As a furniture designer I work in a range of materials but primarily with wood and metal. I like to use a bandsaw for both of these materials but I only have a small woodcutting saw. Aside from a variable speed DoAll, which is very expensive, there aren’t many bandsaws made that can do both. So if you need to cut both, like me, you need two different machines.
Why can’t someone cut both materials on the same saw?
Well, primarily because in order to cut metal, a saw blade needs to be moving slowly through the work, and to cut wood it should be moving fairly fast. We can talk about the different speeds that a blade moves through a workpiece in terms of FPM – feet per minute (sometimes also called surface feet per minute – SFPM). The speed range for cutting wood is wide but a comfortable speed is about 3000 fpm. The range for metal cutting however is around 300.. a big difference. The other difference is of course in the blade itself.
*SAFETY FIRST: the images here show all belts and pulleys exposed. If you build this or something like it, you should also consider building a cover for these moving parts. Exposed they do present a hazard.
The goal here is to build a speed reducing mechanism that will change a wood cutting bandsaw into a metal cutting bandsaw, while maintaining the original function of wood cutting. In order to make this a useful mechanism one must be able to switch back and forth between the two speeds somewhat easily.
The saw is a 14″ Rigid brand woodcutting bandsaw. A 6″ riser block was installed by the previous owner, which allows for cutting taller work by making the distance between the lower (drive) wheel and the upper wheel, longer. This does not affect the speed of the saw, but the length of the blade does increase to 8′ 9″.
The base of the speed reducer/changer sits behind the saw and is about as wide as the saw and the motor together, or about 24″ wide. It is a simple open sided box made of 1″ mdf. There is a small channel or dado on top 1/4″ deep x 1/4″ wide, and a series 3/8″ holes 2″ apart. These details are not critical, but the holes should be made on a drill press if possible and they should be parallel to the channel.
Next there are two plywood boxes which serve as movable housings for the pulleys. Each box contains a 10″ length of 5/8″ diameter shaft, two bearings to house the shaft, and two stop collars which hold the shaft in the box and bearings. The bottom of each box has a spline protruding from the bottom which mates with the channel in the larger mdf base described above. The boxes each contain a plywood plate with a 3/8″ t-nut set into it. This plate & t-nut just act as a nut to make the connection to the mdf base via a 3/8″ bolt.
There are many ways to fabricate the system pictured here, this is just one way.
What is important is:
-that the speed is reduced to a good general metal cutting speed
-that the speed can be easily switched back to wood cutting speed
-that belts can be tensioned enough to drive the saw under cutting load
This diagram shows the pulley layout and sizing that I used. The lines that run tangent to the circles are the belts. The speed that the saw runs when driven through this system of pulleys is a good general cutting speed for most metals, but it still is a little fast for steel. If you want to set up a saw for cutting steel only, you might consider switching the one or both of the 6″ pulleys to 8″ pulleys.
In the video above you can see the bandsaw being used at the reduced speed to cut 16 gauge mild steel, 3/16″ aluminum bar, and 3/8″ cold rolled steel bar. (The blade has a skip in it from being poorly re-welded but you can get a sense of the feed rate.) You will also see the pulleys in motion at the end.
Vintage machinery is a very useful site for calculating rpm / pulley combinations for fpm.
Most of the parts were ordered from mcmaster carr and are listed below:
Twist lock adjustable v belt #6173K37
2” pulley # 6245K62
6” pulley # 6245K73
5/8” shaft collars # 6534K15
5/8″ ground rod (shaft) #8279T28
sealed bearings #6384K76
8’ 9” metal cutting bandsaw blade (14 teeth per inch) #4125A755