Saturday, August 2, 2014

Fourteen More Guitars

I've been doing more CNC routing of Wallace Detroit Guitars. Here are some details of the setup and cutting.

The blanks are made from wood recycled from abandoned Detroit houses. The wood is collected and scanned for metal fasteners by Reclaim Detroit. For the guitars blanks below the wood is sawn and glued up with the endgrain facing upwards. This makes the annual growth rings very apparent and creates a unique look for the guitars.


The blanks vary a bit in width and length. They are all exactly 1.76" thick. I had them panel sanded at B&B Heartwoods to get them just right.

The computer model which establishes the cutting has the origin at the bottom, center-line of the instrument. This allows me to precisely place the cut on each unique blank. Because the blanks are so accurately sized in thickness I cut the pockets as measured from the top. So when they are finished sanded (removing 0.01" of material) the pickups and neck are all aligned exactly right.

In order to set that on the router what's knows as the "NC offset" is used. For each blank the desired center point is measured from the edges.


Then the values measured are entered into the router's controller as an offset of where the 0,0,0 point in the computer model should align.

Sometimes this results in the guitar being centered in the blank.

Other times, in order to get the desired alignment, the shift means it barely fits. This really came down to if I wanted to lamination line to be centered, or a strip between lamination lines to be centered.

To miss defects (or sometimes remaining metal such as screws and nails on the outside of the blanks) I offset towards the top as well.

The variation in growth rings based on species and grain tightness make for some beautiful patterns:

The router has powerful dust collection so very few chips escapes the router bed during cutting. But there is also a very powerful vacuum holding the blank in place on the table. And with a down shear bit some of the chips remain in the cut groove. Even after the blank is removed!

These chips are quite fluffy - and almost no heat is accumulated in the tool.

They guitar is held within the blanks with small tabs. In this case I used 3/4" wide and 1/10" deep tabs. Here you can see two on the sides and one next to the neck pocket. These are easily broken away - especially because they are end-grain up.

Some of the wood is trim, rather than framing lumber. Below is a blank made from mahogany, walnut, maple, and cherry trim boards. It's also side grain so it has a much more conventional appearance.


Here's a sampling of the fully routed guitars:


Here's a video of the cutting process. Parts of the video have been sped up by a factor of 2.

Tuesday, July 1, 2014

Pixelated Vessel

When working with CNC cut Baltic birch plywood it is a very common method of construction to use a stack lamination technique. Normally the edges of the layers are cut smooth by the router, and the discontinuities between the stacked layers are sanded smooth. Thus the resulting piece matches the designed smooth form.

I wanted to try something different:

I wanted to accentuate the fact the form was made from stacking. And to visually demonstrate that the vertical edged stacking is an approximation of the curved form. To make this more apparent I chose to stair-step or "pixelate" the forms horizontal curves as well. Below is an example:

The original form - a simple twisting pedestal:

The typical method of fabricating this form. Section it into layers, cut them, assemble (left image), and sand the edges from one piece to the next smooth (right image):

The stack lamination, "pixelated" version of it. Here the layers are 3/4" thick (corresponding to the plywood thickness) and the horizontal stepping happens in 3/4" increments as well.

For the vessel I wanted to create here's the source geometry I started with. It's a hollow closed polysurface in Rhino:

I wrote a Rhino Python script to section the form horizontally in layers which correspond to the thickness of the plywood. These intersection curves are then "pixelated" on each edge. That is, instead of using smooth curves like the original form, I have it step across in a user specified amount. This makes it look as if it was constructed out of curved edge Legos. 

The filleting of the curves is done so the CNC can cut into the corners. I actually find this less compelling than the more pure, true to concept, square edge pixels. So that's a compromise. However it is easier to sand, and a lot nice to touch once finished!

It would be possible on a 5-axis router to use an endmill parallel to the face of the plywood to clean out the corners. That's an entirely doable but bigger task for another day... This vessel is made by a 3-axis router.

In plan the curves look like this. You can see the implied grid and how they all align, layer to layer:

Here's a single curve shown in blue. You can see how it twists through the grid but will always align with its adjacent layer above and below:

In terms of the form approximation, here are the two forms overlay-ed on one another:

A property of the script controls how the volume of the original form relates to the pixels. You specify the number of corner points of each pixel (a number from 1 to 4) that are required to be within the form for inclusion in the result. In the example above 4 points were required. In the example below only 1 point was required. Thus, in the example below the pixels sit mostly outside the form rather than within it:

In order to align the parts during assembly some points are generated. These become holes which are drilled in each piece to allow dowels to precisely align them up. The points are generated by intersecting curves with each horizontal section. You can see the curves in the 3D form (in red) and the points on the laid out curves (in white) below:


And of course the layers are numbered in the 3D model to indicate which piece goes where.

Fabrication / Assembly

The parts were cut at Taubman College on one of the Onsrud 3-axis routers.

Shown below is the drilling of the piece to piece alignment holes. In each location there's a hole to align with the piece below and a hole to align with the piece above:

A 3/8" compression bit was used to cut the contours. This bit cuts all the way through the material in one pass. Tabs hold the piece in place so they don't move when fully cut. These were removed with a chisel and mallet. Here you can see two of the tabs at the bottom of the cut (circled):

I'm always trying to find out the smallest size I can get away with and still hold adequately. I'm currently at only 0.5" wide and 0.05" thick, two per piece. So it takes a single chop to get a perfectly clean edge. Much quieter, cleaner, and quicker than a trim router.

Back at my shop the parts were sorted during a dry fit. Then edge sanded to smooth them. If you look carefully you can see a few of the tabs still need to be chiseled off.

Assembly was simple. Quarter inch dowels were sliced up and used to align the forms. An easy glue up which could be done in sections.

Finished Vessel

Here's the fully assembled form prior to applying a finish. 

Here's the finished vessel:
 


Tuesday, April 8, 2014

Sectioning and Nesting in Grasshopper

I did some experimentation with the sectioning and nesting Grasshopper components available from RhinoNest. The idea was to see if I could interactively modify a 3D form, and have it quickly broken up into layered parts, then have these laid out on sheet stock.

Here's the test form I used - it's about 22" across and 2" thick.

Here's the Grasshopper definition.Inputs are the object(s), sheet thickness and size, and distance between parts. (The link to download is at the bottom of this post).  

The definition takes the 3D form, breaks it up into layers of the specified thickness, and then nests them on the sheet(s). In the image below a 48" x 48" sheet at 0.7" thick was specified.


This example was sectioned and nested in only 3 seconds. I consider that fast enough to be interactive!

You can download the RhinoNest demo and use it for 30 days. The download includes the Grasshopper components.

This link is to the zip file with the Grasshopper definition and a Rhino sample file: Download Here

Friday, March 21, 2014

Rotating Model Stand / Sculpture Pedestals

I just finished a couple of quick CNC projects. One was a rotating stand for a figure model to stand, sit, or lie on. The other was a few sculpture pedestals to donate to my son's school.

Rotating Model Stand

This stand was built for use in our sculpture studio (West Huron Sculptors) to let us rotate the model rather than have the sculptors move around the model. Here's the design which is meant to break down for easy storage when not in use:

The rectangular top is removable and is used for more stretched out poses. 

An exploded view: Eight milk crates, a single 17" lazy susan bearing, six 1" ball bearing rollers, two 4' diameter baltic birch plywood circles, three Poplar 2x4s, and a plywood top. 


This was very simple 3-axis CNC routing.

Installing the Lazy-Susan bearing and the six 1" ball bearing rollers. That'll support about 500 pounds - and so far all our models have weighed less than that...

These were purchased at Lee Valley and Woodcraft: Lazy-Susan Bearing, Ball-Bearing Rollers

Routing the notches for the rails to overlap the rotating base plate. The router was balanced between the rail and two pieces of MDF along side it.

The corners were sawn out and cleaned up with a chisel and chisel plane.


Here's the stand in our studio with a double stack of crates. The bottom circle has guides to prevent the crates from slipping. First without the top (good for a standing pose):

With the top on: 



Sculpture Pedestals

This project was a few simple 10" x 10" x 5" and 10" x 10" x 45" sculpture display pedestals. These were made to be donated to my son's school for use in the art department. These were CNC cut with a 3/8" down-shear and a 3/8" compression bit.

The taller stands had a few small joints at the top and bottom then a long one in between:


The interesting thing about them is the joint design and the resulting corner condition which gives them a bit of detail rather than being completely straight-forward boxes. You can see how the joints go together here and the corner detail at the top left:


These were painted satin black by my son and his friends and donated to Washtenaw International High School.