Cast Saws Everywhere, and Not One That Cuts
September 2016 Issue
If the facility at which you work is like most others, it probably owns multiple cast saws and there are a few of them that don't work. I work at an O&P practice with three offices and ten cast saws. Each of these saws, which cost anywhere from $500 to $1,200, requires maintenance, which can be expensive, so it's easy to postpone. So when one or three of them stop working correctly it is a huge pain. Here are a few tips to save money on maintenance and repairs. If you are handy at fixing things this will be easy. This whole process only takes a few hours. Make sure to go through all of your saw's parts to see what needs to be replaced before ordering anything-chances are you may have a few parts that need replacing.
First diagnose the problem. If the saw heats up more quickly than normal, it could be that the bearings or bushings are bad. If the saw stops functioning entirely, it could indicate a problem with the switch, internal wiring, a winding, a brush, or the cord. Try turning the blade and then turn it on again. Sometimes when a winding is bad, you can "jump start" it past the bad winding. Cords don't typically go bad unless they are kinked, so it is easy to see if that is the problem. At this point, if it's not a damaged cord, the saw will have to be dismantled.
To dismantle the saw, remove the plastic caps on the sides of the saw and pull the brush assembly out (Figure 2). Check the condition of the brushes. If they are chipped or charred, then there is an issue with the commutator. The brushes can be sanded down past the chip, or replacements can be purchased from an online parts store. For repairs on commutators and brush conditions, read "A Metal Lathe in an O&P Lab?" (The O&P EDGE, February 2015). If the brushes look good, move on. Remove the screws holding the bottom housing to the main housing. Carefully pull the two sections apart. Tapping the housing lightly can help the parts separate. As you pull the bottom off, be mindful of the two washers under the bearing. Remember what order they go in for reassembly (the spring washer is placed against the housing, and the regular washer is placed against the bearing). Then remove the screws that hold the field assembly to the bottom and carefully pull the field assembly out (Figure 3).
Check inside the bottom housing for charred or broken wires. If broken wires are present, splice them together using a crimp connector or solder and shrink wrap. You should be able to find the connectors and shrink wrap at a discount tool and equipment retailer. Sometimes the wires that wrap around the brush housing can slip off. Make sure they are gripping tightly.
If the wires are fine, then the power switch may be bad. Use a multimeter to check for resistance through the switch. YouTube has a host of videos about checking resistance (ohms). If the switch reads a one on the meter, it means that electricity will not flow. Most switches can be disassembled to check the rocker terminals (Figure 4). Be careful when dismantling switches as some contain spring-loaded ball bearings, which are difficult to find if launched across the room. If there is charring on the terminals, clean them well and reassemble the switch. The multimeter should flicker some numbers then read zero if it is getting resistance, which indicates the switch is working. If the switch is sealed, you may find a suitable replacement from an online electronic parts stores. Just search for the numbers on the side of the switch. Once you are sure that the switch and wiring are fine, you may as well do some maintenance since the saw housing is open. Use an air compressor to blow out any dust or debris. If there is any built-up dirty grease inside, clean it out with a cotton swab soaked with paint thinner.
At this point, check the bearing at the base of the armature. A properly working bearing should spin easily but stop quickly because it is properly packed with lubrication. If it spins easily and for a long time, it is going bad. If it is gritty feeling and difficult to turn, it is already bad. To change the bearings, the top of the saw must be disassembled.
To disassemble the top of the saw, remove the blade. Once the blade is removed, remove the top cap screws. The top cap comes off easily. Carefully remove the gasket. Inspect inside the top cavity for signs of wear. Remove the felt band around the shaft; there is a loop spring that holds it in place (Figure 5). There is a tiny bearing around the offset cam shaft. Check to see that it spins properly. These are not grease-packed bearings so they can often just be lubed. But if it is bad, replace it. A cam tool is needed to remove the offset cam. This can be purchased online from www.rpiparts.com (tinyurl.com/hj7v8kd), or a similar tool can be made with some time and effort. Use ring lock pliers to remove the locking ring holding the bearing in place. Some saws have a locking ring on both sides of the bearing. Wrap a towel around the saw and place it in a vise for removal.
Using a hammer and punch, gently tap the armature so it comes out of the large opening. Be careful so you do not drop the armature. If the saw has a second locking ring, the bearing will come off inside the housing. You will need to use a long punch through the large opening in the housing to tap the bearing out the small opening. If the saw does not have the second locking ring, the bearing will be attached to the armature and can be placed in the vise; the armature can be tapped by a punch for removal. An arbor press can be used to remove some bearings, and videos for this can be found on YouTube. Test the bearing and replace it if needed. Check the top cap assembly for wear or excess movement. These parts can be replaced individually if they appear inoperable.
The aforementioned descriptions address the most common cast saw failures. Reassemble the saw in reverse order, lubricating moving parts with general-purpose grease where bearings sit. On the top cap assembly, use a lightweight oil like 3-in-One. Be careful to lay the wiring in a way that the rotation of the motor does not rub it. Once reassembled the saw should function much better.
What follows are a few additional tips to help save your facility money and keep your cast saws in good condition. Cast saw blades vary widely in price. Some companies sell the round blades for just a few dollars and the shaped blades for many times that. Instead of paying for shaped blades, you can shape round blades in just a few minutes. First, use a black marker to trace the shape that you want on the blade. Grab the blade tightly with vise-grip locking pliers and use a metal grinding wheel to grind it to the desired shape. A rotary grinding tool can be used with a cut-off wheel if a grinder is not available.
Don't throw away dull cast saw blades. Use some micro files and sharpen them. Using a triangle file or a double half round-whichever one has the sharpest edge-file in between each tooth of the blade until they are all sharp again (Figure 6). This only takes a few minutes, and it doesn't even have to be removed from the saw. I have sharpened blades many times over to extend their life. Prevent damage to your saws by hanging them instead of laying them on a bench. A pegboard hook or a piece of metal can be bent to hold them on the wall next to your workspace.
I hope you find these tips useful. Some of these procedures can be difficult but should not be too complicated. If you have any questions, feel free to contact me. I have more photos and can point you to videos if needed.
Travis Petersen, BOCP, COA, works at Alpha-Omega Orthotics & Prosthetics, Springfield, Missouri, as a practitioner and technician. He has also worked as a technician for other companies in Missouri. He can be contacted at .