When do you turn carbide insert cutters?

[teakwood]

How do you know when you need a new, sharp cutting edge?
On my 25" one side planer I noticed some tear outs on the wood surface so I knew it was time. But on the 4 sider it's alot harder to tell. The first cutterhead (under side) is only 5hp and I can hear it bog down under a heavy load. On the VFD I see the amps jump from normal 6A to 9A and hear the head lowering rpms. Ok, I turned the cutters, spiral head, and with the new edge the symptoms went away. But now after only 10h of use I hear the head bogging down again   ???. Touching the cutting edge with your finger you can't really tell a difference in sharpness. How do you guys know when to change them?
On the one sider I can use them for months.

[YellowHammer]

Typically, when I see tear out and bogging, although the real indicator for me is noise.  When I can hear them "slapping" and the noise has about doubled, it's time to turn them.

[Don P]

Look down at a new edge in strong light, there will be no reflection back at you from the edge. It comes to a point and splits the light at the edge. Then look at a dull edge in the same strong light and you will see a white line reflecting ligth back at you, a "land", a flat or rounded spot that would take a 10x or better hand lens to see but that white line has a width dimension that you can see with the naked eye. That is "dullness". Pick your pain point, I went with that. I've seen some go with the ammeter, and as YH points out, it gets noisy and what does the finish look like? Running dull is usually no bargain no matter how dull I run it.

[longtime lurker]

Silica is tough on them, and teak has a reputation for being full of silica.

 I'm looking at finish quality, listening to how it sounds, and I might start to feel a little bit of slip in the feed rollers which I assume is poor finish creating more friction with the table.
Amperage is a really good indicator of bluntness, never had the gauge to tell on a moulder but I worked a wide band resaw with an amp gauge and pulling more power to do the same amount of work was a sure sign of bluntness.

Mostly I have no issues telling when the top and bottom knives ( both straight knife heads) are dead. But I've run the pencil round cutters to the point of delamination of the carbide edge a few times. Too much HP, too much noise, not paying enough attention to the parts no-one will ever see.

[longtime lurker]

Another thought. 
It's always my first bottom head that runs blunt first. I usually double pass ( 4 header) for profiled products, the first being to square my blanks and the second to actually do the profiling. I would normally set to run 2mm off the bottom first pass and skip dress the top at 23mm or so. Second pass is 1mm off the bottom and top head set for a 19mm finish. So theoretically the top head is actually pulling off more wood between the two passes than the bottom so it should go blunt first right?

I think it's grit. Boards occasionally get dropped on the floor. Forklift tynes that aren't always spotless when packs are being handled. Packs getting placed on bearers that might have embedded fine sand from being out in the yard and then used wrong side up. And also maybe some recutting of sawdust because the dust extraction from that head isn't as good as the other heads. Maybe something else I havent figured yet.

[customsawyer]

When I was reading the title, my first thought was "Not until I have to". It is not a job I enjoy much but it is a necessary evil. 
I'm with the others in that I go by finish, sound, and amps when available.  I like the light info just passed down.

[teakwood]

When I can hear them "slapping" and the noise has about doubled, it's time to turn them.

Thanks for all the good advice. you can hear a slapping noise on a spiral head, i get it on the straight knife but spiral.

[teakwood]

Another thought.
It's always my first bottom head that runs blunt first. I usually double pass ( 4 header) for profiled products, the first being to square my blanks and the second to actually do the profiling. I would normally set to run 2mm off the bottom first pass and skip dress the top at 23mm or so. Second pass is 1mm off the bottom and top head set for a 19mm finish. So theoretically the top head is actually pulling off more wood between the two passes than the bottom so it should go blunt first right?

I think it's grit. Boards occasionally get dropped on the floor. Forklift tynes that aren't always spotless when packs are being handled. Packs getting placed on bearers that might have embedded fine sand from being out in the yard and then used wrong side up. And also maybe some recutting of sawdust because the dust extraction from that head isn't as good as the other heads. Maybe something else I havent figured yet.

Yes, silica is a problem and i don't expect my cutters to hold up like i would been planning pine wood.
I do 2mm first head and 3mm the upper head and yes i noticed that the last head should go blunt faster because it works a lot more than the first head but this is not the case. also my upper head has 7.5hp and is way far away from me because i'm infeeding , so i first hear the bottom head. also the bottom head has more hp drawn because of a big reference ingroover it also needs to turn

[GBar]

As others have indicated, the finish cut is what counts.  At the risk of being Captain Obvious, "if it is working, keep working."  It is your call on when the finish is NOT good for your application and you need to change to a sharper cutter.

As mentioned teak with silica in the wood, is hell on tooling. About 40 yrs ago, I had a big teak job in San Diego. It was a multi-million dollar remodel.  Milled several thousand bd feet of teak for outside decking around pool, around house to huge deck, and down spiral staircase to beach...oh a teak hot tub too.

The planer was an old Oliver 40"...I think.  it had a rotary grinder on the top on a worm gear shaft.  Could run a small amount of teak until it did not cut smooth and then had to stop... and wind the grinder over each of the four knives.  That was a painfully slow process...as I had to raise each knife after a few grinds.  The worst part was the hand routed finger joints on the end of each board. The fingers were literally the size of the fingers on your hand, about 4 inches long.  I had to route each end with an aluminum guide template with a half inch Whiteside carbide bit. would get to the end of the fingers and the last one would tear out...and start all over again. Put on a new bit and go.  I went through several dozen router bits on that job, which took me over a month to mill all the material. Also had to do a shiplap rabbet with 1/8 gap for the urethane rubber filler. 

So I feel your pain.

The good thing about your setup is the carbide insert cutters, which are normally better than brazed on carbide. Won't get into the ratings of C-numbers (C-1 to C-n), but there are higher C-numbers that do not braze well (a cutter mfg rep mentioned they crack when brazed). Generally, insert cutters can/will use the higher C-number that are tougher for extended cutter life and applications like teak. For example, C4 or better is common in insert cutters. 

Therefore, by you using an carbide insert cutterhead, there is a good chance you have top quality carbide...as long as it is not from China.  So the best solution for teak is to source the best carbide you can find. But that could be costly.