Wednesday, June 24, 2009

Cutterhead Distortion

On one of the forums that I contribute to, there was an interesting discussion about the effect of removing all of the cutterhead knives (from a jointer specifically) at the same time- thinking it might allow the cutterhead to distort.

The original poster said: “Re-reading Duginske's "Mastering Woodworking Machines" on the procedure he mentions that removing all the knives at once can de-stress the cutterhead to the point that it loses its cylindrical shape and becomes distorted.”

The “Duginske” he is referring to is Mark Duginske, who has authored many books on woodworking tools.

My reply to him was: “Personally, I have NEVER seen, or heard, of a cutterhead distorting 'because all of the knives were taken out at the same time' I guess I have to go on record as disagreeing with Mr. Duginske.”

The ‘cutterhead distortion’ possibility comes into play when the knives are put back in.
When you re-install your knives, put one knife in place and only tighten it enough to hold it in place, do NOT fully tighten it in the slot.Then install the next knife (assuming it is a 3-knife cutterhead) and again, only tighten it enough to hold it in place.Then install the 3rd knife, but this one you can go ahead and fully tighten it when you have it set right.Then go back and fully tighten the other 2 knives.The point I'm making is that if you fully tighten a knife when the other 2 slots are empty, you WILL be causing the cutterhead to distort, possibly to the point that you won't be able to get knife/lockbar #3 in place because the slot will be squeezed.

One of the more experienced fellow had never heard of this happening, so I brought him up to speed: “If I hadn't had PO'd customers who actually did this, I wouldn't have thought so either. It got so routine that when they called complaining about not being able to get their knife/lockbar into the slot- our FIRST question was, 'did you take ALL the knives out'? An affirmative answer got them the instructions I gave above.I understand what you're saying, but think of it this way- if you look at a cross-section of the cutterhead, what do you 'really' have? Not a 'big iron hunk of a cylinder', but basically three 'wings' that CAN be wedged apart by using those 5 or 6 "small screws".I've also seen guys chase their tails trying to figure out why one knife is always higher than the others...and it was also a result of the wedging effect.I've got to stand by my advice- either remove, replace one knife at a time OR be very gentle reinstalling them until all slots are full.

As another fellow said- this possibility also exists on planers and it is even more imperative that it is accounted for.