I need to settle an age old technique argument that has persisted over the years and yet I can't come to grips with.

When setting a two knife shaper head there is an old school of thought to set one knife a notch lower than the other knife so that one knife cuts allowing the second knife to follow to "clean up" the cut.

I'm going to give my answer first and hope to get clarity.

I believe this technique works IF the first blade to BEGIN contact with the wood is the lower blade...otherwise IF the first blade to BEGIN contact with the wood is the higher blade then ONLY one blade is TRULY cutting. To me it's a 50/50 chance of this technique actually being effective.

What say you?

I have always set my collar knives set ups as equal as possible.
IMHO only 1 knife hits the wood first the way I understand the other knives chop the waste smaller. So an example would be a 3 wing cutter they are all equal .

Line them up equally, only one is going to do the majority of the cutting anyways. It is impossible to obtain a perfect cutting circle unless using hydro heads and jointing.

You’re the type of person that gets taken out Snipe hunting at night with a flashlight and pillow case!

Set your knives as even as you can.

Yup, set the knives as even as possible. Depending on feed speed, DOC and how tight everything is, you will probably end up with one knife doing more than the other and determining the final outcome, but they will probably both be cutting and the lower knife will help the chip size etc. Also balance is a bit better if the radius of both knives is the same or close.

Unless you want to watch your shaper do a dance across the floor, ALWAYS set your knives equally in the head. Someone once told me that 1 gram of imbalance in a spinning head is equal to 22 pounds of force. Think about the center of balance in that cutterhead with knives not set equally. Dangerous is just the start, it could be deadly. As others have already mentioned one knife will do the cutting without hydrolock tooling and jointing stones. The difference between knives only needs to be .001 or more for a one knife finish.

I’m guessing that lower by a “notch” is that you’re talking about a corrugation in the head? This would put one knife .030”+ out further than the other. Typically not considered a safe practice and as stated is a balance issue.
You really don’t want the lower knife to be more than .003” lower. The closer the better.
Think of it this way, the leading knife cuts even waves like in an ocean. The second knife clips the tip of those waves off. Not quite the perfect example but the closer the two knives are the better the finish and the less work and hp used by the shaper.

One time I was tasked with reproducing some sort of siding that basically had a large raised panel cove cut and ship lap the other side. I did have some large collar knives in my collection and fired up an unmatched set, my PM 26 with a power feed vibrated across the floor and out of its foot print, it was the day I needed a change of shorts int he shop. Be safe.

John,
I would question any other information that is provided by your "set one knife" lower person.

Knives set in a head are either jointed or non-jointed. Jointed doesn't mean they are ground in the same head. It means they are finish honed in that head. They are effectively the same geometry. Serious molding company will joint their heads.

In the case of non-jointed knives, which are the vast majority of knives. Some part of a knife is always protruding .001". It's not always the same knife. It could be half of one and half of the other. If you push one knife further down the head(parallel) the geometry totally changes again.

Do you purposely set your planer or jointer knives at different heights? Molding knives are the same cutting tool, just has more shape.

John
I would agree with the posts about safety concerns.
The practice you're referring to is used by some folks to maximize the amount of linear footage they can obtain between sharpening. When the other protruding knife gets dull, they swap positions having a new knife to do the cutting. It's sometimes done among people that don't have a profile grinder. If both knives weigh the same, the cutter head itself is balanced, along with the gib screws and gibs, then it is statically balanced. However, the dynamic balance would be off by the offset; each cutterhead corrugation is typically 1/16" between one to the other.
I have attached a few articles that might help you better understand industry standards. As one post described, only one knife cuts when you line both blades up because of bore tolerance unless you use a hydraulic cutterhead and then joint them with a jointing stone.
It's all about safety, I always side with the best safety practices. I would always keep them in the same corrugation.

I hope that helps
Russ @ MR

https://www.mr-moulding-knives.com/information/moulding-knife-marks