I recently needed to cut out a square hole, notch corners, and carefully place some tapped holes in a plate for the SLA printer that I am building at home. I have a CNC machine tool that I built from a kit. It has a 2.2 kW spindle with a 3 phase driver and voltage controlled RPM. I have the output of a Gecko stepper controller running motors at 48V, and also controlling the spindle speed. I have used this machine for artistic wood carving among many other jobs.
The plate that I machined holds the resin tank for the SLA printer that I am building at home. The frame of the printer structure is built from 8020 aluminum hardware, so I needed 1 inch x 1 inch corner notches removed. I added tapped holes for two machine screws to affix the resin tank, and two more holes to bolt down the plate to the support frame. All of this could have been accomplished manually, but it seemed like a good application for a CNC cutter.
The reason I am posting a blog about this job is that I added a laser pointer to the spindle mount to enable precise location of the cutting tool. There was a through hole available on the spindle mount, so I added a long bolt and nut that protruded downward but stopped a few inches short of the cutting end of the bit. I strapped a laser pointer on to this bolt with three cable ties, then epoxied it in place in addition to using epoxy to fix the focus of the laser. The laser is in focus near the height when the cutting tool touches the work surface. If the pointing direction of the laser is not identical to the cutting axis, as long as I use a similar height each time for alignment, any offset due to pointing error will be within the tolerance of the overall alignment system i.e. the best I can hope to match the laser dot center to the cutting bit tip is some fraction of a millimeter, and pointing offset with working distance within a centimeter will be less than this error.
I marked a fixed test piece with a pencil at the center of the laser focus. I then moved the cutting tool so that it was centered on the pencil mark. I made a shallow hole at that point and re-checked the laser position with the same X and Y offset. Once I had the offsets recorded, in all future circumstances I could use the laser to mark the starting location of the CNC cut that I wanted to make. I marked this point as X, Y = 0, 0, and then used Mach 3 to move to the offset location so that the cutting tool was precisely at the same origin. I could then re-zero the tool and begin a new cut.
This solution occurred to me after using a commercial CNC mill (LPKF) that had an integrated camera cross-hair system. The system automatically compensates for the offset between the cross-hair center and the cutting tool when cutting begins. My solution was available for the price of a laser pointer and a few accessories. I can find edges and corners by watching the laser spot disappear, and I can also align a work piece by marking an edge and making large excursions in X or Y, noting that the edge is still indicated by the laser spot.
Using this system, my CNC cuts were very successful on this mounting plate. The hole for the resin tank is just the right size and shape, and the mounting hardware fit perfectly.
Note that I mounted the plate for cutting by simply screwing in to an MDF board bolted to the CNC tool bed. I did learn that any fluid touching this material causes it to swell immediately. I used alcohol as a cutting lubricant. I will need to flip over the MDF board for the next job. On the other hand, this stuff is really inexpensive, and makes mounting of a large part very simple. If the cutting tool goes deeper than the work piece, and it did several times, it really does not matter very much as the MDF is essentially disposable.
Here are a few pictures of the arrangement.
Laser pointer indicating edge of plate
Finished corner cut
Cutting tool in action using laser indicated corner location