Why the need for new belts?

K

Karoo

Joined
Jan 26, 2017
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A beginner here.
It is always said to use a new, sharp belt on the important parts of the knife, such as the bevels.
I am not disputing this, but want to know the reason why, please. Is a worn 60 grit not just the equivalent of an 80 grit, etc.?
 
one of the reasons is heat. if the belt is not sharp, it makes much more heat from the friction. if handle material becomes too hot, it can discolor from the heat, as well as the epoxy bond being ruined. also grinding the handle bolts with a dull belt can get them too hot and and leave dark circles around them on the handle material.
 
Karoo said:
A beginner here.
It is always said to use a new, sharp belt on the important parts of the knife, such as the bevels.
I am not disputing this, but want to know the reason why, please. Is a worn 60 grit not just the equivalent of an 80 grit, etc.?
Click to expand...
Depends on what you are doing with the belts. Nice sharp belts cut faster and cleaner than dull ones. If you are trying to clean a blade up after heat treat, then a dull belt may come in handy. However, be careful not to ruin your heat treat because dull belts overheat much faster.
 
A worn belt isn't cutting as well it just rubs whatever you are grinding. The thinner the steel the quicker it heats up and you can ruin your temper.
Also a worn belt isn't going to give you as even of a finish.
 
Karoo said:
Is a worn 60 grit not just the equivalent of an 80 grit, etc.?
Click to expand...
I see where you're going with this, but no, the grit doesn't wear down evenly as you grind and you can still have the odd 60 grit chunk that will leave a deep scratch. But yeah, it'd be great if that's the way the world worked.
 
You also want to be careful with other materials. Not just metal. Paper Micarta burns very quickly and if you aren't using a fresh belt, you could ruin it. I've had this happen with other non knife projects with clear epoxy, as well. When in doubt, use a fresh belt or only hand sand.
 
Belts don't often wear the same across the belt and it can effect your grinding. If youre trying to make a knife with a crisp grind line it will be very difficult with a worn belt. I've found myself chasing and chasing a smudgy grind. Then changing the belt and suddenly it's crisp and clean after just 1 pass.
 
I have a buddy who also makes knives and when he comes over he is constantly rooting through my garbage can to scarf up any worn out belts. I’m not the be-all to end-all on every knife discussion, but I always tell him he’ll never get as good as I am now (after a long learning curve) trying to get $10 worth out of a $2.50 belt. More projects have been ruined or screwed up due to using dull belts, or other cutting instruments, than most would care to admit.
 
You need realize that belts are cutting tools and once they go dull they no longer cut. They can still remove material but it starts to come from a high friction rubbing action. There is a in between area where they still cut somewhat but also give a lot more heat. To cut the belt must be able to bite into the material so the hardness of the material is a factor so a more worn belt may still cut well if you increase the pressure or drop the hardness of the material. Many belts are made to be self sharpening and are designed for the grit to fracture but this takes speed and pressure. When the grit fractures it is not as sharp as it was when it was brand new. For knife makers this can make a difference because it is now going to take more pressure to cut at the same rate. This often isn't a big deal when hogging of steel on a quarter inch blade where a bit more heat isn't going to hurt anything and you can lean in as hard as the miter will let you. On the other hand extreme you have a chef knife out of a steel like CruWear at 66Rc and just a few thousandths thick out of thin stock that is hard to put in a lot of pressure and once the sharpness is gone they will tend to glaze belts. The only way to get around that is to increase pressure and for us that means surface area since we don't have a flood system to keep things cool while we load up a weighted sled to hold it down. Some people who use lighter pressure switch to zirconia belts because they fracture easier. You have to play with the way you grind since the combination of pressure, material hardness, size and surface area of your blade, if a platten or wheel is used, how the blade is held in the platten, how comfortable you are with different types of belts at different points of wear and probably much more is very unique to a person and then you compare that to your budget and the surface finish and type of grind you need as well as the equipment you have and if that's the equipment you want vs the equipment you can afford and you eventually get different answers to the same question. I will aslo say that if you rotate belts so you can use certain levels of sharpness to different task you can probably get a bit more use out if them but once again that's up to how you are ok with doing things.
 
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