Dorkiphus.net - Any EE/tool geeks with a Miller Mig machine?

This is a (long winded) build and install description of a post flow controller for a Miller 135XP Mig welder. Some time ago I made and used a simpler system for flow control while doing a bunch of spot welds. It was a build in 10 min and black tape to the side of the box kind of thing and was disassembled right after the job was done. I found that with some gas flow control you can make Tig like stack-O-dimes welds on very thin metals with a Mig. It also seemed to reduce the heat affected zone and warping by hand trigger pulsing.
I designed and built this one after some folks mentioned an interest in having this kind of control on the simpler Miller Mig machines. As built you can weld in normal mode, with preflow only, post flow only or full control using pre and post flow. After a quick look at the wiring diagrams of some of the other standard Miller Mig machines it appears that this device can be used on them with little or no change other than how it is hooked up.
Please take note. There are many ways to do what this device does and this is only one way. I used the solid state cube timers rather than true process controllers as the timers are, simple, easy to find, robust and inexpensive. The only thing I can guarantee is it works for me on my machine. What you do with/to your machine is up to you. Let the smoke out of your welder don’t come crying to me, you were the guy that made something he found on the internet.

PARTS
1 One Single pole single throw toggle switch
2 Two single pole double throw toggle switches
3 One On Delay timer. Granger Part# 2A561*
4 One Off Delay timer. Granger part# 6A857*
5 Two double pole double throw 120V relays
Granger part # 5CJ04* and two bases part# 5KN18*
6. Something to mount everything on including the switches.
7. Wire, male and female 1/4 spade crimp connectors.
8. Zip ties to keep the added wires out of the cooling fan.

* There are less expensive timers than the Granger sourced components that are the same in function available from HVAC supply houses. I used Granger items to ease others finding similar components (and I had them laying around. ) All told this is about $50 from Granger.

Notes on function.
Preflow works by delaying the start of the wire feed motor. The welding control contactor will close and the filler wire will be hot but the wire feed motor will not run until the On Delay timer cycles and the preflow relay closes. The wire motor will then run at the panel set speed and will stop as soon as you release the trigger as normal.
Post flow works by giving timed 120V power to the gas valve after normal control power has shut off. The relay marked isolation or post flow works as an isolation relay to prevent the post flow system from bootstrapping the preflow side back through the gas valve control.
One down side to the cheap cube timers is that the time adjusters are small and easy to mess up. I recommend that you find a small flat head screwdriver that fits the little adjuster just right and leave it with the timers. Be careful when turning the timers up and down. I found that once set to average delay and postflow times I rarely need to adjust them. Unlike Tig, where a function like post flow is cooling the tungsten based on amps, in Mig all you are doing is keeping the weld pool covered until it solidifies. 1 or 2 seconds of post flow is more than enough for most applications. Big welds on smaller parts that load up with heat might need a bit more. .5 seconds of preflow seems to be enough to produce a shield gas envelope in most cases. If you are running only pre flow set the post flow timer to .5 seconds or it will interfere with timing via the isolation relay being energized until the Off Delay timer clears.

Notes on building and mounting the device.
I apologize for only posting a picture of a hand drawn wiring schematic.
Be sure to keep an eye on the switch toggle functions and on the relay connections. All wiring connections can be made via the relay base connectors as they are double lug. As an example you can connect main power to the isolation relay then to the common of the isolation switch and on to the preflow timer by using the lugs only.
No holes need to be drilled and no wires need to be cut to install. You can remove the device and leave no trace that it was there. I used a high bond double stick tape to mount the timer inside the wire feed compartment. I have a plastic cover (not shown) that protects the timer from getting shorted out if the filler wire bird nests.

Notes on hookup.
Splice into 120v power at the on/off switch for the Uninterrupted power. Add a 3 amp fuse if you like.
Neutral is easy to get at the contactor on the 120V machines as there are extra 1/4 spade taps.
The easy place for the trigger controlled 120V is at the gas valve. Remove the 120V wire from the valve and use a male 1/4 spade connector to connect the Interrupted power wire.
Connect the gas valve wire from the isolation/post flow relay to the now open spade on the gas valve.
The wire motor is controlled by interrupting the power to it only. The wire speed control system is not changed and the wire motor does not get power from the preflow system. Remove one wire connector from the wire motor and connect it to one of the 2 wires from the preflow control using a 1/4 male spade connector. Connect the other wire from the preflow control to the now open male spade connector on the wire feed motor.

Notes on bench testing.
Main power switch should pass trigger power to the preflow timer and isolation relay coil in On position and to the gas valve in the Off position.
Preflow switch should short across the wire feed motor control wires on the Off position.
Post flow switch should interrupt the trigger from the preflow relay.
Gas valve control should go hot with main switch in Off position.
Gas valve control wire should go hot as soon as trigger is hit and hold on after trigger is released and wire motor stops for Off Delay timer setting.

Remember you saw this on the internet and it was designed by a holder of a GE-Degree not an EE-Degree.