Kerf thickness' effect on yield -- and profitability

      How profoundly does the thickness of blades used in sawmilling affect yield, costs, and wise use of resources? June 20, 2000

From Gene Wengert, forum moderator:
It might be helpful to consider, for a moment, the effect of lumber and blade thickness on yield. Assume that you are cutting 4/4 hardwood lumber and have an average thickness of 1-1/8 inches, plus a 9/32-inch sawkerf (sawtooth total width).

This means that each piece of lumber requires 1-13/32 inches of wood. Let's also assume that in a log, you will get 12 pieces of lumber if you cut from face to face (live sawing, which is not recommended).

In sawing the log, you will have 11 kerfs (two other kerfs are in the slabs which are waste anyway; thus we only have 11 kerfs for 12 pieces of lumber.) The net result is that it takes 16.6 inches of thickness to make 12 pieces of lumber.

Now assume you have a thinner kerf, 5/32. It will take 15.2 inches of thickness to get the same number of pieces. The difference is 8.4 percent less wood, or about 2.1 percent savings per 1/32 inch reduction in kerf.

The numbers are nearly the same for savings in thickness; cutting 1/32 on each of the 12 pieces will save over 2.2 percent.

If lumber is worth $500 per MBF on the average, then each 1/32 improvement in kerf or average thickness is worth over $10 per MBF. For a one million BF a year mill, that is over $10,000 per year!
Plus it is certainly much wiser use of our natural resources.

Note: "Savings" does not necessarily mean more pieces of lumber from every log. Rather, the lumber will be wider and longer; once in a while there will be an extra piece, too.

I hope that this example points out two things: (1) kerf is very important and (2) accuracy of setting up is very important. When considering new equipment or maintenance of old equipment, consider this as well.
Gene Wengert



What I like about thin-kerf blades is that I can fill my customers' orders and I don't have to saw as many logs to get the pieces I need. I have gained as much as 50 percent or more over scale when sawing at 4/4 on larger logs (more saw cuts equals more gain). Also, my custom-sawing customers can't believe how much lumber they get out of their logs. This is with a thin-kerf bandmill.


I have received several e-mails about this posting, asking about "chainsaw" sawmills vs. bandsaw mills.

If a chainsaw has a kerf of 5/16 inch and a band has 1/8, the difference is 6/32 inches, which is $60 per MBF more income from a bandmill. If you saw just 100 MBF per year, that is $6000 per year.

If you plan on using your mill for 3 years, the savings is about $18,000 over the three-year (depreciation) life of the bandmill. (Hopefully you will use it for many more years than three!)

Can you afford a 1/8-inch kerf bandmill vs. a thicker kerf (5/16 inch) mill, even if the band mill costs $18,000 more? Based on the above, the answer is clearly YES!

(P.S. If you are considering getting into sawing, see a lawyer and set up a limited liability corporation (LLC). Then get a loan for the hardware, saving your cash for log costs, inventory, advertising, etc. Pay back the bank as soon as you can see that you are making money, but always keep some reserve to carry you through deer season, Christmas, snowstorms, etc.)
Gene



On most logs, the board gained will have a hard time being worth the 500-dollar price range.

I will give you that yield will increase. However, the extra yield comes from the center of the log, where the defects congregate. Dimensional lumber has the greater advantage in realizing thin-kerf savings.



The extra wood comes from throughout the log, not just the center.

The defective center is still the center -- as I stated, the gain is not just more pieces of lumber, but many pieces will be wider (especially pieces near the outside of the log).

Further, outside pieces in a tapered log can be longer. Instead of 10 feet 8 inches, the piece could be 11 feet long. A little here and there can add up.

The $500 represents log run values. Therefore, it is the correct number to use for many species.

If you are still not convinced, consider that the first piece of lumber is sawn in the same location with any mill. However, with a narrow kerf, the second piece sawn will be closer to the outside (further from the defective pith). This then has the potential to be higher in grade, as it will be closer to the outside, which is clearer wood. I did not even include this benefit in my earlier calculations.
Gene



What Dr. Wengert says is very true. However, if you are cutting high-end products (we cut aircraft-grade sitka spruce), the numbers are astonishing. Every time I make a cut in a 12-inch flitch, 20 feet long, I "lose" $50 in sawdust.

(.125" kerf x 12" x 20'/12=2.5 board feet. @ $20.00/bf = $50.00)

This amounts to an average sawdust value of over FIVE HUNDRED DOLLARS for every 12 x 12 timber we resaw into 1-inch material.

You bet that we look at ways to reduce these losses.



One should also consider the uniformity of one's product prior to making much ado.

How uniform a sawing system will perform and how much maintenance is required to keep that unifomity are considerable factors, which must be taken into account in any system.
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  • KnowledgeBase: Primary Processing: Sawmilling

  • KnowledgeBase: Primary Processing: Woodlot Management

  • KnowledgeBase: Primary Processing: Yield Formulas

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