Thicknesser crowning timber

[Pintglass]

Hello

Im having a problem with my scheppach planner thicknesser, for some reason the thicknesser is crowning the timber, not in length but in its width. The planner side is perfectly flat so i don't thick its the blades.
Anyone else had this problem?

Thanks Pintglass

 

[Steve Maskery]

No, but it implies that your thicknessing table is a bit dished. The pressure roller is pushing it into the dip, the knives cut, the workpiece springs back afterwards.

How much is the error?

I don't have a solution, I'm afraid.

S

 

[JoinerySolutions]

This is not a problem I have ever come across, any chance of some photo's of your set up and the machined timber.?
Any further clues/ info may ring a bell in another member reading your post.
One further thought, what is the timber and how dry is it. Sometimes timber will move when planed (or sawn) as stresses are released. It is common practice when thicknessing to alternate the side being planed to reduce this risk. A photo of the end grain of your timber may well put us on track.

Rob.

 

[OPJ]

If your thicknessing bed is dished or slightly hollow, one temporary fix would be to use a false auxiliary bed. Something 18mm thick (ply or MDF) with a hook on one end (so it can't be pulled through the machine) should be fine. To the underside, you could add a thin strip so that it doesn't get pressed down in to the hollow by the rollers.

Of course, when setting the cutting depth, you'd have to account for the extra thickness of the false bed, if you cannot reposition the pointer on the scale by as much.

 

[JoinerySolutions]

Steve & Olly, the OP stated that the wood is coming out Crowned across the width. I cannot see that as a result of the bed being dished.
I think this is a stress in timber and not alternating the thicknessing cuts scenario. Without seeing the situation first hand my gut instinct is as stated.
The term 'crowned' means that the timber is thinner at the ends than in the middle or that the natural bow in the timber means there is a gap under the middle of the board when both ends of it are in contact with a flat surface. So crown up = bow up when determining which way up to install joists for example.
If it is crowning across the width then we are on different tracks of thought.
In that scenario the edges will be thinner than the middle of the board, not in length but width, however, if the surface planing is all okay and the knives therefore set correctly, it could only be a timber movement issue. The more I think on it the more I am sure of it. More info would maybe alter that thinking, but based on what is posted, that's all I've got at the moment.
Regards Rob

 

[Steve Maskery]

Rob
You may well be right. And so could I.

If the table is dished the cente will be further from the knife than the ends are, so when it comes out, the centre will be thicker.

I agree it could also be a movement issue, but if it is on all boards and is consistent, that would seem to be one heck of a coincidence.

S

 

[9fingers]

Does this happen with very thick timber?

My guess is that thick timber will come out flat and the dishing is only occurring with thinner stock that will flex.

Bob

 

[devonwoody]

No problem, just put it over the planer afterwards

 

[Pintglass]

Thanks for the replys guys,

The timber i'm using at the moment is 2" douglas fir 1 3/4 finished, the problem is more evident on wider boards, 8" for example, i've checked the thicknesser bed and there is no hollow in it but when i checked the planner tables there is a slight hollow in them. so i'm now doubting myself as to weather it is the thicknesser or the planner.
If the planner was planing crowned thought you would think my thicknessing would be hollow, but i don't think the pressure rollers would effect thick timber.
Can timber crown from stress on one side but not hollow on the other?

Thanks Pintglass

 

[JoinerySolutions]

Thanks for the replys guys,

The timber i'm using at the moment is 2" douglas fir 1 3/4 finished, the problem is more evident on wider boards, 8" for example, i've checked the thicknesser bed and there is no hollow in it but when i checked the planner tables there is a slight hollow in them. so i'm now doubting myself as to weather it is the thicknesser or the planner.
If the planner was planing crowned thought you would think my thicknessing would be hollow, but i don't think the pressure rollers would effect thick timber.
Can timber crown from stress on one side but not hollow on the other?

Thanks Pintglass

No, not in my experience.
Have you faced up a board and checked it with a straight edge?
Douglas Fir can sometimes have quite a bit of tension in it, usually in the form of the board going like a banana when ripping for door stiles!
If the timber was thinner than your using I'd still think cupping due to stress released.
Are the edges of the boards coming out the same thickness? Are they square to the flat face?

Rob.

 

[devonwoody]

I would put another sort of timber through your thicknesser and see if it is still happening. I doubt it, I bet its the DF.

 

[jasonB]

Are the blades getting blunt in the middle and just squashing the timber as it goes under them

Jason

 

[WellsWood]

HAve you checked that the knives are ground straight? If they are (and sharp) and there is no discrepancy in bed flatness then it must be timber movement when stresses are released.