How do you cut pieces of wood to be the same shape?

[DK123GWR]

I know this sounds like a stupid question, but I have just had a very frustrating afternoon trying to build the prototype for a set of modular baseboards. This is the process that I followed:

1) Use a rectangular template lined up to the edges of a piece of wood to draw lines parallel to the long edge

2) Do the same on the short edge - This created a pattern with 9 sections - the top in the centre, the sides and ends adjacent to the top, and the corners to be discarded

3) Verify that all lines are the correct distance from the edges and parallel to them

4) Cut along the lines, ensuring that the same point on the saw blade follows the line

 

The result, one would expect, would be two side and two end pieces of equal height, and yet while the two end pieces were identical and the two side pieces also identical, the height of the sides and the ends were different. In the past I have had similar issues with the casing for a controller, although in this case I had drawn the pieces as a net rather than using a template to ensure (in theory) that the dimensions were the same. Can anybody suggest what I might be doing wrong? While I feel like I have taken all variables into account, clearly this is not the case as I cannot acheive consistent results. Thanks in advance for any advice you may have.

[DCB]

How are you cutting the board?    Normally I would use a bench saw, circular saw on a bench blade upwards, with a guide which should reproduce the same width time after time to about + - 1 or 2mm.  (My saw just threw its armature windings)

With a hand saw it difficult to saw down a line accurately, easy to be 1mm out which adds up to 2mm when one is plus and the other minus. Easier to saw beside a line, or between a pair of lines,   Otherwise join the rest of us with Baseboards which are flat ish on top and all  shapes underneath where you can't see. 

[AdamsRadial]

I think the issue here is what is called the Kerf, the width of the gap left by the saw when it cuts. You are cutting along the centre of a line, so the kerf is being divided between the two resultant parts. In joinery one of the first thing apprentices are taught is "saw to the line, not through the line". The problem with through the line is you would have to position the saw cut so it was evenly divided either side of the line, but cutting "to the line" such that the line remains on the piece of wood to be kept and not the waste, means no such problems with dividing the kerf between two adjacent parts. Of course, if you are using a power tool and a fence or other guide you can do what you are trying to do, but you should really work out what the kerf is and so where you have been drawing one single line, draw two spaced such that the distance between them is equal to the kerf.

 

ETA what David said, he got there before I did and used less words

Edited November 28, 2020 by AdamsRadial

[PMW]

Personally I never like using a template to mark out, use a marking gauge. 

 

Determine the height of your sides and ends and run the marking gauge around all four edges of the board to get your cut lines, you can run a pencil in the score line then to highlight them. Use a straight edge then to guide your saw. You can buy saw guides but large ones capable of dealing with 8' sheets can be expensive. 

 

You mention this is a prototype, if you are making a lot of these I would consider making a jig which would allow you to repeat accurate cuts time after time

 

I assume you are using a circular saw to cut?

 

 

[AdamsRadial]

Perhaps I can give a better example of what problems the kerf gives us.

 

Imagine we have a piece of ply 4' x 2', and we want to cut it into three pieces, 2' x 1'4". 

We establish that the 4'x2' is accurately cut with all four corners at right angles.

We mark along one top edge at 16" and at 32", and using a square, we draw lines across the board, and check at the opposite long side that they again intersect at 16" and 32"

We saw accurately through the two lines we have drawn, and put the three pieces together.

We find that although the two pieces from each end of the board are exactly the same width, the piece in the middle is slightly narrower.

 

The reason being is that the two outer pieces have each only one sawn edge, and so are the nominal width of 16" less half the width of the kerf.

The middle portion however has a saw cut on each side, and has therefore lost the full width of the kerf.

 

[AndyH]

As previous replies, every cutting tool has a kerf, even a laser cutter. Sometimes there is also a question of tolerance when marking out. Sharp pencil or knife? Even down to the question of a left handed or right handed marking knife. The old adage ‘measure twice, cut once’ (or maybe that is me) comes in to play too. Accept wastage if you want accuracy by not trying to fit too much on a sheet too.

 

edit, also buy a good tape or rule and a good square too. We have a £26k saw at work with digital read out, and the installer said ‘you have just bought a precision tool, then you use your £5.95 tape to check it, guess which one is accurate...’

Edited November 28, 2020 by AndyH

[doilum]

So you accept that you cannot have two pieces of 24" out of 48" and plan accordingly. The wood butcher's method is to cut one master piece and use this to measure out subsequent clones. When hand sawing we were taught to cut slightly to the outside of the pencil line and then sand back until the line disappeared*. If your hand saw isn't a joy to use, bin it. Most DIY stores have saws for £10 or less. They will not last for ever, but at this price they don't have to. If you intend building many boards a chop saw is a good investment, better still is a radial arm saw but it may be difficult to justify the extra cost.

* Buy a pack of abrasive belts for a belt sander. Cut a bit of scrap 18mm ply that just fits inside to create a most useful tool.

Equally important to length is square. A large 24x18" flat square can often be found in budget tool stores quickly becomes indespensible.

Edited November 28, 2020 by doilum
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