The Unknown Known (Bandsaw)

Today’s post title comes from a humorous, if not infamous, Donald Rumsfeld press conference:

I have two Hitachi bandsaws, as regular readers of this blog have likely observed. One is the CB75, which I have set up with the optional blade guides for small blades. Thus the machine is dedicated to using blades to cut curves. My other Hitachi is the larger brother to that saw, the CB100FA, which is a dedicated resaw and accepts 4″ wide stellite-tipped blades. They are both excellent machines, though I have had some unsatisfactory experiences with them at times as well in terms of cutting issues. While I thought I knew bandsaws well enough and could set the machines up to work well, I’ve now come to realize that I was relatively clued out about certain aspects.  In life, if one is open to learning, then, as the saying goes, the more you know, the more you come to know that you don’t know. Thus it is for me with bandsaws and a whole lot of other things I’m sure.

I thought I was having problems with blade drift on the bigger resaw, and was finding myself fighting the machine a bit while in the last stages of work on the Colgate project. so, over the last couple of months while I was laid up, I spent some time researching this issue. I wanted to know, if I might be cheeky, if others caught my drift.

Well, you can read about and watch videos on people dealing with bandsaw blade ‘drift issues’ until the cows come home. I’ve been reading articles on this topic at regular intervals in woodworking magazines until, in the past few years,  I have stopped paying much attention to woodworking magazines – and magazines in general for that matter. I used to go to bookstores with some frequency, but that habit has gone by the wayside. Anyhow, bandsaw blade drift has been, and continues to be, a common touchstone of discussion and analysis.

Some people get into quite complex undertakings when addressing the topic of blade drift. This video would have to be the most over-the-top example of that:

I don’t suggest watching the entire video, but it’s your life.

If you search on Youtube for ‘Bandsaw Blade Drift’, you will discover quickly that the term should be added to the list of uncountable nouns. It’s unbelievable how much energy and analysis is devoted to this topic.

So, for years I had accepted the info I had come across regarding tuning your bandsaw to deal with drift as being gospel. Brand new blades somehow drifted all of their own accord and you need to adjust the machine to the blade’s peculiarities whenever you changed blades.

There were two videos which opened my eyes however, to see that maybe drift was not all it was cracked up to be. The first was provocatively titled “The Myth of Bandsaw Blade Drift…”:

If you didn’t find his argument convincingly demonstrated by making a good resaw cut with a bandsaw having the guides pulled away and the blade tension slackened, then you weren’t paying attention.

Obviously, the narrator wants to sell that resaw carriage, which is an item of little interest to me, but the point he makes applies all the same. What I learned was that it was likely that the wooden extension fences I had attached to both of my bandsaw fences, had likely led to my last saw blade getting worn so that they started to drift. It’s not so easy to get a 4″ wide blade to drift, but believe me it can be done.  The cut begin wanting to drift away from the fence, toward its sharper side. Indeed with my resaw this was the exact problem I had been battling, though I had carefully adjusted my fence to the line of cut and had done other things to adjust for drift. The extension fence, what I thought was an added extra to improve stock control and cutting was in fact having a negative effect.

You see, even after you set the blade fence, etc., to deal with drift, with every new stick you run through that happens to release stress  and opens up after the cut, will give more of the same: pushing the blade once more against the near side teeth, wearing them more on that side and thus increasing the tendency of the blade to wander away from the fence because  it is now the sharper side. Got it.

The next time I got to my shop I removed the secondary wooden fences from both of my bandsaws. The factory metal fences on both machines end right as the blade starts cutting, so maybe Hitachi had some good sense in their designs that I might have been better off leaving well alone.

The second video I watched which was highly educational and convincing was one taken of a talk given at a woodworking show by Alex Snodgrass, who has been working for Carter Products for many years:

For me, the gem of information in that video concerned the set up tricks for small blades, in which you use the upper guide’s back wheel to push the blade forward, then use the tracking to pull the blade on the upper bandsaw wheel back into position, teeth gullets centered on the tire.

So, when I got back in my shop after having absorbed the content of the Snodgrass video, I went to set up my smaller bandsaw accordingly. All was well until it came time to adjust the lower guide, where I ran into a problem I had noticed previously with these guides. For small blades, the lower blade guide has a mounting arm which is too short. At full reach forward it it sits too far from the correct position, and this problem is exacerbated by following Snodgrass’s approach of loading the small blade forward in a bow, as it moves the blade even further out from the guide.

I contacted Carter Products and asked if they had a guide package for my saw, and as it turns out, they did not, which I found a little surprising considering that the CB75 certainly sold in adequate numbers to make it a more or less common machine.

So, I decided to modify my lower guide. Not the first time. While I praise Hitachi for their excellent machines overall, when it comes to this factory-made guide kit for smaller blades, there are some shortcomings. The biggest problem with the lower guide in particular is that the mounting post is too short, but another issue I had with it is that when you try to tilt the saw table, a portion of the table casting runs into a portion of the guide. I rectified that issue by milling a small clearance out on the guide last year, so to revisit this guide for more work is kinda like saying hello to an old friend now.

I figured I could extend the post by milling up a piece of brass and bolting it on there. Found the perfect piece to start with:

Here’s where the rotary table on the mill comes into its own:

The flat is machined:

The piece is separated from the base with some tedious hack-sawing, then I deck off the mating surface on the mill:

Combining a vise with a c-clamp gives the required alignment for the two parts:

Perhaps time to look at renewing the tape on the jaws, as it isn’t doing much anymore.

Next, I transfer the bolt centerline over to the guide post with a transfer punch:

After drilling and tapping the end of the guide post, I attach the extension piece with a cap screw:

Done:

A basic repair – sure could have done something fancier/better to join those parts – and I was tempted to t&g and silver solder them – but at the end of the day, it wasn’t merited for something like this. The guide itself was not a shape lending itself to easy fixturing as well, so chewing up hours of time for something like this didn’t make sense to me. There’s just not much load on it as the bolt for securing the mounting post in position bears much more closely upon the other end of the bar, far from the extension piece.

The lower glide is slid into position and here you can see the brass extension poking out of the rear opening, and you can see the location of the aforementioned fixing bolt:

I was able to put the new blade in – I’m trying the 1/4″ blade form Carter Products – and set it according to the method shown in that Snodgrass video.

I then grabbed a chunk of cedar and proceeded to freehand scroll cut the block, going every which way on the grain, mimicking the sort of cuts Snodgrass showed when making a band-sawn nested box in front of the audience. There result was impressive to me, and I was left considering that I did not have this functionality on the saw previously, all due to ignorance as to how to best set up smaller blades:

I was unused to being able to smoothy swing around and scroll cut the block that it was as if i was learning the technique for the first time:

I felt the cut quality was excellent, even being shiny in several places:

The blade is cutting well so far at least. We’ll see how it goes.

All for this time. Thanks for visiting the Carpentry Way.