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Optimizing dimensional lumber to minimize waste

Bill Grider

Posted on May 31, 2022

 
A question about optimizing dimensional lumber and minimize waste. 
 
I am trying to minimize waste when working with different length parts and fit get them fit onto stock lengths of lumber available at the store. In the US we use 2x4’s (actually 1.5’ x 3.5”).  I can readily get 8 foot and 12 foot lengths of the type of lumber I need. 
 
I’ve found that the materials section will allow me to define that I have 8’ and 12’ lengths but when I click on cutting diagram it wants me to choose which length I want to work with, and will not use different sizes. 
 
I tried to trick the system and use NO standard lengths in the materials list and add them in the ‘offcut’ section in the cutting diagram.  But it seems that if I enter 8’x3 and 12’x3 it will use the smaller pieces first even if using the 12’ material has less waste.  For example: (note that I have the trimming size & blade thickness set to zero). 
 
2 parts that are 4’ long and one part that is 3’ long.   Tricking the program and using the Offcut field and telling it that I have 3 offcuts at 8’ and 3 offcuts at 12’.  The solution with the least waste is to use a single 12’ offcut. 4+4+3=11 = 92% usage.  
 
The program seems to want to use the 8’ pieces first and give me a result to use one 8’ long piece with cuts of 4+4 and a second 8’ long piece and use only 3’ of it, leaving a 5’ waste piece and using only 68%. 
 
I’ve tried ordering the offcuts in different orders, thinking that it takes what it sees first – that didn’t seem to change it. I’ve tried using NO standard lengths and tried using 8’ standard with offcut trick and 12’ standard and using the offcut trick.  I’ve always double checked that I used the foot mark (‘) so the program didn’t think it was in inches. 
 
Here is a 2nd example: 
Material available in 8’ and 12’ lengths 
Need parts 3 @ 4’, 3 @ 3’ and 2 @ 1.5’ 
The most efficient way is to use 2 x 12’ lengths and have zero waste. 
But the program wants to use 8’ pieces first again 
4+4 
3+3+1.5 
4+3 
1.5 (with a 6.5’ scrap) 
Thus only using 75% 
 
Now, I don’t ALWAYS want it to chose the 12’ pieces if that isn’t the most efficient. 
If I needed 1 @ 5’, 2 @ 4’ and 1 @ 3’ the most efficient would be to use 2 x 8’ for 100% usage. 
With the same parameters that I tricked it before to get the most efficient from the 12’ pieces, now it doesn’t help this. 
 
The solution to these scenarios are pretty evident, and they only have a small amount of parts but I’m planning on doing ones that are much more complicated soon and need to be sure the scenarios are maximized.  Am I missing a setting somewhere? 
 
Any suggestions on what I may be doing wrong or is it simply a limitation with the program? 
 
Thanks all! 
 
 
 
 

Martin Mueller

Posted on May 31, 2022

Hi Bill,

First, thank you for giving so much detail!
Second, you haven't missed anything.

The case of dimensional lumber is somewhat special (apart from the fact that the first 2 dimensions are nominal and the length is actual).  Dimensional lumber comes in a few standardized lengths like  8', 10', 12', 14', 16' and sometimes even 20'. It may appear as a huge saving to use one or the other size, but you may have to cut 1" from the end of a board, so that super efficient calculations may simply be ruined by the available lumber. Dimensional lumber is construction lumber and as such may have more or less defects, but cannot be compared to higher grade boards.

As for the algorithm, having to select between different board lengths to obtain the least offcuts is not always the best solution, because longer boards tend to produce less waste (like in your 2nd ex), but that is not always the case. Longer board may not be cheaper per foot than shorter boards because taller/better wood has to be used. 

When using offcuts, the shorter boards will be used first, regardless of efficiency, because we assume that they are the least reusable.

Adding additional complexity by allowing the selection of multiple standard sizes and asking the algorithm to make a good choice from 5 to 6 different lengths, possibly mixing them, does not really make sense. Think about going to the store with a list of 4 different lengths, only to realize that they have only 3 of them. For my part, I would always buy a little more lumber than I really need. I know this because I have made a lot of cutting mistakes and I hate to go back to the store telling them my story. I prefer to buy a couple more boards.

I am sorry if the answer sounds disappointing to you!

Bill Grider

Posted on May 31, 2022

 
Thanks! 

I totally agree with you about the lumber and the quality and end cuts. 
 
I’m precutting sheds for a community support project, so rough lumber is what it is. 
 
When I did this before I didn’t use sketchup, I did it all by hand.  This time I was able to model in sketchup and the simplicity of going directly from model to cut list is AMAZING.  When I helped with this similar project 5 years ago I used a program that would go through and take the stock lengths you tell it to (8’, 12’ and 92.25”) and optimize it - but it wasn’t integrated into sketchup – I had to manually enter each piece into the other program and label it which created double work and another layer for mistakes (in addition to my miscutting). 
 
All that to say, I’m glad it was a software limitation and not a dumb mistake that I was making. 
 
Thanks for the help!  
👍️  1

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