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Splitting hairs: Basic components to maintain tolerance

May 6, 2024  By Josh Bergen

A guide inspection. Photo courtesy Precision Machinery.

He walked into my shop looking angry. 

“Are YOU competent?” he demanded.

While it’s not something I’m often accused of, I could tell that was not the answer he was looking for. 

“Well sir,” I replied. “If it involves your drivetrain, this shop has a 5-star rating.” It wasn’t a lie either, I’d written the review myself. 


“Come look at my truck,” he said, turning and walking out the door. 

I scurried after him, a bit worried about what we might have done to cause his abrasiveness. 

“Look!” he said, pointing. “I’ve done two alignments and this thing still bounces and shakes all over the road!”

“Ummmmm… It looks like your driver’s side tire is worn down to the wires, and your passenger side is almost flat…”

“So what?” he barked. “The problem is obviously alignment or a worn front end. I’ve been wrenching on stuff for 30 years. I KNOW!”


I have similar feelings walking into many sawmills. Management and maintenance pour hours into saws, rolls and other high wear parts. All while ignoring worn or poorly maintained saw guides that are the main support component for accurate, repeatable lumber manufacturing. 

Many times, I’ve heard, “We can’t afford to replace these guides right now.”

You can’t afford NOT to. Saw guides are at the heart of lumber production and reliable sawing. They are no different than saws. 

Guides are a consumable item, not a capital component.

It is easy to overlook guides because of the incredibly small tolerances they must be manufactured and maintained to. It is often impossible to visually see a problem, specialty tools and knowledge are required.

When discussing guides you will hear the term “thou”. Often, we forget how small a “thou” is. 

A “thou” is one-thousandth of an inch. Remember grade school math class? One-thousandth = 0.001-in. 

To put that in perspective, a human hair measures around four-thousandth of an inch thick (0.004-in).

Now split that hair into five equal pieces, each piece will be eight 10-thousandth of an inch thick (0.0008”). 

That number is the tolerance our tradespeople must work within to maintain proper saw support.

Our lumber mills are running higher feed speeds, thinner saws and more production then ever before. 

Additionally, they’re doing it with less manpower. Where we used to need one guide change, we now need three. Where two-thou deviation ran acceptably, saw life is now a problem. 

Accuracy matters, and we need to give our tradespeople the proper tools for the job we expect.

If you go to any technical training institute and inform them you need to maintain a half-thou tolerance, they will say you need a temperature-controlled environment, dust control, hair net and white lab coat. 

Sawmills are obviously not that type of environment. Even so, there are things we can do to maintain tolerance. 

While it’s not easy, the basics are simple:

  1. Ensure you have a workspace set up for success. It should be clean and tidy with important tools close at hand. A granite block, micrometer and dial indicator are necessities. These tools should be readily available for use DAILY. If you must dig your granite block out from under other equipment, your guides are not being properly inspected often enough. We go into a lot of shops and often find these basic inspection tools in disrepair or missing. Each year we find an average of two to three inspection blocks that are worn, unknown to the mill. Micrometers are often out of calibration and give false readings. Dial indicators are the wrong graduation, or not present.
  2. Hold your vendors accountable. If you ordered saw guides with a guaranteed tolerance, check the product is as promised. Request inspection reports and then check the product matches the reports. If you don’t do a full inspection of all parts, spot check a minimum of 30 per cent against reports. If your vendor can’t provide a report, they are not providing a toleranced part.
  3. Train your people. In many saw shops, the entry level job is grinding knives and maintaining saw guides. We are handing the highest tolerance part in our sawmill to the least experienced guy in our shop. A few months ago, I asked a young fellow to show me three-thou with his fingers (0.003-in). He held up two fingers about a quarter inch apart. It wasn’t his fault – he’d never been properly trained. The fault lay with his supervisor and the leadership that allowed the situation to happen.

While these basic components may seem like common sense items for any shop, I can tell you from personal experience very few places are doing all three.

 Josh Bergen is a certified Red Seal millwright and a founding partner of Precision Machinery. You can reach him at jbergen@pgmr.ca

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