Best mig welder to buy for a complete beginner?

[Richard T]

OK so two questions spring to mind from this discussion so I'll ask them here rather than start a new thread :
1. It seems to me that both MMA and MIG welders are basically a current source, with a 'ground' clamp, and an electrode which is either a plain welding rod in a holder (MMA) or a more sophisticated "cable" which carries a gas supply and a conduit for the welding wire (MIG). So, if you're designing a MIG machine to sell, it is almost trivial to make it also do stick by providing a simple connector for an MMA cable, taking this from the current source before it gets to the MIG connector. Why don't more manufacturers do this ? Rohr seem to, and R-Tech, but it's not as common as you'd think. I'm looking for a "first welder" and I want to start with MMA to get the basics into my head, but like the idea of being able to do MIG later.
2. AC versus DC. Leaving out clever stuff like aluminium, people talk about loving traditional transformer technology, for reliability and simplicity - which is presumably AC - but also rave about the controllability and "lovely arcs" on DC with the newer inverter boxes. I've seen people put simple high-current rectifiers on the output of buzz boxes but that's not really DC is it - it's rectified AC, kind of 100Hz pulses. So, (a) is DC actually nicer to weld with, or it is just that DC usually means inverter which is more controllable; (b) does AC limit you in any other ways, and (c) does simply rectifying AC actually improve anything, and (d) Some people put large capacitors onto rectified AC but this sounds a little bonkers to my basic electronic brain because they're a completely uncontrolled current source and you could get truly huge current spikes across an arc. Are large inductances a better bet here ?
Apologies for the list, just trying to sort out all the Qs buzzing around my head !
Oh, and does this site have any sections for hooking up people looking for a day's one-to-one hands-on instruction, with people looking to provide that for a suitable sum ? feels like that would be a good thing to have !
Richard

 

[a111r]

R -Tech 180 for me
I have one at home as well as one at work
Very good on stick as well
Option of alloy spool gun
Even decent on scratch start TIG

An unused one was listed on Ebay today:

R-Tech MIG180 180A 240V Portable Inverter Welder - Blue 616320509934 | eBay

Condition is classed as "Used". BUT IT HAS NEVER BEEN USED.

www.ebay.co.uk

 

[paulo1974]

From 3 mm up a stick welder is the best budget bet. The mig option will be on the table as best budget if the material was 2,5 mm less. The mma welder will be quicker to setup.

 

[paulo1974]

OK so two questions spring to mind from this discussion so I'll ask them here rather than start a new thread :
1. It seems to me that both MMA and MIG welders are basically a current source, with a 'ground' clamp, and an electrode which is either a plain welding rod in a holder (MMA) or a more sophisticated "cable" which carries a gas supply and a conduit for the welding wire (MIG). So, if you're designing a MIG machine to sell, it is almost trivial to make it also do stick by providing a simple connector for an MMA cable, taking this from the current source before it gets to the MIG connector. Why don't more manufacturers do this ? Rohr seem to, and R-Tech, but it's not as common as you'd think. I'm looking for a "first welder" and I want to start with MMA to get the basics into my head, but like the idea of being able to do MIG later.
2. AC versus DC. Leaving out clever stuff like aluminium, people talk about loving traditional transformer technology, for reliability and simplicity - which is presumably AC - but also rave about the controllability and "lovely arcs" on DC with the newer inverter boxes. I've seen people put simple high-current rectifiers on the output of buzz boxes but that's not really DC is it - it's rectified AC, kind of 100Hz pulses. So, (a) is DC actually nicer to weld with, or it is just that DC usually means inverter which is more controllable; (b) does AC limit you in any other ways, and (c) does simply rectifying AC actually improve anything, and (d) Some people put large capacitors onto rectified AC but this sounds a little bonkers to my basic electronic brain because they're a completely uncontrolled current source and you could get truly huge current spikes across an arc. Are large inductances a better bet here ?
Apologies for the list, just trying to sort out all the Qs buzzing around my head !
Oh, and does this site have any sections for hooking up people looking for a day's one-to-one hands-on instruction, with people looking to provide that for a suitable sum ? feels like that would be a good thing to have !
Richard

1 The mig works as constant voltage and the stick welder as tig welder as constant amperage.
2-The traditional transformer unit has an ac output that is worst than a constant current (dc).
The inverter technology is more complex and more prone to failure that the simple transformer ,but the last don´t have a constant probe that checks the output ,so the welder can change the frequency to match an constant output .
There are also transformer rectifier that have an dc output and are better than the ac buzz box.

 

[hooblah]

Refer to my recent thread on the subject. If you're ONLY looking for longevity (100+ years), buy a transformer mig. For £250, an inverter mig beats a transformer in every other way. Theoretically there's more to go wrong, but I haven't heard of any breakdowns yet.

 

[Longhorn6120]

I've seen people put simple high-current rectifiers on the output of buzz boxes but that's not really DC is it - it's rectified AC, kind of 100Hz pulses.

All welders I've ever seen plug into AC mains, so to achieve a DC output (whether from the factory or as a DIY mod) I would have thought there has to be a rectifier in there somewhere. Plus there're likely smoothing capacitors in there to iron out the wrinkles.

Maybe I'm wrong, I often am.

 

[eddie49]

OK so two questions
1. if you're designing a MIG machine to sell, it is almost trivial to make it also do stick by providing a simple connector for an MMA cable, taking this from the current source before it gets to the MIG connector. Why don't more manufacturers do this ?
2. AC versus DC..... but that's not really DC is it - it's rectified AC, kind of 100Hz pulses......is DC actually nicer to weld with....Some people put large capacitors ..... a completely uncontrolled current source .... Are large inductances a better bet here ?

As paulo1974 said, the MIG welding process requires a constant voltage ( CV ) power source, whereas arc welding needs constant current ( CC ). Additionally, MIGs work at 15 to 35 volts and 20 to 200 Amps. For arc ( MMA ), the open-circuit voltage ( OCV ) is typically 50 to 80 volts, and upwards of 70 Amps. These differences are not trivial.

Using a transformer, these output characteristics are obtained by selecting the correct turns ratio, wire thickness and length, number of tappings, and magnetic core design. A single transformer-based machines can not fulfil both these roles, but inverters can. The feedback for the pulse-width modulation controller that drives the gates of the MOSFETs or IGBTs in the power section of an inverter comes from either a current-sense pickup coil on one of the output leads, enabling CC mode for MMA, or from a voltage comparator across the DC output, providing CV ( MIG ) mode. Inverter-based welders that have both of these control methods, plus a wirefeed assembly and Dinse sockets for electrode holder and work return cables are dual-capable.

Is DC better than AC? To obtain correct melting of the MIG wire and heating of the workpiece, the MIG torch needs to be positive. For this, the power source has to be direct current; swapping polarity 100 times per second ( i.e. alternating ) is not desired. Some specialised arc welding electrodes are also polarity-dependent, and require an MMA welder with a DC output.
Connecting a full-wave rectifier to the AC output of a transformer does provide DC - the resulting output is uni-directional.
Sometimes this is referred to as pulsed or pulsating DC.
Transformer-based MIG welders mostly have a DC output. ( Some of the worst imported hobby-level transformer-based "MIG" welders omit the rectifier...) Some transformer-based arc welders have a DC output, so they are no longer buzz-boxes. All inverter-based machines have a DC output, and in addition are more controllable.

Capacitors? These are added to smooth and fill-in the 100Hz peaks after rectification. The output is much smoother, with just a slight ripple at the top, almost pure DC. The OCV is raised to near the peak value ( 1.414 x rms ). Typically large high-current low equivalent series resistance ( ESR ) "computer grade" electrolytic capacitors are used, at about 250 - 300uF per Amp of welder output.
The current flowing if a charged capacitor is short-circuited is limited only by the ESR of the cap - 0.1 Ohm or less. When capacitors are installed in MIG welders it is common to wire a bank of multiple "small" caps in parallel - e.g. 5 x 10,000uF rather than one 50,000, so the total ESR is even lower. However, the arc in a MIG welder is not a dead short, it has resistance, which varies with the length of the arc, so peak currents out of the caps are limited. There is also an inductor in the circuit - see below.

Inductors? Most transformer-based MIG welders have an inductor, usually in series with the work return lead, on the output side ( after the capacitors, if they are present ). Having an inductor in the MIG welder output circuit helps with flow-out ( wetting ) of the weld pool. When the wire touches the workpiece there is a dead short. Since MIG welders use a constant-voltage power source, current rises very quickly. The inductor slows the rate of current rise, which also helps to protect the rectifier diodes. It alters the time ratio between the short, which melts the wire, and the arc, which heats the weld pool and the metal of the workpiece.
More explanation of inductance in MIG welding here:
https://www.twi-global.com/technical-knowledge/faqs/faq-what-does-inductance-do
https://www.weldingtipsandtricks.com/mig-welding-inductance.html
Professional-grade transformer-based MIGs may offer a choice of inductance settings, usually by having more than one Dinse socket for the work return cable. Inverter MIGs sometimes have a "variable inductance" control on the front panel. I can only guess that this potentiometer alters the hysteresis of the constant-voltage feedback circuit.

 

[Richard T]

Many thanks all for the comments on my questions above (doing MMA and MIG etc) - especially eddie. Understanding the electrical-level differences between the two processes makes things much clearer. I guess the great thing about inverters driven from very controllable transistors is that you can make the output do just about anything, provided you have enough power. I think a very common phenomenon when electronics its way into a previously "old-style" environment is that getting things basically working at a functional level is cheap and simple for the manufacturer, but making electronics that survive in a new and relatively hostile environment takes considerably more money and time. Consumers tend to be seduced by marketing and not good at telling the difference between cheap and good, so you end up with highly variable experiences and feedback. The great thing about these forums is they let you learn.

Can anyone recommend a good book that goes into detail on welding electricals and chemistry / metallurgy ? Not so much "here's how to lay a good weld" but "here's what's going on in the machine and in the arc" ?

 

[Paul OCallaghan]

If your budget will stretch to it, this would be ideal.