Thicknesser Issue With Elm - Wavy Surface

[Nomad]

I tried running an elm board through my thicknesser the other day, and the surface came out wavy or undulating. Other bits of wood (like maple) seem okay.

The bit of elm is slab sawn, 1m long, about 9" wide and 45mm thick (sorry for the mixed units). It was planed flat before I started. The thicknesser is the portable Clarke 10" one sold by Machine Mart. I was taking pretty light cuts - was creeping up on it with 1/4 and 1/2 turns of the handle with a view to skimming the surface to see how the wood planed.

The undulations are across the grain (parallel to the blades), about 1mm deep, and have a wavelength of about 50mm. The transition from high to low is quite smooth - not like snipe at the end. They're also only in the middle of the length of the board - the first and last 10" or so are flat. I was trying to support the board as it went through to avoid snipe, but not consciously pulling it upwards, especially during the main pass, where I just let the machine drive it through.

This is the heaviest bit of wood that I've put through the machine. Other bits that have gone through have been maple, either thinner, shorter, wide boards, or longer, narrow bits of similar thickness. No undulation on any of those, and supporting it when it's planing near the ends seems to help reduce snipe.

What could be causing this?

 

[custard]

Maybe it wasn't actually planed flat, and it's very slightly bowed with the upper face (as it passes through the thicknesser) convex along the length. As the board gets towards the middle the pressure/feed rollers are trying to press it flat, they succeed for a second or two and then the springs are overpowered and the board rises up into the cutter block, the pressure rollers then try again, and so on and so on.

Test your initial planing with a straight edge and winding sticks.

 

[marcros]

has the first face been planed? I may be misunderstanding, but it sounds like you are trying to skim the surface with the thicknesser to see how the grain looks, rather than doing the same with a planer? The former will result in a board of constant thickness, but not necessarily flat, whereas the latter will produce a board flat on one side, but of constant thickness.

My guess is that you dont have a planer that will take a 9" board, and are trying to use the thicknesser that will!! Obviously, I have never tried to do that before...

 

[Nomad]

Yes, my planer is only 6". I tend to agree that it could be some sort of pressure thing, but struggling to understand how it would work. Also, I didn't mention that the undulations have a smooth surface - no tear-out.

The board was bought planed on 3 faces - one waney edge (which doesn't catch in the thicknesser - the overall width with the waney edge is about 9.5"). The board looks very flat with a 1m engineer's straight edge, and I can't see any twist with winding sticks (or very minimal, if any). However, although it's hard to be sure after running it through the thicknesser, it appears to be bowed outwards across the width on both sides. This is less obvious in the troughs of the undulations. Another thing is that it looks like the undulations don't have the same depth across the width. Shining a torch down the length of the board (both sides), it looks like they are deeper along one edge than the other, and the increased depth is at the same edge on both sides (the planed edge as it happens).

It's starting to look like there has been some sort of tilting motion across the width as the board has passed through. I think it would be worth sticking a couple of thin strips of wood along the outside edges to lift the bowed centreline away from the machine bed and seeing if the opposite face planes flat.

 

[custard]

Don't put anything through the thicknesser unless you're sure the bottom face is reasonably flat. It doesn't have to be perfectly flat, you can easily manage with a hollow in the centre provided the board is true further out towards the edges, but you don't want it bowed like a banana or in wind (twisted).

It's easy enough to prepare the surface with a hand plane, when I had a DeWalt lunch box thicknesser it would take me no more than five minutes to true up a four for five foot long board so that it was good enough to pass through the thicknesser. But you do need a straight edge and some winding sticks in order to do a final check.

You can use wedges and a hot melt glue gun to fasten your board to a thick, flat plywood platform and send that through the thicknesser, but most times it's less faff to sort out any problems with a bench plane.

Good luck!

 

[Mike Jordan]

You say that the wavelength of the faults is about 50 mm, this would fit with a chipping or other item stuck to one of the table rollers and lifting the board a little at each revolution.

 

[Nomad]

Thanks all - now sorted. Putting the thin strips of wood along the long edges allowed the opposite surface to be planed flat. So, the lesson learned is to check the wood before working it.