jointing bowed timber

[transatlantic]

I've always wondered how you straighten a piece of bowed timber that is longer than the size of the jointer bed.

for example, this scenario :

I've been researching it for a while and the only real solution I came across was to make a long sled for your thicknesser, which works particularly well for twists (from what I have seen)

I guess like this (which can of course be scaled down) :

But then I came across this on wikipedia.

To straighten a piece of bowed timber, the guard is temporarily swung out of the way. The machine is switched on and the timber is slowly lowered to the machine table, with the concave side down. A few cuts are made out of the red section "A". The timber is turned end for end and the same procedure is done to the section "B". This is repeated as required with the operator sighting along the length of the timber from time to time to check on straightness of the timber. When the timber is almost straight, the guard is replaced and the last cut is made in the normal way.

Twisted material is treated in a similar way. The operator lays the timber on the bed of the machine and rocks it slowly from side to side to estimate the amount of twist. If there is, say, 20mm of twist in the board, he holds the board level and takes 10mm off one end, then repeats it for the other end.

Has anyone tried this technique? or has perhaps a different technique?

 

[RobinBHM]

Your wikipedia technique is a common way to reduce a bow.

On a very bowed board when you start pushing it through the planer, the bow soon gets the wood away from the cutter so once it stops cutting take the wood back and push it through again repeat until a fair bit of the bow has gone from the end. Then reverse and do the same at that end, until flat enough to make a full pass This is the easiest way, not the SAFE way though as the wood can snatch when removing it.

The reason a board much longer than the planer beds wont flatten is that the part overhanging can never be referenced, the technique above removes that material.

Twisted timber is very difficult to flatten with a surface planer, very long beds are the answer. In practice it isnt that common to try and flatten out a twist on a long piece of wood -simply because the piece of wood is likely to end up too small by the time its flattened. Best just to keep twisted boards for short components.

 

[Beau]

Yes I do that from time to time with very bowed timber. Most of the time I just pass it over the beds as normal with guards in place just adjusting presure to compensate for any twist.

 

[MatthewRedStars]

Yep, take 2 or 3 cuts on the planer from either end. Reassess. Repeat.

 

[transatlantic]

Does anyone know of a video of this technique?

And are you guys talking about starting the cut with the bow over the blade like in the picture? or running the board through as normal, but stopping when it is no longer cuts (before halfway say) several times, flipping over and doing the same for the other end. Is there a video for that technique also?

 

[custard]

I don't disagree with anything's that been written. However, the best solution is to not find yourself in that position in the first place. Pick your boards in person and filter out the badly warped stuff or pay a bit more at a decent yard and get useable stuff. In either case make sure you've bought enough for a reasonable degree of waste and that you've priced that into the job.

The truth is that if a board's not behaving itself at the outset there's every chance it'll never be all that stable.

Here's a nice board of Tiger Oak, typical of the stuff you might discover in a small, out of the way timber yard. Personally I'd buy this because of its unique character, but I'd know from the get go that the waste will run pretty high. I'd be cautious before ripping off the clear section on the right, as there's every chance it will always be inclined to go crook at the mid point. Neither would I cross cut at the white marks, as by squeezing that extra bit of yield you run the risk that the ends of those boards would always be inclined to ripple or cup rather than laying flat. Instead I'd cross cut at the orange marks or even further out, and just accept that there's a fair bit of firewood in that board.

Good luck!

 

[Jacob]

Cut to length first!
After that plane it convex side down.
You put your weight near centre of the board so that as you pass it over, the ends will be too high to cut, but you will take a slice from the middle - getting longer with each pass.
The logic of this is that given a regularly bent and/or twisted board (as most of them are) then the middle of the board will be closely parallel to the best possible flat face you can get out of it, so if you start at the middle you are on the right tracks.

PS there's another advantage with this: you are removing waste from the middle of the board and it's possible that when you thickness it it will be too thin in the middle but full depth at the ends. But for some purposes this is fine - you can joint the ends but the back (with saw marks) will be out of sight e.g. under a table top or inside a sash window chassis.
This means it's also a technique for hand planing - if you don't want the ends of the board to be too thin after thicknessing

PPS This'll work with long timber over a short bed but you have to pay attention to what's happening and adjust your handling accordingly

 

[Racers]

I would love that centre section of that tiger oak!

Pete

 

[custard]

I would love that centre section of that tiger oak!

Pete

PM sent

 

[Rorschach]

I would love that centre section of that tiger oak!

Pete

Don't blame you, it would make a lovely rustic stool!

 

[deema]

Well my solution is slightly different. Passing the timber over the surfaced is a solution, but long heavy boards are difficult to handle even with two people. The danger is that you end up tipping the board and actually cutting too much of the ends.

I take a string and tap a nail in one end, and then stretch it to the other end so that its just skimming the wood in the centre. Now I measure the depth from the tip of the board to where the string is. This gives me double what I need to take off the ends. I could run it from end to end and measure the centre, but that needs two people or another nail!

I move the nail down half the distance I measured and with a chalk string line the side. I grab the trusty scrub plane and after a few minutes I have it reasonably flat. This now goes over the surfacer. I take out twist on big heavy long stuff the same way. I find it's quicker and more controlled than using the surfacer alone. Equally I know before a lot of effort if I can get the board I need out of the stuff.

 

[Jelly]

Cut to length first!

THIS!
Are we seriously saying that dropping on, on a Planer is anything other than an unsafe practice?
Especially when common sense solutions like the one above abound.

 

[Jacob]

Who is recommending "dropping on"? Not me!
What I describe isn't unsafe at all, though like any process there's a limit beyond which you can't go.
The trick is to keep the centre of the board firmly down on the bed with each pass.
I've been doing it for years. If it seems unsafe then possibly you haven't quite understood my explanation.

PS I understand "dropping on" as lowering a piece onto a planer (or a saw) - a desperate measure! but that's not what I do. I pass the workpiece from infeed table to outfeed in the normal way, all the time firmly held.

 

[Jelly]

Who is recommending "dropping on"? Not me!
What I describe isn't unsafe at all, though like any process there's a limit beyond which you can't go.
The trick is to keep the centre of the board firmly down on the bed with each pass.
I've been doing it for years. If it seems unsafe then possibly you haven't quite understood my explanation.

PS I understand "dropping on" as lowering a piece onto a planer (or a saw) - a desperate measure! but that's not what I do. I pass the workpiece from infeed table to outfeed in the normal way, all the time firmly held.

Sorry Jacob, I think I've articulated myself rather poorly here.

I was trying to say that your suggested method of cutting to length then planing convex side down (using a push stick to apply pressure in the centre of the bow at the balance point), was the most appropriate way IMHO; sometimes I think it's safer to establish a flat at that balance point first using a jack plane, if the desired finished size is quite large.

By contrast, I'd say that the other process discussed which involves removing/backing off the guard then placing the timber down over the cutter block, is "dropping on" as I was taught... It creates an unnecessary risk of inadvertently placing the board onto rather than over the cutters, and to my mind is an unsafe practice.

I don't think on re-reading, that my point was obvious in the previous post.

 

[Jacob]

...sometimes I think it's safer to establish a flat at that balance point first using a jack plane, if the desired finished size is quite large.....

Yes that too. And two push sticks all the time, one holding down, the other down and in.

 

[transatlantic]

Cut to length first!

THIS!

I forgot to mention that the scenario in question is one where the boards need to be long pieces (for a table or something)

 

[sawdust1]

I've used both methods, and would favour Jacobs way. Feed the board over the cutters as usual, it will
miss the front, cut in the middle then miss the back, once done a couple of times you will soon have a
flat section in the middle, which will get longer the more passes you make.

 

[transatlantic]

I've used both methods, and would favour Jacobs way. Feed the board over the cutters as usual, it will
miss the front, cut in the middle then miss the back, once done a couple of times you will soon have a
flat section in the middle, which will get longer the more passes you make.

But surely it's so much more difficult that way as you're having to apply firm pressure to balance the board. Where as convex side up, it supports itself?

Having thought about this some more. With long boards (say 5ft+), it only makes sense to plane out bows if the bow is only minor, else you'll lose too much material in the process. But then if the bow is only minor, and the board not so thick, surely it's flexible to clamp out anyway (with cauls)?

 

[custard]

There'll be cases where there's an advantage to be gained by planing either way, but all things being equal I prefer concave down/convex up. Basically because I want plane with as little downward pressure on the infeed table as possible. Once you're down to really thin pieces you need some downward pressure on the infeed table to stop the workpiece "fluttering", but as long as your knives are sharp you should only apply downward pressure over the outfeed table. In fact excessive flutter is one of the cues that say it's time for a knife change.

Good luck!

 

[Jacob]

I've used both methods, and would favour Jacobs way. Feed the board over the cutters as usual, it will
miss the front, cut in the middle then miss the back, once done a couple of times you will soon have a
flat section in the middle, which will get longer the more passes you make.

But surely it's so much more difficult that way as you're having to apply firm pressure to balance the board. .....

It usually tends to balance itself roughly in the middle. You just have to hold it down in that position as you pass it over the cutters.
Think of it like floating a boat - your weight is in the middle - the bows go over above the cutters which then cut along the water line, then the stern goes over! if you see what I mean. :shock: