Getting a good surface finish milling foam with cnc

Hi, I mill site models for an architecture firm, the scale is 1:500.

The foam is white and closed cell, and have a density around 40kg/m3,
the finish is good enough, but it could absolutely be better, especially the area where they
paint by hand, (some roads/parking lots++), it is alot of paint work getting an even finish.

Higher density urethane foam will create a better finish, but is harder to work with using hand tools if they decide to remove a building during the planning process.

One idea that I have is to mill the site model, then remove it spray paint it with several layers
of primer, and then put on the cnc table and rout it out again routing into the layer of primer.

Any ideas welcome, thanks.

I have found when cutting foam, that using the largest diameter (given the design) at the highest rpm, and cutting through the material at a reduced speed (test several) will give you the best results and the smoothest finish. I don"t use spiral bits for foam, just two straight blades and sharp.


I use 4mm tool diameter for the finish pass due to the small scale.

I have tested 7-8 different tools in a methodical way.

rpm is 24000

I use 8 mm fort stock removal,

It seems that the problem is related to the material itself,
that lower density foams have air pockets that get visible when you apply paint.

Well, I hate to say it but the best way is to eliminate the foam entirely - you’ll never get a top quality result with it - and start machining in something that has an inherently good surface, such as the urethane composite blocks specifically made for milling models. These have a density of 0.35 to 0.7 (i.e. 350 to 700 kg/m3) - much higher than your 0.04 density foam. The material will be quite a bit more expensive, but the milling quality is excellent, and the work required to finish it is reduced to nearly nothing - just a little sanding, one light coat of primer, any retouching and then a finish paint coat…

Most of the modelmakers I know here have given up light foam for site models, it doesn’t machine precisely and is a pita to finish, and on top of that creates a lot of disagreeable abrasive dust.

My 2¢… --Mitch

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that might work. (though make sure the foam and spray paint are compatible)

i do something similar with MDF except instead of paint, i use a mixture of water & white glue (i.e. elmer’s).

but yeah, the problem is the material and it’s unlikely you’re going to find a straight machining method/setting to give a paint ready surface with the type of foam i imagine you to be using.

Yes,I am looking into denser urethane foam, I ordered some samples last week.

In this particular example they like to “edit” the site model by hand which I guess will be quite tedious with Urethane foam?

I get a file from sketchup, convert it to mesh in Rhino and create g-code using Rhinocam.

They 3d print the new buildings that they are working on, and put it into the routed model (flat areas drawn in Sketchup) , and sometimes they remove buildings by hand.

The model is sort a “work in progress” that they use for discussing placement from what I understand.

They also show the model to clients.

The foam I use are quite costly at around 100 dollars or 800 Norwegian kroner for 6001200100 mm and are sold to architects for model making using hand tools, the foam has no abrasive substance in it.

Yes, I might experiment spraying on a primer and rout out again, have to take it of the table of course, and use dowels for repositioning.

Where did you order the samples from? I can check with my colleague where he got his material from. He’s done some urethane foam milling, I would think he got it from Plastkompaniet.

Hi, I`ve ordered samples from Trident foams in the UK, the product line called “mhd”, interesting to hear about a Norwegian distributor of urethane foam.

take a look at Obomodulan also, they have variety of density.
I have used Obomodulan 300 (pink) which was hard enough for making thermoforming moulds. I remember testing their samples pack by which contains the whole range.

Depending on the density, this stuff can be worked with most hand woodworking tools - saws, chisels, gouges, files, sandpaper etc. It is a little like synthetic MDF. If you know how to work with such hand tools, you can “edit” pretty easily.


Apparently the material he got is called Ebaboard and you can also get it in different densities. He got it from Svas Kjemi AS.

Edit: The original producer also supplies some different properties to the different densities. Might be useful:[produktgruppen]=40&tx_ebaltatoni_pi1[klartext]=ebaboard%20-%20board%20material&L=1

But one great advantage over MDF is that the particles aren’t stratified so features won’t split off. It’s fantastic on the CNC!

if you sand it after milling, does it get better?
because if so, you probably should just sand it.

idk, i don’t think i’ve ever used a cnc created surface as a final (visible) surface when fineness is required without giving it a little bit of love… this is pretty standard/normal procedure in fabrication world.
and yes, it could be hours worth of tedious/boring sanding.

(or, pay someone else to do it :grin:)

Yep, no problems with humidity or varying density between surface and center either… --Mitch

We always used to use chemiwood for anything architectural like that or pretty much any model making. The lower density products are perfect for machining and hand finishing.

Not sure where you are in the world but the link above is for the EU. I went as far as the foundry board for one private job but regretted it. It chattered and chipped during the 3/12 axis but turned lovely on the CNC lathe.


When you’ve spent days and weeks filling, sanding, priming, filling, sanding, priming trying to get a good finish on bog standard MDF because the job “didn’t warrant” at least Medite or some form of PU composite you know it makes sense to invest at the front end if you can!

Once you’ve broken the surface you may as well use Weetabix…

Lots of good info here, appearantly lots of different brands available in Europe, having a good supplier of urethane foam will come in handy either way.

OK, I see

The editing is done in an office environment without proper dust extraction, so they preffer to only use a scalpel.

It might be a solution to rout the light density foam slightly undersized on the cnc machine, apply SIKA biresin m72 or similar product and let it cure , and finally rout the finished site model out of this outer layer of harder urethane foam.

This way it will only be a thin layer with high density foam, making it easier to manually with a scalpel.

edited post, cleaned up text.

Thanks Siemen, thats really helpful info, I`ll contact Svas kjemi AS next week.[quote=“siemen, post:12, topic:41119, full:true”]
Apparently the material he got is called Ebaboard and you can also get it in different densities. He got it from Svas Kjemi AS.

Edit: The original producer also supplies some different properties to the different densities. Might be useful:[produktgruppen]=40&tx_ebaltatoni_pi1[klartext]=ebaboard%20-%20board%20material&L=1