use a ball end mill on my machine and a ball end follower matching the size.  It would be nice to have a CNC machine but I don't know enough about them to convert...

  My modest but functional blade duplicator...

Its done me well for quite a few years although you still have to hand carve the first blade.  Some of them I've kept the pattern for future use.

Can't wait to see a couple blades from your "sit back and watch the blades be made" machine...

Have Fun

Windstuff Ed

[jacquesm]

do you have a picture of your foam cutting head ?

Is that handbrake to switch the router on and off ?

I'll post some pictures later today.

  Jacques

[windstuffnow]

   My foam cutter is about as primitive as it gets...  I don't have any pictures of it but if you can imagine a 1" x 1" square piece of wood with 2 3/8" holes on a slight angle in the ends.  Now add 2 lengths of 3/8" wood dowels in the holes and drill the ends just larger than the wire going through it.  String the wire through and tension it by slightly bending the two 3/8" dowels.   By adding enough tension or bow to the dowels they will flex back and forth as current is applied maintaining a reasonable amount of tension on the wire.  If you set it up with out a tensioner ( either spring or bowing the wood ) then the wire will go slack when its heated.

   I use it for making small parts, its only about 2 ft or so.   I've seen some extreemly nice ( and expensive ) CNC foam cutters and there are alot of plans on the web for them.  

   I used it to cut foam "D cells" for the wings I was making at one time...

Have Fun

Windstuff Ed

[jacquesm]

ah Ok, I get it. I thought you were cutting your entire blades out of foam with that. I'll have a look at what's available online. The noise is simply driving me nuts (we have an office above the workshop with 4 people working there during the day), and they are all just about ready to lynch me.

[kurt]

http://www.charlesriverrc.org/articles/tools/brengmancutter.htm

http://www.hhhh.org/~joeboy/resources/hotwire_foam_cutter/hotwire_foam_cutter.html

http://webpages.charter.net/rcfu/ConstGuide/FoamWing.html

[troy]

I sympathize with your "tweeking" pains as you sort out your very cool software/hardware.  But without significant risk, there is rarely significant gratification.  Once you DO get it sorted out, you get orders of magnitude more satisfaction from the process and the finished product. Buying a turnkey solution has it's advantages, but nothing like this.

And if you have a nice way to make foam patterns, then you have an "easy" way to get into lost foam aluminum casting.  Make a foam pattern 4% larger than you want, plus allowances for the finished surfaces, stick a sprue on it, bury it in sand and pour molten aluminum in.  Out pops a lovely casting.  Of course, I grossly oversimplified some of the steps, but you have all the skills to do that.

Just to tease you, here's a fun link to a guy that did a home brew software driven hot wire cutter to make foam patterns to make linear bearings to make more cnc router toys.  Whew.

http://www.buildyouridea.com/cnc/hblb/hblb.html

Nice to see you making real progress.

Good luck and have fun!

troy

[jacquesm]

I've seen that page, it was quite impressive. 'Kurt' sent posted a bunch of links and one of them led there. I spent hours just reading what he's done and how he has gone about it. very impressive.

One thing I have noticed, and for which I have no explanation is that after cutting the core loose from the billet it curves downwards. Very strange behaviour, it is almost as if the foam density isn't equal everywhere and as soon as the foam is cut out the higher density foam expands a little bit (it can because the 'other side' is no longer pushing back.

I still have to experiment a bit to verify that that is really what is causing this.

[troy]

I had to do some "precision" work in blue high density styrofoam for a small low delta stirling engine I built a couple of years ago.  The guy who wrote the instructions suggested that you either need to keep both surfaces of the board, or machine both surfaces off, because the skin tends to be under tension, and if you machine off just one skin, the opposing tension can cause warps, etc.  HTH

troy

[jacquesm]

pictures of your stirling ? I love those little machines. I have one here that is solar powered (very small, but high delta-t, runs of 4 shaving mirrors!)