How Many Table Setups can a Lineal Saw Support?

Let’s talk about a plant built around a single lineal saw. Is it practical for some kinds of plants, but not others? To answer this, we’ll attempt to estimate how many truss assembly work stations a lineal saw can support, recognizing that different plants have different mixes of residential, commercial and agricultural work.

This will be highly simplistic, but may shed some light nevertheless.

Creating Cut Pieces on Lineal Saws

How many pieces an hour can a lineal saw cut? Clearly, having an automated infeed system and a full time person catching the lumber on the back end of the saw will make a considerable difference. Some of the literature from manufacturers claim the saws can cut 450 pieces per hour (7.5 per minute.) Certain sales reps, those practical fellows that have a reputation to consider, have been quoted as saying about

300 pieces per hour (5 per minute.) To be safe, let’s also consider we might only get half that number (2.5 per minute.) We’ll look at all three numbers (best, most likely, and worst case) in creating estimates.

Consuming Cut Pieces on Tables

Since production figures vary so greatly I want to start with an extreme case. Imagine a plant that never has to set up a truss, it simply builds 28’ fink trusses all day, every day. If that crew, working on one workstation, can build one truss (10 pieces) a minute, then it is pretty easy to calculate that there is no way, even in the best case scenario, that the saw (7.5 pieces per minute) can keep up with the table (10 pieces per minute.) But one truss a minute is p

retty fast. What if it takes five minutes for each truss? Now the table is consuming 120 pieces (12 trusses x 10 pcs/trs) per hour and a lineal saw would have no trouble keeping up – it probably would even be able to support two workstations building these 28’ finks.

Conclusion: In this ultra-simplistic scenario, if your workstation can build one truss a minute, a lineal saw cannot keep up. If that workstation takes five minutes a truss, the saw can keep up – no problem.

Let’s look at the same idea, but with a more complex truss, a cathedral truss with 14 pieces. In this case, if the table can produce a truss in less than three minutes, the saw will have a hard time keeping up.

In these charts, the red boxes indicate “the saw cannot keep up,” even in the ‘best case’ scenario. Orange or lighter means the saw will keep up – if our ‘best guess’ for lineal saw output is correct. To create metrics using units that may be more familiar, note that our fink has 65.33 BF; the cathedral 71.33. The BF/MH column indicates what a plant, with a three man crew, would produce per man hour producing nothing but finks or cathedrals with no setup time.

So much for “theoretical” truss plants. I know of a plant in the Far West described as “highly efficient” that produces pretty chopped up work at a rate of 110 BF/MH. These man-hours include only the three guys working the tables. If this is a “highly efficient” plant, let’s speculate that a plant that is merely “efficient” can produce 100 BF/MH. To keep things simple, let’s borrow the ‘number of pieces’ and ‘board footage’ from the cathedral truss above and assume that those represent a typical truss at this plant. This would mean that each workstation at this ‘efficient plant’ produces 300 BF or 4.2 trusses per hour, or one truss every 14 ¼ minutes. Going a step further, we can arrive at these numbers:

If our assumptions (3 men per table, average truss is 71.33 BF and has 14 pieces) are correct, then a lineal saw at a 100 BF/MH plant can support at least two workstations and perhaps as many as four.

Summary:

·Lineal saws can cut between 1200 and 3600 pieces per shift. This roughly equates to 6,000 – 18,000 BF depending on saw efficiency and mix of 2x4, 2x6, etc.

·A lineal saw can almost certainly keep up with two assembly workstations in all but the most efficient lines

·For custom residential work, a single lineal saw may be able to supply as many as four workstations

If you want a copy of the spreadsheet I used for his analysis, just drop me a line.

Dividing Work Between Saws

How do divide up your work among your saws? Chords to one, webs to another? Or do you find it easier to send certain types of jobs to your "A" saw, and other types of jobs to your "B" saw?

Before Automated Saws

Prior to the development of highly automated component saws (around 1995) the conventional wisdom suggested that two saws were best because:

1. Saws are the production bottlenecks and we need at least two saws to keep up with one 100' gantry.

2. If one saw breaks down, we won't be totally shut down.

Before 1990, there were large saws for chords and a smaller ones for webs. The web saw was more compact, meaning less distance for the sawyer to cover between blades to set them up.

The Current Landscape - Different Saw Lineups at Different Plants

Today, a relatively small number of component operations cut their pieces primarily using a single saw - either a component saw or lineal saw. One MiTek account manager I know is convinced if you are going to build your operation around a single saw, that that saw should be a component saw. Component saws are popular, of course, and have been the workhorse of the industry for many years.

In spite of this, in recent years several startup plants have chosen to begin with a single linear saw. Several ideas have driven this decision:

1. The lineal saw is better suited to a 'just in time' production process

2. Lineal saws can cut "chopped up" (high 'setup to run' ratio) work more efficiently

3. Lineal saws produce less waste

4. There is very little a lineal saw cannot cut, meaning a simple manual radial arm saw is the only "backup" saw needed. A plant with a single component saw is best complimented by a more expensive, semi- automatic radial saw.

5. In some cases, a lineal saw is less expensive than a component saw.

Most truss operations have two or more "major" saws, either multiple component saws or a component paired with a lineal saw. Let's look at the first of these two, the multiple component saws shop, first. As mentioned earlier, component saws used to be more specialized than they are today. Some were more clearly "chord saws" and others "web" saws. Today, component saws are designed to be equally proficient at either. As a result, although some plants parse their cutting lists into "chords" and "webs" and send them to different saws, many simply alternate - sending complete jobs to each saw.

If your truss operation has an older, semi-automatic saw and a newer, fully automatic component saw then sending the chords to the "old" saw and the webs to the "new" saw makes a lot of sense. Another strategy is to split the work between the two based on quantity. For example, If there are 10 ore more pieces that are identical, send them to the "old" saw, and all quantities less than 10 to the "new" saw.

The other 'multiple major saw' scenario is the component and lineal saw combo. Although arguably the "best of both worlds," there is by no means a consensus about how best to divide work between the two. Here are some of the ways people divide work between their component and lineal saws:

1. "Straight run" jobs go to the component saw, "chopped up" jobs to the lineal saw. This is a variation on the "quantity-based" strategy mentioned in the last section. The advantage here is simply keeping the entire job on one saw.

2. Chords to the component, webs to the lineal, a modern-day version of the "two saw" strategy of twenty years ago.

3. Attempt to keep the saws equally busy by implementing the "quantity based" strategy, but changing the quantity as one saw gets ahead of the other. As an example, you give your component saw all trusses with quantity over 5, and your lineal saw gets the rest. You find you component saw getting ahead, and so you change the rule and start to give the component saw all trusses with quantity over 4.

Reviewing, we have seen several strategies for dividing work among multiple saws:

1. Job by Job - I use my judgment and send some jobs to "A" and some to "B." I do this on the basis of either the type of work it is, or which saw is currently "ahead."

2. Role - Each saw plays a role, such as "Chord saw," "Web Saw," "Panel Saw," and work for that role is sent to that saw.

3. Quantity - By splitting my cutting lists using quantity of pieces or trusses, I send work with more "setups" to the quicker (to setup) saw, and work with more "runs" to the slower (to setup) saw.

Thinking through these "utilization of saws" discussion, many questions suggest themselves:

· At what point will a single linear saw be unable to keep up with a plant's work?

· Is it worth keeping an old, inefficient saw in production when the work could be done on a more efficient saw?

· Is there a way to decide with numbers which strategy is best?

We will tackle these questions in future newsletters.

Saw Batching Strategies

This is the first in a series on the evolution of strategies for cutting lumber efficiently. In future editions, I’ll eventually get to my favorite saw subject – defining the proper role for your lineal saw.

Batch cutting was already well-established when I began creating cutting lists in 1982. The concept was simple: Component saws take 5 to 10 minutes to change from one setup to another, so let’s minimize the number of setups! Minimizing setups is accomplished by batching a large number of trusses (the more, the better!) and then sorting the pieces, by cuts, then by length. This then is our first strategy:

Strategy #1: Minimize Sawyer Labor
Best for: saws that take a long time to change from one setup to another
Number of trusses in the batch: as many as possible
Sorting logic: similar cuts, length
Advantages: minimizes setup changeovers
Disadvantages: lumber picking must include many different lengths, stacker must stage pieces from a large number of trusses

The Minimize Setups strategy was batch cutting for decades, but as saw manufacturers made improvements that reduced setup time, production managers longed to reduce the time it took to pick lumber. It seemed so inefficient to travel to several bunks and hand pick a certain number of pieces from each bunk and bring them to the saw. And if a piece needed to be culled, then another trip to the bunk was needed. The solution?

Strategy #2: Minimize Infeed Labor (or "Eliminate Picking")
Best for: semi-automated saws that have room for live deck infeeds
Number of trusses in the batch: as many as possible
Sorting logic: size/grade/species, stock length
Advantages: minimizes infeed changeovers
Disadvantages: more saw setups than Strategy #1, and stacker must stage pieces from a large number of trusses

During the heyday of the Minimize Infeed Labor era, a variety of creative live decks were designed, and many are still in use today. One bunk after another is moved into position in front of the saw and the sawyer cuts everything he can from that bunk before moving on to the next one. Saw manufacturers pressed on, and suddenly setup times were reduced to from the 1-2 minutes of the semi-automatic era to the current standard of 25-30 seconds. Saws were cutting so much that a second stacker was added to help organize all the pieces being cut. Enter the third strategy:

Strategy #3: Minimize Stacking Labor
Best for: automated saws
Number of trusses in the batch: a small number of trusses that will be built together (a table production group)
Sorting logic: varies, but one simple one is, "tops, bottoms and webs."
Advantages: keeps things simple on the back end of the saw
Disadvantages: lumber picking must include many different lengths, lots of saw setups

In very broad terms, the strategy you use is based on which part of the sawing operation you want to favor, picking, sawing, or stacking. Note that creating small bundles that can be taken directly to the assembly line can also help the people setting up and building the components. You may find in considering this discussion that your current strategy is based largely on "This is how we have always done it." If this is the case, you are not alone. A strategy that worked in the past will not be abandoned quickly, even after new, faster equipment has been purchased.

The final strategy is the extreme case of the "favor the back in of the saw."

Strategy #4 - Cut One Truss at a Time
Best for: lineal saws that are dedicated to one or two production lines
Number of trusses in the batch: one
Sorting logic: whatever optimizes the material on the lineal saw
Advantages: cut it, then build it; minimal material tied up
Disadvantages: If the saw goes down, you are stopped in your tracks

This strategy is the logical evolution of a 'just in time' production process. It raises challenges that will be discussed in a future article.

A Self-Test
If you want to analyze your own strategy, answer the following questions:

1. How many trusses are in a "job" sent to my saw department? (size of batch)

2. How many different lists do I create for each job? (parsing)

3. In what order are the pieces in each list sorted? (sorting)

4. How many people are needed for the overall sawing process? (fractions are OK)

Incidentally, if you are not using software to create a batch cutting list, you are not alone. But make no mistake, you are using one of the strategies nevertheless. Letting the sawyers themselves figure out how to cut a set of trusses usually means you are using Strategy #3 and probably using some variation of "cutting one truss at a time."

For each of the questions above, find out the answer and then write down exactly why you do it that way?" If you can't put it into words, they you are on your way to learning something important about your production. It is essential to understand the "why" for each part of your sawing operation. By putting it down on paper your will really be forced to think through each decision.” Once you have done this, evaluate your process and see if it makes sense based on your current equipment. "We have just always done it that way" is a sign that we really don't know why we are doing what we are doing - a sure sign that we may be able to be able to improve our efficiency.