Month: June 2017

New wire current contact

I have changed the way current is transferred to the moving wire. In my previous setup I used a carbon brush, which seemed like a nice idea, but the resistance of the brush turned out to be too high (1 ohm) which would introduce large losses and heat dissipation.

Instead, I now use a piece of HSS CO-8 very hard steel from a broken mill bit that I saved after a milling accident (never throw those away, they always come in handy!). The EDM wire drags over the steel making contact. The power is connected to the HSS piece by a silver soldered brass nut. I’ve seen a similar solution in Jaakko Fagerlund’s machine and Mariano also mentioned this solution in a comment on a blog post.

EMC Integration issues

I’ve tested all systems (wire control, water control, arc power, arc control, motion control) individually and they all work. Upon integration (putting everything together) I ran into EMC issues. This is something I expected that could happen. EDM machines are extremely noisy.

Upon activation of the arc, the wire tension control system gets noisy. The force control looks like it has a bad case of Parkingson’s and the display shows characters I’ve never seen before. The water conductivity display also gives the wrong value if the arc is turned on.

It figures, for the arc control enclosure I deployed every possible EMC countermeasure I know and it works fine.

For the wire control system however I’ve been less strict and it clearly shows. I thought I would get away with it, but the EMC from the arc supply is very unforgiving. So now I have to deploy several EMC hardening tricks. This is quite some work which I hoped was not necessary, which is why I omitted it in the first place (drive for quick results). I’ve learned my lesson, EMC counter measures on EDM machines are an absolute must. Here’s the list of additional work (sigh….).

For the mechanics:

  • Hook up the main frame to ground (that’s a no-brainer, forgot that)

 

For the arc power system:

  • Add EMC power line filter with high attenuation to block conducted emission to other parts of the system (was planning on adding this anyway, already purchased the filter but did not install it yet)

 

For the wire control system (I already implemented this whole list for the arc power and arc control systems):

  • Conducted immunity hardening: Change the mains input filter to a two-stage filter with a higher attenuation
  • Radiated immunity hardening: Connect all chassis parts together to a single earth point, grind away paint on the inside connection points to make sure the ground paths have a low impedance.
  • Radiated immunity hardening: Make sure the metal frame of the display (which is connected to the GND of the display) does not touch the enclosure by 3D printing a plastic bracket to mount the display in that provides good isolation with a creepage distance of a few mm.
  • Radiated immunity hardening: Cover the display hole in the chassis with a stainless steel see through mesh, that is connected to ground
  • Radiated immunity hardening: Make sure all cables are 100% shielded over the full length. This is going to be a challenge, especially the cables on the C-arc are difficult to shield over the full length.
  • Conducted/Radiated immunity hardening: All connector casings should be metal only and connected to the shield of the cable on one side (to prevent ground loops) on the other side of the cable the connector casing is then connected to ground via the chassis of the recieving end.
  • Conducted immunity hardening: Only ground the shield of the cables on one side, to prevent ground loops.
  • Radiated immunity hardening: seal any openings and/or gaps in the enclosure with copper EMC tape.

 

Pump control system:

  • Conducted immunity hardening: Change the mains input filter to a two-stage filter with a higher attenuation
  • Radiated immunity hardening: Make sure all cables are 100% shielded over the full length.
  • Radiated immunity hardening: seal any openings and/or gaps in the enclosure with copper EMC tape.

 

 

 

 

Pump control system finished!

I’ve finished all internal wiring of the pump control system and finished all cables from the pump control system to the pumps, float switches and water conductivity probe. Some small details left to do to finish the entire water system:

-Motor capacitor mounts

-Strain reliefs for cables of the water system

-Change pullup/pulldown resistor values from 10k to 470 Ohm, add decoupling capacitors (found a bug with noise. Inrush currents by motor on/off switching could cause a faulty reset trigger of an error condition)

 

Water system progress: Enclosure for pump control system

I found an old enclosure with the right dimensions for the pump control system. I machined and spray painted a new front panel.

The front panel will receive:

  • A power on/off switch
  • A switch for the de-ionisation pump
  • A start switch to enable the automatic running of the dirty and clean water pumps
  • A stop/hold switch for the automatic running pumps
  • 3 bi-color red/green LEDs to indicate the status of each pump
  • A water conductivity display
  • A running hours counter. I had one lying around and it might prove handy to estimate when the filters need replacing. When the filter really needs to be replaced will be indicated by the LEDs as an error. It is nice to have an indication when this will occur, the running hours counter will help with that. As a future update I might add flow sensors to the output of the pumps to check and report how full a filter is (pressure sensors won’t work as water is nearly in-compressible).

Water system progress: Start of pump control system

I’ve made an electronic design for controlling the dirty water and clean water pumps. Its simple yet effective. It could have been simpler by just using some relays, but since the pumps will switch quite frequently (especially the dirty water pump) solid state switches are a more elegant solution. I want to use some 10mm bi-color LEDs in the UI panel, to indicate the current pumping status, so I added suitable outputs for that as well. There is an opto isolated input to connect the motion control platform to the pumps, this will allow the platform to enable/disable pumping. Next to that, there is also an opto isolated output, so the motion control platform can act when an error in the pumping system occurs (global shutdown).

Here’s the schematic (click the pic for PDF):

Here’s the resulting PCBA on a proto board. I tested it and it works well.

The small 5V SMPS PCBA is salvaged from an old mobile phone charger.

I’m not sure if the deionisation requires automation. I’ll be using a OTS water conductivity sensor and display I found on Ebay. At first I’ll just manually operate the deionisation pump by looking at the display. If that requires frequent manual handling, I’ll figure out a way to automate it with a solid state switch and some modding of the display to obtain a switch signal for a given microsiemens setpoint.

Black anodization for C-arc

I’ve anodized the C-arc part that is going to be submerged. The C-arc is electrically isolated from the wire and is also isolated from the machine. To prevent electrochemical corrosion even further I’ve anodized the part. The oxide layer is a good isolator. The black dye finishes it off. It not only looks cool but also has a function. Damage to the oxide skin can be easily seen when the contrast is high.

Water system progress: mechanics finished

I’ve finished the first filter stage and all the water hoses that connect the pumps to the filter and main tank. I also finished the water level switches that will detect when the dirty water pump should turn on and off, and one switch that should turn everything off in case a filter is clogged so I won’t swamp my workshop when (preventing the first filter stage from flowing over). With those things done, the mechanics of the water system are finished.

The next job is to make a solid state switchbox that will control the pumps and that will signal the rest of the machine when to stop in case of a filter problem.

Water system progress: Frame finished

I’ve TIG welded a custom aluminum frame for the water system. It has 4 wheels to move it around the shop, since it will carry about 100 Kg of weight.

The motors are mounted to a subframe, which is suspended inside the main frame by rubbers for acoustic isolation. The rubbers are pieces of a bicycle inner tube.

I buy almost all my raw material from the salvage yard. The wheels were salvaged from some old machine and I purchased the aluminum for 3 Euro/Kg (scrapped new stock). I bought 12 Kg and ended up using about 9 Kg for the frame.

In case you’re from the Eindhoven area in the Netherlands, this scapyard is a goldmine for cheap raw materials: http://www.metaalhandelteunissen.nl/

 

Begin of construction new water system

The water system was the first thing I made for this machine. Now, ~1.5 year after I finished that system, I’ve gotten new insights into how to correctly filter. Next to that, the capacity of the work piece tank has gotten quite a bit bigger than I originally anticipated. Because of this, I’ve decided to start all over again and make a new water system. I’m re-using the water pumps of the original setup. These were salvaged from old dishwashing machines. The rest is going to be new.

I’ve sourced all the required parts. Including a gigantic 70L water tank (usually used for showers in camper-vans).

This is my old setup:

These are the parts for the new setup.