Bandsaw blade width - theoretical discussion anybody?

[ian maybury]

These damn band saws are addictive!!!!! I've posted before about how I think maybe the ability of a carbide tipped band saw blade to cut/plane a little off the side of its teeth may be a factor in both it's improved ability to cut a true line, and at the same time deliver a good finish.

Van and others I think have posted on not just of the benefits of the additional beam strength delivered by a wider blade when re-sawing, but also of the downside this brings in terms of the need this brings for a saw capable of applying much increased blade tension.

The situation (it struck me while hand sawing some wood blocks I needed when building a rack over the weekend because my machines are not up and running yet) is not even as straightforward as the above.

One further problem with wider blades it seems to me is that once one gets deflected off track, the effect of the deep back is going to be to keep it on that line - making it much harder to get it back once the deviation exceeds the clearance available in the width of the kerf. You see the effect with a handsaw where getting a straight start is all important to sawing to a line.

Another is that since the the blade is flat and cuts off its leading edge the effect of too much thrust loading will eventually be to produce twisting - which will also take the blade off line. But this will presumably be influenced by the effectiveness of the guides too.

The positive of the wide blade is of course that (provided the kerf isn't providing much clearance) the back of it will tend to be guided by the already completed part of the cut - although this isn't a self correcting system of guidance as such.

I wonder if it's possible that (if we were dealing in ideals) that the optimum band saw re-saw blade might be a circular section blade of very high tensile strength cutting equally off all sides - combined with a saw frame/chassis capable of applying so much tension that beam strength in the blade was unnecessary to take thrust loads. That way it would rely entirely on the fence or whatever for guidance, and be completely non-directional. (it'd be hard to guide to a line by hand though)

It seems to me that blade tension is probably one variable where there's a fairly clear 'more is better' situation (frame strength is probably another) - at least until it starts to produce side effects like blade breakage, frame twisting or undesirable vibration harmonics or whatever.

The ideal is not going to arrive any day soon I guess, but maybe it does suggest that the optimum re-saw blade is one that is just wide enough to be strong enough to handle the maximum tension the saw can apply, and also add enough additional beam strength to get the job done without the blade deflecting/twisting under the thrust loading.

Kerf clearance and side cutting ability (a little like a circular saw blade) is an interesting variable too, in that there's potentially arguments for both less and more.

What this really brings out for me is that a band saw to work well has to be designed and set up so that it's sitting in a 'sweet spot' where all of these multiple variables (and many more - including stuff like depth of cut, feed force etc) line up. Also that it's so easy by changing or blunting a blade or something to slip out of this zone.

The zone becomes very narrow indeed on badly designed, flimsy or under specified saws I guess. We tend to lose interest once we have something that gets our job done, but I wonder how well optimised even larger professional saws and blades truly are? What might it take to get to a point and shoot band saw that once set up would stay that way, and not need to be cajoled into behaving itself for every job?

:o Pardon me if this is all to esoteric, please just ignore it if that's the case....

ian

[Van Huskey]

Quite frankly I agree with most of your hypothesis and reasoning BUT I am far from an engineer. The idea of a cutting "cord or rope" is interesting but the structural integrity of the saw would be seriously challenged. I occasionally wonder why we don't see two spined saws which would make tensioning most anything a non-issue, then I always remember we have them, just in horizontal bandsaws. I suppose with the vertical variety good enough is good enough. I must say watching veneer come off a horizontal resaw bandsaw is amazing big 2+ inch carbide blades with enough strength in the frame to snap them if one so chose.

I think one thing to keep in mind about bandsaw design is that the ubiquitous 14" cast clones were designed by Delta for a 1/4" blade so frankly they are only "weak" in the modern respect. I have seen one with a 1/4" bimetal blade tensioned at the 3/4" mark on the saw cutting paper thin veneer that was quite uniform and well finished. The other thing to remember is large resaw heights are a relatively new fad. Look at the venerable Tannewitz GH series saws 3,000 pounds of steel and iron, 36" steel wheels and "only" 20" under the guides. Many of the "real" bandsaws from the past weigh FAR more then the 750# of my MM20 but lots of them only have 12" of resaw height. We are asking for more and more of the dimension that is most difficult to engineer and even with the higher end "hobby/light commercial" bandsaws like Felder, Laguna, Minimax and Agazzani we are paying far less than price adjusted bandsaws of near their size from the past.

To touch back on width I still am firmly in the wider is better camp up to the point that a particular saw can no longer properly tension the blade, as I have said I have seen excellent results with a 1/4" blade but for me the best, most consistent, easiest to setup and requiring the least skill resaw blade is a wide resaw blade. That said your suggestion is very interesting.

As for how well professional saws/blades are optimized it depends on the situation. As told to me by blade company engineers the hardwood flooring industry drives the resaw blade market. Often the resaw bands we use have been designed specifically for that market, variable pitch and carbide shapes apparently have trickled down from them. From talking to the Lenox guys this industry takes resawing VERY seriously and even more so in the last few years as hardwood flooring in the US has become a "requirement" in new construction and remodeling. So if you are speaking of production I think the blades and saws are tweaked pretty well, if you are speaking about vertical bandsaws in commercial custom furniture shops that depends on the guy running the saw and likely as you mentioned once "good enough" he no longer tinkers with it.

[ian maybury]

Thanks for the feedback Van. I couldn't disagree on your rule of thumb regarding blade width, in that I don't have that much experience, and anyway it seems to be a good way of approximating a reasonably optimum balance between blade tension and width. It's perhaps a way of saying 'by all means use a wider blade, but make sure that at minimum your saw can wind it up to at least 28,000psi (or whatever the number is) tension, or you risk running into problems with the blade deflecting sideways or twisting'. My suspicion is that in absence of monster tensioning possibilities and ultra high tensile strength blades (30,000psi isn't too far from the yield strength of mild steel) current saws probably depend to a fair degree on the beam strength of the blade when re-sawing.

My gut feel meanwhile on the sort of band saws we use is much like what you say. That there's a lot of custom and practice/'it's good enough' for what people typically did with them, but that we're now into an interesting era of evolution towards more demanding applications. With re-sawing being the current fad as you suggest.

I wonder if there's enough scope for development in them to take them from being the pernickity and slightly cranky machine they are to the sort of point and shoot device the table saw is???