Foamy Season is here - Milling Depron Foam -> A How To

[Matchless]

The ETOC is always the best place to see the 'Cool New Thing' in foamy electrics. Last year was no different. So what was so different last year? One of the biggest differences were the severely lightened depron airframes.

The method for lightening that several competitors used was strategic milling of the depron structure using a CNC machine. Pockets were milled out in the structure to make them as light as could possibly be. Typically contestants milled the foam out with a CNC router. That method is fast, accurate and allows for quick changes if necessary. The guys at Fancy Foam and at Insane Foamies showed their engineering prowess by setting up their machines and producing special airframes for their sponsored pilots.

Some also used a more labor intensive method to do this themselves (I know AJ was busy) This method is effective as well and gets the job done and gives the same benefits.....

So why does it work? Depron is a foam like most others, however a treatment given the foam during production produces a skin on both sides. This skin gives depron much of its strength, but also is where much of the weight is concentrated....

You've read on this far, you might as well see how you can do it at home- right?

[Matchless]

This process will work on any foamy design of any size. The benefits are relative to the amount milled out and the thickness of the foam used. For this thread's sake, I'm going to stick to 3mm foam for a typical "Shocky" sized foamy.

You need to start by having the fully cut out foamy ready. You'll also need:

#11 exacto blades & handle
Dremel tool with router attachment
Dremel routing bit
Scrap of plexiglass
3/8" brass tubing
Scrap 3mm depron
6mm depron
Straight edge
Foam Safe CA

[Matchless]

The first thing you need to make is your cutting tool. I modify a regular Dremel router attachment to make this job easier.

I modified the attachment by gluing a piece of 1/8" plexiglass to the base. I just used regular foam safe CA to do this.

Drill a hole in the plexiglass where the bit will exit. Drill the hole to fit the brass tubing you'll glue into place in a moment.

Next up, I glued in a short piece of 3/8" brass tubing. Diameter is not terribly important, but your router bit needs to fit inside. Glue the piece so that 3-4mm sticks out of the plexiglass. Make sure to glue it from the topside. You don't want the glue fillet on the 'working' side.

Finally, screw the modified router base on to your Dremel.

Why on earth did you add this plexiglass base and tube? The plexiglass allows you to router out larger areas, and the tube allows the router to follow a pattern.

[Matchless]

Next up, you need to make yourself a pattern to guide the router. This step allows you to mill out airframe parts quickly and with good repeatability. It takes what can be job which fills an hour or more, and turns it into a 10 minute job with very repeatable results - especially important if you plan to build more than one of the same foamy.

I like to use 6mm depron as a guide. Its relatively cheap and pretty strong. As an alternative, you could use 1/4" balsa as well.

I start by laying my cut out part on the depron and tracing around it with a ball point pen. Take a moment to mark the hingelines, bracing points or anything else important on the 6mm depron.

Once you have your parts traced on the 6mm depron, you can begin to draw in the 'structure' that will remain once you've milled the parts. There are varying ways to do this, however, triangles are always strong design elements.

Do yourself a favor and use a straight edge to draw in the lines. Doing a sloppy job here will result in a sloppy milling job every time you use this milling pattern.

I like to leave the root of horizontal stabs, as well as the nose and landing gear areas on the fuse unmilled.

[Matchless]

One more note about drawing in the 'structure' - Don't forget the offset between the outer edge of the router bit and the outer edge of the guide tube in your router base. Forgetting it will mean that you won't cut out as much material as you thought.

Now that everything is drawn in, its time to cut out the holes. I use a fresh #11 blade and a steel straight edge to make this go fairly fast. Do this right, because sloppy work here will result in a sloppy looking routing job.

Not much more to say on this step, just get cuttin!

[Matchless]

Finish creating your milling pattern by shimming it for use.

Shim the pattern with scrap depron the same thickness as the parts you will be routing.

I cut some 3mm depron into 1/2" wide strips and glue it around the perimeter of the part you drew on the 6mm depron. Glue them with foam safe CA and give them time to dry before you test fit the part in there....

Glue a few scraps in the corners of the pattern sheet as well to support it while you work....

[Matchless]

FINALLY!!!!! Its finally time to put all of this to work.

Clear your workbench of everything. If you leave a washer on the workbench, you'll have a great little circular indention in the foam when you're done...

Before you cut good parts, put a scrap of depron under the pattern and cut a test cut to ensure you have the depth right. Milling ultra deep does not seem to eliminate a ton of weight, so go 1.5mm - 2mm deep into the foam.

Once you are satisfied with the depth, lay your cut parts on the bench arranged so the milling pattern will fit over them and lay the milling pattern in place. Get the parts arranged inside the alignment shims you glued onto the milling pattern so they will stay put.

Start up your router and drop the cutter in the center of the hole to be cut out. I like to trace the outline of the hole with the router then move in a zig zag pattern across the face of the hole with the router to 'clean it out'.

Move your way accross the entire part until you've milled out all your holes.

You've done it! You have milled a part and reduced its weight

[buttface]

Tell me more....

[Matchless]

Now, finally, some secrets to success, and some hard numbers.......

First off, make sure the alignment shims on the milling pattern are fairly tight against the part. In this example piece, I allowed the shims to be fairly loose, and my part shifted during milling, so the milling is a bit off to one side (you can see it if you look closely at the last pic).

Don't mill out too deep or use a dull bit. Milling out too much will run the risk of 'tearout' and ruining a perfectly good piece. A dull bit can give much the same result.

You should not be afraid to add a little depron back to the milling pattern or remove a little more. This is a $4 template.... not expensive to replace!

There is a strength/stiffness penalty for doing this process. Think that through when you plan out your milling. You may need to add more to the pattern so you cut less, or you may need to think through getting rid of one or more hole.



Now, some hard numbers - What do you really get by doing this?

The airframe I am milling this evening should result in an airplane which is approximately 130 grams ready to fly.

The depron in the airframe makes up 45 grams of the total weight or approximately 35%.

Milling this airframe took out 7.6 grams, or reduced weight of the depron parts by approximately 17%

Doing so reduces the all up weight by 5.8% overall.

Does not sound like much, however if you put a 5.8% reduction in a giant scale frame of reference, that would be like taking 1 3/4 lbs out of my 30lbs Carden 35%!!!!!!!

So imagine you could take 1 3/4 pounds out of your 35% airplane for a total cost of $8 (total cost of the parts for my router modification and a sheet of 6mm depron). You would jump at the chance and do it in a heartbeat.

[asher]

Very Nice Matchless, Your stuff looks a good as the stuff we did here at InsaneFoamies with the cnc. And I was a little frustrated with the amount of work it took to do it with cnc. Just wanted to say looks great!!! Heres some pics of Johns winning bipe from the etoc. We milled it paper thin. You could see through it. Big weight savings!

[Matchless]

Thanks a bunch Jason! Ya'lls work was definitely part of my 'inspiration'.

Now its time to finish this proto!

[HuckinChikn]

Jeremy, You Rock! I've been slamming my brain trying to figure out a cheap
and easy way of milling foam without the CNC....THANK YOU!
I'm going to start my dremel router mod right away