mklotz
Well-Known Member
Over the years I've finished all of PMR's machine tool models. My wife keeps bugging me to build the dollhouse scale (1:12) shop to contain them and operate them with scale overhead line shafting. I never seem to get around to it. (Besides, if I build it, I won't have any place to store it.)
Nevertheless, to demonstrate to kids how machines were powered before the ubiquitous electric motor, I built a small demonstrator using an old circuit board test jig as the skeleton.
The (adjustable) overhead shaft hangers are available as kits from PMR and they work very well.
The milling machine model uses two 3/16" brass balls as part of the sliding universal joint that transmits power to the table leadscrew. These balls have to be drilled with two orthogonal 1/16" holes to accept the yokes that make up the universal joints.
To accomplish this, I made up the little jig shown below. A chunk of 1/2" steel is drilled and threaded to accept a matching 3/8" lock screw that has a centerdrilled hole in the end. This conical depression, along with the other cone formed by the tap drill used to thread the chunk will serve to center and clamp the brass ball when it's inserted in the jig.
Here the ball is shown mounted and clamped by the lock screw. At this point, the chunk is stuck in a collet in the lathe and a 1/16" drill passes through the hole in the front of the chunk, through the ball and into the centerdrilled cavity in the lock screw.
Prior to mounting the lock screw, I measured from the face of the chunk to the back of the ball with my mike. After the first hole is drilled in the lathe, the chunk is transferred to the mill and, using that dimension, I can locate the center of the ball and drill the second hole secure in the knowledge that it will be perpendicular to the first.
When I built the demonstrator, I quickly discovered that miniature belting doesn't work like full size. Full size belts have enough weight to them that they will "hang" on the pulleys and grab well enough to transmit power even if they aren't absolutely tight. The miniature belts are much lighter and need to be snugged up carefully or they'll slip.
Rather than shim the models in order to tighten the belts once glued, I built this jig to allow me to mount the belts, tension them in place, and then glue the ends.
It consists of two tiny carriages with male dovetails on the bottom which ride in a matching dovetail in the base. Two brass strap clamps secure the two ends of the belts to the carriages. A long screw passes through one carriage and is threaded into the other such that, when the screw is tightened the two carriages are pulled together thus tensioning the belt. Once properly tensioned, a couple drops of super glue and a tiny clamp (not shown) secure the two belt ends.
Nevertheless, to demonstrate to kids how machines were powered before the ubiquitous electric motor, I built a small demonstrator using an old circuit board test jig as the skeleton.
The (adjustable) overhead shaft hangers are available as kits from PMR and they work very well.
The milling machine model uses two 3/16" brass balls as part of the sliding universal joint that transmits power to the table leadscrew. These balls have to be drilled with two orthogonal 1/16" holes to accept the yokes that make up the universal joints.
To accomplish this, I made up the little jig shown below. A chunk of 1/2" steel is drilled and threaded to accept a matching 3/8" lock screw that has a centerdrilled hole in the end. This conical depression, along with the other cone formed by the tap drill used to thread the chunk will serve to center and clamp the brass ball when it's inserted in the jig.
Here the ball is shown mounted and clamped by the lock screw. At this point, the chunk is stuck in a collet in the lathe and a 1/16" drill passes through the hole in the front of the chunk, through the ball and into the centerdrilled cavity in the lock screw.
Prior to mounting the lock screw, I measured from the face of the chunk to the back of the ball with my mike. After the first hole is drilled in the lathe, the chunk is transferred to the mill and, using that dimension, I can locate the center of the ball and drill the second hole secure in the knowledge that it will be perpendicular to the first.
When I built the demonstrator, I quickly discovered that miniature belting doesn't work like full size. Full size belts have enough weight to them that they will "hang" on the pulleys and grab well enough to transmit power even if they aren't absolutely tight. The miniature belts are much lighter and need to be snugged up carefully or they'll slip.
Rather than shim the models in order to tighten the belts once glued, I built this jig to allow me to mount the belts, tension them in place, and then glue the ends.
It consists of two tiny carriages with male dovetails on the bottom which ride in a matching dovetail in the base. Two brass strap clamps secure the two ends of the belts to the carriages. A long screw passes through one carriage and is threaded into the other such that, when the screw is tightened the two carriages are pulled together thus tensioning the belt. Once properly tensioned, a couple drops of super glue and a tiny clamp (not shown) secure the two belt ends.
.... let's talk about authenticity here 
