Most people find it difficult to say I messed up but this post has probably taught a lot of folks a most valuable leson. Thanks for a visual warning, besides the shaft failing the rest looked great
diyfixman (Oct 15, 2019)
I've heard you should never side load drill press bearings. I mill with a 3 jaw chuck in my mill all of the time. Not a problem. I have collets for my mill too. Just not enough. The last slot I milled the day before yesterday I put an end mill into my drill chuck. I don't have a 5mm collet for my mill. I needed that slot milled though. I didn't know if that end mill could cut steel, or not. It was an el cheapo import jobbie I'd bought for my CNC engraving machine. I have all of the collets for it. It only has an ER11 holder though. And there's little chance it can mill steel. I never tried and I doubt it can. It's a soft materials machine. Wood, plastic, stuff like that. Maybe some aluminum. The OZ25 on my mill is a bit harder on the wallet to fill out the set on. I'm thinking about picking up an ER32 holder for it. I could get the holder and a 9 collet set for about what 3 OZ25 collets would cost me. I feel burned either way so I haven't done anything. I've heard some bad things about some of them ER32 holders too. Which makes me leery to get involved. I know my OZ25 holder is good. I've been using it for decades now. Collets for it are just so damned expensive though.
I don't think the speed was too fast for routing, just too fast for your under spec'd connection with a too heavy lump of non-concentrically turning metal. Tiny, short shaft, loosely fitting sleeve, held on by set screws. Couldn't expect that to resist all the forces trying to tear it apart. Because of the length of your chuck and bit combination, the effect of the first little bits of deflection, due to imbalance and the load on the cutter, rapidly became magnified and ran away. Lee
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.
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?
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.
Last edited by Claudio HG; Oct 22, 2019 at 02:26 AM.
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