Bill Oberpriller's Minnie

Minnie 2 is a freelance 2-4-2 live steam locomotive designed by Bill Oberpriller.



Bret Kueber
Bret Kueber published the following photos of his version of Minnie in 2000.


 * Bill Oberpriller provided us with fantastic tech support via Email, and his products are first rate. Allen Models, What can I say, Gene has been extremely helpful, and my order ALWAYS arrived promptly. We never had a problem with any of the castings we purchased from him.


 * # 13 (2-4-2) Coal Fired Columbia Class steam locomotive (Built 5/1999)

Jim Cook
Jim Cook published the following build log for his Minnie in 2002.

Frame
This design first appealed to me because of its general shape. To me, it looks as if it is an engine from the last half of the 1800's. The diamond stack, large headlight and cow catcher gives it character.

From a first time builder's point of view, the 2-4-2 configuration fits well. All the castings will be from the tried and true Allen Models. The designer is attempting to reduce the number of parts. This should help keep the cost and time necessary to build the engine down.

I purchased the first kit. It is for the frame. The quality of the punched parts is very good. I was able to put both side rail together during a Saturday. The rear bearing housing, front bearing housing and equalizer and the front and back frame spacers have been received and completed. In the first picture you can see the front axle mounted in the equalizer bearing boxes.



The second picture shows the rear axle in their bearing boxes. These boxes are attached directly to the frame.



Note that the the axles (1 inch diameter) were machined on the ends to accept the drivers at this time.

I ordered the four driver bearing assemblies from a local bearing supplier and received them within a few days. The castings were ordered from Allen Models. I received all the wheels and valve rough castings by the end of October.

This is a view of the frame as of the end of the second week in December. The drivers have been machined and mounted on the axles. This is taken from the rear right.



I am now working on the valve gear. I have not finished the four eccentrics and straps. I have a Sherline lathe and mill that I use for the small stuff. I just got a 9 inch swing Jet lathe and I'm setting this up for the larger items.



To give you an idea of scale, the main drivers are 8.5 inches in diameter. I was able to turn these on my friend Jerry Johnson's large lathe. He also helped me broach the key slot and quarter the wheels.

The trailing truck has a nice set of spoked wheels whose axle runs in Oilite bearings. There is plenty of movement, left and right and up and down.



Boiler
Here the boiler has been set in place. There is a plate welded to the front face of the fire box that anchors it to the frame. The front portion of the boiler has a sliding fit into the smoke box. This allows movement of the boiler as it expands and contracts.

Note the fittings at each lower corner of the firebox. This will allow proper cleaning of the mud ring.

The smoke stack has been mounted but the top half has not been machined.

Ron Thibault
Ron Thibault published the following build log for his Minnie in 1998.

Tender Trucks
After talking to Bill, I decided to start construction of the engine with the tender trucks. He recommended Cannonball's Archbar trucks. After also looking at other companies castings, I agree! Cannonball has a good reputation, and their kit is also the least expensive! I like that combination!! The complete kit with all materials required is only $175.00 (they are also available RTR at $355.00). In addition I will order a wheel gauge template for 7.5" track, and the 29/32" drill needed for the bearing pockets. The drill is a standard drill that you can get elsewhere, if you desire. This brings the total to $199.00 plus about $10.00 shipping. Figure 1 is the picture of a completed truck from Cannonballs catalog. Note: My wheels are the plain back type rather than the fluted ones. This made machining for a novice (me) a little easier and should not be noticeable on the finished tender. In addition the trucks can be built with a lathe, drill press, and file. I do not have a milling machine, so this combination fits my available resources.

When I was inspecting the parts after they arrived I noticed that the lower "bar" on the cast frames was much thinner than the other "bars". This seemed strange, I have not noticed this in pictures of other company's kits which I thought had all the bars the same thickness. I pulled out my copy of Meyer's "Modern Locomotive Construction 1892" and looked up tender trucks. Lo and behold, the trucks in the book also had thinner lower bars!! So these castings are quite prototypical! The rest of the parts were all of good quality, with no visible defects.

Tender Truck Sideframes
I started on the trucks with the cleanup and filling of the truck frames. I clamped a casting in my trusty WorkMate, and started filling (Figure 1). I've never filed cast aluminum before and quickly discovered the difference between it and steel! The file loaded up with globs of the soft aluminum at intervals between one and "X" strokes! After consulting the Live Steam E-mail forum I'm on, the following solutions were recommended: Coat the file with motor oil or Kerosene, use chalk as a lubricant, and clean the file with a file card. As I do not wish to get petroleum products on my "woodworking" WorkMate, I tried the chalk and card file route. I tried blackboard chalk, but most of it fell off with the first stroke. So I applied a very light coat of oil then applied the chalk. This worked a little better, but frequent use of the file card is needed. I understand that the type of chalk used for chalk lines sticks better, but I did not want to get blue chalk dust everywhere in my shop. Do not use the RED variety, it is a permanent type that will stay on whatever it contacts. I could not find any of the white type.

I found the file card at Sears in the tool department. It is a block of wood with one side covered with steel bristles, like a wire brush, but with the bristles bent on the ends. You run this over the file, pulling it. So the wires travel in the direction the tips point toward. Use a fair amount of pressure.

This initial filing was to remove the parting line flash, and enough of the pattern draft to make the "bars" look like flat bar. I did not file them dead flat, as the more material left the stronger the final piece.

Truck Wheels
The cast iron wheels have a very tough skin that I would recommend using carbide tool bits on. Even with carbide I reserved one for the initial cutting and used a second bit for the finishing cuts. Also a 5 inch three jaw chuck is the minimum required to hold the wheel during the initial machining. The jaws on a 4 inch chuck might hold a wheel, but the jaws would be only minimally engaged with the scroll. This might over strain either the jaw threads or the scroll itself.

Cannonball recommends three different ways to mount the finished wheel to the axle:


 * 1) Press fit between the axle and the wheel hub hole.
 * 2) A non press fit using Loctite to secure the joint.
 * 3) Two setscrews 90 degrees apart with matching flats on the axle.

With any of the above a consistent hole size in the wheel means that the axles can be turned to the same diameter not individually tailored to match a particular wheel. To insure a consistent axle hole in the wheels, I used a 9/16ths inch reamer for the final cut. For now I plan to go with the press fit option for six of the wheels, with two of the wheels held by Loctite, one each, on the axles that the water pump sprockets will go.

A wheel was placed in the 3 jaw chuck with the front face against the jaws. I then used a carbide AL4 bit (left bit in Figure 13) and trued up the circumference of the flange area taking a cut about 0.010" deeper than the lowest spot on the back of the wheel at about 200 rpm (in backgear), using the bit I had designated for the roughing operation. By under cutting the skin on the flange edge first I was able to eliminate the interrupted cut that occurred at the start of each facing operation without doing this.  This interrupted cut was caused, of course, by the slightly out of round edge of the casting.  By taking a deep enough cut to get below the lowest spot the truing operation cut without an interrupted cut.  With this edge trued up the facing operations that followed started on a machined surface, not a rough casting surface.

With the outer circumference machined I next a facing cut about 0.010" deeper than the lowest spot on the back of the wheel. Starting at about 200 rpm (in backgear), using the same roughing bit.  When I had cut about halfway across I stopped the lathe and set the speed at the next higher setting, about 300 rpm. After the first cut had removed the skin, I exchanged the first bit for the one reserved for the finishing cuts. I then continued facing the back (at 300 rpm) with 0.015" to 0.020" cuts until the total thickness of the wheel was a little under 0.880".

Next I center drilled the back of the wheel, for the start of the axle hole with a #5 centerdrill at about 400 rpm. I followed this with a 7/32 inch drill for a pilot hole. I then step drilled the hole with a 1/2 inch and 17/32 inch drills. Then with the lathe in the lowest backgear speed (28 rpm on mine), I brought the hole to size with the 9/16ths reamer, held in the tailstock drill chuck. I finished by using a D4 bit (right bit in Figure 13) to debur and chamfer the opening to the hole. The burrs on the edges of the flange area were smoothed with a file. The edge away from the chuck was smoothed with the lathe running (about 300 RPM), and the other with the wheel turned by hand. The hand rotation was done because that edge was to close to the jaws to safely run the lathe under power.

Next the wheel was chucked in the 3 jaw with the machined back against the jaws. The front of the wheel was then faced to bring the total thickness down to 0.800". The Cannonball drawing shows a range of between 0.812" and 0.750".  I left the wheels on the "fat" side in case I make a mistake later, and need a little metal left to correct it.  The axle hole edge was chamfered and the inner and outer edges of the wheel face smoothed with the file (under power).

This was repeated for the other 7 wheel castings.

The Wheel Arbors section describes the fabrication of arbors for the Tender, Trailing Truck, Pilot Truck, and Drivers.

The next step was to clamp a wheel to a stub arbor and rough turn the tread area, then using a form tool finish turn the flange. Finally the tread area is coned to the 3 degree angle. After getting advice from several fellow model railroaders I decided to do all the machining operations on the wheels before installing them on the axles.

Alternately you could rough turn the wheels. Then after the wheels are in place on the axle the wheels are finish turned with the axle held between centers.

As the Wheel Arbor holds each wheel concentrically, the wheels were exchanged in turn until each operation was completed for all the wheels. Then the next machining operation was performed in the same manner.



The first operation is to turn the tread area of the wheel. In this case I turned the wheels all the way down to the 4.125 (4 1/8") dimension, but not all the way over to the beginning of the inner flange radius. This area will be removed when the coning operation is completed, so this is really still a rough turning operation.

Each wheel for all operations was mounted with the back of the wheel facing the headstock. The wheels can vary slightly in overall width, but the flange must be machined in a fixed relationship to the wheel back, in order for proper tracking on the rails. Mounting the wheels in the above manner insures that the above relationship will be assured for all the wheels, if the proper stops are setup during the machining. The flange tool that will be used for the final cutting also assumes this mounting orientation.



The first step in turning the tread is to set up a fixed carriage stop for the locating the point between the tread and the first rough cut for flange (Point "A" in Figure 12). With this stop in place only the final diameter of the tread has to be watched during machining. The bottom of the way is protected by a piece of craft (or popsicle) stick. While the first wheel was machined I found that the chips built up quickly between the stop and the carriage and were difficult to remove due to the limited space. So I changed the setup to that shown in Figure 14 (again with a craft stick used to protect the way). With this setup almost all (or frequently all) the chips fell away from the edge of the block, requiring only the occasional clearing of the chips.

With the stop set the first wheel was turned to the desired diameter measuring carefully for the last cut. With this final cut finished the crossfeed dial was set to 0 (zero). The rest of the wheels were then turned with the final cut done at the zero setting, rather than measured. Doing the final cut this way requires that the toolbit always be the same size (no wear allowed). The skin on the cast iron wheels is quite tough and chewed up a HSS bit almost immediately, so all the following operations (until noted later), were done using an indexable carbide toolbit and holder. This bit showed no appreciable wear, and if it had, going to a new cutting edge does not require resetting the zero point and stop location.

Figure 15 shows the tread cutting operation in progress. You will note that the bit is being fed in with a left-hand bit parallel to the lathe axis. With the triangular bit this cut the part from under the skin (as is recommended practice), reducing greatly the wear and cutting force required. With the above setup the skin is mostly broken away from behind rather than cut through.

After the final diameter was reached the tool bit was cranked out to cut the "vertical" edge, again removing any skin from below and establishing a fixed point "A" for all the wheels.

The next operation is to cut an angled face in preparation for roughing in the flange to tread fillet. With this cut all the cast iron skin should be removed, allowing a HSS tool to be used to cut the rough fillet.

For this cut I used a right-hand bit, once again run in point first. The topslide was set over 30 degrees clockwise to produce the 60 angle desired (Figure 17). While there is probably some well accepted method for locating the tool to end up at the final cutting point, I do not know it. For these cuts I locked the carriage and used only the cross and topslide feeds. To get the proper settings I carefully cut the first wheel and when I finished the cuts and the point of the tool was sitting at point "A", I zeroed the crossfeed and topslide dials.

To make the cuts the cross slide was advanced and then the tool bit run in until it broke out at the end of the cut. As the last few cuts were made I took care not to run the topslide past the zero mark. When the carriage dial reached zero the last cut was being made. Figure 18 shows a wheel after the final cut was finished.

Locomotive Frame
Minnie's frame differs from the "standard" construction methods traditional to the Live Steam hobby. The two traditional 1 1/2 inch scale methods (at least for American prototypes) are to either mill the frame from 1/2 inch steel flat stock, or to build the frame from sections of barstock. The frame from flatstock requires that you either have a decent size milling machine, a good supply of endmills, and several days of time, or that you have $100 or more to have them cut for you. The barstock method does not need a milling machine (though one helps tremendously), but does involve many precise cuts and accurate drilling to many pieces of barstock. In either case you still need to mill the journal boxes.

The major components of Minnie's frame on the other hand requires no more complex a machine than a drill press! Even this could be dispensed with a by talented person with a hand drill. I fall under the drill press standards, though. The frame consists of a sandwich of two punched 1/8 inch thick steel plates, with 1/4 inch barstock between them. All the required openings and (pilot) holes for all the bolts are already punched in the 1/8 inch plates on a CNC punch press! All that the builder needs to do to produce a strong square frame is drilling, countersinking (for flat head screws), taping, and noncritical trimming of the barstock! The bearing retainers for the axle bearings and the front axle equalizer are also assembled from punched plates. Additionally the crosspiece and outer bearing plates for the front equalizer assembly are jig welded before delivery! The accuracy of the frame comes from the accuracy of the CNC punching, and makes it much easier for the first time (or tenth time) builder.

The total time for me to assemble the first frame side was 6 hours, start to finish! I did not trim the barstock to final size, but that was because I do not yet have the full set of drawings. I left the trimming until I am sure I'm not cutting off something I may need later!

Some precautions do need to be taken before construction begins. The punching process can cause some warpage. To account for this the plates are punched in pairs. By matching the pairs, any warpage will be canceled out. Therefore I compared the plates and selected and marked each one as to its final position in the assembled frame. The pairs were matched so that they looked like Figure 1 from the top. When these are bolted together the bowing in each piece will be canceled out. I did not want them to look like Figure 2. As the warpage there would be reinforced by the plates when bolted together!.

With all the holes deburred the frame was assembled using #10-32 screws and nuts and is shown in Figures 15 (an assembled corner). You will note the markings on the frame sides. The closer one means "Side 2 Outer", the one just visible behind it is "1I" or "Side 1 Inner".

These parts will have to be disassembled from time to time in the future, so no thread locking compounds were used at this point. In fact I used low grade screws and nuts at the corners, as these were cheap and available locally. I will replace them with high grade hardware when the parts are all attached to the frame and I can determine the exact lengths required for the screws.

Rear Axle Mounts
The pieces for the rear axle bearing mounts are shown in Figure 1. The smaller pieces, shown below the larger rear bearing mount plates, fit on either side of the lower frame bar at the bottom of the axle opening. These fit in the area where the frame plates were punched out when they were made.

Repeat the above steps for the other side, but additionally use a piece of axle stock fit in both bearings in all the steps, to insure that the bearings are lined up, with all the plates fitted.

When you are finished installing both rear axle mounts, disassemble the mounts and both side frames. Clean out all the metal chips, and reassemble the parts. Because the side frames are hollow and open in some areas, you may have to do this a few times as the locomotive construction progresses.

Equalizer
The frame for the front axle equalizer, minus the supplied bearing plates, is shown in Figure 1. The equalizer comes already welded and with the holes pre-punched as shown. In some of the pictures the equalizer shown is an earlier design (the bottom of the outer plates are closed rather than open). Figure 1A shows the old design with the bearing plates installed. The new design uses the same plates. All the frame kits will have the new design as delivered, according to Bill Oberpriller. The two designs are otherwise identical and the construction steps the same. For this reason I did not reshoot the pictures.

Before starting on the fitting of the axles to the equalizer the top bar on the frame must be notched out to match the cut-out in the frame so that the equalizer will pivot freely up and down approximately 3/16 to 1/4 inch. This is done easily on a mill, but with only a lathe, it is more difficult. I plan to clamp the bars (one at a time) to an angle plate mounted in place of the toolpost, with the bottom of the bar facing the headstock. The bar centerline will be at the lathe centerline. A 1/2 inch endmill will be gripped in the chuck. This will leave concave ends in the slot, which will be cleaned up with a file.

The next steps are to fit the equalizer journals to the frame. As with the Rear Axle Plates, you will need to have the Driver Axle Bearings first!

First the bearing journal plates are fitted to the bearings as was done for the rear axle. Next the front equalizer needs to be fit to the frame. This should be a free but not sloppy fit. The equalizer should be free to pivot, with the sliding fit to the frame not allowing any great front to back motion. Bill Oberpriller used a belt sander to reduce his to fit. I may do that, or maybe turn mine down on the lathe, assuming I can come up with a good way to mount it to the faceplate.

Rear (Cab) Frame
Minnie's frame comes in two major parts. The first is the main frame already detailed in the previous sections. The second is the rear frame that supports the engine cab and the rear of the boiler, and additionally makes up part of the trailing truck assembly. Figure 1 shows a picture of the completed rear frame.

As with the front frame kit, care must be taken that all the parts are square and true, otherwise the trailing truck will not move freely as it should.

Trailing Truck
Minnie's Trailing truck frame is part of the rear frame. The truck bearing plates ride up and down in the slots at the bottom of the rear frame. Figure 1 shows the punched plates for both bearing assemblies. The four plates at the top are the outer plates and serve both to capture the bearings and as the outer slide plates. Figure 2 shows one of these longer plates just below the slot in the rear frame, with one of the shorter bearing retainers below it.

For the outer plates I plan to mount them so the slightly radiused edged side is toward the frame. This will provide a smoother surface to slide against the frame. Deburr all the plates before fitting the bearings.

Pilot (front) Truck
The upper front apron plate (left) and the pilot truck hinge plate (right) are shown in Figure 1, The apron plate serves as one of the supports for the front truck and as a means to hold the frame square. The hinge plate bolts under the frame and serves as another frame squaring device as well as serving as the hinge mount. Both are bolted to the frame with 10-32 by 1/2 inch screws as detailed in Figure 2.

The holes (except as noted later) for the apron need to be of the blind type. If a standard (hardware store type) tapered tap is used, the hole would have to be quite deep. The first several threads of a tapered tap are only partial (shallow) threads, they then taper up to full thread depth. The end is also generally pointed, further extending the region of partial threads. This feature helps in starting the threads in a straight manner. For threading a blind (closed bottom) hole you need to make the hole deeper by the length of the point and tapered area to get the depth of full threads you need. To solve this problem you can use a bottoming tap. This has full depth thread all the way to the end of the tap, and a flat tip. It is almost impossible, though, to start the threads with such a tap. So you start with a tapered tap then finish with the bottoming tap, which can be screwed into the threads left by the first tap, and then run all the way to the bottom to cut the rest of the threads. There is also another type of tap called a plug tap. This tap has a flat point and a short section of taper. This allows you to cut threads further into the hole than the taper type tap. These, however, are more difficult to start straight by hand.

One problem with these taps is that your local hardware or auto parts store is not likely to carry either of them. You can, however, get them from an industrial supply house or by mail order. An alternative solution is to buy another taper tap and convert it to a bottoming type. This can be accomplished by using a cutoff disk to cut off the tapered thread area (and by default the point), leaving just the portion with the full threads. I used a cutoff wheel in a Dremel tool. After cutting the tip off, I used the disk as a grinder to true up the bottom and remove any burrs. Figure 3 shows a standard taper tap and one that has been made into a bottoming tap.

Internal Stevenson Valve Gear
Minnie uses internal Stevenson type valve gear to operate the slide valves. This type uses four eccentrics and a link that is raised and lowered to control the timing of the steam admission and exhaust to the drive cylinders. Each cylinder is controlled by two eccentrics on the front axle, one for forward and one for reverse operation. Thus the need for four eccentrics total. Allen and other locomotives that have a reciprocating axle pump for feeding water into the boiler use a fifth strap and eccentric to power this pump. Minnie uses a different setup, and the fifth set is not needed. Figures 1 and 2 show the completed valve train. ''Note that this is a picture of the Bill's original prototype of Minnie, before he redesigned it for her present configuration. I will replace this with one of my locomotive's when I finish machining the parts.''

Figure 3 shows most of the Allen castings used for Minnie's valve gear. Not shown are the castings for the eccentrics themselves and the castings for the straps (or yokes) that run on the eccentrics and impart a reciprocating motion to the rest of the gear train. I am going to turn the eccentrics from 2-3/4 inch HRS bar, and have not yet bought the strap castings. In the upper left is the Lifter Arm (part # M191). At the center top are the four Rocker Arms (part # M190). In the upper right is the Reverse Arm (part# M196). Bottom left are the Lifter Links, and the Rocker Shaft Bearings are at the bottom right.

Figure 4 shows more of the valve gear parts. The four plates on the lower left corner are the side plates for the rocker box (part of the bearing the shaft for the rocker arms that drive the valve slides). These are punched plates supplied by Bill Oberpriller. The round and square stock need to be bought locally. I'll list the sizes and uses, check Bill's page for the relevant drawings.

The 3/8 square stock is for the saddle links and the 1/2 square stock for the rod ends. The square stock shown is CRS, but keystock, which many hardware and some auto parts stores carry can be substituted. These are generally galvanized, though, and some additional steps will be needed when the parts are painted.

The round stock is also CRS, and they really have to be as they are used for the various shafts in the gear train, so smooth close tolerance stock is needed. They can be turned from the next size larger HRS round stock, but the CRS is a better choice. The sizes needed (in CRS) are: 1/2, 3/8, 5/16, and 3/16 inch. A foot or so is sufficient.

Figure 5 shows two of the castings for the eccentric shafts. I purchased the standard 5 straps, to provide me with the option of using an axle pump for the boiler feed, rather than (or in addition to) the pressure washer setup Minnie was designed to use.

Valve Gear Eccentrics
I chose to turn the eccentrics from 2-3/4 inch HRS round stock, rather than using the Allen castings. Figure 1 shows the drawing for an eccentric. The main reason is that I had the steel available. Even if I had had to purchase it, the place I bought it from sells by the foot, and for this size charged $10/ft. + $2 for the cut. After turning the parts I still have a decent stub and another 7 inches of material for some future project.

The face of the spine on the strap side is the finished surface, and the other side will be placed against the top of the chuck jaws to align the inner face of the strap end for final facing. The spine is left wider than the final thickness to allow for any correction that may be needed to bring the strap bearing surface to the correct thickness. The spine will be machined to final thickness when the collar is turned. The diameter of the spine is not critical. It serves to keep the strap from wandering off the eccentric, so as long as the diameter is close, it will operate satisfactorily.

Once the initial turning is completed (Figure 11), the bar is removed and the first 3 (three) eccentrics cut off. Return the bar to the lathe and chuck it by the end with the remaining eccentric (Figure 12). You will notice that I forgot about the need for brass shims to protect the eccentric surface during this operation. The reusable ones I made are described later. Face and turn the circumference of the former stub, truing those surfaces up. The stub is now ready to be used in some future project, rather than ending up as "a too short to use chunk" in the scrap box. Alternately it can be used for the the simple 3 jaw eccentric turning jig (described in the section so named). Remove the bar and separate the last eccentric.

To keep from marring the finished surfaces I made three simple reusable brass jaw covers as shown in Figure 13. These should be made so that they are shorter than the jaw height, so that the eccentric spine can set down on the jaw top.

Return the eccentrics to the 3 jaw chuck and face each side (Figure 14)(Note: Again before the brass jaw protectors). Starting with the strap end, bring the strap bearing surface to the correct thickness. Machine some of the spine thickness away if the area is too narrow after the facing operation. Reverse the part and face the collar end. You can either turn the spine to the proper thickness now, or wait until the axle hole and collar are turned later.

Valve Gear Eccentric Jig
See Turning an Eccentric Hole in a 3-Jaw Chuck

Video
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