Tolerances for grinding a 43" cutter blade

I work for a printing company, and our paper cutter has developed an
unusual problem-- the blade cuts OK at both ends, but not in the middle.
I think the sharpening service is suddenly not grinding them straight
anymore. Before I accuse them of screwing up our blades, I'd like to
know what a typical tolerance would be for a blade that's 43" long and
3/8" thick.
I measured one of the blades with my Chinese digital caliper, and found
that it was .02" wider at both ends than it was in the middle. Even
assuming some error in my measuring, that seems like too much,
considering that some of our paper is only .003" thick. The blade cuts
into a sacrificial plastic "cutting stick" which is replaced
frequently-- I found a note that says it should cut into the stick by
about .004". I'm sure we're setting it deeper than that to get a clean
cut, and it's making the blades get dull faster.
The blade is adjustable for angle, so it's OK if it's slightly tapered,
as long as the edge is straight. The last person who changed the blade
told me that when she adjusted the angle so it cut at both ends, she
could still see daylight under the middle of the blade. I haven't
checked the cutter's table with a straightedge, but I'd be surprised if
it was warped that much.
Any guesses as to how they managed to grind a curve into our blades?
If I'm right, they've done this more than once, over a couple of months.
Reply to
Ron Bean
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Depends on how they're doing the sharpening.
(I happen to own a similar paper cutter, a Triumph 4810A, with some experience machining.)
Given that the intersection of two planes is a straight line (that is, your cutting edge), and you don't have a straight cutting edge, one or both of your planes has gone curved.
Given that the blade tends to wear faster in the middle, perhaps your servicer is just following the wear pattern, such as by freehand sharpening, instead of using proper fixturing in a surface grinder? Freehand, or any non-rigid method, is going to generate worse and worse curves, instead of planes, as time goes on. You need the rigidity and precision of a fixture and a surface grinder to maintain flatness.
Ask him exactly how this sharpening is done, what tools, what process, what metrology. I suspect not the right ones.
Ask him exactly how he measures that the results are in spec. This is absolutely required of a precision task such as this. You can't just wing it. I suspect little or no quality testing is being done.
Reply to
Richard J Kinch
Sounds like their grinder is badly worn out
If the grinder table is sagging badly..it will be thicker on the ends than in the center. So whichever one of the grinders is giving you that bad grind..needs to be identified and not used on your blades
Gunner
Rule #35 "That which does not kill you, has made a huge tactical error"
Reply to
Gunner
Damned straight.
er....ah..well....shit..you know what I mean
Gunner
Rule #35 "That which does not kill you, has made a huge tactical error"
Reply to
Gunner
snipped-for-privacy@shell.core.com (Ron Bean) wrote in news: snipped-for-privacy@corp.supernews.com:
The thickness difference might be the problem, if the sharpener is not using a standard magnetic based blade sharpener. If they are not grinding the thickness to true the blade up, and clamping on the ends only, the blade may be bowed up. After the edge is ground, when the blade is released from the clamps, it's got a bow in the edge.
(Used to grind a few of these for guillotine type CNC paper cutters years ago on a purpose-built blade grinder) You might be better off doing what the company I worked for did and just purchase the grinder and have the maintenance staff do the grinding (after training).
Reply to
Anthony
That type of paper cutter is like a Guillotine. Only one blade slices thru the stack. Not like a shear as is used the trim sheet metal where two blades scissor the stock. I have used both types alot over the years. In the metal shear if those blades aren't true you will get a bad burr. I usually adjust them so they will cut newspaper cleanly. Because I was cutting small nameplates from .020 aluminum or thinner, no burr was acceptable. The paper cutter blade only needs to be straight along its length when sighted end to end. If it has a hump in it when you look at the face of it you will just go thru more of the sacrifice strip. Most people lay a few sheets of scrap under the stack anyway just to make sure the bottom sheet gets cut cleanly.
Reply to
daniel peterman

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