A new idea for keeping PCB material flat while milling artworks
The vacuum plate mentioned elsewhere on my blog serves me well when milling printed circuit boards on the Tormach PCNC440. It keeps the PCB material flat and makes the cut widths repeatable when using V cutters.
Idle hands and brain during social distancing has produced a possible solution that might be of interest and stimulation to others. It consists of a circular pressure ring that sits around the spindle chuck and tool. There is a second ring that sits on the spindle body connected to the lower ring with rods which have coaxial springs pushing down on the lower ring. The magic is to use mini ball transfer units on the lower ring to press down on the PCB and glide friction free around the PCB as the cutter does its stuff. The assembly is held in place on the spindle with 3 gripping screws. The downward pressure is adjusted by 3 screws that press against the spindle mounting frame.
The prototype was made using 3D printed rings. There is an image below. Apologies for the yellow PLA but finding any PLA at a decent price is very difficult in the present circumstances.
The idea seems to work and has produced some good consistent quality PCB prints. It does have disadvantages in that you need to have a larger PCB blank to allow for the larger footprint of the pressure ring. It is probably only of practical use for PCB milling but then the problem of flatness is less critical in drilling the board and routing the profile.
After a lot of editing I think the attached document will give an in depth understanding of how to use FlatCAM based on Version 8.5. The document is based on our experiences and a steep learning curve. We now have a repeatable process for milling PCBs from Gerber and Excellon files exported from a PCB design package.
The document may well have mistakes and we would appreciate feedback good or bad.
One of our group of ‘silver experimenters’ is building an Arduino based celestial camera tracker. This will be deployed in the garden and he needed all control to be routed back inside the house. The garden installation consists of a USB webcam mounted on a servo controlled platform all powered by 12V DC.
We pondered long on how we might remotely connect to the garden. The crucial thought was that the Arduino servo board was a two wire interface using the I2C format data exchange. Given that the USB needed four wires and the DC supply two wires we had a need for an eight core cable connection. It seemed like a length of CAT5 cable would do the job and we could elegantly use standard CAT5 sockets.
The PCB was designed in Design Spark and milled on the Tormach PCNC440 using FlatCAM.
There is a problem with running USB over more than 5m but I did some tests at 10m and all seemed fine which should be adequate for the application.
The breakout boards had a male and female USB connector fitted and the connections had to ‘cross over’ on one of the breakout boards to maintain continuity. We also paired the Data + and Data – connections with the +5 and Ground twisted pairs in the CAT5 so the Data + and Data – were not twinned together.
Nothing technically magic but a simple solution to a project need.
While I have been quiet for the last month or so there has been some intensive work by Dave and myself on FlatCAM. We are in the throes of doing a formal document to help others get to grips with the process and techniques for milling circuit boards.
I need to also put my hand up and admit going over to yet another dark side by experimenting with Arduino technology. This came about as a follow on to the work on silencing my Bill Smith Gravity Arm Gearless Clock. With the help of another colleague we have replaced the discrete timer logic board with an Arduino. I have learned quite a bit in the process and more details will follow.
Finally like many others round the world, myself and my wife are socially distancing ourselves at the moment but every cloud has a silver lining and this does mean I am spending even more time in the workshop doing ‘stuff’. It has also been a good time to look at the workshop and make plans to tidy, organise and structure things better. Some of the accumulated odds and ends are getting sifted and sorted and binned as appropriate.
Previously on Woody’s Workshop … I had spent time trying to get a consistently level PCB blank clamped to the tooling table ready to mill the traces using CNC. The results were not too bad but being anally fussy it left a bit to be desired, particularly if the board had a large area which magnified the variations.
Any variation of the level of the PCB top surface will produce variable width cuts when using a V shaped cutter. I had machined a clamping plate which was a simple open frame with a clamping step equal to the PCB thickness (1.6mm) and a sheet of MDF or hardboard as a sacrificial backing board.
Despite having more clamping screw holes than a magazine burst from an AK47 I still ended with the corners of the PCB being a few thou lower. Results were shall we say ‘variable’. I had reason to run a new prototype board this week and once again hit the same frustration. In the end it was a sit and look at it and have a think session.
The resulting revelation was maybe the sharp edges on the step are applying too much pressure ? What if I were to be more gentle with the clamping ?
I cut some strips of thin foam rubber and put this into the step such as to push down on the PCB. As a quick test I only fastened the frame down using the four corner holes.
Absolute magic. The PCB surface hardly moved the Hamer needle at any point on the surface. Milling result was an artwork to be proud of.
Issues – the current step on the clamping frame is meant to clamp to a hard stop based on the sum of the PCB thickness and the sacrificial material thickness. Adding the foam meant I had to do away with the sacrificial board. The frame step therefore needs to be deeper. The sacrificial material is essential to allow drilling to take place without breaking the drill as it runs into and potentially damages the tooling table. (For the board in question I drilled to only 1mm and then over drilled by hand off line to the mill).
So a worthwhile bit of experimenting and hopefully a better result going forward