Knife Weight Balancing Tolerances and Scale Choices

      Paired knives on a moulder should weigh within a tenth of a gram of each other. Here's advice on measuring and achieving that. October 19, 2013

Question
I'm curious as to what guys who grind their own knives are using to balance them? I've read many times about the importance of balancing the knives but have not found any info on how to do it.

I occasionally re-shape knives for my Euro head and I've been thinking I should really try to balance them. The head is pretty small, and the shaper pretty big - but I figure better safe than sorry! So what type of scale should one look for? Would a cheap kitchen scale do the trick, or do I need something more accurate/costly?

And I guess more importantly, what tolerances should I shoot for? I realize there are a lot of specifics that may come into play. So let's just say for the sake of this thread a 2" tall, 4" diameter Euro or corrugated head, run on a 1-1/4" spindle heavy shaper.

Forum Responses
(Architectural Woodworking Forum)
From contributor D:
You can use a Triple beam or a balance scale to determine they are the same regardless of actual weight. A digital scale as long as it weighs grams would be the easiest.


From contributor M

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On moulder heads you need your knives balanced to within 1/10 of a gram spinning at 6000 rpm's. I would think you would need the same for a shaper. The issue is the forces exerted on the bearings of the machine with an out of balance tool body. An Ohuas digital scale that reads in 1/10 is what I use for my moulder.


From contributor L:
We do the same as contributor M with the same balance scale. They aren't very expensive. We also balance to 1/10 gram, though I suspect you wouldn't do a lot of harm to a heavy shaper if you were off somewhat more. Turning the spindle speed down quickly reduces the forces. You then have to feed a little slower to keep your knife marks where you want them.


From the original questioner:
Thanks! So kitchen scales are out since they only go to about 1 gram. The balances are fairly reasonable, but I'm ordering one of the Ohuas triple beams as it seems like it may be more useful in general over time. More money than I'd like to spend, but probably cheap insurance over the long run.


From contributor K:
I use a triple beam, then grind off the back end of the knife to get them equal.

From contributor D

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Of the 500 plus knives we have at work, most were never balanced before I started working there. Now armed with a triple beam, I am weighing each set before setup. I am finding differences on average of 2 - 10 grams. One set was off 50 grams. The first thing I check is to see if steel was cut to the same length. Then I find the true end and grind from the other. Only found one knife so far with an extra row of corrugations. 1/10 of a gram is what I shoot for.


From the original questioner:
Wow, 50 grams... Seems like that would be noticeable when running!

Luckily all my knives are starting out pretty good. They're either knives for a Euro head I've modified, or corrugated steel that's been professionally ground and I'm just fine tuning a bit.

This question arose because I had to alter a set of knives this week for some molding. Worst thing is the knives were new, I just changed the way I wanted to run them and had to grind the ends a bit for a relief. I ended up removing about 1/16" of profile length on them as well and while they cut fine, (benefit of having a really big shaper), I know they're out of balance by the sound. Kind of a harmonic wobble, for lack of proper terminology?

So I think the $130 the scale costs will be much less expensive than the $500+ new bearings would set me back.



From contributor W:
As a side note, you will also find holes part bored to remove weight, even in modern cutters and assemblies.


From contributor R:
For a small shop a simple O'haus balance beam works best. The triple beam will work just as well but is not as efficient, as in more time consuming. You don't need to accurately weigh them, just make sure they weigh the same. Ebay.


From contributor C:
There are decent enough digital jeweler scales on Ebay for $20 that will read 1/10 gram - look for .1x3000g, up to 6.5 pounds.

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