Using D/P MIG as MMA

[dafydd]

Sorry about this but my posts seem to be longer thany anyone elses.

Anyway, I was trying to flog my Sip Migmate Pro 170 to a mate last night. When he asked "does dual purpose mean it can do stick as well as mig"?

It suddenly dawned on me that if I modified the machine by puting a dp/dt switch on the thing to permanently switch on the trigger and switch off the wire feed you could use the +ve & -ve terminals on the front for stick arc welding.

We tried this by hooking up arc cables to the terminals, loosened the wire feed roller and jamming the torch trigger on and on full power it happily stick welded 5mm plate.

Thousands of welders must have tried this before now, but does anyone know what the consequenses might be? Can a 170amp mig transformer & rectifier designed to melt 0.8mm wire withstand melting 3mm rods?

Any advice would be appreciated as I don't want to do this mod only to find that after ½ an hours welding the machine is completely burnt out.

 

[irondarren]

The difference between mig and mma is voltage. There are units on the market that run always mig tig and mma, normally inverter based units so its easy to change base voltage. As for what damage it will do to a normal transformer machine i have no idea.

 

[rtbcomp]

The advantage would be that you'd get DC welding current.

IIRC MIG untits produce a higher voltage than stick, MIG current is limited by wire thickness and speed, rods are a lot thicker then wire so maybe the MIG unit will try to supply too much current causing an overload.

If the transformer and diodes are thermally protected you may be OK.

It's only a SIP - run it to destruction and tell us what happened!

 

[Drains]

Dafydd, the basic difference is that MMA is constant current, variable voltage, depending on the distance between rod and workpiece (and coating type), whereas MIG is constant voltage with varying current, depending on gap, wirespeed and wire dia.

If you google "MMA drooping curve", or "MIG flat curve" (you can include "characteristic" with those too), there are lots of articles describing the difference, how they work and why. I came across a comparative article the other day which spelled it all out beautifully, but I can't find it now, sorry.

Sorry, I couldn't tell you if you're running a risk of burning out the transformer. Possibly not if the thermal cut out works. For MMA, you won't be getting the ideal volt/amp characteristics, but nice idea, and if it works, why not?

BTW, what diameter electrode were you using on your 5mm plate?

 

[shenion]

Si is right, the curves are different. You could also damage the machine. There is a difference in a mig wire momentarily touching the work and causing a high-current load than a heavy stick electrode touching work.

You might be able to get away with it. If you managed to get a rod to stick for a few sec, then you might blow the rectifiers before the thermal cut of has time to heat up.

You should be able to set the machine at is highest voltage. Then make a variable saturation coil to control the current.

BTW, stick machines produce a higher voltage (35-45V), my mig MMA/TIG beast outputs 90V with no load. Drops quickly under load. MIG is usually 12-20V for small units have very little drop.

I have a big CV power supply with adjustable slope (8-different taps on the reactor). Would be interesting to try it on stick on the steeper slope setting.