Sunday, January 15, 2012

Skeleton Wall Clock - test cutting in plywood

I was playing with trying to get the template of the plates into BobCad so that I can machine the parts under CNC. Thought of making them by hands, as shown in the book, but I remember that I can't follow a line with even the bandsaw, not to mention the piercing saw.

This is how a piecing saw looks like, taken from

I'm supposed to paste the template provided in the book, saw out the shape, and clean it up. The same is done to the back plate using the completed front plate as a guide. The 2 plates are to be fastened together for this operation. I'll try doing that after successfully making the first one or if I fail to put the template into BobCad "correctly".

I scanned the template as a jpg and loaded it into BobArt. I tried to vectorize the outline of the template but couldn't get useful result. BurrMan, the guru over at BobCad forum jumped in as usual to help, when I posted. He produced a few tiff files and dxf for me to try out. He even make a YouTube video demonstrating how it can be done. According to Burr, the outlines need to be "blurred" in order for vectorization to work well in BobCad.

The resulting vectors from the process represent a solid line as 2 vector lines (note: I may be using the wrong terms here - pardon me). Simpler to look at the pic:

The blue lines are the result of vectorization. The black solid line is the outline I want. What I really wanted is the vector to trace the outline in the middle.

I wrote to GeneK after I posted on BobCad forum with the scanned image as he mentioned before in a comment to one of my blog post that he is into clockmaking. He responded the next day and forwarded me a dxf file. The lines in the file traced the outline of the template which is exactly what I am after. At almost the same time Burr also sent me the file tracing the middle of the outline.

I loaded up the 2 files and saw that GeneK's is a little undersized and Burr's a little oversized. I believe the size difference is due to the image I scanned. When I forwarded GeneK the image, I sent him a resized version to seek his advice on which are the critical dimensions to keep. This resulted in a tiny image when loaded into BobCad which he scaled up.

I decide to try out the smaller sized version from GeneK to avoid having to worry too much on setting up in the mill. More work had to be done to get the geometries into the right size for the real work.

After assigning the features to the inside and outside of the shape to cut, I cut up a piece of MDF and a piece of 4.75mm thick plywood on the bandsaw to 200mm square. The MDF will be the sacrificial base.

The wooden pieces were fastened to the mill table with a pair of 1-2-3 blocks to raise their height. Otherwise, I'll run out of Z travel before I can cut through the plywood.

Running the gcode. Lots of wood fiber was produced. I wonder if faster spindle speed will solve this problem. If yes, I'll mount the Proxxon IB/E on the Sherline as it has spindle speed of up to 20,000 rpm. Sherline spindle maxed out at 2,800.

The first part of the inside shape done.

Machining the outside. I was worried that I may hit the hold down clamp but all went well.

From this angle, you can see how close I was to the vertical bed. This has always been a dilemma for me; if I don't raise up the part, I don't have enough Z travel, but when I raised up the stock, I sometime do not have enough clearance for bigger stock... Sigh...

Job done. Quite some clean up to do with the sand paper.

One strange observation: though the image used for this is slightly smaller than the template, the machined plate is even smaller.

Comparing the part against the template.

More work to do on BobCad and I also need to figure out why the part turned out undersized as compared to the drawing in BobCad.

That's all for now. Fever is back. I need to wrap myself with thick blanket to sweat it out. Lots of things to do tomorrow. Can't afford to be ill.

- Posted using BlogPress from my iPhone

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