I just received a package containing a sawguide sample from our newest customer, and I had to write this article right away. They were having issues maintaining clearance between their babbitt pad and saw blade. In other words, they had no gap control! As I have mentioned previously, maintaining a tight control of the gap at 0.0015” between the babbitt pad and the saw blade is critical to the production of high quality lumber. It is definitely not the only critical factor, but gap control is a main cornerstone of lumber manufacturing.
The customer sent us one of their spare sawguides so we could measure it and make drawings so that we can manufacture their new guides. When I opened the box, I found two pieces inside. One piece was the body of the saw, and the other piece was a spacer. I sat in silence for a moment as my memory travelled back in time to the literally hundreds of conversations that I’ve had in sales calls and at trade shows about spacers. They all seemed to stem from the same question, “Do you make spacers for sawguides?” This question is easy to answer, “Yes!”
If I said this, I could stop writing this article. Most manufacturers would have the same easy answer. But I can’t say this easy answer in good conscious, so here goes the rest of this article…
A sawguide with a built-in spacer is manufactured together, and the critical surface is held to one tolerance. Let’s say that tolerance is +/- 0.0005”. It’s what other manufacturers hold and makes the math easier. This means there can be a total variation of 0.001” (from -0.0005” to +0.0005”) from sawguide to sawguide. This can cause problems when trying to hold a gap of 0.0015” because a variance of 0.001” will open or close the gap by a significant percentage.
Then let’s talk about a sawguide and a spacer that are manufactured separately to +/- 0.0005”. That means in a worst case scenario you will have a total variation of 0.002” – on one side the sawguide is twice -0.0005”, and on the other side it’s twice +0.0005”. This is what we call ‘tolerance accumulation’ in the machining industry. When your sawguides (the guide and spacer) vary by 0.002”, it is impossible to maintain a proper gap control of 0.0015”.
To sum up, tolerance accumulation is bad for gap control. Without proper gap control, producing quality lumber is even more difficult if not impossible. I guess I could stop the article here again by just saying that the use of spacers will cause sawing problems, so you should think twice about using them. But I think we should dive a little deeper here for a complete understanding of the issue.
I have asked other people why spacers are being used in the first place and there is only one argument I’ve heard so far to justify using a spacer. That argument was: “The sawguide is so big and heavy that it’s virtually impossible to get it in the edger in one piece!” I can see how using a spacer in this one case might be justified.
The other arguments I’ve heard revolve around the fact that with a spacer, a sawmill can change target sizes easier or cheaper. I guess this is true to a certain degree, and has been used as a sales tactic by some manufacturers/suppliers. If you take into consideration the costs incurred from downtime and poor lumber caused by a lack of gap control, the savings promised from using the spacer method are not there. Actually in the end, using spacers causes a lot more costs than they save – usually costing many multiples of the theoretical savings they’re supposed to provide.
You can use spacers if you want to, but I thought you should know the truth about them, and the costs associated with using them. I would love to sell lots of spacers for financial reasons, but what I actually want is for our customers to be productive and have great quality lumber and you just can’t do that using spacers.
Using spacers may save some money in the beginning, but downtime and lumber quality issues will kill any savings you may experience, and may end up costing you a lot more money in the long run. Don’t be penny wise and pound foolish, as the old saying goes!
Author: Udo Jahn
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