planer knife setting jig

[MickCheese]

re setting the blades a thou or so above the outfeed - I had a read and the guy knows his onions. But unless I missed it he doesn't explain why he does that. I have always set dead level to the outfeed and my problem is it works fine for me, so I'm reluctant to change without understanding why, and I don't get the logic of it. If anyone can cast light I'd be keen to understand, then I may give it a go.

Douglas

I'm with you, I have tried it above the outfeed table and just got tapered cuts. I would need convincing and some rationale as level works for me.

Mick

 

[TheTiddles]

How can having the knives above the outfeed table be anything but wrong? It will always snipe.

Aidan

 

[Jacob]

How can having the knives above the outfeed table be anything but wrong? It will always snipe.

Aidan

There is a rationale. Theoretically if the setting is dead-on then every little shake, tiny ripple, piece of dust etc will lift the workpiece a touch - but one way only, so it will be accumulative. You can see this exaggerated if the blades are too low - the workpiece will lift onto the out-feed and float off the in-feed and not cut (if it's being held down on the out-feed that is).

If it's just a tiny gnat's over this is less likely to happen and miniscule errors will cancel out.

 

[condeesteso]

O.K., may try it next time but what I do works anyway, for me. I just had to replace the blades this morning (what a pain anyway) so I'll leave it for now!
Small point (on the Scheppach 260ci) - there are 5 lock nuts, and however you try, when final tightening any one of them it raises the blade a small fraction as it locks down - because the axis of the lock-nut isn't tangential to the block, it is slightly outward.
I think set the centre one first and lock it (not full tight but closed up), then set the 2 ends. If you start one end and work along, when you go back to the start it is almost always out again.
Not my favourite passtime :evil:

Oh yes, and Jacob's ruler moving 3mm trick (or some dead-straight hardwood maybe?) makes a lot of sense, will surely try that one... next time.

 

[chipmunk]

Steve Maskery advocates setting the planer knives a smidge above the outfeed table...

http://www.youtube.com/watch?v=mHfjSatKITQ

along with a typically lucid exlanation as to why (about 3-5 mins).

Jon

 

[sometimewoodworker]

Steve Maskery advocates setting the planer knives a smidge above the outfeed table...

http://www.youtube.com/watch?v=mHfjSatKITQ

along with a typically lucid exlanation as to why (about 3-5 mins).

Jon

There is a good explanation as to why the out feed table should not be too low, but I couldn't see anything as to why it should be just lower than the knives other than "it works"

All the web sites also say the blades should be about 0.1mm above the outfeed table. So "it works" may be what you mean?

 

[chipmunk]

Jerome,
Ok, I think you may be right there.

As I understand it, a planer effectively cuts a shallow scallop rather than a flat shaving like a hand-plane. So the reason for having the outfeed table slightly lower than the blade is to permit the point where the adjacent scallops meet to skate along the outfeed (and infeed) tables rather than the top of the scallop.

For a 4mm drag from the drag test this equates to a 4mm long scallop.

Editted Due to dodgy physics - The drag is the chord across the edge of the circle and not a length of the circumference but the outcome's the same

Assuming a 75mm diameter knife block the scallop depth would be approximately 37.5 * (1 - cos(asin(4 / 75)) = 0.05 mm.

HTH
Jon

 

[condeesteso]

Hi Jon - that's about 2 thou then, I think. Sums good but an allowance for feed rate needs to go in. This theory is right if the following cut enters where the previous left... quite some feed rate I would say.
I'm sticking with true to the outfeed - as I have said it has worked for me for years, it seems to make sense, and I am not sold at all on making the blades higher, if only by a thou or two.

 

[sometimewoodworker]

Hi Jon - that's about 2 thou then, I think. Sums good but an allowance for feed rate needs to go in. This theory is right if the following cut enters where the previous left... quite some feed rate I would say.
I'm sticking with true to the outfeed - as I have said it has worked for me for years, it seems to make sense, and I am not sold at all on making the blades higher, if only by a thou or two.

First: If it is working for you then I would not try to change your mind.

However unless you can maintain constant pressure on the work piece (I know I can't) if it ever lifts off the Infeed table then I'm not cutting.

My empirical research has proven that with my machines operated by me that when the knives are level with the out feed table they don't cut. But if they are fractionaly above they do cut.

FWIW on my main planer both in feed and out feed tables are adjustable so it was easy for me to check. That seems to be unusual from what I have read.

 

[chipmunk]

Doug,
You may well be right that my analysis is a bit dodgy if each cut requires a 4mm feed, which I agree is too fast.

It makes me wonder whether the short drag is a pragmatic way of ensuring that the blade isn't lower than the outfeed table but at the same time represents an easy way to ensure that both blades across their length are parallel to the outfeed table?

A drag of zero after all would be indistinguishable from a miss. A consistent 2 thou protrusion for me is probably as near as damn-it flush to the outfeed table, isn't it? :wink:
Jon

 

[condeesteso]

Yes Jon - I think we end up splitting a hair or so! Having replaced the blades on my Scheppach 260 yesterday, I find it more frustrating that the act of final closing the lock nuts (even when they are backed off just a touch, 20 degree turn say) moves the blade up a frustrating and unpredictable amount, so you end up trial and error. If i get within a couple of thou I should be content. Can't speak for other planers though - there must be better systems out there surely?
On that Scheppach point I note the allen bolts that sit under the blade to adjust up or down are off-the-shelf bolts, so the tops are not very smooth or flat. I plan to remove them next time and put them in the pillar drill, try and get them smooth and flat - see if that helps, as a tiny turn of them at the moment can make unpredictable differences to blade height.
Is it time to save up for a Hammer, I wonder??

 

[Streepips]

I doubt if any of this has helped the original poster decide his approach to what to do when its cutter change time. Lots of theories on the subject here but little of it showing a true understanding of how a olaner iron does its work, and the function on the relative positions of the tables and the cutters reference position to the outfeed.
So what I propose Lumberman, is that when you find its time to change cutters, let me know and I will come along and show you how, without any jigs. Just have a fresh set of irons ready!
I am only over the Bridge just past Barton, so its no real bother for me.