Woody's Workshop – Page 55 – Keeping the Grey Matter active

February 15, 2018February 5, 2020

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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February 14, 2018February 22, 2018

TIG and MIG Welding and a Jasic AC/DC inverter

We have had 5 weeks of our welding course so far and have focused on MIG only. It has been a steep learning curve but we are now able to produce reasonable looking joints. Coping with so many variables takes some getting your head round.

Talking to the other guys on the course they all commented that for what I am doing I might find TIG more appropriate. The tutor on our course suggested that TIG was like using one hand to pat your head while the other hand was rubbing your stomach and you were jumping up and down all at the same time. I can now see what he means.

After many hours of YouTube watching I have invested in a Jasic TIG welder that does both DC and AC so I should be able to do aluminium joints. It is a nice piece of kit and I am slowly learning its functionality.

So it’s back to practice and more practice but I quite like TIG. It is gentle and focused without all the sparks you get with MIG. It reminds me of my school days when I was doing gas welding.

No projects as yet but it is only a matter of time.

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February 14, 2018February 19, 2018

Magnifier Lens Scratches and Polywatch

Working on small items requires me to use optical magnifiers such as the Rolson one with swap out lens. These inevitably pick up scratches and other marks.

I recently bought some Polywatch polish from my clock parts supplier to polish a plastic clock dome which was looking pretty rough. Polywatch transformed it to the point of where it looked almost new.

You can buy it on Amazon.

Today I tried Polywatch on my favourite Rolson Headmagnifier lens and I can now see what I am doing once again. Worth remembering.

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February 6, 2018February 19, 2018

Finishing the surface of 3D Models

Over Christmas I had 3D printed a clock frame for my granddaughter’s birthday. The size ended up being three separate prints which I later glued together. All three prints were done ‘face down’ on the Sindoh bed and all printed on a raft. Once the raft had been removed the finish was not very elegant as it showed the striations of the raft on what would be the viewed surface. The print was in PLA and I initially did some tests on scrap prints to see if I could rub a surface down. This was not an ideal approach and I was not too keen to start on the clock in this manner.

Next idea was to use Gorilla twin pack glue cut with Meths. This created a very runny adhesive mix that could be brushed onto the surface of the print. Surprisingly enough despite being ‘modified’ the glue went off and was quite hard to the touch. It did not rub down all that well.

Reading up on the net there is a product called XTC-3D which is made just for this purpose. It is expensive but does stretch a long way in use. It is a twin pack mix in 2:1 ratio and once mixed it has a setting time of around 20 minutes so you need to get your act together and be well prepared. The makers recommend using a wide foam brush to apply the compound. This was quite good in that it ‘scraped’ across the tops of the striations and the filler settled like levelling compound does on a floor.

I left it to harden off overnight and then rubbed down wet with 400 grit paper on a flat surface. This kept the dust damped. The finish was excellent. I then undercoated and glossed with acrylic spray paint.

The final clock frame looked very good with no obvious signs of the printing artefacts. While pricey, I would recommend the XTC-3D product for this purpose.

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February 5, 2018February 19, 2018

Another Haimer tip bites the dust or at least the mill bed

When I was at NYC CNC I saw that John was selling posters as shown below.

I didn’t realise the significance until I started doing CNC …..

I had been doing OK on breakages of late then like buses, two came along at once. One was excusable (to my conscience) but the second was a disaster always likely to happen.

I had the Haimer held in a collet in the Myford VMB manual mill. This means you need both hands occupied to contra rotate the collet and spindle when locking and unlocking. I just overdid the realise pressure and the Haimer dropped out onto the mill bed. Crunch, expletives, gloom, expense. The tips are not cheap at GBP20+ each but Amazon do sell them.

There is the same issue to a lesser extent on the Tormach when releasing the power drawbar even though you are holding the Haimer the sudden momentum of the drop from the collet can sometimes catch you out if the mill table is close.

While discussing my gloom with a friend he suggested putting some sort of protection around the tip when loading and unloading from the spindle. The Haimer is not an ideal shape for gripping something along these lines given its curved surfaces. However after some thought a solution was found. I drew a simple cylinder in Fusion that was 40mm long, 38mm OD and 32mm ID and 3D printed it. Note these dimensions are for the Haimer FH 3D Taster version.

The cylinder sits over the probe and rests on the outer rim of the tip mounting. The 32mm ID is just tight enough to squeeze the Haimer rubber gasket and so hold the tube in place.

Not only is this useful for mounting and dismounting the Haimer but it also works as a general protect guard when not in use.

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