inverter welders --what are they?

[brian mitchell]

Hi,

I'm currently trying to do some necessary bodywork repairs for an upcoming MOT and having some difficulty because the arc welder I have

--even on its lowest setting-- keeps blowing holes in the metal (to think we commit our lives to these rolling biscuit tins!).

I can't run to a professional MIG welder but had a look at what else is available. I had in mind a no-gas MIG welder from Screwfix but find its lowest output is 60 amps, which is higher than the 45 amps my current welder goes down to, so that doesn't seem useful.

What I also found was an inverter welder with variable output from 5 to 90 amps, which seems a much better bet, but I don't know what an inverter welder is. The info says it has an input of 230 volts at 10 amps, so what is the inverter business?

Can anyone guide me through this enigma? Or anyone experienced at welding thin metal give me the benefit of that experience? MOT rushing up towards me at alarming rate!

Many thanks,

brian mitchell

[Andy Burns]

[John Rumm]

Firstly I am no welding expert, and there are sure to be people along shortly to give you better information - so take this as background info only.

Arc welding and car bodywork is so "not the way to go" - for the reasons you have found. It *can* be done by someone very skilled at it - but it really is doing it the hard way.

MIG is far better suited to the task, and is what is most often used for thin metal repairs. Don't read too much into the current - mig units often have far less control that the typical arc ones, but that is less relevant since you can vary the wire speed as well, and you don't have the range of electrode sizes to deal with.

Rather than using a 'kin big transformer to step down mains to a low voltage high current supply that pulses at 50/100Hz, you basically build a high current switch mode PSU that uses capacitor banks and high frequency switching to drive the (much smaller and lighter) output transformer. Hence much better control, and small light weight units. These are better at arc welding thin stuff, but still not as good as MIG IIUC. (I presume you can get inverter based MIG units as well)

Based on my attempts in the past, stick to arc welding nice solid stuff!

[Tim S]

brian mitchell coughed up some electrons that declared:

I've done a bit, a long while back...

The NoGas is very expensive to run, unless you feel you really can't stretch to even disposable gas cylinders, which are still expensive to run compared to a small refillable. If you really have got a couple of patches to do and that's it - NoGas might be sensible.

The worst bits about a crap MIG welder are poor wirefeed IMHO. Duty cycle (how long you can weld vs how long to let the machine cool down) is less of a problem for small jobs and is one of the factors that seperates a pro machine from a diy one.

Don't know anything about the inverter types. All I can say is I had a Cebora in 1993 ish (sold to a mate). It wasn't that expensive. Prolly 90 amps IIRC - but it was brilliant. Very very smooth and repeatable and quite suitable upto a couple of mm steel - more if cut the joint carefully so you could fill it.

Cheers

Tim

[meow2222]

I think every car welder struggles with blowing metal first time round. You have to cut the metal back further than you think, to where its thick enough to be welded. Bear in mind you're melting steel onto sheet steel, so if that sheet is too thin its just not going to survive. True with any welding set.

NT

[Rick... (The other Rick)]

Have a look here

(The other Rick)

Science and sound engineering will always prevail in the end "for nature cannot be fooled" [Feynman]

[Dave Plowman (News)]

Sounds like you're a welding novice. I am too. ;-)

There are plenty of domestic MIGs on the market that will give good DIY service. And with skill all will cope with the thicknesses found on car bodywork.

Gasless ones can be useful when working outdoors - but are much more expensive to run than gas types and more difficult to get good results with. You can get ones which give you the choice of gas or gassless.

My limited skills mean I find it's pretty easy to seam weld a patch over damage which will satisfy the MOT people - but not so easy to let in one (butt joint) for an invisible repair.

[John]

Is Brazing an option?

[Harry Bloomfield]

brian mitchell laid this down on his screen :

It can be done with an arc welder, but it needs lots of practice, the finished result is not very good and your car body is not really the thing to practice on.

I have not tried a no gas type, but I have got and used quite a lot a small gas mig. It is very easy to use, perfect for car bodies, but it uses a throw away gas bottle. This last point made it expensive to use, the regulator was not very satisfactory and it also leaked the gas away

- I'm looking convert it to a refillable bottle.

[Andrew Mawson]

rushing

Brian, please don't take offence - certainly none intended, but I think you should be questioning if this is not a job better done by someone with experience. As others have said, arc welding is not the way to go with thin sheet metal - mig or oxy-acetylene are the usual choices. The implications of welding on vehicles are fairly wide and deep - it's not only your life at risk but that of other road users. I know that you say bodywork, but much bodywork is structural on modern cars, and some are high tensile heat treated steel that change properties if overheated. Have fun learning to weld, but not initially on cars.

AWEM

[brian mitchell]

Brazing IS an option and I've been doing some of that as well, although I've found it has its own difficulties in awkward, confined spaces and its consumables are also quite expensive.

[brian mitchell]

Thanks for that very clear explanation.

[Fred]

I haven't done any sums recently, but being a hobbyist MIG welder I found bottled gas the most expensive consumable, whether in large BOC or noddy canisters. NoGas wire is about £50 for 5kg (if you look hard) which I hope should keep me going for a couple of years or more. It welds outside better and doesn't leave too much slag which can be easily removed with a brush on a angle grinder.

Also it welds just as well with the wrong electrode polarity where I'm using an unconverted conventional MIG machine.

[John Rumm]

I was under the impression that brazing was not acceptable for MoT repair work on structural stuff?

[Cicero]

========================================= Use a joddler on the edge of the patch before inserting. The joddler gives a stepped edge so that the bulk of the patch lies level with the surrounding area requiring minimal cosmetic filling after welding. The joddled patch can be trimmed (before inserting) to give a patched area that's almost a butt seam but is as easy to weld as a lapped patch.

Cic.

[Andy Dingley]

MIG welders are 300 quid for a decent, usable one (aluminium windings, try Cebora), 500 for a good one with copper windings. They are rated by output current, but not _adjusted_ by output current as they're a fundamentally different operating principle from manual stick (constant voltage output, not constant current).

Although you haven't a snowball's chance of fixing car bodywork with a stick welder, any half-competent MIG set ought to handle it easily.

Cheap MIGs usually become a pain because their wire feed is temperamental. If build quality is OK you'll get the job done, but they're hardly pleasant to use or productive. No-gas welders aren't the greatest things (you can no-gas weld with most gas-capable MIG sets, if you use the right wire, rollers and liner) but they're OK for car repair.

If you use gas, use the right gas. Pub CO2 is the wrong stuff. posts passim.

Don't buy, borrow. Surely most people reading this ng have friends with the right kit? As you're talking about machine, helmet, gas, consumables, hammers, dollies, grinders, a whole lot of other general workshop kit and in this weather a decent working space, it's not a pratical proposition to tool up to do a simple one-off. Nor is it much fun to bodge through it without most of these tools.

You don't need, and can't afford, an inverter. That's for TIG work, or for _very_ serious MIG use. Off the radar for today's discussion.

[Dave Plowman (News)]

I have a joddler - but in practice there's lots of places where it can't be used - or distorts the panel to be fixed.

[Cicero]

========================================= True, but nothing is perfect. Still worth using if appropriate.

Cic.

[chunkyoldcortina]

Similar principle to an inverter microwave, I would guess.

[Bob Mannix]

An inverter is the opproite of a rectifier, ie it is a solid state circuit for turning DC into AC (an "inverted" rectifier). Inverter welders allow for software based control of welding characteristics (cheap) and a high frequency main power transformer (efficient).

The incoming power is rectified and then this DC voltage is inverted to AC at about 10-20kHz using an IGBT based inverter circuit switching at this frequency. This feeds the efficient, high frequency main power transformer, the output of which is rectified and this does the welding. Controlling the power output at the inverter stage is far easier than trying to control the main power transformer and the welder can use voltage sensing etc to create easy arc strike conditions (for example).