Stretching FlatCam and PCB milling on Tormach PCNC440

Tormach M Code Expansion Card

I have got a project underway to use the Tormach USB M Code expansion board in association with an A axis rotary table.   Details of this will follow in due course.   The expansion card when added to a Tormach mill allows the operator to embed M Codes in their CNC program which will operate up to 4 dry contact SPCO relays or accept 4 inputs as handshaking acks.

My Cabling Masterplan

As part of this project I need to have cables from the USB expansion board to various devices and in a rush of blood to the head decided to use standard readily available Micro USB cables for this. Now the cable connectors are pretty small and the PCB mating socket is even smaller with its 5 connections.   Glibly overlooking this I asked a colleague to produce a PCB layout for the connector to a breakout connector strip.   Dave duly produced a layout and a scratching of head resulted.   How was I going to produce the PCB and how was I going to solder to the connections assuming I could see them ….

FlatCam to the Rescue

Elsewhere in my blog there is mention of the use of FlatCam to create a CNC GCode listing from PCB Gerber and Excellon files.   This program works really well and many successful PCBs have been produced but I have never attempted to mill such fine PCB tracks. A number of problems needed to be addressed to make this successful.   The PCB sheet needed to be held very flat on the PCNC440 tooling table and the correct milling tool with its associated feeds and speeds needed to be chosen. In the past I have used strips of aluminium to fasten the PCB blank down on the tooling table.   This is never perfect and leads to variations in the pressure around the edges of the board.   With single sided PCB there is a natural curvature of the board material as a result of the surface tension of the laminating process.  A single sided blank has a concave surface on the copper side.  I needed to create something more repeatable.

Milling Window Restrictions

Before I bought the Tormach PCNC440 I had a discussion with John Saunders at NYC CNC and he recommended going for the biggest machine I could fit in my workshop.   I could have squeezed the 770 in at a push but I would have had to sell off my Myford VMB which I was reluctant to do.   My order therefore went through as a 440.   With hindsight this decision has been justified on two counts.   I rarely need a larger working area than the 440 offers and the VMB gets used very regularly for quick jobs that don’t justify CNC.   This project was an exception. I wanted to make a frame that would clamp the PCB blank down onto the tooling table.   In order to get the maximum working area for the PCB blank the clamping frame would have to sit outside the machining area.   How was I going to manufacture it ? Fortunately my tooling plate was designed to have a mix of M8 clamping holes and 3.7mm tooling holes and I was going to use this to advantage.   The clamping frame would be symmetrical.   By adding some matching tooling holes in the frame I could cut just over half of the frame and then flip it round 180 degrees and cut the second half. Here is a picture of the CAD showing half of the machining on what will be the underside of the plate when in use. The outer holes are for the M8 clamping to the table and the four smaller holes are the tooling holes.   Being tight with my materials I did not want to just mill out the centre of the plate and have a mountain of swarf (chips).  Instead I designed it with two slots as shown,  one for the clamping surface and one that almost cut through the stock.   The partial cut was to ensure the central piece did not flip out once cut free and damage my cutter. First one half was drilled and cut and then the plate was rotated 180 degrees and the second half cut.   This left the central island just held in place by less than 0.5mm of material.   This was easy to hand cut through to liberate the central area.   The plate was then turned over and the cut edge cleaned using the same tooling position and doing the same 180 degree rotation. To my surprise the rotation process on the tooling pins worked very well with only a minor step transition at the overlap point on all cuts.  This was probably more down to my 3.7mm tooling pins being not quite concentrically turned from 4mm silver steel. With this finished I now had a much more robust clamp for the PCB material.   I had made the clamping step 4mm deep so I could put sacrificial backing boards behind the PCB being run.   This would allow drilling through as needed.   Checking the flatness of a clamped PCB blank with my Haimer showed variation of a few thou in the top surface of the PCB Z position. The worst case variation in Z was at dead centre where the PCB’s natural bow was most dominant.

Tooling and Feeds and Speeds

The next problem was the milling tool and feeds and speeds.   I experimented with various V shaped routers but was not happy with the results.   The 5 thou tip on a 10 degree V tool was incredibly fragile.   Also because the tool was V shaped, any residual bow on the PCB surface lead to a variable width cut.   In the end I opted for Think & Tinkers 15 degree, 2 flute tapered stub (P/N EM2E8-0051-15VC).  This has a 5.1 thou cutting tip which is parallel for the first section so depth variations have no impact on the width of cut.  I ran the program at 10,000 RPM (PCNC 440 maximum) and at 150mm per minute feed rate. The PCB does not look particularly beautiful after milling as there are burrs and shavings present but a gentle rub over with a fine wet and dry removes this and leaves a remarkably clean cut tracking.   The images below show some of the results.   The fine tracking for the USB connector connections is shown on the microscope with a scale for reference.   This shows the five fingers occupying 120 thou with fairly similar track to gap widths of around 15 thou. So now I just have to solder the connectors in place …. I will let you know how it goes.

Overview shot of the clamping plate in position on the my tooling plate on the PCNC440
Finished clamping plate in position on the Tormach PCNC440 holding down a 6″ square piece of single sided PCB.
Tracking on the USB micro connector mounting
Zoom shot on the USB connector tracks with the graticule giving an idea of scale (small divisions are 0.5mm)

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Milling Circuit Boards Update

I have made some good progress on taking a PCB design Gerber copper and Excellon drilling files into CNC.  I think it is worthy of a full write up but while that gets put together here are some comments.

First of all the conversion process using FlatCAM is very straightforward and I like the fact that you can default save your GCode startup and end routines along with other default settings.  Note that I had to scale the drilling data by a factor of 10.  Apparently this is not unusual.

The fun starts once you have code ready to run on the CNC.  The board design I was working on was a single sided copper design.  Single sided board tends to always have a curvature with the copper on the inside of the curve and the fibre glass outside (if you see what I mean).  This is probably the manufacturing process with the copper and its adhesive ‘pulling’ the board.

Double sided PCB tends not to be so bad in this respect and the effect is balanced out by the two coatings.   My board was therefore much more bowed than a double sided one.  (Incidentally FlatCAM allows for double sided board designs).

If you think about the geometry of what is going on it is critical to make sure the PCB material is flat on the milling table.   The greater the included angle of the milling cutter tip the worse things get if there are variations in surface height.  A height variation equates to a widening of the tool cut.  See the image below. (Not to scale).

I initially used 6mm MDF as my sacrificial backing board to protect my tooling table.   When I checked the MDF for flatness with my Haimer I was disappointed with the result.  Increasing the MDF to 12mm made a huge difference and good enough for the purpose.   This could have been a different manufactured MDF so the change of size is not definitive.

Initially I clamped the PCB to the MDF with a number of woodscrews around the periphery.   On checking with the Haimer this was not good with visible variations that I could impact by pressing on the PCB surface.

Next step was to replace the woodscrews with strips of 10mm square aluminium with a 1.5mm step on one edge.  These were screwed to the MDF on all 4 sides of the PCB blank and this dramatically improved the flatness to a point were it was adequate.  Pressing the board surface did not change the Haimer readings.

Flatness having been solved I addressed the cutter problem.   I had ordered some 10 degree included angle cutters from China but while they were in transit I got to talking with Think & Tinker in the US.   They were incredibly helpful and suggested that I should consider a 60 degree included angle cutter with a 5 thou tip.   They also suggested I try their lubrication to improve the cut quality and to also help protect the tool from wear.   Their tools also come with a fixed collar which means you can change out the cutter without having to reset your Z zero.

This 60 degree cutter worked a treat and the results were startlingly good.   I did not use the lubrication from T&T but instead used my normal FogBuster fluid (QualiChem ExtremeCut 250C) on a gentle repetitive puff.  This seemed to work and kept the dust damped as well as improving the cut.

While I could run the spindle at up to 10,000 RPM, I kept it down at 6,000RPM with a cutting speed of 3″ per minute (75mm).  I had a Z clearance of 0.1″ and depth of cut of 0.005″.  (Sorry for the mixed dimension standards but PCBs tend to be designed in Imperial but I prefer to work in Metric).

After the milling of the copper was complete I drilled all holes at 0.6mm (24 thou) using a carbide drill sourced from Drill Services of Horley (UK).  This was simply a change of tool, registering the tool length and loading the drilling GCode produced by FlatCAM.   The drilled holes were spot on dead centre in the copper lands.

In closing I would like to say how impressed I have been with the Tormach.   I had milled the copper one day and switched off for the night.  Next day I switched on the mill and absolute referenced XYZ and put the drilling tool in the spindle and hit go.  The holes were smack on dead centre in the lands without having to tweak anything.

It has been an interesting challenge that my friend had set me and he has gone away with a good looking PCB and my knowledge base has improved which is what it is all about.

        

A more detailed write up to follow.

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FlatCAM Update and experimental copper cutting

I’m quite pleased with progress converting GSpark PCB gerber files to CNC with FlatCAM.   The conversion process is straightforward and the resulting GCode looks OK.

I have tried cutting copper using the cutters I have to hand but quickly realised I need to get some much finer ones in carbide.  My tests with modified dental burrs does not work or at least not for very long before the burr goes blunt and the cut width degrades.

I have ordered some 10 degree included angle 0.1mm wide cutters but they won’t be here until February.

The picture shows two runs.   The left hand run used 30 degree cone shaped burr and the right hand run used a modified teardrop burr.   The initial cut on both was where the stars are.

The left hand cut left severe copper burrs which were easily removed using a scalpel blade flat to the surface of the board.

The right hand side was clean of all burrs but gradually degraded in quality as the cutter became blunt.

The teardrop burr was ground to half diameter in an attempt simulate a more normal engraving cutter profile.  I must have drawn the temper in the grinding process.

More experimentation needed once the better cutters arrive.

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Welding and being well and FlatCAM

Welding Course

Despite being laid low with flu since New Year I’ve attended two of the evening welding classes much to the contrary advice from my wife.  With hindsight she was right – I should not have gone the first night such was my state of health.

The focus so far has been on MIG welding and I am slowly getting the feel of things.  Too many variables to start with what with power, angle of attack, closeness to the job and of course wire feed rate.

How anyone did welding without the modern auto dimming helmets astonishes me.   They are amazing.

Not much else to report given the state of health but clearly I have now started to creep back into the workshop so health and state of mind must be improving.  Made a note in the diary to get a Flu Jab next year.

FlatCAM

Anyway more interesting stuff – my friend has asked if I can mill a prototype printed circuit board on the Tormach for him.   My initial thoughts were to bring the design into Fusion and do the CAM there.   However a conversation with a friend in Namibia lead me to FlatCAM which is a wonderful little program.   It takes the Gerber files from the PCB design package and converts them to GCode.   It is simple to use and produces nice code listings.   It gives you options to add your own GCode initialisation strings.    There are a few YouTube videos out there if you are interested.

More results to report when we have milled some copper.

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