Tips & Tricks

Welder Buyer’s Guide

Farms & Farm Machinery - - Contents -

Weld­ing is de­fined as “a joint where pieces or parts have been fused to­gether”, usu­ally by heat. Most com­monly welded ma­te­ri­als are met­als, but the process can in­clude many plas­tics too. The heat is gen­er­ated by an elec­tri­cal spark or a gaseous flame.

With min­i­mal train­ing, pretty much any­one can join bits of me­tal to­gether, but re­ally spe­cialised skills are learned – and prac­tised – over years.

There are var­i­ous ways of cre­at­ing welded joints – and when you’re mak­ing me­tal hot enough to melt to­gether, you need some spe­cial­ist equip­ment. Yes, we’ve seen the bush me­chan­ics who can weld us­ing a car bat­tery and some fence wire, but they’re sel­dom per­ma­nent re­pairs!

There’s no sin­gle ma­chine that can re­ally do it all, but de­pend­ing upon the ma­te­ri­als be­ing joined to­gether, the most com­monly used equip­ment will fall into one of three main types:

ARC WELDERS

Arc welders join two met­als by gen­er­at­ing an elec­tric arc (spark) be­tween a cov­ered sac­ri­fi­cial me­tal elec­trode and the met­als to be joined us­ing high volt­age. Think of it as hand­held light­ning and you’ll have an idea of the process.

The heat pro­duced by the arc be­tween the pos­i­tive and neg­a­tive elec­tri­cal charge melts the par­ent me­tal, which mixes with the molten de­posits of the coated elec­trode – or the wire and gas shield.

The most com­mon arc welder is the good old ‘stick’ welder: the elec­trodes – the sticks – are about 250mm long and look like stout bits of wire that were dipped in ce­ment.

More cor­rectly called ‘shielded me­tal arc’ weld­ing, stick weld­ing works bet­ter than most other meth­ods on dirty or rusty ma­te­ri­als and in less-than-per­fect con­di­tions.

De­pend­ing on the met­als to be joined, there are many types and di­am­e­ters of weld­ing elec­trodes, but with age the flux (the ‘ce­ment’) can ab­sorb mois­ture or crum­ble off, mak­ing fresh rods de­sir­able.

Weld­ing rods are kept in ‘hot boxes’ on some big job­sites to pre­vent mois­ture in the flux.

For big weld­ing jobs, stick weld­ing is com­par­a­tively slow, since the con­sum­able elec­trodes must be re­placed ev­ery cou­ple of min­utes and be­cause slag (the residue from the flux) must be chipped away af­ter the weld is com­plete.

Arc weld­ing cur­rent

Stick welders can be of AC (al­ter­nat­ing cur­rent) or DC (di­rect cur­rent) types.

DC weld­ing of­fers ad­van­tages such as eas­ier starts, fewer arc out­ages, less stick­ing, less spat­ter, bet­ter-look­ing welds, and eas­ier ver­ti­cal up and over­head weld­ing. Plus, with DC it’s eas­ier to learn how to weld, and you get a smoother arc.

DC re­verse po­lar­ity (elec­trode pos­i­tive) pro­vides about 10 per cent more pen­e­tra­tion at any given am­per­age than AC, while DC straight po­lar­ity (elec­trode neg­a­tive) welds thin­ner met­als bet­ter.

AC has ad­van­tages if weld­ing on ma­te­rial that has be­come mag­ne­tised from fric­tion, such as when hay, feed, grain or wa­ter con­stantly rubs against a steel part.

In that case, a DC out­put doesn’t work well be­cause of

‘arc blow’, where the mag­netic field blows the molten filler me­tal out of the weld pud­dle. The AC suc­ceeds be­cause AC out­put al­ter­nates be­tween po­lar­i­ties, thus en­abling weld­ing of mag­ne­tised parts.

A ma­chine rated at be­tween 225 to 300 amps will be ca­pa­ble of han­dling most jobs that a de­cent me­chanic will need to weld: be­yond that, you might be in for a trip to a spe­cial­ist.

In fact, most stick-weld­ing pro­ce­dures re­quire less than 200 amps. To weld ma­te­rial thicker than about 10mm, sim­ply make mul­ti­ple passes – this is what the pros do, even when weld­ing on 1-inch (25mm) struc­tural steel.

When buy­ing a ma­chine, the welder’s ‘duty cy­cle’ is im­por­tant. This refers to the num­ber of min­utes out of a 10-minute cy­cle a welder can op­er­ate.

For ex­am­ple, Miller’s Thun­der­bolt XL cre­ates a 200-amp DC out­put at 20 per cent duty cy­cle. It can weld con­tin­u­ously at 200 amps for two min­utes, and then must cool for eight min­utes to avoid over­heat­ing.

Duty cy­cle and am­per­age are in­versely pro­por­tional. Op­er­at­ing at only 90 amps, the Thun­der­bolt has a 100 per cent duty cy­cle, mean­ing you can weld with­out stop­ping.

Ex­ceed­ing the rated duty cy­cle can dam­age the ma­chine or cause it to over­load and trip out.

Arc weld­ing rods

The de­tails for weld­ing rods are printed on the end of the stick if you no longer have the packet. These des­ig­na­tions were stan­dard­ised in the United States by ASTM and WIA, with the first two dig­its rep­re­sent­ing the me­tal’s ten­sile strength. For ex­am­ple, 60 se­ries rods have a max­i­mum ten­sile of 60,000psi (415MPa) and the 70 se­ries 70,000psi (485MPa).

Com­mon elec­trodes used for gen­eral work in­clude 6010, 6011, 6013, 7018 and 7024, each of which has spe­cific prop­er­ties: 6010 elec­trodes pen­e­trate deeply, while 6013s pen­e­trate less.

For much bet­ter bead ap­pear­ance and work on higher strength steels – say for an im­ple­ment hitch – use a 7018 rod. For bet­ter pen­e­tra­tion on thick ma­te­rial, grind open the joint to a 30-de­gree bevel (leave a 2mm width ver­ti­cal land on the bot­tom of the groove) and make mul­ti­ple passes.

Al­ter­na­tively, make the first pass with a 6010 rod, then make a ‘cap’ with a 7018.

The 7024 rod is per­haps the eas­i­est to use.

This is also known as a ‘drag rod’, mean­ing the elec­trode’s thick flux au­to­mat­i­cally main­tains the cor­rect arc length, which al­lows you to drag the rod di­rectly along the work piece. That’s pretty handy for bet­ter-look­ing welds.

Hard-fac­ing rods can pro­vide im­pact re­sis­tant or abra­sion-re­sis­tant welds – or both, de­pend­ing on the ap­pli­ca­tion.

Be­cause the type of rod re­quired de­pends on the type of en­vi­ron­ment the tool will ex­posed to, it’s prob­a­bly bet­ter to ask your lo­cal weld­ing sup­plier for ad­vice.

IN­VERTER WELDERS

In­verter welders have some dis­tinct ad­van­tages over more tra­di­tional welders.

Both re­quire a trans­former to con­vert in­com­ing cur­rent to suit­able weld­ing cur­rent, but an in­verter welder does this far more ef­fi­ciently and is much smaller and lighter — and uses less power too.

Be­cause they’re more ef­fi­cient, their duty cy­cle is also higher. Us­ing mod­ern solid-state elec­tron­ics, In­verter welders lose very lit­tle heat in com­par­i­son to tra­di­tional ma­chines, giv­ing the smaller in­verter ma­chine the abil­ity to use nearly all of its in­put cur­rent – where an older-style trans­former could lose up to 20 per cent ef­fi­ciency to heat loss.

The elec­tron­ics make it pos­si­ble to main­tain an arc where a con­ven­tional trans­former welder would al­low the elec­trode stick to the work.

Big 5mm di­am­e­ter rods can be run on an in­verter ma­chine and it’s prac­ti­cally im­pos­si­ble to make them stick to the job.

The higher fre­quency of the out­put cur­rent and com­puter soft­ware to mon­i­tor and ad­just cur­rent and volt­age while weld­ing pro­duces a con­sis­tent, smooth arc that’s eas­ier to strike and to main­tain.

When run­ning on do­mes­tic sin­gle-phase power, most in­verter ma­chines do need a 15-amp power out­let.

Weld­ing sup­plies such as elec­trodes, weld­ing wire and shield­ing gas typ­i­cally last longer than when us­ing a tra­di­tional weld­ing power sup­ply.

Ad­just­ments to cur­rent and volt­age can be made for dif­fer­ent ma­te­ri­als and thick­nesses, giv­ing the op­er­a­tor tighter con­trol over the weld­ing process.

The small size and weight of these welders make them pop­u­lar – where a tra­di­tional trans­former welder is too bulky or uses too much power – so they are pop­u­lar in main­te­nance fa­cil­i­ties, gen­eral fab­ri­ca­tion shops, con­struc­tion sites and farms as por­ta­ble, light­weight units for on-site re­pairs.

TIG WELDERS

Tung­sten in­ert gas (TIG) weld­ing, some­times known as gas tung­sten arc weld­ing (GTAW), re­quires more skill than most other weld­ing meth­ods but is ex­tremely ver­sa­tile, ca­pa­ble of join­ing a huge range of met­als in vary­ing thick­nesses.

The re­sult is neat and pre­cise, giv­ing a beau­ti­fully fin­ished weld bead where ap­pear­ance is im­por­tant.

In the TIG process, the arc jumps be­tween a non-con­sum­able tung­sten elec­trode and the work piece, gen­er­at­ing high tem­per­a­ture. The in­ert gas shield, usu­ally in­clud­ing ar­gon or an­other no­ble gas, pre­vents weld ox­i­da­tion.

There is no spat­ter or slag, and thin met­als are read­ily welded, and with less dis­tor­tion.

The most ob­vi­ous dif­fer­ence be­tween a TIG unit and an AC/DC stick welder is the non-con­sum­able tung­sten elec­trode which is held in the han­dle.

The shield­ing gas is di­rected around the weld by a ce­ramic noz­zle or cup de­signed to with­stand the heat of weld­ing.

Dif­fer­ent sized cups are used: smaller cups for smaller welds and reach­ing dif­fi­cult ar­eas; larger cups for im­proved gas flow and cov­er­age for big­ger weld beads.

While there’s no slag to chip off and clean up, TIG re­quires a clean pre­pared sur­face for qual­ity welds. Any type of for­eign mat­ter, in­clud­ing mois­ture or clean­ing sol­vents, will con­tam­i­nate the weld.

Apart from the higher skill re­quired with TIG, there are some other con­sid­er­a­tions, such as cost and speed.

Al­though there are cheaper units avail­able, a good TIG welder can cost sev­eral thou­sand dol­lars; in con­trast, an in­verter TIG can be bought for around $400.

That’s still twice more than a be­gin­ner needs to spend on a mod­est stick welder, but its com­pact­ness and the ease of strik­ing and run­ning a smooth bead are im­pres­sive once you’re used to hav­ing both hands en­gaged.

Fi­nally, TIG weld­ing is a bit slower than other arc weld­ing meth­ods, so the time spent TIG weld­ing is rel­a­tively high.

Where time equates money, an­other process might be more cost ef­fec­tive.

MIG WELDERS

MIG stands for me­tal in­ert gas, and this process is also known as gas (shielded) me­tal arc weld­ing (GMAW).

This process has a con­tin­u­ously fed wire elec­trode that melts in the arc to form a weld bead. Like TIG, the weld is pro­tected from sur­round­ing air by a shield­ing mix­ture of gasses, usu­ally in­clud­ing ar­gon.

Al­ter­na­tively, tubu­lar flux-cored ‘self-shield­ing’ wires are avail­able, which need no ex­ter­nal gas. Ven­ti­la­tion is needed be­cause of fumes, and the slag formed on the weld has to be re­moved be­tween passes or be­fore paint­ing.

MIG welders with solid wire are more com­monly used with in­dus­trial ro­bots and on heavy, thicker work be­cause the con­tin­u­ously fed elec­trode wire and arc length are au­to­mat­i­cally con­trolled.

MIG weld joints are stronger, more ductile, have less dis­tor­tion than other weld­ing pro­cesses, and are ideal for thin me­tal work, al­loys and alu­minium.

How­ever, weld­ing wire isn’t cheap and there’s a bit more to con­sider with MIG, like weld­ing me­ter amps and volts, wire speed and po­si­tion.

There’s a gas­less wire op­tion with MIG that’s a lot cheaper than bot­tle rental but it isn’t as nice as run­ning a MIG on gas, and the power sup­ply must be con­sid­ered – ide­ally a con­sis­tent cur­rent with very lit­tle drop in volt­age is needed.

Duty cy­cle is also a con­sid­er­a­tion: the ma­chine will need to be watched to avoid over­heat­ing. For ex­am­ple, a 200-amp welder with 30 per cent duty cy­cle should be op­er­ated for no more than three min­utes in 10, al­low­ing seven min­utes for cool­ing.

GAS WELDERS

More of­ten than not, gas weld­ing uses a com­bi­na­tion of (usu­ally) oxy­gen and acety­lene as the heat source, which is why it’s also called oxy weld­ing or oxy­acety­lene weld­ing.

As for tech­nique, if you’re learn­ing to weld us­ing the stick process, re­mem­ber­ing these five points will im­prove your tech­nique. Think CLAMS: Cur­rent, Length of arc, An­gle, Ma­nip­u­la­tion and Speed.

1 & 2. Arc weld­ing 3. Gas weld­ing

4. In­verter weld­ing

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