Sheared Edge C Channels 1

[Design-fab]

Hi Folks,

Small time trailer builder here. We use off the shelf hot roll formed C channels for crossmembers. The tolerance of these is not the best, especially the squareness of the flanges, which causes various minor issues during fabrication. They are also locally only available in a zinc coated finish which is not desired as we are in a low corosion environment and the zinc impairs weldability.

Since the length of the crossmembers is always less than 8ft, we are considering the alternate option of having the channels formed from sheet stock on a heavy pan brake. This would hopefully produce a channel with tighter tollerance and allow us free choice of material (no zinc). The opertunity to use non off the shelf sizes would be a huge advantage as well.

It all sounds like a great idea but my concern is that the shop with the pan brake would typically cut the sheet stock on a guillotine therefore leaving the channels with a sheared edge. Is the sheared edge is going to cause a fatigue issue and come back to bite us in the a** later down the track?

I should say that obviously I'm not expecting a definitive answer here but any hints or tips would be greatly appreciated.

John.

 

[TugboatEng]

You can debur the edge by shot peening or vibratory tumbling. There are also chemical/electrical methods of deburring that are similar in practice to electropolish. If your production quantity is large enough these options are economical.

Stress raisers aren't likely to be an issue on such ductile material that it can be brake formed.

 

[Design-fab]

If only deburring the edge is required to mitigate this then it sounds like a quick pass along the edge with a flap wheel or belt grinder could be a possible option? That would be quick and inexpensive.

We size the channels very conservatively anyhow so it may not be a huge issue. I just have a habit of thinking about things a bit too much sometimes!

John.

 

[WKTaylor]

Are You familiar with some of these amazing sheet metal forming books?

https://www.bing.com/search?q=sheet...ANNNB1&refig=d51c0a07227742e3bac6d7aa3391ad91

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

 

[Design-fab]

No, but always happy to add to my library. Any one in particular you would recommend given the above applicattion?

John.

 

[3DDave]

Is it thin enough to hem the edge? Alternatively, run the strips through a light edge shave with a milling cutter. A 0.010 cut might be enough.

 

[MintJulep]

It's a valid concern. This has caused expensive problems for a former client and their supplier.

A secondary operation to clean up the edge will reduce the chances of cracks starting.

Hemming the edge will help with the stress risers, but depending on orientation, may introduce a corrosion problem.

 

[Design-fab]

Dave the material is usually 1/8 or 5/32 thick so if I understand what you mean by hemming correctly (folding it back on itself) that won't work. But I was thinking about adapting a beveling tool to perform an edge shave (rather than a bevel) like you suggest. It would be fairly quick and easy, especially at such small cutting depth. Probably much more consistent and foolproof than grinding.

John.

 

[MintJulep]

Water jet would be a good option if your local supplier has the right capability.

https://www.techniwaterjet.com/waterjet-cutting-quality/

 

[Design-fab]

I've previously searched in regards to fabricating other parts and haven't found a local shop with water jet capabilities unfortunately. I agree it would be ideal if available though!

John.

 

[TugboatEng]

Waterjet is VERY expensive...

 

[EdStainless]

I have seen sheared strips pulled through a device with carbide knives on the sides to shave the edges.
With this you could put a full radius on them or double chamfer.
Whatever you grind into the carbide.
There are hold down rolls to keep the strip flat and a drive with pinch rollers.

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P.E. Metallurgy, consulting work welcomed

 

[TugboatEng]

Include an edge finish and flatness tolerance on your proposal and see what the fabricators return with. There are many solutions to this problem but if you specify a solution directly it may be outside of that particular fabricators ability and you'll be refused.

 

[Design-fab]

Interesting, in my mind the fact that someone went to the trouble of building that machine confirms my suspicion that the sheared edge is indeed a concern.

I think we'll do a trial build using folded channels with manually finished edges first. If it proves viable I'll probably look to set up something similar.

John.

 

[Design-fab]

Good point Tugboat, we're probably getting ahead of ourselves here. Now that it is established the edges need secondary finishing I will definitely consult with the shop first.

John.

 

[EdStainless]

SS strip is used a lot in architectural applications.
The edge finishes are important for looks and if they are in places where they can be touched.

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P.E. Metallurgy, consulting work welcomed

 

[Metalfixer]

We used a slitter, but only for 16ga. No burrs on edges.

https://www.alliancesteel.net/coil-slitting/

This company will slit .010-.500
Larry

 

[EdStainless]

Slit material always has a burr.
If there is no burr there has been a secondary operation to remove it.
The edge is about half sheer so some burr is unavoidable.
One thing is that both edges should have the burr in the same direction.
Then you can form parts with burr edges in.

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P.E. Metallurgy, consulting work welcomed

 

[Design-fab]

I had always thought slit edge was equivalent to sheared edge, or is that not the case?

Edit: sorry Ed just saw your post. Makes sense.

John.

 

[EdStainless]

No, it is part sheer and part break.

https://www.thefabricator.com/thefa...g/how-to-avoid-slit-in-coil-slitting-problems

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P.E. Metallurgy, consulting work welcomed