A Metal Lathe in an O&P Lab?

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By Travis Petersen, CPOA

Injection molding, resin casting, milling, lathing, and other procedures may be used when fabricating O&P devices and components. But what happens when a part is needed posthaste and it's backordered or doesn't exist? One solution is to make the part yourself. Most of the tools needed to fabricate unusual items are already on hand in O&P fabrication laboratories, but a metal lathe might not be-and it is quite handy. A savvy technician or assistant will find that he or she can use a metal lathe to fabricate components, such as the ones described in this article, and even more.

Helpful Hints

Figure 1

Figure 1

It is often possible to purchase a metal lathe secondhand, such as on Craigslist. It is not a cheap tool, new or used, but it may pay for itself after you've had occasion to use it on one or two projects. There are many types and sizes, but a 7-in. x 12-in. tabletop lathe is sufficient. The brand is not that important, but I recommend researching the features of the lathe before you purchase it. There are a few additional tools that you may need along with the lathe, such as cutting tools, calipers or micrometers, safety goggles, a centering gauge, and maybe a box to keep all the parts organized. Lathes produce shavings, and it is a good habit to clean it after each project. Figure 1 is a typical metal lathe.

Project One: The Locking Pin that Doesn't Exist

Figure 2

Figure 2

Figure 3

Figure 3

There are many gel liners and locking mechanisms, but not all are compatible with each other. For example, a prosthetic arm may require a very small gel liner with a very small lock, but what happens if the threads on the pin don't match the liner? A solution is to alter a pin to give it the proper threads. Figure 2 shows the pin in the lathe ready to be shaved. This pin had serrations that were too large for the lock, but the lathe makes quick work of that. After turning it down to the proper circumference, a little polishing finishes it. The pin on the left in Figure 3 is the altered pin with the correct threads.

Project Two: Plate Adapter

There are differences between adultsize and pediatric-size prosthetic components. For example, the four-hole pattern is different, the screws are different sizes, and some adult components don't have standard four-hole patterns. There are adapter plates available for purchase, but occasionally you may encounter a situation where a standard adapter doesn't offer the solution you need. An example might be when you need to bond components together for a check socket. An adapter plate can be fashioned on the metal lathe out of ¼-in. aircraft-grade aluminum to match the standard four-hole pattern with whatever screw hole pattern is needed. Figure 4 is a drawing of such a plate. The appropriate tap is also needed to achieve the proper threads. This project is, of course, up to the discretion of the practitioner, because the newly fabricated part will not be ISO tested. However, when you are in a pinch for a check socket, it could be a lifesaver.

Figure 4

Figure 4

Figure 5

Figure 5

You can also alter this plate to be a suction plate adapter by lathing a ring around the center to fit a rubber O-ring.

Project Three: Making a Dummy

It seems that sometimes the lamination dummy doesn't get ordered, is on backorder, or just isn't in the package, and there may not be time to reorder one. Dummies are typically chunks of metal or plastic that are machined to take the place of the real component during the device molding or laminating process. Some are simple to lathe from scratch or can be altered to work in place of other components, and many times the materials are already in-house. The dummy in Figure 5 was made from a ½-in.-thick blister mold sheet of polypropylene.

Project Four: Cast Saw Repair

One of the more expensive tools used daily in an O&P facility is the cast saw. When one breaks, you must decide whether to repair it or buy a new one. The repairs can cost nearly as much as a new saw, and it may take weeks or months to get it back.

The parts that are most likely to go bad are the switch, the brushes, the bearings, the commutator, the armature, and the windings. If the saw seems to be running with less power than it used to, the commutator or the brushes are the likely culprits, and can often be repaired using the lathe. This one project can make the lathe pay for itself.

First, remove the brushes, which are often covered by two plastic screws on either side of the housing. Examine the brushes; if they are chipped or charred, the commutator may also be damaged. Remove the screws holding the housing together; there are often three near the switch. Carefully remove the housing. A flat spring may fall out from a recess where the shaft goes. This spring will need to placed back in position when reassembling the tool, so don't forget about it. Examine the commutator for any damage. (Author's note: For more information about troubleshooting issues with the brushes or the commutator, visit www.oandp.com/link/300.)

Figure 6

Figure 6

If you find that the commutator is the problem, it can be repaired using the lathe. To remove the shaft and armature assembly, the top of the cast saw will need to be disassembled. Typically there are four screws under the blade. Remove the screws and the oscillating assembly. Inside the housing you will see a post that is offset from the shaft (Figure 6). On some models, this has to be removed for further disassembly. Use a flat punch to tap the shaft out of the housing. Be careful when removing the armature as damaging the copper wires may force you to send the cast saw out for repair after all. Now place the armature in the lathe and begin reducing the commutator until the contacts are smooth.

Be careful not to shave off too much: There are recesses between the copper contacts that are filled with plastic, and it will cause issues if the contacts are at the same level as the plastic. Use a scalpel to clean the copper from between the contacts and polish them with fine sandpaper. Now the cast saw can be reassembled.

This repair is complicated and should only be undertaken by someone who is confident in his or her abilities and has studied the procedure. To prepare for this repair project, view the video found at www.oandp.com/link/301. This video shows the procedure on a larger lathe with a different type of motor, but the principle is the same.

Have fun!

Travis Petersen, CPOA, works as an assistant and technician at Alpha-Omega Orthotics & Prosthetics, Springfield, Missouri. He has also worked as a technician for various other companies in Missouri. He can be contacted at .