When I lent my services to a collegue who was working on a film about 12 years ago, I was introduced to the world of cnc. I remember watching this huge machining centre milling out a slab of mdf and turning it into a fantastic set of gears. I knew that it would have taken me hours to achieve the same thing with traditional power tools. I decided then that I would invest in a cnc router for my own business Oxenham Design. At that time I could turn on a computer, but even to check email seemed like a crazy set of operations. I persevered and learned every piece of relevant software I could get my hands on. I am now fortunate enough to be using Vectric's ASPIRE software, and Techno cnc routers, which has helped us to create some amazing projects, both in part, or in full. I thought that this blog would be a great place to share "behind the scenes" adventures with the software, materials and equipment we use, as well as the projects we build.

Monday, 24 February 2014

Slicing Made Easy!

Since we finished the butterflies, we have been busy making a couple of video game characters for an exciting new game! These guys are full size at 78" tall and made almost entirely from HDU! I can't post anything else about it until the NDA is up in March, but I have been documenting it as I go.
We also have some pretty cool film props that also have an NDA, so I'll post them later as well.

In the mean time, I thought I would do a helpful post on slicing 3d models, and getting the cnc to clean up the glue joints.

There are a million reasons why you would want to slice some model parts on the router.
One of the biggest for us, is that we are sometimes limited by available cutter lengths or material thicknesses.
For example, when were cutting a model over 3.5" thick, but the available material only comes in 2".

One way is to have Aspire slice the model, machine the parts separately, then glue them up. But then the seams require work to clean-up after. I much prefer letting our Techno do as much work as possible, because it does it faster and better than I ever could!

So here's a 3d model replica I made of the Excel gum's bad breath coffee guy like in their commercials.
As you can see, the front half of the model come in at 2.4998" thick on the Z axis. The problem is my
HDU material is exactly 2" thick. I also have a piece of 1" material I can use as well. I could glue them together into a 3" thick slab, but the other problem I have is that my 1/4" ballnose cutter is only 2.5" long. So after it's slid into the collet, I only have a total of 2" of cutter length available! I certainly can't do it all in one shot...........or can I? (dun, dun, dun) That was cliffhanger music you just hummed..............

First thing up is to slice the model into layers I can use.
In order to overcome any material variances, I slice my model just a hair under the material thickness. In this case 1.95". This will let the cnc plane off .05" creating a dead flat surface to glue the 2nd layer of the model to.
Here's our model, sliced at 1.95" thick, and the second layer turned off in Aspire.

I had Aspire create a vector boundary around the whole 3d model slice, then offset it outwards just over the .250" ballnose diameter we'll be cutting this with. In this case, .300" This will limit the toolpath so we don't machine the whole 12" material block.

At this point, it is important to add some small tabs to hold the model in place for the second layer machining. I simply drew some rectangles, and made the .250" thick. Make sure to set the tabs to "MERGE" or they'll pop out the model, looking weird. This should hold it while it cuts.

When we set-up our material block in Aspire, we want to set the Z zero to the machine bed/ bottom of the material block. It get's to be too much of a math game for me to set it to the top of the block :)

So this is the roughing pass done with a .250" ballnose cutter. I'm a little lazy, so I usually rough and finish with the same size cutter to eliminate a manual toolchange.
And the finishing path as well! As long as you don't do a cutter change between the roughing and finishing pass, you can use the same vector boundary for the finish machining. Aspire calculates differently between an endmill or a ballnose, A ballnose gets calculated to the centre of the cutter, while an endmill calculates to the edge of the cutter. So if you plan on using different cutters, you'll have to adjust the vector boundary accordingly.

Tomorrow I'll cover the rest of the process...........

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