internal thread stress in thin walled casting

[1980c3]

Hello all,

I have a scenario in which I am tapping m10x1.5 threads into cast iron close to an edge - ~0.120". (imagine a 2x2 that is 6 inches long, with a hole drilled in the end faces off-centered such that the edge of the hole is .120" from the sides).

This distance should not move - it's pretty darn controlled dimensionally.

I am a touch worried about the small remaining wall thickness... I know that there are a few "rules-of-thumb" out there with respect to minimum distance to edge, but they are not always practical and arguably excessive. However, how would I quantify the local peak stresses I would see in the internal threads with a engaged thread length of about 20 mm? these are metric threads so there is no radius callout at the root of the thread, so I'd have to assume something geometrically...

Due to the clamp force, I don't really expect that much transverse forces imposed on the threads, should be mostly axial.

I have tried using FEA detailed such that the thread geometry, but it's not very clean at all, as you can imagine (levels around 400 ksi σ1 but I am fairly convinced these are just singularities)....

Anyways, any suggestions to what ails me above?

TIA!

 

[rb1957]

M10 is 10mm, about 3/8" ... right?
0.12" is "tiny" edge distance.
I hate the idea of tapping a casting.

If you are worried about it, or if it is important, then maybe run a test to get an allowable load ?
If you run FEA, make sure to do NL (400ksi is clearly a linear FEA).

another day in paradise, or is paradise one day closer ?

 

[1980c3]

hi rb1957, thanks for the reply.

Yes, the analysis is linear... I suppose I could run the analysis NL and compare strain levels. I usually i am a bit concerned with NL analyses and castings since they are so brittle (not much room for plasticity) but in the case of singularities, I expect them to sort of 'average' out in a sense..

as silly as it sounds, how else would you affix something to a casting? you cannot weld it (easily) but i do agree that tapping something so brittle is asking for trouble, yet engine castings are full of tapped holes.

I look forward to your response

And I think paradise is one day closer lol.

 

[rb1957]

true you can tap castings, as you say engine blocks do it all the time, but what is their process around this? special control of casting process ? very low stress level (not considering the thread 'cause that'll light up like a christmas tree) ?

Maybe look at the nominal stress in your part (without the thread) ?

Again, if worried or if important, test.



another day in paradise, or is paradise one day closer ?