Here is another thought experiment. Let’s say we have one saw that takes 20 seconds to set up, and another that takes a minute. We are now dividing up the work between the two of them, to keep both as busy as possible. Would we be better off retiring the old saw and just using the new one? I’ll present a method to answer that question and invite you to apply that method, using your own numbers, to your shop.
Let’s quantify several things about each saw.
1. How long does it take to set up a new piece, including the time between when the last piece has cleared the saw and the operator (or computer) initiates the next setup?
2. How long to set up a new piece if the new piece is also a different size (2x4 to 2x6, etc.)
3. How long to run the last piece through the saw before the next setup can be started?
Once we have these saw metrics, let’s look at the work we are doing. When we do, let’s make sure we get the number of:
1. Pieces
2. Length changes
3. Size changes
The two together will enable us to estimate how long it will take to cut the work on each saw.
We will make up some numbers for our ‘thought experiment’ saws:
The “sample job” MiTek distributes with its software has about 50 different roof trusses in it. Using the default sorting routines, the job breaks down as follows:
Let’s assume this is a typical job, but perhaps it isn’t a typical job for you! The only way to know is to run the numbers on your own jobs and see what you get. Moving on… Let’s see if we can come up with a plausible way to estimate the time it will take to cut this work. We’ll use numbers from the Old Saw first.
· The job begins, the saw sets up (30 seconds)
· The saw cuts (3.1) pieces (on average) and each one takes (2 seconds) to cut
· The saw takes (20 seconds) to clear
· Total = 56.2 seconds
This is repeated for each new piece (303) so the total is 303 x 56.2 seconds = 17028.6 seconds. Add to that 4 size changes for another 120 seconds (4 x [60 seconds for the size change – 30 seconds for the length change already accounted for]) and you get 17148.6 seconds or 4.76 hours.
Now, we will work the problem using New Guy numbers.
· The job begins, the saw sets up (15 seconds)
· The saw cuts (3.1) pieces and each one takes (2 seconds) to cut
· The saw takes (20 seconds) to clear
· Total = 41.2 seconds
303 new setups x 41.2 seconds is 12883.6 seconds plus 20 more for the four size changes and you have 12906.6 which is 3.59 hours.
Presented as a chart, this looks like:
About 33% less cutting time on the New Guy compared to the Old Guy. If they split the job up 50-50 (151.5 piece setups each) then the New Guy would be done with his work in 1.8 hours and the Old Guy would finish up in 2.4 hours. In order for them to finish at the same time you would have to give New Guy about 175 of the setups and the Old Guy about 128. They would both get done after about 2 hours. Putting this all in a chart:
In conclusion, it would seem that the least expensive way to cut the job is to have the most efficient saw cut it – and shut down the older saw. This is only true if the one saw can keep up with the shop. The two saws, working together and splitting their work more or less in proportion to their efficiency, would be able to cut four jobs like the “sample” job in eight hours. The New Guy saw, working alone, would only be able to cut 2¼ “sample” jobs in the same time.
We’ve only scratched the surface here. What about comparing costs when we give the Old Guy only “quantity greater than 10” work and give the rest to New Guy? Hopefully this method can be the starting point to your own investigations and lead you to new ways of thinking about how you use your component saws.