Aerospace Looking to settle a dispute with my boss about fillet/corner/bend radii

1. May 14, 2012

brr407

The question seems a little esoteric, but who knows. Also, we are making spares and replacements for the Gov. These parts were designed in the 50's and 60's. So I can't just ask the customer. Also this issue comes up a few times a year. So I'd like to find some information to finally settle it.

My boss says that a fillet radius only has to be tangent to both surfaces if the drawing says it is a full radius. I say that I have never seen a radius value given with a full radius because full radii are used in places where they want a radius around the entire material thickness. So a radius value is not needed since it is controlled by the material thickness (tangent to three surfaces).

The question is when a radius value is given, is the expectation that the radius is tangent to the two surfaces or is the expectation that you only need to put in as much of the radius as you feel is needed, leaving sharp corners that would then be smoothed out a bit, but resulting in a a much tighter radius at the end of the larger radius? If doing this is allowed, then I need to see a spec that shows how much of the larger radius needs to be put on, and how much of the small unspecified radius is allowed to get it to work.

This image shows the two methods I am talking about.
img24.imageshack. us/img24/9307/fillets.png (remove the space. I can't post links yet)

The .50 radius on the right is tangent to both surfaces, while the .50 radius on the left is not. If the radius on the left is allowed, I need to see the spec that tells me how I'm allowed to do that. The following is a description of the actual issue. In this case whats going on is that I am modeling an angle (a long narrow piece of sheet metal bent at a 90 degree angle.) There is also another bend in it that makes this part trickier to form, but that's besides the point for this question. At one end of the angle, a tongue sticks out, but the way the flat pattern is laid out with the radius that is specified, that tongue, which should be flat, gets partially bent by the radius.

My boss is like, just don't make the radius "full" or tangent and I'm like, the spec for forming this type of sheet metal says that the minimum radius is the one on the drawing, if I don't make the radius "full" then I'll have a tighter radii on there, and that would seem to go against the forming spec. He's like that doesn't matter, and I'm asking him to show me a spec that explains that. He says it's just understood. I think that's BS.

2. May 14, 2012

Mech_Engineer

Just call out the radius per ASME Y14.5, and specify on the drawing that the dimensions and tolerances are per ASME Y14.5. Problem solved.

3. May 14, 2012

brr407

Thanks for the response. I have ASME Y14.5 and in section 2.15 it would seem to suggest that you can't do what my boss is saying, though its not at all explicit.

Also, I am not making a drawing, I am interpreting a drawing. So I don't think the problem is solved.

4. May 14, 2012

AlephZero

IMO the drawing shows arc radius 0.5 with its center position .46 from the straight edges. The two arcs radius 0.1 are tangent to this arc and the straiight edges.

That is enough to define the centers of the R 0.1 arcs, and therefore they are not dimensioned on the drawing.

Presumably there is a reason why somebody specified the radius as 0.1, rather than just saying "break sharp corners".

Last edited by a moderator: May 6, 2017
5. May 14, 2012

brr407

I made that drawing to show two ways to execute a .50 corner radius. I'm not asking how to interpret that drawing.

6. May 28, 2012

kkaliber44

Being a design engineer I can tell you that the design intent is to have that radius tangent to the straight lines. Otherwise you could just do a tiny edge break with the big radius and e.g. you would not be able to assemble this part later, because you wouldn't have enough clearance at that corner.
Also (but this especially applies to inside radii) you can easily create high stress concentrations and lead to the failure of the part if you do what your boss is telling you to do.

My experience with old drawings is that they are not defined as precisely with some things being implied. Maybe at the time everyone would interpret it your way, whereas nowadays people like your boss are trying to find loopholes in the system.

Most importantly, I WOULD ask the customer. They should have a design engineer, who knows or is supposed to know the part and they could find out what will work from the assembly, durability, and other important standpoints.