Interesting! Being that it was made in China, I was leaning towards sloppy manufacturing rather than rational and intentional design. As I noted, it seemed to work OK with just 3 solidly connected bolts. BTW, the stripped one is to the operator side. All the static load would be on the fasteners on the other side. Dynamic stresses would basically have to overcome the weight of the head before putting much load on the front-side fasteners, no?
Not necessarily. If you stop thinking about overload stress failure and think instead about fatigue from cycles, an excessively loose front fastener can fail. Since you don't have the original failed bolt, you will never know for sure, but it is possible to check for other evidence. A stress analysis would require modelling and strain gages - which are more work than its worth. If it were me, I'd just duplicate whatever the factory did.
A few comments. I bet that head weight can be easily lifted by the quill crank. How much down force was being applied by the previous owner?
Cyclic stress and strain does not need to lift the head to fail the bolt. A front bolt could fail from fatigue without ever being overloaded.
I have no idea what the design limits were. Nor could anyone but the designers themselves. I am only pointing out that the whole situation might not be so simple.
I'll give you a classic example. Have you ever seen a failed wheel stud on a car or truck? They happen all the time. Most people assume that some gorilla over torqued the bolt and it snapped a few days later or maybe going around a corner. This is NEVER the case. It is ALWAYS the result of undertightening. The nut or bolt will always strip before it breaks from overtorquing. And it will almost always snap in fatigue failure if it is under torqued.
Anyway, it is not very hard for me to imagine an excessively loose bolt failing on a mill that does so much bouncing as it cuts. This was actually one of my hates about my mill drill. It was never very solid or rigid compared to either of my big knee mills. I've had that thing bouncing around like a male rabbit way more often than I should have.
So before everyone piles on, be gentle - I'm old. These are all just pure conjecture on my part noodling the possibilities on something I have not seen in person.
My best guess is still the gorilla on the wrench overtorquing a steel fastener in a cast-iron hole. But that is just my best guess.
In all reality, I'm still on the page of installing a few helicoils only in the failed location, use original size and grade of fasteners, and torque to specifications - no more no less. Drilling, tapping, and installing helicoils is pretty easy to do with hand tools because everything follows the existing hole and the existing threads.
I'll head out to the barn shortly and I'll let you know what is on mine. I'll also do a little noodling on the what-ifs while I'm looking at it.
FWIW, I've learned that
@RobinHood is pretty good at forensic failure analysis. If I were you, I'd be interested in his opinion on this one too.
