Tack welds...

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steveo
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Tack welds...

Post by steveo » 05 Jan 2017, 10:43

Having finally got to the point where I'm cutting steel, I'm going to need to start tacking.

The thought suddenly hit me, where to tack? The corner of the square tubes or the middle? Look at the pic below:

Image

Is it better to tack each corner or to tack the middle of each face? What are the pros and cons? Or is there a better way?

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Re: Tack welds...

Post by Midlana1 » 05 Jan 2017, 11:12

For this stage, as long as your tacks feed in a minimal amount of heat, distortion will be a non-issue. Just keep in mind that a weld will always try to pull the metal towards it, so when you start welding for real, as long as you keep alternating sides you'll be fine.

A race car fabricator once told me that if you can get the tubes within 1/4" of where they're supposed to be, you're doing well.

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Re: Tack welds...

Post by steveo » 05 Jan 2017, 12:26

Makes sense! Thanks!

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Re: Tack welds...

Post by Def » 05 Jan 2017, 13:42

Tacking the corners is a bit easier to keep things square, as it's going to pull the tubes together on the edge vs. along one plane. Things do tend to stay a bit straighter doing it on the edges.

That said, as Kurt mentioned, the heat input on a proper tack isn't much, so things shouldn't pull much.

But I personally always try to tack on the edges of square tubing. It's also less noticeable as you weld over it IMO.

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Re: Tack welds...

Post by rennkafer » 06 Jan 2017, 08:22

^ I'll second what Def said... also less likely to have a bad weld if the tack is at the start or end of the final weld.
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Re: Tack welds...

Post by Fuzzydicerule » 06 Jan 2017, 18:23

Third for tacking the edges. However tacking the center makes cutting and repositioning tubes MUCH easier. But, like stated, its much easier to weld between two tacks at the ends of your finish weld and have it look good and be strong, than to start and stop in the middle
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Re: Tack welds...

Post by svltrack » 28 Jul 2017, 22:14

If you can clamp, this will minimize almost any distortion. I like corners as they take the least heat to puddle, and generally no rod is needed. I think you'll find that a lot of the weld is dictated by what is easily accessed.

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Re: Tack welds...

Post by stankoprowski » 29 Jul 2017, 08:31

Back in the day people like Lotus, Lola, Cooper etc. didn't weld-weld their frames they used nickel or silicon bronze welding. This appears to be a cross between welding and brazing. You get a bead but you don't melt the base material so there's less heat and less distortion. The resulting weld is supposed to be stronger than the base material. Some modern folks like Ariel Atom(?) still use it on parts of their frames. I've been trying to figure out if you could weld thinner walled aluminum extrusions into what today is called an aluminum space frame. Conventional welding produces a weaker joint and requires the whole frame to have thicker walls (heavier) to make up the degraded joints. You have to design to the joint strength rather than the material strength. Any comments?

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Re: Tack welds...

Post by rennkafer » 29 Jul 2017, 14:03

While you could build an aluminum chassis, you probably don't want to. Fatigue life is a real thing with aluminum, and not really an issue with steel chassis.
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Re: Tack welds...

Post by svltrack » 30 Jul 2017, 10:00

stankoprowski wrote:Back in the day people like Lotus, Lola, Cooper etc. didn't weld-weld their frames they used nickel or silicon bronze welding. This appears to be a cross between welding and brazing. You get a bead but you don't melt the base material so there's less heat and less distortion. The resulting weld is supposed to be stronger than the base material. Some modern folks like Ariel Atom(?) still use it on parts of their frames. I've been trying to figure out if you could weld thinner walled aluminum extrusions into what today is called an aluminum space frame. Conventional welding produces a weaker joint and requires the whole frame to have thicker walls (heavier) to make up the degraded joints. You have to design to the joint strength rather than the material strength. Any comments?
With regard to your question on silicon bronze welding, it boils down to the strength of the HAZ of the component, versus the strength of the brazing material. Yes, some of the substrate will get pulled into the braze joint. But it is not providing any real strength.

To be clear, since I think someone would read your post the wrong way, when you are referring to the "joint", I assume you are indicating the HAZ around the weld, and not the actual weld itself?

The way I look at it is this, even when brazing, a good bit of heat is being put into the part. I'd much rather have the strength of the weld, versus just the brazing. Also, if you are really worried about it, there are ways to not input a ton of heat into the part, and still get excellent penetration. Pulsing, different waveforms, etc.

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