OScale Trains

Steel Crankpinsfrom “Scratch”Pressing new steel crankpins into out-of-production drivingwheels can be a challenge. They are often fragile; a little toomuch stress and the driver is gone forever. Here is at least oneway to avoid over-stressing those old castings while securelyanchoring removable steel crankpins. With this method, wecan do this ourselves in very little time with tools found in mostmodelers’ inventory, and at virtually no cost. Starting with ordinary,inexpensive, hardware-store steel machine screws, we’llflatten some of the threads to form the bearing end of the crankpins.The remaining unmolested threads can then be used toanchor the pin in the driver.The photos illustrate the process. I used a drill-press, an inexpensivedrill-press vise and a small keyless chuck held in placein the vise. You could certainly use a lathe if you have one. Onthe other hand, this could evenbe done with a regular vise anda hand-held power drill, but thatwould require someone withsome real skill!Start by cutting the headoff a screw that will becomea crankpin (I used 1-1/4” long#6-32 machine screws). Eitherease around the cut so that thescrew can be threaded into a nuthead-end first, or run it througha matching die.Chuck the screw, head-endfirst, about 3/8” into the keylesschuck. Grasp (snugly, but nottightly) the protruding end of thescrew in the drill chuck. Lowerthe drill chuck about 1-1/2” andsecure it at this level.Set the vise on the drillpresstable and raise the tableto where the vise will securelygrip the keyless chuck stem.Carefully align the vise with thestem. Tighten the keyless chuckstem in the vise. Avoid stressingthe screw. Securely fasten thevise in place on the drill-presstable carefully, checking to makesure you still avoid stressing thescrew. Now the tools are alignedand the work is in place to formthe first crankpin.Release the press’s chuck andlet it rise away from the work.Put a drop of cutting oil onthe upper threads. Put on yourleather-palmed gloves and lowerthe drill-press so that the press’schuck, when loosely holding thescrew, covers about 3/8” of theNick Pulskamptop threads. Lock the press’s chuck vertically at this level.Turn on the press at a slow speed. CAREFULLY and GEN-TLY close your gloved hand around the drill-press chuck sothat the chuck closes gently to rub – not grip - the threads. Wewant to burnish these threads almost out of existence; that is,we will rub them down to a smooth surface without removingany material. We’re using the smooth inside surface of the jawsof the drill-press to re-form part of the surface of the screw’sthreaded shank.Be patient. Do not allow the chuck to grab the screw. Keepthe work cool with your cutting oil. Actually, the oil will feed tothe work from the bottom of the thread. This phenomenon willwork to our advantage again on the engine. Back off (up) occasionallyto observe the progress. There will be a shrinking channelat the bottom of the thread corresponding to the growing flatsurface where the thread tops used to be.How small this channel becomes is up to you. It will holdlubricant on the engine so it’s probably useful to keep somethingof a channel, however small. Also, you might settle on afavored diameter for the crankpin, in which case the channelsize will take care of itself. Just keep an adequate bearing surfaceto minimize wear on your rods later.With the screw still in the keyless chuck, put your nice shinynew 2-56 tap drill in the presschuck. Still running slowly, drilla tapping hole down into thescrew about 3/8” deep. Easydoes it here; forcing the drillcould deform the outer surface.We now have what could becalled a crankpin blank, readyfor fitting and final tapping toreceive the screw and washerthat holds the rod on the crank.Drill and tap a #6-32 holethrough a spare driver or otherpiece of flat stock that is as closeas possible to the thickness ofthe drivers you will use on yourengine.Thread your blank crankpin into the spare driver as far asit will go. The smoothed-thread part of the pin will stop thethreading. Don’t torque it down now; you will use a tap for thatnext. Using the skills learned through countless constructionarticles, tap the hole inthe pin as shown, in thiscase #2-56. Again, easydoes it. Forcing the tapcould deform the outer(bearing) surface. The tappingtwist will drive thepin into the wheel to an“optimal” depth withoutover-tightening.Grind or file the pin tothe desired length. CleanSept/Oct ’06 - O Scale Trains • 45