Burr spec for tubes?

[happystamps]

Morning all- I've recently won some work designing some tube assemblies, and would like a spec for burrs & edges on tube ends. Does anyone know of anything?

Cheers
Simon

 

[tbuelna]

What are the tubes used for? What is the diameter and WT of the tubes? Is the tube end being prepped for welding or brazing? Is it be prepped for swaging? Is the tube part of a fluid circuit where surface smoothness and flow conditions are a concern? Are there fit/clearance requirements at the tube end mating interface?

 

[MikeHalloran]

How big a burr do you want?


Mike Halloran
Pembroke Pines, FL, USA

 

[tbuelna]

MikeHalloran- that's the first thing I thought of, but I was too polite to say it.

Regarding edge breaks, every single machined part drawing I have ever produced, and there have been hundreds of them, had a general note stating something like "Break all sharp edges .005 to .010". And often there are more specific requirements for edge breaks shown on the drawing.

With a tube assembly, where the tubes have very thin walls (ie. around .020") the dimension/tolerance control of edge breaks can be very critical, so that you are not producing a knife edge. There are also situations where it is critical to make sure the sharp inside edges of a hole thru a tube wall is rounded to a specific profile, so that it produces a desired flow coefficient.

 

[MikeHalloran]

I was trying to say that I'm not aware of a universal general spec; you have to tell the fabricator what you will NOT accept.
... and that depends strongly on what mates with the tube end, which is why a general spec couldn't be universal.

E.g. if the tube is to be butt welded, cutting it with a laser may not be a good idea. It doesn't leave burrs, but it does leave a HAZ which will cause the tube to 'weld funny'.

Laser cutting would also be contraindicated for use with a Lenz O-seal fitting, because an o-ring has to ride up and over the tube end. In that case, you'd want a chamfer, or the sort of partially swaged end you get from a hand operated tube cutter.

In any case, you usually want to deburr the inner tube end.
Luckily, there are simple, fast tools, manual and powered, that can do a decent job of chamfering inside and outside, sometimes both at once. Be a little liberal with the tolerance if you can. Nobody wants to pay for measuring chamfers with a micrometer.




Mike Halloran
Pembroke Pines, FL, USA

 

[tbuelna]

I have some experience with very critical fit-up requirements (edge breaks, gaps, etc.) for thin wall tube joints that were butt-welded using TIG and no filler on hypergolic propellant systems on the Space Shuttle. The tube ends had to have almost perfect fits, no internal burrs or mismatch, and almost no edge break.