This has been an intermittant problem for years, but having just experienced it again today, I figured I would ask all you experts “wuzzhappnin?”
Today – milling out ~150 BF of 8/4 rough sawn African mahogany. When both flattening and edge jointing, I would periodically end up with one or two knives rendered ineffective due to chips jammed between the cutting ege of the knife and the top (coved) edge of the knife retaining plate. I could always tell when it occured because the sound of the cut would change (louder).
I would clear the jam using my trusty Leatherman, being careful to not contact the cutting edge of the knife, but all too soon, it would happen again, and I’d have to shut down, clear chips, etc……
The jointer is a PM Model 50, perhaps 20 yrs old. The knives have been recently replaced, having been sharpened by Forrest Industries. I can’t tell you precisely how heavy a cut I was taking, but it was certainly less than 1/16, possibly as little a 1/64. It was difficult to tell whether it was always the same knife(ves), and the jam was sometimes the width (or nearly so) of the material, sometimes only 1/2″ or so wide. Sometimes only one knife, sometimes two, never all three.
It is my guess that the problem is caused by either excessive feed rate, depth of cut or a combination of the two. Or (I guess) it might be a knife misalignment problem, but as the quality of cut when it was cutting properly was universally excellent, I doubt it.
Anyone want to weigh in on this? Thanks in advance.
Replies
Mark the cutterhead with a sharpie so you can tell if it's always the same knife. I'm wondering if there's some foreign matter between the blade and the chipbreaker.
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How far do the knives hang out past the cutterhead?
Thanks for the replies.
Interesting that there is no confirmations (yet) on my suspicions.
The knives were set at the PM specifications above the cutterhead.
There is no DC on this machine. While there is some accumulation of chips inside, there is (was) also a large pile of chips on the floor below the chute, as one would expect, considering the BF being milled. Certainly most of the chips are escaping - perhaps not enough? I dunno.
Sounds to me like either a dust collection problem or poorly designed blade clamps (can you file them? Is there grease on them attracting debris?). Even if you are using good dust collection is there a major clog just within the machine that you can't see from the outside?
Brian
I don't know if this applies or not, so take it for what it's worth. I have an older jointer which I retro'd with dust collection. I have had the inside casting of the jointer completely pack with chips, so there was no place for the chips to go except out the top of the jointer. I solved it by allowing more air to flow through the jointer's body by opening up a gap around the belt shroud. I think the problem arose when face jointing boards which completely covered the top of the jointer. Don't know if that helps...
I am with David in thinking it might be a shortage of air intake > flow.
On the front of the knife is high air pressure, but just behind is ####very low pressure, which is pulling the chip into that area due to the fact that there is just too many chips in the air down there.
Are the knives installed with the flat side toward the infeed table? Also, does the bar that holds the knives have the concave edge next to the knives?
Life is what happens to you when you're making other plans .
"Life is not a success only journey." Dr. Phil
Yes to both.
I am sorry I misread your first post, so I am sure my earlier post sounded dumb. If the chips are jamming between the knife and holder, I would suspect that maybe the holder has been dropped, and now has a ding along the edge. There should not be any room for chips to get in there. Next time you have the knives out, I suggest that you hone the holder on a flat plate. If there is no ding, then there must be a slight rounding in on the holder. The wood is splitting out ahead of the knife, and finding a place to jam in that V. There is an ideal cutting angle for each kind of wood, and sharpness angle and feed rate. If you were in an industrial situation, you wouldn't want to use as high a cutting angle for hardwood as you would for softwood, day in and day out, so the heads can be bought to suit either one. However on this level of tool, there isn't that choice unless you bought one of new heads with segmented knives. Another option is to sharpen with a face bevel. My jointer is an old Brown and Hall 12". The knifes are set at 30º, which is too sharp for hardwood, so I grind a 20º face bevel, leaving a 10º rake angle. This does require a more powerful motor, but I can surface the wildest grain hardwoods without any tear-out. This would also give the added benefit of deflecting the chip more forward of where it is jamming.
Thanks everyone for your replies.
Back at work today, I took a careful look at the knife/retaining plate assemblies. There doesn't appear to be any degradation of the retaining plates; the knives look and feel good, as they should, being recently sharpened as I mentioned before. The cove recess in the plates is not particularly well machined: it's a bit wavy, but doesn't appear to be so bad as to cause any "stickiness". There is no gunk in the recess between knife edge and retainer.
I believe the knives are sharpened at 40 deg.
When the problem occurs, there are no chips jammed between the knife and the retainer, they simply build up under the knife. A pocket knife blade run lengthwise in front of the jointer knife will clear out the jam, leaving no remainders to be coaxed out of any cracks or crevices. This can take a bit of effort, however, as the chips are tightly compacted.
I'll describe the problem in a different manner: it's like when you've scratched a chunk of wax or something similar - the material gets packed up under your fingernail, the fingernail in this case being the jointer knife.
Any further thoughts?
Magnetic wood? ;-) Sorry, couldn't resist!Work Safe, Count to 10 when your done for the day !!
Bruce S.
Refer to "WORLD WOODS IN COLOR" Linden Publishing Co. Fresno, Ca. copyright 1986 Page 158 - "Mahogany - African"
"Working Proprerties" There is a moderate blunting effect on tools, and
tension wood or brittleheart and interlocked grain can cause wooliness. To avoid tearing the grain a reduced cutting angle from 15 to 20 degrees
is desireable.
This so called "wooliness" may be the cause of your problem if your knives have been sharpened at 40 degrees as you stated.
I have no experience with african mahogany - I work mostly with walnut,
cherry, ash and oak.
Interesting.
This being the first time I've milled any quantity of mahogany, African or otherwise, it's worth further study.
That said, I refer you back to my original post. This problem has occurred several times in the past, when working with domestic hardwoods: cherry, maple, birch and red oak. I can't tell you if one of these species was more problematical than another, though, just that it has been a problem prior to the mahogany session. It's just now that it has occurred to me to seek advice in this forum.
There’s really no question in my mind that those who suggested that the chips were getting caught in the chute and forced back out through the blades had it right. The jointer is the one piece of equipment that I feel needs dust collection the least, however, it’s a nuisance when it gets packed full. It usually gets stuck in the area where the top of the stand meets the bottom of the jointer. You have to reach way up in the chute and clear it out with a stick or your hand. Turn it off first! My current jointer gets packed more easily than my last one, but the last jointer I had was used in an arid climate where wood rarely exceeded 6% MC. I would guess that the wetter the wood, the more easily it could get packed full. Even though chips may be still coming out of the chute, they are not coming out in the area where it is packed. Species of wood doesn't make any difference.
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