Cutting shapes out of 4mm gilding metal

Cutting shapes out of 4mm gilding metal

For the enamel cross project I needed to cut out sections of the cross that could be placed together with a high degree of accuracy. The shapes are quite complicated having an inter-locking jigsaw-like joint, see below. For this purpose I decided to use my CNC Sherline mill and program some tool paths.

 

In this post I'll document the centre of the cross, which is the most complicated of all the sections.

I wrote the program for the tool paths directly in gcode, I just don't have any CAD or CAM software! Luckily for me I know a bit of maths, so the shape sections of the cross were abstracted, mathematically, and described by a set of parameters. This way I could reference the parameters for all the pieces, ensuring all the sections were consistent. For this particular section, I ended up writing a function for doing one side and then found I could do a 90 degree coordinate rotation (that's the g10 l2 p0 x0 y0 r90 bit) and re-use the same function to do the next side, woo-hoo! I'm exposing my geeky side! Here's a sample of it:

gcode example

The program that finally materialized follows tool paths that incrementally get closer to the final shape, here's a schematic of the tool paths where each tool pass gets 1mm closer to the final shape. In the actual routine I used 0.1mm increments but they're too close to show up in a photo, so this is just to illustrate:

tool path diagram

 

The cone is the location of the cutter and the lines show the path it will take.

In retrospect this program was not the best idea since the tool spends a lot of time not cutting any metal at the start of the routine, with only the inward semi-circle making any contact. A better approach would be to shave off the excess metal in straight lines first, then perhaps do the inward semi-circles next and finally finish with the routine above, but just without so many cycles. Anyway it worked but next time I think I can do better!

After checking the machine backlash, making fixing holes in line with the design, securing the blank metal piece to the milling table, making sure everything is aligned and axis zeroed etc, I ran the routine. It's always a bit nerve racking at first since, if anything is wrong, the cutter can easily crash into something. In this case I'm using a 2mm diameter carbide cutter and any crash results in the cutter just snapping off...as I found out! Here's the mill in action cutting out the cross middle section:

sherline mill cutting

I used a feed rate of 300mm/min and, as mentioned above, a depth of cut of 0.1mm. The machine coped very well with this making good sized chippings.

The joint between sections of the cross have a 1mm flat section in between the two semi circles. The largest cutter I could use was therefore 2mm diameter. However this still leaves a radius whereas, for the joint to fit snugly,  I need a right-angle; so after the routine was finished I remounted the piece to cut the right-angle:

cutting the right-angle

And then finished off by hand:

filing by hand

The other sections were done in a similar way. When my camera recharges I'll put a photo of all the pieces together with a little gap in between each piece to highlight the join. Hopefully I'll remember to do that!

I did remember! Here it is:

cross pieces laid out showing gap

And here's with all the pieces together:

cross showing all the pieces and the joins

Pingbacks are closed.

blog comments powered by Disqus