I still need to make the dust shoe setup for my machine, but the rest of the dust collection system is now complete. Nothing fancy - just a shop vac and a Dust Deputy cyclone plumbed up like this:
small diameter hose from machine → cyclone → larger diameter hose → vacuum
Home Depot had 18 AWG stranded bare copper wire for $0.15 per foot, so I picked up 25 feet of that plus a package of 22-18 AWG bullet connectors. The 18 AWG wire is small enough to fit through a 1/16" hole, so that’s what I drilled wherever it needed to enter or exit a component (2 hoses + the cyclone). I left a 4-6" pigtail at each entrance/exit and crimped an appropriate-gender bullet connector on the pigtail to make the system easily disconnectable when needed:
The wire inside the cyclone is just kind of there (not necessarily following the walls tightly). I did use a ring terminal to mechanically restrain it to make sure it was running all the way to the bottom before going back up and out. Here’s a shot from the underside:
To ground the wiring, I took it through a 1 mega-ohm resistor to the ground wire of a 3-prong cord that gets plugged in along with the vacuum. The terminal block that holds the resistor and wiring is hung from the vacuum’s top handle using a piece of coat hanger and some scrap wood:
Thanks! So far so good - I vacuumed the floor with it and wasn’t getting any static. I’ll have to test it with larger flow rates of particles next time I cut something on the machine.
The usefulness of a concrete floor as an earth potential will probably depend largely on how much moisture is in the concrete, cleanliness where a connection is made to it, and the contact pressure at that connection.
The idea with having a wire running along the entire dust collection path is that static charge can build up on all surfaces exposed to particle-laden flow and you want to drain that charge away from all those surfaces.
The materials are pretty cheap. Couple bucks for the wire. You can get a 10-pack of 1 mega-ohm resistors on eBay for a few dollars. Wire nuts can be used in place of the bullet connectors. The resistor could be soldered in instead of using the terminal strip. An old 3-prong cord can be used for connecting to electrical system ground or you can get a replacement plug for cheap and just wire to the ground prong like this:
I used to have static in the vacuum hose but since the installation I can’t feel it any longer.
It has been a while, but I believe this was in the instruction for the Dust Deputy.
I’ve always gone by the premise, “if it ain’t broke, don’t fix it”.
Should I have any future issues I’ll post it here or try and look up this post.
Thank you for your input. It may help others having problems with static.
“Air dried concrete has a resistivity in the order of 10 k ohm-m”
So the dust deputy (I assume it’s one of those with the metal can) grounded to a bolt in the floor will be leaking away charge through what will appear to be about 1/1000th the resistance of the commonly used 1M Ohm to electrical ground.
This assumes that your concrete slab is on the ground, not on an insulator or hovering.
That would give the DD itself a pretty good route to bleed away static charge.
As you say, if you were to have static problems on the machine, it might be worth doing more things, maybe a semiconductive hose to the dust boot, but the DD would appear to be, well grounded (pun absolutely intended)
Resistivity (specifically, volume resistivity in this case) is a material property, not an absolute resistance. Note the units of ohm-m being different from ohms.
To calculate a resistance, you’d also need to know the length and cross sectional area of the path that the current is taking. Path length is easy - it’s just the thickness of the slab. Area is trickier (if not impossible) to determine, as you really don’t know the effective area of the washer that’s in contact with the slab (or the area that the current is taking through the concrete). This gets into contact pressure, micro asperities in the two surfaces, cleanliness of the interface, etc.
Also, as you mentioned, the slab could have factors that further insulate it from earth potential, like vapor barrier plastic sheeting and/or a layer of gravel.
I think the washer thing could provide some path to earth ground, just not as consistent as hooking into the ground net of your electrical system.
I went and read the Dust Deputy site and it seems that they mould the plastic cyclone out of an anti-static resin so, yep, the grounding to dump static all seems to be sensible.
To avoid a ground loop which could cause additional interference and comms problems between your Shapeoko controller and PC
For safety, should you touch something AC live and the dust collection at the same time, if there is a large resistance to ground you are (slightly) less likely to be injured
The regular Dust Deputy DIY page (specifications tab) says it’s made of polypropylene and claims to be anti-static (if you mouse over the words “anti-static”, it has a pop-up that mentions surface resistivity of 10^9 to 10^12 Ohms per square). The material appears to be natural semi-translucent polypropylene, which leads me to question what sort of polymer additive they’re using to lower resistivity. Page 4 of this document says polypropylene (assuming unmodified) has a surface resistivity of 10^13 Ohms per square.
Oneida also has a static-conductive version of the Dust Deputy DIY, with the specs page saying 10^3 to 10^6 Ohms per square for that material. The black color of the material is indicative of the carbon additive used to achieve that lower resistivity.
It controls how fast the static charge can bleed off / discharge. Limiting discharge current protects sensitive electronics.
As Liam mentioned, it protects the operator by providing a higher-resistance path to ground. If you get in the circuit path between some decent voltage and ground, that high resistance limits the current through your body. There are many factors involved in electrical injury, but current going through the body is the major one.
