Machinery Masterclass archive
Monday 7 July 2008
Alan Holtham starts a new series on understanding and using machinery, beginning with the prime workshop must, the versatile bandsawError loading Partial View script (file: ~/Views/MacroPartials/cwsGalleryImages.cshtml)
The bandsaw is without doubt the most useful machine in any woodworking shop, simply because it can cope with such a sheer variety of work. But although it is a relatively simple tool in both design and use, there are some important points to look out for if you are thinking of buying one or are struggling with it.
One of its best features is its relative safety, because all the reaction forces generated by the blade are pushing downwards onto the table and there is little chance of the dangerous kickbacks associated with, say, a circular saw. It is also much quieter, which helps to inspire confidence; even a novice user will soon be happily working quite close to the blade, photo 1.
The other big feature is its massive depth of cut, with even a modestly priced machine cutting through 150mm (6in) material; you would need an impressive and expensive machine to cut this depth with any other type of saw.
For serious cutting, a two-wheeled machine with large-diameter wheels is essential, photo 2. These bigger wheels can accommodate thicker-gauge blades which will cut true against the fence and will not break. However, thicker blades require more tension, so the overall construction of the machine must be heavier and therefore more expensive. Also, the bigger the wheel diameter, the bigger the machine throat will be.
What to look forUse as a determining factor the maximum depth of cut you think you will need, but always add a bit on to give yourself some spare capacity, photo 3. Remember that the distance under the guides is only the theoretical depth of cut and may not actually be attainable in some timbers.
Blade sizeWhat you can do with a bandsaw is also determined by the size of blade it will take, so you will need a machine that will take a range of blade widths from 6mm (1/4in) to 16mm (5/8in), or even up to 25mm (1in) if you anticipate doing a lot of deep ripping, photo 4.
Wheel qualityAlthough the blade is a vital factor, the performance of the bandsaw also relies on the quality of the wheels. For smooth, accurate cutting these must be running true in all planes and should have some provision for balancing, which is usually carried out by drilling or adding weights, photo 5. It is impossible to over-emphasise the quality of the wheels, so do look carefully at their construction. If the wheels are out of true,
the tension varies continuously as the blade rotates, setting up a vibration which shows up in the quality of the cut.
Motor powerBecause a bandsaw blade generates only a small amount of friction, motor power is generally far less than that of a sawbench. For a 150mm (6in) depth of cut you need a motor size of about 550W; anything bigger is a bonus, but will be reflected in the cutting speed in thick timber.
Single-speed machines run with a blade speed around 2,500 feet per minute, which is fine for most woodcutting jobs. Machines with two speeds are still only meant for woodcutting, usually offering the slower option for very hard timbers, photo 6.
FenceThe bandsaw is ideal for ripping thick material as it is quiet and generates very little waste, but for this it does need a good fence for the cut to be accurate. It must lock solidly in place without moving as you tighten up, and shouldn't distort unduly when the unsupported end is pressed, photo 7.
Mitre guideA mitre guide is useful but not essential. Accurate mitre cutting is not possible on a bandsaw and the guides are only really useful for crosscutting small pieces or for incorporating into homemade jigs, photo 8.
Successful setting upFive operations must be carried out every time you change the width of a blade, and they should also all be checked whenever you change a blade for one of the same width. The adjustments are quite simple but must be carried out in the following order:
a. Set the blade tension
b. Adjust the tracking
c. Set up the thrust bearing
d. Align the side guides front to back
e. Align the side guides side to side
TensionWhile some tension is necessary to give the thin blade its rigidity - what engineers call beam strength - tension is not that critical and people who are struggling with it are usually trying to get a wayward blade to cut straight. Winding up the tension will have no effect and if a blade does not cut straight there is only one remedy: change it for one that does.
Start a new setup by moving all the guides well away from the blade, both above and below the table. Slip on the blade, centre it on the wheels and then gradually wind up the top wheel until the blade starts to become tight, rotating the wheel slowly by hand as you do so to distribute the tension evenly, photo 9.
Some poeple check the amount of deflection by pushing sideways on the blade but this is subjective, photo 10. When trying to gauge the correct tension don't rely on the tension indicator if your machine has one because these can be unreliable; you will soon get an instinctive feel for the correct tension.
You need enough to stretch the blade taut so that it doesn't slip on the wheels, but not a lot more. If blades start fatiguing and breaking well before they are blunt, you may be over tightening, which will gain nothing apart from crushing the tensioning spring in the top wheel adjustment. This spring is vital to provide a cushioning movement for the wheel as it rotates, ensuring smooth running and prolonging blade life, photo 11.
TrackingThis entails tilting the top wheel backwards and forwards to get the blade running where you want it on the tyre. I prefer to track it in the centre as it makes the blades more stable, particularly if you start backtracking, photo 12. The blade may or may not run in the same position on the lower wheel but this is unimportant. Tracking adjustments should always be carried out as the wheels are being spun by hand.
To move the blade forwards on the wheel, tilt the wheel forwards by slackening via the tracking knob, and vice versa. The adjustment is very fine so go steady and spin the wheels a fair bit after each alteration to allow the blade time to move before you alter it again. An over-tightened blade may be unresponsive.
Thrust bearingsOnce the blade is tensioned and tracked correctly, set the back-thrust rollers. These resist the backward pressure of the blade during a cut. Set the bearing so that it is just clear of the blade when it is free running; a gap of about 1mm is ideal, photo 13.
Keep these bearings in good condition as they will help to prolong blade life. At full cutting speed they will do a lot of work and eventually wear out, so be prepared to replace them occasionally. Don't forget that the guide system is usually repeated under the table as well, so both sets must be adjusted together.
Side guidesSide guides keep the blade running true and stop it twisting during the cut. For maximum effect they should be set to provide support right up to the gullet of the blade teeth, so will have to be adjusted for each different blade width, photo 14. If you don't reset them the blade may move back between the guides, knocking all the set off and ruining the blade in seconds, photo 15.
Guide blocksPlain blocks or bearings, these must be moved in until they are again just clear of the blade. The thickness of a piece of paper is about the right clearance, photo 16. Line up top and bottom guides vertically too.
Blade choiceNewcomers can be bewildered by the sheer variety of blades, but with careful thought about the type of work you anticipate doing you can get away with quite a small selection. A number of variables dictate your final choice, such as straightness of cut, speed of cut, quality of finished surface, depth of cut, grain orientation and hardness of material.
If you anticipate doing a lot of a particular type of work it is always better to change to the proper blade for the job. How you actually choose it can be simplified into three categories: width, tooth pattern and tooth pitch.
Blade widthThe blade width, as opposed to the thickness, is the single most important factor, photo 17. If you are cutting curves the radius of the smallest curve will determine the most suitable width. If you are cutting straight then choose the widest possible blade your machine will take.
Tooth patternThis really determines how quickly you want to cut and the quality of the resulting surface, photo 19. For general-purpose wood cutting the skip-tooth pattern has always been accepted as the norm, but a new generation of blades offers more aggressive teeth profiles and thinner gauges to minimise kerf wastage.
Tooth pitchThis is the spacing of the teeth, normally expressed as teeth per inch (tpi); basically, the fewer teeth the coarser and rougher the cut, photo 18.
Blade maintenance tipsOnce you have your new blade installed and tensioned properly a good tip for easy running is to smooth off the back and slightly radius the corners; this also minimises burn lines, particularly with the narrower blades. Use a diamond lap applied gently to the back of the blade as it is spinning under power, photo 20.
A clogged blade will stop cutting or cut off line. Resist the temptation to give it a good wire brushing; this would just ruin the sharpness. Instead take off the blade, fold it up and spray it with oven cleaner, photo 21; this softens the resin, which either dissolves or can be wiped off.
It is often said that a bandsaw cannot cut accurately, particularly against the fence, but if it is set up properly with a decent blade you should be able to rip veneers in thick hardwood with ease, photo 22.