How to apply a Tolerance to a Bend Radius

[JRKH]

Lot's of good info in this thread. Palmer and Kevin Mader covered about everything I could have thought of.

Basically if the drawing uses a "title block" tolerancing and not GDT, go with the Title block. If GDT is indicated go with that.

I always took the position that they can't reject a part that is made using their tolerancing.

Although, that being said, we DID have a customer occasionaly reject a part that was in print by their title block but when they ahd made it they "knew better" and applied a tighter tolerance to some feature. You know - The old issue of old drawings never being updated, cause the guys on the floor knew what was needed. Then when the part was farmed out to us, all we had to go by was the drawing.

Anyway - Long story short. If you stick with the best toleancing available - that which is documented on their drawing - you can hardly go wrong.

Peace
James

 

[bobdoering]

Basically if the drawing uses a "title block" tolerancing...go with the Title block.

Just a word of caution: Be careful about title blocks. Sure, it is the default print requirement, but sometimes you will have title blocks developed for machining (for example), and applying it to bending or welding may not be applicable (but nobody verified that there might be a problem!) That is why it is good to go through a print with a fine tooth comb before quoting and starting a job!

 

[JRKH]

Just a word of caution: Be careful about title blocks. Sure, it is the default print requirement, but sometimes you will have title blocks developed for machining (for example), and applying it to bending or welding may not be applicable (but nobody verified that there might be a problem!) That is why it is good to go through a print with a fine tooth comb before quoting and starting a job!

Of course by the time I got to looking at these things it was usually after the darn thing was already quoted and started and one of tow questions come up. "We don't have that radius can I use this one?" or it is already made and tehy want to know "if it will work".
In other words flying by the seat of their britches.....

Peace
James

 

[Kevin Mader]

ScottS,

My fellow Covers helped pick me up! Triggered another thought, although I think that you were looking to make the radius larger. Especially with the aerospace industry, radii have expanded importance. If you stretch the metal or coin it, you could bring dimensions into print, but ruin the material properties. I recall our trying to reduce a radius for a part made out of aluminum but ended up cracking the material on the bends. We had to grow the radius beyond the drawing dimensions to avoid this condition, which for this part would not have led to a catastrophic failure of a key component, but it could have for another.

Using a toleranced profile is not a bad suggestion. If you are lucky enough to have good back lighting or face lighting that allows you to catch the profile, an overlay on a comparator or toolscope might be a good way to verify your radii. In the end, its really about the tooling in your toolbox drawers. What can you achieve without investing in a new tool? As I noted earlier, it's best to work these details out up front. It may not matter, it may be cosmetic, or it could mean whether you get paid for the part or not. Engineers tend to put down ideal dimensions for features (ideal to them) without thinking about the cost it might bring (e.g. tightly toleranced vs. Form, Fit, and Function practical). They're human after all, and capable of oversight.

Regards,

Kevin

 

[palmer]

Kevin brought up a thought from me as well about material damage.

In some cases, we heated the material to prevent cracking or tearing.

In other cases we formed radii in steps to keep the material consistent. In this case, the steps didn't require the material to form all at once which let it relax before being stretched again.

In both cases there was a lot of trial and error before we acheived consistency.

Hot rolled and cold rolled metal are one thing, SST and alum are a whole nother animal when it comes to bending. We had #3 grain in our SST and had to maintain it throughout the process which required bending and welding. It had to look like there never was a weld.....:truce:

We also would form 2 1/2 radii and weld them together to place the weld in the corner to prevent warping across the flat part. Finishing the weld (making it look like no weld at all) seemed to be easier as well as putting the grain back in the material (It was SST)

For not being an engineer I sure have performed miracles with some metal.... LMAO

 

[palmer]

To all that care to help,
We have a forming (bend) radius of .35" on our print but we only have a .375" punch for our pressbrake. The title block gives us a generic tolerance of .030" + or - for all two place decimal deminsions. There is no special note on the print covering radius deminsions. Our question is, does the title block deminsion cover the radius deminisons or is there a special rule that covers radius deminsions? Thank you kindly in advance. Scott and Robert

Scott and Robert,

Ask the engineer also. Or the company that you are making this for. .35" radius is a strange dimension for a radius but it may be mating up to something that has that radius.

The engineer may not have knowledge of bending and used a formula to place a dimension for it. The material thickness plays a part in this.

We were always fighting with engineering about wanting a 1/16" radius in 10 ga. material. To do that you were putting an indentation where it was formed and tearing the material on the outer radius......