Thread: The "Gap" between two pieces being welded together?...

generally its so you get better penetration depends on material being welded as in thickness

But in saying so had a TAFE teacher and tradie boilermaker who were sticklers for grinding edges so everything was butt tight fit then weld so which is correct

From my TAFE course, it's always advisable to have a gap for butt welds to get full penetration. You want full penetration for butt welds for strength. Without it, you basically have a "crack" on the back side.

The rule of thumb amount of gap is approximately the width of the rod you are using. This is because the width of the rod determines the width and height of the weld.

Thicker material, you make a V joint. Double V or U joint for the super thick.

Dear Ray and Ed,

Thank-you for the replies. Reading between your lines, I'm taking it that welds can easily end up being unnecessarily wider than they should be, at the expense of not achieving correct depth, and hence need a bit of "shaped" help that allows them to "channel" the flow of molten metal in the correct direction. I'm a bit worried about the finish on the other side of the pieces, so I'll probably go for a small-chamfered "V" at the top of the joints in question, with the bottoms abutting. But I will nonetheless experiment first with a straight-out face-to-face butt, as well as a straight-sided gap between the pieces...

Many Thanks,
Batpig.

Weld gap preparation is appropriate to the application of the welded article.

Penetration is controlled by correct arc length,correct polarity and angle of electrode. Get this right first before doing too much with the above.

Given that a weld application is fit for its purpose, I would not stress too much.

Most of us are capable of depositing a competent 4mm leg length fillet weld on say a piece of 6mm flat and it could hold the weight of the full 6 cyl family sedan. No prep needed here.

Given that amps are set properly and there are no undercuts or inclusions, a 25mm length of weld each side will hold the weight of your falcon or commodore.
For most DIY blokes most of us are not going to to weld too many things that will require that gap preparation.

Perhaps a trailer might be the extent of the max required quality.

The gap if needed is relative to the thickness of metal to be joined.
A bit of a rough guide for you :

• Thickness to 6mm Square butt prep.
• 6mm to 12mm Plate Single vee butt
• Above that double vee prep.

If have more on this ,I will follow up with tomorow.

Grahame

Nice post Grahame.

Dear Grahame,

Yes - thanks very much for responding. The join will certainly not be subject to much in the way of loading, but it is still handy to know the rationale behind the particular practice in question. I must have missed your reply when I was trawling through the "New Posts" list the first time after you wrote it...

Best Wishes,
Batpig.

It occurs to me that in the thicker welds as above chamfers and gaps might be appropriate.

But when you are working in thin material and pussyfooting around trying to get a nice clean weld and not blowing thru a gap in the weld , particularly when the gap is near the material thickness, might well be the worst thing in the world.

cheers

http://www.tpub.com/content/construc...s/14250_51.htm

Above an illustration of some forms of welding.

From my (limited) experience, I would leave an opening between the parts to be welded only if the material is too thick for the machine I am using. In this way the opening will allow more penetration and a flatter fillet. If I have the proper machine and rod or wire for the job, a butt joint with both parts to be welded touching, should be no problem and the heat melt all the way to the other side.
If you have a 3 phase or diesel powered rotary DC that can pull a 6mm rod you can butt weld 10mm plate with no gap nor grove. A lot of fan.
If you don't then you resort to groves and openings and welding both sides