Machined radii's strength

[MazatrolMatrix]

If you surface a radius instead of it being formed by the tools cutting edge already having the correct radius, and the purpose of this radius is to prevent cracks in the part, is there any meaningful difference in the radius ability to divert forces?

Provided reasonable surface finish of course?

 

[Milland]

Presuming no random scratches or pressing due to edge conditions, no, I wouldn't think so. But there are tools made to function like burnishers to "roll form" or press a radius into final shape, which if used correctly would increase the hardness and fatigue strength of a radius/fillet. I have a surplus Ford tool from one of their crankshaft plants that did this.

 

[Nerdlinger]

As Milland said: with the exception of cold forming the surface with a burnishing tool of some sort, the only difference between the two methods you asked about would be surface finish...and that WILL affect fatigue life. I would think a short dwell (or using exact stop mode) with a tool of proper radius would be smoother than a smaller radius tool interpolating the shape of the required radius. Again, it all comes down to surface finish when all else is kept constant. Of course, a larger radius will help, too. Good luck!

 

[ViktorS]

If you mean a radius between a top surface and wall, and you're interpolating it in parallel with a regular endmill that leaves steps than yes it will affect the strength. Sharp corners are stress risers, no matter if small. Better to interpolate with a ball- or corner radius endmill in that case.

 

[MazatrolMatrix]

Presuming no random scratches or pressing due to edge conditions, no, I wouldn't think so. But there are tools made to function like burnishers to "roll form" or press a radius into final shape, which if used correctly would increase the hardness and fatigue strength of a radius/fillet. I have a surplus Ford tool from one of their crankshaft plants that did this.

As Milland said: with the exception of cold forming the surface with a burnishing tool of some sort, the only difference between the two methods you asked about would be surface finish...and that WILL affect fatigue life. I would think a short dwell (or using exact stop mode) with a tool of proper radius would be smoother than a smaller radius tool interpolating the shape of the required radius. Again, it all comes down to surface finish when all else is kept constant. Of course, a larger radius will help, too. Good luck!

If you mean a radius between a top surface and wall, and you're interpolating it in parallel with a regular endmill that leaves steps than yes it will affect the strength. Sharp corners are stress risers, no matter if small. Better to interpolate with a ball- or corner radius endmill in that case.

Allright, thanks guys.

 

[Orange Vise]

Anything's better than nothing.

Optimally, hit it with a ball end or large corner rad endmill. Surface finish doesn't need to be crazy. Your goal is eliminate sharp inside corners. Light scalloping from a ball endmill doesn't produce inside corners.

Larger fillets spread stress concentration better than smaller fillets, even if the larger fillets have a higher RA.

and the purpose of this radius is to prevent cracks in the part

Not so much to prevent the cracks. Micro cracks already exist in the material. Reducing stress risers prevents existing cracks from opening up under load and causing part failure.

 

[guythatbrews]

The simple answer of surfacing vs. forming is the best surface finish rules. This is a complex topic and i expect for critical applications there is not a simple answer.

The purpose of the radii is to reduce the stress concentration factor, and hence increase fatigue life of ductile materials, or decrease catastrophic failure for brittle materials.

The bigger the radius the better. Cold working, such as roller burnishing or shot peening, adds compressive stress to the surface and so increases fatigue life.