Monday, February 22, 2010

Making cut paths using Revit

We have been cutting wood with our CNC router
for several years, both in two axis applications and three axis. However, we have rarely cut other materials and I suspect few people realize that we
can cut almost anything. I decided to do a project with aluminum to show off the capability of cutting other materials. Besides, anything you make out of aluminum will look beautiful.

The project is to design a canopy over a train station platform that would be 240 feet long. Build a model of the structure at ½” = 1’. The models will be ten feet long which is an awkward size to move around. Consequently, the models must be manufactured in two 5 foot sections.

Aluminum cost us about $215 per sheet of 3/16” thick by 4’ by 8’. It is not cheap but it is not prohibitively expensive either. I used grant funds to buy a sheet for each of my students, and some for me too.

Cutting aluminum is just like cutting MDF or plywood except the particular settings for revolutions per minute and linear feet of movement are different. Chuck called his suppliers and other experts and also experimented to find good settings. Slow is good, plus we used a blower to air cool the bit and push the shavings away.

One of the problems that we encountered was that the bit lifted the aluminum sheet off the bed and then mangled it. We designed the locations of drill holes in AutoCAD so that we could screw the sheet firmly to the sacrifice board without damaging the pieces of the model.

Just like cutting MDF or even laser cutting, I needed to create a cut path sheet in AutoCAD to load into MasterCAM for running the CNC router. The AutoCAD drawing needs to be the accurate size of the model (120’ in the design should be 5’ in the model) and all lines need to be polylines (PLINES). It is helpful to have a 4’x8’ rectangle drawn to represent the sheet of stock material.

My process involved making the model in Revit. I created sections through the model at the location of every part profile that I would need. I then used a sheet template to create a new 4’x8’ sheet and laid out all of the parts on the sheet carefully to avoid wasting material.

There are numerous steps to convert the Revit sheet to the DWG file, but it is really pretty easy. The sheet can then be exported to a DWG file. The file will then open in AutoCAD 2010 as a Paper Space sheet. This peculiar command will save a Paper Space DWG into a new Model Space flle.

Each part exists as a BLOCK, so it must be EXPLODEd. The first time through, the EXPLODE command produces SPLINES, ARCS, LINES and other objects. The PEDIT command will convert a SPLINE into a PLINE. I recommend converting all SPLINE segments into PLINES and then using PEDIT Join to recreate closed loops. If you join a spline into a PLINE then the spline will not become a proper curve. So convert the spline to a pline first and then join it.

I used the OFFSET command to create new paths to cut holes in the largest elements to make them lighter and more elegant.

The DWG drawing was then saved as a DXF 2004 file because that is what MasterCAM wants to import. It imported without errors and Kyle rotated it in MasterCAM to the orientation that the router expects. He set the tool cut depth, linear speed and revolution speed and designated the order to cut the profiles. When designating the order, it made sense to first cut all of the holes so that we could put screws in each hole, and then pick parts in a proximate pattern to avoid moving the head needlessly. Cutting at first produced rough and rather nasty edges, but after adjusting the cut settings and adequately securing the stock the cuts came out beautifully. Here are the parts laid on tables awaiting assembly. The scrap is also beautiful and reminiscent of a dressmaker’s remnants.

It is all pretty easy and straightforward. I am dying to see 110 feet of aluminum canopy models lined up outside Langford Architecture Center!

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