How to build a STRAIGHT strongback.

All Materials used in the building of my strongback are my measurements and are not standard. You can use whichever lengths you can get.

Overall measurements for my strongback are 5400mm long, 204mm wide, 200mm high.

Since the longest single piece of lumber was 3000m I had to join various lengths of various materials together. Joins were staggered in order to maintain a semblence of rigidity.

Materials Used:

1. 4 x 22mmx44mmx2800mm Meranti planks (#1)
2. 4 x 22mmx44mmx2600mm Meranti planks (#2)
3. 6 x 12mmx200mmx1800mm Plywood board (#3)
4. 4 x 12mmx180mmx2400mm Plywood board (#4)
5. 2 x 12mmx180mmx600mm Plywood board (#5)

Taking two #4's and one #5, lay them end to end in a straight line.

From an edge that is known to be straight, draw a CenterLine down the length of each board.
Using a 2m aluminium straight edge ensure that your boards are laid out STRAIGHT.
I took care to ensure I had a straight edge on each of these boards and that they were cut to equal widths. Thus both left and right edges lined up straight with the straight-edge.

Using one #1 plank and one #2 plank, line these up along one edge of the boards.
Attach these with wood screws every 20-25cm. Ensure these planks are flush to the edge.

Repeat this process for the second edge of the boards.

Ensure that your CenterLine remains straight during this whole process!!!
Note: Bearing in mind that your planks could be warped slightly, compromise on the edge flushness, but not on your CenterLine. If the CenterLine is straight, the board edges will align straight too.

This complete side becomes a "bottom". Duplicate this to create a "top".

Using three #3's, enclose one side as shown in the following picture. Try to ensure that your edges remain flush.
Remember that every time you attach a new board, use the straight-edge (clamp it into postition if you have to) to ensure you have everything STRAIGHT.
Check and double check straightness at every opportunity!!! Once you have screwed in wood screws, it is difficult to re-align a board. You will be moving screws to new holes, leaving old holes all over the place.

Now, using three #3's enclose the last open side of the strongback.

And there you have it.
It sounds simple.
It is simple.

But it's even easier to get it slightly skew. And once you have something skew, the end result is skew and everything escalates from there until you have something that you cannot work from.

Take care,
Measure everything,
Measure again,
Be patient.