homemade Wood Router - project failed And almost got me seriously injured- READ DESCR

[diyfixman]

After a test I did I came to the conclusion that the converter was broken because it was very thin about 6mm and it was broken just in connection with the wheel

[pfredX1]

You mean the adapter was damaged?

[diyfixman]

What is broken is the axis of the engine
Tomorrow I'll take a close-up of what's broken

[pfredX1]

Oh you mean the motor shaft? The bit that sticks out of a motor and spins is the shaft. Though it does rotate on an axis I suppose. That's bad if it broke. The chuck must have really been out of balance.

[diyfixman]

Yes, probably everything was balanced by the time I touched the tree

[pfredX1]

You are right about that. Touching the tool to the wood may have balanced it good for you.

[pfredX1]

Even if it worked them return springs were too light. I know stock they're too heavy but what I saw here is too light. I got three commercially manufactured plunge routers. You have to push them pretty good to get them to go down. Just how they are.

[Big Sexy]

I am curious if the motor shaft had a flaw in it. By putting the chuck on it with a sideways load, it wasn’t designed for, that caused it to fail. One of the 2 would be my guesses. But I am not an engineer.

[CanBeDone]

The failure was predictable. Here is the offending frame from whence the chuck flying off the shaft was inevitable:

You see here the chuck being fastened to the shaft using an Allen key.
Thus:
1. there is no means on the shaft to prevent the chuck travelling along the shaft, except friction.
2. When cutting, the shaft will flex, setting up a pattern of (rotary) vibration. This vibration is enough to cancel the friction force holding the chuck in place. --> BOOM

I have had the same issue on several of my routers when using 1/4" tool bits. None of the bits flew off, but they dug deeper and deeper into the wood - until I noticed and switched the machine off. Tightening the collet harder did not help at all. Sharpening the bit did help, however, at least for a while. Only one action eliminated the problem entirely: use bits with a 1/2" shaft.
So far the observations. Once you consider that halving the diameter of a shaft makes it flex eight times as much, and that chattering of a tool arises whenever the forces acting on your tool times the lever arm exceed the stiffness of your setup, you have it.

There is no indication in the video that the shaft actually broke, nor that it bent either. Thus, I take it that the shaft came out unharmed, and that the use of "broke" is an exaggeration ascribable to the shock of the failure happening. Further, there is no reason for Makita engineers to specify a steel with 0.8% carbon or even more to make a shaft that can fail by brittle failure. Why use steel with a yield stress of 2000MPa (about 60 Rockwell C) if 500 or 600 MPa, or twice that of mild steel, is more than enough? But there is plenty of reason to make the shaft thinner than 6mm, 5mm perhaps, or even 4.5mm. Maybe diyfixman can provide us with this kind of information?

[Claudio HG]

There is no indication in the video that the shaft actually broke,

If you read carefully at the end of the video you see the shaft is broken. Even the OP confirmed it in a previous comment.
But you have a point about the allen screw fastening (I didn't thought at it at first sight): that certainly induced a vibration (an oscillation of the moment with the fulcrum at the bearing) that made the shaft subject to a pulsed shear/bending force exacerbated by the contact of the bit with wood that, because its veins, induced further variations in the moment.
This could have been coumpounded and potentially resonate, increasing on some instants the peak of said shear force.

About the motor shafts, these are usually thinner in universal motors to reduce instabilities and increase lighweight when they are meant to hold axial forces only. In the industry it is usual to make these shafts with a particular grade of steel (that vary among manufacturers) able to widthstant the axial force despite the reduced section. Also AFAIK often they haden the steel (not as a file, though) at the location where the shaft will be in contact with bearings: I've seen a manufacturer of wiper motors that uses an induction coil to harden the steel where the shaft will mate with bearings.
I am not an expert in metallurgy neither a mechanical engineer, but my experience was that motor's shafts generally are harder that C45 steel and brittler, far less than a file though. So I do not dispute your reasoning about the choice of steel, actually I second: the steel will be likely somewhat around 700-900 MPa, thinner than what it should have been if it were meant to withstand mixed, pulsed forces, and this is a further reason to see it broken when a pulsed shearing force is applied to it.

EDIT: Grammar.