welding tips please

Post by **Alec** » Thu Nov 18, 2004 1:12 pm

Cam,
each to his own, but any welding, particularly seam welding (which is now mandatory in certain areas) distorts, and with gas you save time as there is little or no grinding to do. Certainly when putting in sections of outer panel it is possible to weld using virtually no filler metal and so requires less after work.

Alec

Post by **Cam** » Thu Nov 18, 2004 1:17 pm

Oh yes, gas is much neater but it DOES distort the panel much more than MIG as MIG is a lot quicker and more localised. I always seam weld and I do it in sections spread out to minimise distortion. You are heating up the panels a lot more with gas so you will distort the metal more.

Don't get me wrong, I'm not against gas welding at all, but MIG seems a lot more popular these days as it's fast, cheap and easy. OK, you have to dress the welds sometimes, but if it's nice clean steel and you have good light and access then the welds can be neat too and don't require dressing..

Post by **lowedb** » Thu Nov 18, 2004 1:21 pm

It's right that the voltages during welding are not that high: I was told around 50 volts. However this is plenty enough to damage electronics which are only usually protected to 24 volts (for truck jump starts). Also, the arc means that the voltage is quite spiky, which tends to upset electronics.

Regarding the theory of putting exhaust gas in the tank, I don't see how it helps significantly. If the tank has liquid petrol in it, there will be so much vapour in there that you'll never get it to burn. The problem occurs at any point that the vapour can get out and mix with the air to create an explosive mixture. Even if the tank is full of exhaust gas, getting it hot will encourage vapour to vent from the filler, and you've potentially got a problem. That's why electric fuel pumps can be run quite safely inside a petrol tank, without any form of shielding on the brushes.

Post by **rayofleamington** » Thu Nov 18, 2004 1:31 pm

welding voltage is typically 90 volts open circuit and less while the arc is sustained.

Capacitance - it can do funny things! but yes, I meant a few hundred, not a few thousand

That only refers to the transients, but even the lower voltage during welding is still more than a lot of vehicle electronics is designed for. By the late 90's most vehicle electronics would survive 24 volts for short periods as mentioed already - in case someone jump starts the car from a 24v truck.
Reverse voltage protection is even lower than 24 as it is based on someone connecting the 12v jump leads the wrong way round.

Post by **brixtonmorris** » Thu Nov 18, 2004 8:42 pm

[quote="Alec"]Hello Brixton,

what sort of welder are you using?

"you will put a few thousand volts to the cars electrics instead"

no, just disconnect the earth strap from the battery and connect the earth strap to the return of the welder. now tell me how that goes through the cars electric circuit.

as to my welding machine its from shiremore compressers, near newcastle, its pritty big, they have a great range.

Post by **Cam** » Thu Nov 18, 2004 8:46 pm

Well.......... it doesn't!

BUT the electromagnetic flux generated due to the high currents involved can mess with delicate electronics.

Post by **Alec** » Thu Nov 18, 2004 9:13 pm

Hello Brixton,
I did not say that it goes through the car's electrics. But the basic principle remains, the welder's earth clamp should be as close as possible to the area being welded.

Alec

Post by **brixtonmorris** » Fri Nov 19, 2004 8:24 am

i didnt realise there was delicate electronics on a minor.

if you disconect the battery as you supposed to then the electric circuit is switched off and no emf forces will flow through the wires of the car

as for the earth point being near the weld, i suppose if you use low powered welders it may matter, but no.

try measuring the resistance at points close to each other and the resistance from one end of minor to other, you wont see any differance in resistance, apart from whats generated in the thin wires your using on the volt meter. cross sectional area, imagine the cross sa through a minors body. the csa is huge, tiny resistance.

the car electrics. as there is no potential differance, the circuit is broken due to the battery beeing disconected, so nothing will flow down there, and noting can, be sucked out so no effect on delicate electronics,
got to go cya
the potential differance on the northern line has been disconected this morning, so the current wont flow and i am going to be late for work.

Post by **Cam** » Fri Nov 19, 2004 1:32 pm

brixtonmorris wrote:i didnt realise there was delicate electronics on a minor.

There isn't on a standard Minor, but a lot now have radios and other upgrades.

as for the earth point being near the weld, i suppose if you use low powered welders it may matter, but no.

You have to use low power anyway or you will blow holes in automotive bodies!! Even huge current welders have to be turned down to low power for automotive use or it will be a right mess.

At very high current (such as welding curent) the closer the earth then the better the weld as (amazing as it may seem) the CSA does make a difference. I would have said it would not but I have experienced it myself.

try measuring the resistance at points close to each other and the resistance from one end of minor to other, you wont see any differance in resistance, apart from whats generated in the thin wires your using on the volt meter. cross sectional area, imagine the cross sa through a minors body. the csa is huge, tiny resistance.

That's because the meter is generating a tiny current, so the resistance measurement will be small. If you were using a huge current to take the measurement then you would see a difference in resistance (not massive though I grant you). Using a 'hobby' multimeter that you would find in a garage would not show this.

the car electrics. as there is no potential differance, the circuit is broken due to the battery beeing disconected, so nothing will flow down there, and noting can, be sucked out so no effect on delicate electronics,

Well, that's not exactly how it works. electronic devices have internal inductances and capacitances which basically act as a 'resistance to change' or AC impedance due to the reluctance of electrons or 'holes' to change direction rapidly due to their inertia. If you weld a piece of steel, you generate lines of electromagnetic flux in between the welding tip and the earth clamp (and the wires too obviously! - ever seen iron filings gathering in patterns around the welding wires??). This flux is sporadic as the weld 'crackles' so acts like a AC. If there are any wires in the vicinity of the flux then voltage will be induced into them and due to the internal capacitance of the devices connected to them there will be a 'resistance to change' and a very spikey EMF will be present which can damage components.

Obviously if you disconnect the battery then it will vastly reduce the chances of damage but it does not remove the possibility all together (although the chances are small).

Post by **brixtonmorris** » Fri Nov 19, 2004 2:28 pm

ohms law

Post by **bigginger** » Fri Nov 19, 2004 4:02 pm

bless you!

Post by **Cam** » Sun Nov 21, 2004 12:18 am

brixtonmorris wrote:ohms law

Yes, what about it??

Post by **Alec** » Sun Nov 21, 2004 8:15 am

Hello Cam,
just a quick question as you say "so acts like a AC", (which is quite correct for a DC circuit, otherwise your DC coil would not work) but I was under the impression that MIG sets were AC (output) sets anyway?
When you look at the price of TIG sets, which are DC output, I somehow doubt that MIG sets are.

Alec

Post by **Cam** » Sun Nov 21, 2004 6:34 pm

Alec, I was referring to the 'electric arcing' effect which creates 'airborne' AC magnetic flux wether the welding machine is AC or DC. It does this as the wire arcs onto the molten pool, then melts breaking the contact then arcs again as more wire is brought closer (hence the blue constant arcing). So from a flux point of view it does not really matter if the set is AC or DC as the resultant flux is AC.

Post by **Alec** » Mon Nov 22, 2004 7:30 am

Hello Cam,
Ok, but that is not what I am interested in, I just wondered but I think that they must be an AC output set.

Alec

Post by **Cam** » Mon Nov 22, 2004 3:45 pm

Alec, yes I think they are an AC output set too.

Post by **lowedb** » Tue Nov 23, 2004 1:25 pm

There might be some AC Migs about, but mine has a rectifier in it, and as it's a cheapy I don't suppose they'd have put it in unless needed. I also seem to remember having to change polarity on some machines between gas and gasless, though I don't know why. Ordinary arc (stick) welders are AC, though.

On to the point about the resistance of the body Cam is dead right. The resistance of the body is small, yes. The currents involved are large, though. Someobody mentioned ohms law, and it becomes important when either the resistance OR current are high, hence the reason bad earths on batteries / engines only really give problems when you try to draw a big current.

Post by **Alec** » Tue Nov 23, 2004 2:06 pm

Hello Lowedb,
I could understand a rectifier being used in the wire feed system to give variable speed control. I would be surprised if the output was rectified, on a cost basis.
On a slightly different tack, there now are modern industrial arc welders which are very small, light, d.c. output but expensive, relative to hobby arc welders. Such is progress.

Alec

Post by **Cam** » Tue Nov 23, 2004 3:46 pm

a DC arc welder apparently gives a better arc and weld, but obviously is more expensive than an AC one.

Mind you is that pure DC though?? or just 1/2 or full wave rectified AC?? which is VERY ripply and although unipolar is not REALLY what I would call proper DC....

Post by **Alec** » Tue Nov 23, 2004 4:25 pm

Hello cam,
some applications require a D.C. welder, such as Aluminium (MMA) and I'm certain that there are many more. The other reason that the portable machines I mentioned are D.C. is that they usually have the option of being fitted with a TIG torch. Obviously, the better quality machines are full wave and smoothed, but even generators don't produce true ripple free D.C., the only source of that is a battery.

Alec