Bandsaw Conversion

By Brian Simpson

One of the many problems one encounters making scale model horse drawn vehicles
is cutting metal. Mostly it is converting sheet material into sized strip but also cutting round and square section steel.

A Delta or similar bandsaw will cut brass using a suitable toothed blade but all the teeth disappear when cutting bright mild steel. This is due to the blade speed which must be slowed. The bandsaw blade speed of the Delta is 1400 rpm which is obviously too fast and must be slowed to around 75 rpm.

The Delta saw is two wheeled with the bottom wheel mounted and driven directly on the motor spindle.

To modify the speed, therefore, remove the motor and mount the bandsaw drive wheel on a new spindle held on a pair of Plummer blocks. The motor should then be remounted in an open ended wooden box hinged to the side of the bench top in order to allow adjustment to the drive belt tension by rotating the box.

The formula for speed changes to pulley set ups is:

                            G = M x Dn
                                    Sn

where: G is the desired final speed in rpm
           S is the diameter of the pulley attached to the bandsaw drive wheel spindle.
           M is the motor speed in rpm
           D is the diameter of pulley on the motor spindle.
           n = the number of pulleys on the motor spindle (D)

EXAMPLE: If the largest diameter pulley for which space can be provided is 6 inches together with two small 1¾ inch pulleys applying the formula the speed is:

                             G = 1400 x 1¾ x 1¾ = 119

                                            6 x 6

An additional idler spindle can be mounted in Plummer blocks between the motor spindle and the bandsaw spindle using additional pulleys. Appropriate sized drive belts must be fitted, of course.

In the example the 1¾ inch pulley on the motor drives the 6 inch pulley fitted on the idler shaft on which the second 1¾ inch pulley is also fitted. This pulley then drives the bandsaw. (see diagram).

Although the final speed obtained with the set up described is 119 rather than the ideal 74rpm it has been proved that this set up will cut bright mild steel sheet easily using 24 and 32 teeth per inch blades. Two inch diameter round bright mild steel has also been cut, without difficulty, using a 14 tpi blade.

The only down side is that it is too slow to cut wood efficiently. The way round this is to swap the small pulley on the idler with the large pulley on the bandsaw spindle which will put the speed back to 1400 rpm.