Fine-Tuning Log Scale Estimates
Log scales are a tried and true standard for estimating board feet. But here's a discussion on improving the accuracy of specific estimates for particular jobs at the bandsaw mill. August 3, 2009
Question
I would like a formula for calculating log volume in board feet to better estimate the production from a bandsaw mill. This formula would have as variables log length, small diameter, large diameter, and kerf with of course some assumptions. I’m looking for an actual formula with these variables which I can use in a spreadsheet (Excel or Numbers) to make my own calculators. Does anyone know where this formula has been worked out?
Forum Responses
(Sawing and Drying Forum)
From Gene Wengert, forum technical advisor:
The basic rule is the International 1/4" rule, which assumes a 1/4" kerf. I have seen an Internationl 1/8" rule in table form. You might have to use a table look-up. I have not seen a formula that incorporates 4/4, 5/4, 6/4, 8/4, cants, etc, all which will change the yield as well. And to be very accurate, sweep might also be included. If there is none in existence, it would be something that a university or government agency could fund and work on.
From contributor A:
Ok, I have to ask why would you need it?vThe 1/8 International or even the 1/4 scale will get you really close. Putting everything in a spread sheet has nothing to do with putting a blade in the log. Not trying to be a pain, just a wondering how close you are trying to get to something. I see these new scanners in the trade magazines and how they increase production. It did increase production, but the problem was quantity and quality.
From the original questioner:
Yes you're completely right. I said I would like it, and that's to make my estimates a little more accurate. Using a spreadsheet makes calculating a bunch of logs from sizes a customer gives me, quicker, easier and error-free. Does anyone know the formula for the 1/8 International scale?
From Gene Wengert, forum technical advisor:
Because hardwood lumber is typically 1-1/8" and there is a 1/4" kerf (total 1-3/8 or 11/8), then for 1/8" kerf, why not use the International 1/4-inch formula and then apply a 1 out of 11 correction (9.1%). Similarly, if you cut your lumber to 1-1/16, apply another 4.5% correction.
From contributor S:
Formulas only go so far, you need real world experience. If you use international and saw mostly 13" and under logs into all 4/4 and then trim and cut for highest grade, on some species and log qualities you'll make scale on some you won't. If you saw all 18" 12' poplar veneer rejects into 10/4 you're going to overrun the scale even after any trimming for grade. One formula isn't going to work for all applications. If you just use international and saw on bandmill it will all even out, + or - 5 to10%.
From contributor A:
I would stay with the 1/4 scale rule. Day in and day out it will come closer to "guesstimates". There is not always an overrun but with 1/4 scale you’re safe to say that it will be within 15% of the estimate.
From the original questioner:
I'll use Prof. Wengert's suggestion for adapting the 1/4 scale to specific situations. Also compare and correct for real results taken after the saw actually went into the wood.
From contributor D:
I would have suggested actually tallying the footage from different logs straight off your mill to determine you bdft return, but had assumed you didn't want to go through that tracking process. You apparently intend to compare your formula results to your actual sawmill production results. If you have the actual results from the mill - base your log volume calculations off the actual numbers and forget the initial formula - would be my suggestion, unless I'm missing something here.
From the original questioner:
No one should doubt the value of log scales in predicting the board content of logs, or of the underlying formulae which define these scales. My post was about refining a scale's formula to take into account the bandsaw's thinner kerf. Testing such a refined formula in different situations helps validate it or refine it better, and I enjoy such things when I'm waiting for customers to call. Not unlike tying flies in the winter. I'm interested in making available to the customer who asks for it, for each new job, my best prediction of board content of his logs, before I saw the wood for him.
From contributor D:
What you're attempting makes perfect sense. What I maybe didn't express correctly was that I thought that you could change the order of what you’re doing to get to your end result quicker. I was "hearing" that you wanted to:
1. Take an accepted scale.
2. Modify it.
3. Test that against your actual sawmill return.
It all works in the end. I've been using the 1/4" scale, and have been tracking my production to determine how much over run I can expect from different logs. I too am doing this, partly, to better estimate how much I saw for customers.
From contributor A:
He would need a reference to start with. So by starting with the known scale he would have a target to shoot for. If he just started sawing he would have to do many loads to get a good medium to work with.
From Gene Wengert, forum technical advisor:
To maximize the board footage recovered, cut as many thick pieces (8/4 vs. 4/4, for example) as possible, maybe even a 6" cant. Also, open the log with a 4-1/2" wide opening face on the poorer faces and 6-1/2" on the better faces. Further, make sure that your lumber thicknesses are the best possible. For most hardwood sawing, this means 1-1/6" green (1.0625"). Make sure that your customer will find this size ok. For each 1/32" you save in thickness, you will have the customer get almost 3% more yield. Similarly, each 1/32" saved in kerf will save nearly 3% in yield.
To maximize the footage of the higher grade lumber, without losing overall yield, cut any good faces parallel to the bark. Take out the taper in the low grade. Also, turn the log to the next face when the faces adjacent to what you are now sawing promises higher grade. When selecting the faces, try and maximize the clear area in the faces and also put knots of the edges and not the center of a face. Finally, after opening the log, when it is time to go to the second face, turn the log 180 degrees, not 90 or 270. Following these suggestions will increase value by over 20% in many cases and increase yield by 2 to 5%.
