Weld tolerance and “how big a gap can you weld?”

Oh boy.  I’m sure I’ll get flak from someone for tackling this one, but here goes anyway.

Let’s talk about tolerances for welding.  For this post, we’ll focus only on the weld joint and follow the common theme of customer-vendor interaction.

Customer: “What kind of gap range can you tolerate in a weld?”

Vendor: “What kind of weld?  Process? How deep? How wide? Material?  Filler?”

Customer: “That doesn’t matter yet, I just need to know how big of a gap you can weld and how much it can vary.”

Like the last blog post, this is a trap!  For the welding folks reading this, I KNOW you’re cringing with understanding.  It’s like nails scraping on the chalkboard (if those are still used).  For those of you looking for help interacting with your weld shop, please allow me to explain:

We work with welds that range from invisible to the unaided eye, to heavy section pressure vessel weldments that you can see from across the room.

That pressure vessel can, and does, tolerate a ¼” gap with a +/- tolerance of 1/16”.  That’s a gap difference you can literally see from an arm’s length away, and it’s perfectly fine.  That project uses high power, huge filler, and is relatively insensitive to sloppy tolerances.

Let’s look at the other extreme – that “invisible weld.”  We have worked on a project forming a cylinder from shim material.  This part is made with no filler, and at this scale we discuss gaps in the butt weld as a percentage of weld joint thickness.  This material was not particularly sensitive to stress or heat- related cracking, so we were in a fairly good position to offer up to 25% of the weld joint thickness for localized gaps, with a length of no more than 20x the joint thickness.  Sounds quite generous until I tell you that cylinder was .001” thick.  Cue the stuttering, frustration-laden, concerned call with the designer.  

Customer: “Those tolerances can barely be measured!”

Vendor: “The weld can barely be seen…..”

Ultimately, customer and vendor worked through the tolerance issue and developed a cut process that kept the edge tolerance well within the prescribed band, and all was right with the world.

I’ll cut this short at this point, but there will be a part 2 (and maybe 3) to discuss weld joint thickness, mismatch, and tight penetration requirements.

Welding gaps and “Just weld it”

One of the most dreaded phrases we can hear on the welding job shop side is “Just weld it.”

This is a trap, no matter the good intentions behind it.  It’s also a recipe for disappointment and hurt feelings. To avoid that disappointment and save some time, money, and aggravation on your welding project, let me explain.

“Just weld it” is a phrase that often comes up after a lengthy discussion about parts in-house that just don’t fit together well.  By that, I mean gaps in the weld joint.  I’ll leave that term vague for this post, because the details of gaps and their specific effects will be touched on at some point in the future.  (Maybe even linked to at the end of this blog, if you’re reading this in the future).

Why is “Just weld it” so bad?  Well, quite simply, I’ve yet to encounter a scenario where that didn’t also have an unspoken “….and make it perfect” tacked onto the end.  It’s that unspoken caveat that creates the hard feelings.  Here’s the general flow:

  1. Parts received and inspected.
  2. Conference call with customer discussing fit-up issues and gaps in the weld joint where there should be none.
  3. Customer: Good natured – “Just weld it.”
  4. Part(s) welded – various defects/anomalies in weld due to fit-up, but they are “just welded.”
  5. Parts shipped to customer.
  6. Conference call discussing the unusable part(s), and how anybody with weld experience should have known how to make a good weld.
  7. Us: Mad at customer for their ignorance.
  8. Customer: Mad at us for our incompetence.
  9. Us and customer: Both worse off for the experience.

This is not a finger pointing exercise – both parties messed up.  We should have pushed back and said no.  The customer should have considered the implications of ignoring best practices.  A slight delay to address the issue would have prevented hard feelings and ultimately saved time and money.

Was this blog a touch vague overall?  Yes, I know.  That was intentional – I guess it’s really more of a parable encouraging open dialogue between customer and service provider. 

The moral: unless you’re a large sneaker manufacturer, don’t utter the phrase “Just do it” about anything unless you’re ready to “Just deal with it.”

10 benefits of laser welding

We’ve touched on some pretty heady stuff in the past couple of blogs, so maybe something more straightforward is in order.  Here goes:

Why use laser welding?

  1. It’s LASER! Isn’t that enough???
  2. Makes very small welds (microscopic)

  3. Also makes big (deep) welds – ½” deep in some circumstances

  4. Highly repeatable

  5. Very fast – travel rate to hundreds (in some cases thousands) of inches per minute

  6. Low heat input

  7. Precisely controllable

  8. Few size constraints – parts can be enormous (as long as welds fall inside the laser’s capabilities)

  9. Easily integrated with off-the-shelf motion products: CNC platforms, robots

  10. Very cost effective when compared to other processes

    Good enough?

    That was a little too brief, so how about a little piece of trivia: LASER is an acronym for Light Amplification by Stimulated Emission of Radiation. 

    Another piece of trivia – the laser was conceptualized by Charles Townes.  I had the pleasure of meeting Mr. Townes and speaking with him several years before he passed.  We’ll talk about Mr. Townes and some specifics of his work in an upcoming post.

    As always – to get the nitty-gritty about any of the benefits of laser welding – contact us at 860.653-0111