Crazy Idea?

4DThinker said:

If the toolpaths had been projected onto the parts before we cut them we would have easily spotted this misalignment.

Click to expand...

This does exist as a commercial product:
http://www.carterproducts.com/2d-lp-hfd-laser-projection-system

Pretty pricey though.

My solution for this is to create a simple “outline” toolpath that shows the extent of the cut area (i.e. offset the cutout profile to reflect the diameter of the tool I will use for cutting the profile). I create a simple tool path cut along this outline. I name this toolpath “clearance”.

On my cnc I place my stock. If starting with rectangular stock, I usually have it butted up against dogs in 2 directions (X=0, Y=0). I usually have my clamps loosely in place.

On my machine I have the ability to leave my VFD/Spindle powered off while the rest of the machine is powered on. Leaving the spindle OFF, I chuck a laser in the collet. Now there are $125 lasers designed for this (Laser Center/Edge Finder), but I always prefer my cheaper ways of doing things. Those of you who have rifles may have used a boresight laser to do an initial zeroing of a scope on your rifle. It so happens that the diameter of a .300 win mag round (.489) chucks perfectly into a .500 collet. You can find this type of laser boresight for about $20 on Amazon, I found mine for less than $10 on Walmart.com. I chuck my “bullet” laser into my spindle and run my “clearance” toolpath, usually a few inches above my stock (the laser “bullet” is much shorter than the commercial laser center finder, so it works better if you have thick stock or limited Z height). As the laser travels around, I can make sure it doesn’t hit any clamps. I tighten my clamps and am ready to cut.

I have never (yet) cut into a clamp. And I sometimes tend to use nearly all my material.

The site for the commercial laser center finder has some additional uses that also work with my $10 version, namely verifying that my adjustable angle table, and adjustable fence on it are at a perfect 90 to the spindle axis. It is also useful to line up the center point if that is what you use for the reference on your carving.

The only problem I have with laser projected dots or an X is precision. They are fine when a part is being cut out of a larger board and there is plenty of room around the part. Much of what I cut is for joinery where a few thousandths of misalignment can be seen and felt when a joint is assembled. I tried out a laser X that was intended for drill presses, except I mounted it on my CNC's router body. The center of where the red lines crossed was easily 1/32" wide though when best focused. I can get within .0025 just using thin paper between bit edge and wood.

Thinking about how Glowforge solves this problem I realize that they have a lid which always lines up in the same place over the project bed. A camera in that lid has known coordinates relative to the bed, so any pixel in the image it captures can be mapped to a known x/y location. No lid over the CNCs I use, and they are all on casters so we can move them easily to clean underneath. Mounting a camera on the ceiling above will not work for reliable/repeatable image to coordinate alignment. I've seen microscopes mounted next to the router that let you see when a mark or corner on your project board aligns with an X in the image. Offsets are then used to tell the control software where the bit is relative to what the microscope sees. I suspect pretty good precision can be found using this method. I'd invest in trying it out if I had any clue how to incorporate the microscope image into linuxCNC.

4D