Constructing your project square

[transatlantic]

I just finished up building a very simple bench made from CLS, mostly using lap joints and an MDF top. When Putting it all together though, I noticed it's a little wonky. From a birds eye view, there is a slight parallelogram, and when looking from the side, there is a slight twist. Nothing major, but annoying as I know it's there.

Now, I didn't really think about this as it was only a very rough bench for rough work, but for future reference, I'd like to know how I should have put it together to reduce/fix the issues.

As I was using CLS, I'm going to assume the error was introduced by the wood not being perfectly straight/flat as well as user error in the joint accuracy, but this is almost always going to be the case as nothing is perfect.

I know I could have checked the first issue (parallelogram) by measuring from corner to corner for square, and I could have checked for the second issue by using winding sticks. However, what should I have done to correct it? If it was something smaller, I guess I could have applied the glue, clamped it up, measured using the above techniques, and then tightened the appropriate clamps to bring things into line. But what do you do for bigger things? In this case, I didn't actually use any glue, I just used screws as I wanted something pretty quickly. But should I have clamped it up, measured for square and then screwed?

 

[gregmcateer]

Lots of lookers, but no posters - so he's a friendly post to show we care!!
I've got no idea what you need to do, but you looked so lonely with 256 views and no replies!!

 

[Sheffield Tony]

I think a bit more detail about the methods of joinery might make it easier to pass helpful comment, but as a starter, making sure the timber is square and not twisted is the first thing; even if your joints are perfect wonky timber will muck you up. I say this since you mention CLS, which I'm guessing is construction grade wonky.

One of the first pieces I made when I was getting back into woodwork was a small bathroom cabinet. I started with beech, which I had planed up by the supplier. Mistake, shouldn't have relied on it being square and straight; one length had a twist down it that the planer had nicely followed, and it happened to be the bit I made the door stiles from. Not much of a twist, but enough to annoy me for years.

Edited to add: yes to a trial run clamping up. Certainly when you are glueing. Then you can see if you have a problem, whether it can be fixed by tweaking the clamping, or whether slight adjustment of the joinery is needed - maybe a skim off a shoulder.

 

[Dee J]

Check and adjust each component and sub-assembly as you go. Check for twist. Check diagonals. If you are screwing together rectangular frames use lap joins rather than butt joins..... easier to adjust , one screw at each corner, get the frame square and the follow up with a second screw. As you gain experience you will find you can take short cuts.... but starting out ... Check and adjust.

Further to twist issues. If for example you are assembling two legs with two cross rails... working on a flat surface (yes you need to find that - even if it's the kitchen floor). Lay the rails across the legs and check the joins are flat and fully in contact. If they're not then you need to correct the fit. Chisel, plane or select other timber. On that basis right at every stage leads to a right result.

 

[Droogs]

While CLS is easily available it is, in general, quite high in moisture content. It's usually around 12 - 18% (average CH house is around 6 -8%) and this is useless when it come to making furniture. It is normally stored in awful conditions for the woods future stability. Yes it is planed and has machined coners but this is done with no real consideration to "wind" the twistiness of the wood, as when it is machined the planer just follows the twist.

So you get it home and into a different temperature and humidity environment and it immediately starts to move in order to try and come to an equilibrium with its new surroundings. I'm guessing you cut your joints as accurately as you could but also quite quickly after you brought the CLS home. This has meant that the newly exposed areas have just exacerbated the movement of the wood. You need to remember that in all woods the growth rings want to be flat and will try their damnedest to become so. When using this sort of wood or any salvaged wood you really do need to leave it in situ in an environment that is as close to the final resting place as it were that you can for at least a month in order to allow the wood to do its thing. Then you need to rework it so that it has any twist removed. It does mean you will end up with a much thinner piece but at least it will be more able to accomodate what you are trying to do.
hth

 

[AndyT]

A picture of the bench would help a lot as so much depends on how it was constructed and how the legs were fixed.

Something which may or may not be helpful - when I have made tables, I have found that to get all the legs at right angles to the top, rather than just relying on the squareness of the tenon shoulders, it helps to cut four pieces of scrap, which are an exact fit between the legs at the top, and re-position them at the bottoms of the legs. So, if the table is standing the right way up, put them at floor level. They should be a tight fit - if not, one or more legs is splaying out. Clamps at top and bottom help here.

The other essential in this situation is a flat board to stand it on. If the floor is not flat where the bench is going to sit, you can adjust legs in situ after the glue has set.

Another general tip when using big sash cramps across large work pieces - take the time to cut some suitable pieces of packing wood which will spread the load out nicely without adding twist of their own. David Charlesworth covers this well in one of his books, showing that you often need something which is a trapezium in cross-section, narrow where it goes on the edge of the boards, wider where it touches the broad metal of the cramp.

 

[transatlantic]

Thanks.

This question was more for the general case, rather than my specific build. In that I don't think my joints are ever going to be perfectly square due to many factors (material, my skills etc), and the best way around clamping stuff up so that the end result is plum/true/square.

 

[Ttrees]

A flat reference surface is essential in my opinion
probably the no 1 tool in the shop as it has so many uses.
The only other ways I can think to achieve flatness is with jointing with three master edges and comparing whatever to it.
Or with a line (never tried this approach)
I don't know how you'd expect to build something true without it .
The first bench I built I used a level and chalked the wall, and built a frame to it.
I then put a fire door on top and another composite on top of that.
I then was able to use the plane with accuracy and be able to trust the bench .
I can use the bench like a machinist would use Prussian blue to mark high spots .
This saves time, effort and minimises waste I find