How do you know when to replace jointer/planer blades

[Brian Runau]

Most often I replace my blades because they get a nick in them, despite my best efforts for this not to happen.

If blades are not noticeably nicked, how do you know when it is time to replace your blades? Mine are full size single blade set of 3 or 4.

Thanks.

Brian

[Kevin Jenness]

When the knives dull cutting is louder, hotter, takes more effort and it becomes more difficult to mill a straight edge. A sharp edge will catch on a fingernail drawn across it, a dull one will slide. Pretty much the way you know if a hand tool is dull, except the condition will creep up on you over a longer period of time with high speed steel.

A nick in a sharp knife can sometimes be canceled out by loosening and sliding the knife sideways.

[Mike Kees]

Everything Kevin said. I will add that if you buy quality HSS like T1 or 18 % tungsten that they stay sharp and wear way longer than the cheap knives most all machines come with. I sourced knives through my sharpening service.

[Tom M King]

They make more noise when dull. When they sound like they're shearing wood, they're sharp.

[Stewart Lang]

Dull knives are dull, but there's a huge range to describe "dull". It's largely opinion based. One person's dull is another person's "I could run that for another 6 months."

The best way is install a new set of knives, and keep a close eye on the cut quality in the beginning so you have a standard. Over time, once you get unhappy with the cut quality, get new ones again.

[Warren Lake]

if you have a good light and good magnifier you will see the edge as well if its sharp or round but what the other guys said is good. Fresh knew sharp knives are so quiet. Does anyone have an amp meter installed on their machines so it shows when its drawing more.

[Bill Dufour]

When the knife is shorter then the gib when it is set up for a cut. The gib has to bear on the knife at the top and bottom edge of the jib. I would say once the knife has lost about 1/3 of it's original height.
Bill D

[Mel Fulks]

Everything Kevin said. I will add that if you buy quality HSS like T1 or 18 % tungsten that they stay sharp and wear way longer than the cheap knives most all machines come with. I sourced knives through my sharpening service.

Mike, thanks. I will just add ,don’t buy knives hyped as having “same hardness “ as M2.

[Kevin Jenness]

When the knife is shorter then the gib when it is set up for a cut. The gib has to bear on the knife at the top and bottom edge of the jib. I would say once the knife has lost about 1/3 of it's original height.
Bill D

I thought the question was about changing out knives when dull, but you are absolutely right about replacing a set when they are worn down so far that the gibs won't hold them securely.

[Brian Runau]

Thanks to all. Brian

[Warren Lake]

sorry read wrong replace I thought fresh replace you meant changed out. Not the first time.

[Richard Coers]

You can use the old hand plane or chisel trick. Put the face of a finger nail on the cutting edge. Pull slightly towards you. If it's sharp it "grabs" the finger nail. If it's dull, the nail easily slides. Of course you never push down with any force and you never bring any flesh near the knife.