Boiler - Smokebox Design

3/8/2013                    Subject To Revisions

Overall Boiler Design
The boiler including the firebox of the prototype locomotive is about 36' long, roughly the wheelbase.  The model is 1/24th scale so the boiler would be about 1.5' long.  That is much too large for the printer.  The largest dimension is about 10".  Consequently the author elected to build the boiler in sections.  Arbitrarily these are the smokebox, sanddome, transition, steamdome, and firebox.  The firebox might be done in two halves or one piece depending on how well the other sections come out on the 3D printer.

Smokebox

The design of the smokebox includes three parts, the boiler smokebox section, the smokestack and the smokebox front.  The author contemplates glueing the smokestack in a mating hole in the boiler smokebox section and attaching the front using five 00-90 screws.

3D CAD Model of the Boiler Smokebox Assembly

The above illustration shows the smokebox assembly.  The front is held in place with the five small black 00-90 screws while the smokestack will be glued in place.  The smokebox assembly will be used to attach the front of the boiler to the cylinder assembly on the locomotive model with access to the hold down screw through the smokestack opening.   This section is 3.86" along it's axis, 3.75" in diameter and about 4.4" tall including the smokestack.

Boiler Smokebox Section Design

The boiler smokebox section illustrated above is essentially a tube 3.53" long and 3.75" in diameter.  The tube wall is 0.111" thick.  A hole on the top will accommodate the smokestack part.  In the inside bottom is a boss for the 4-40 front boiler hold down screw that threads into the cylinder assembly on the locomotive chassis.  Five small screw holes around the front will be threaded for 00-90 screws that will attach the front to the cylinder.  The back will eventually be glued to the boiler sandbox section.  Dummy rivets are placed around the front and rear where the cylinder is riveted to the internal rings on the prototype.  The front ring is used to provide attachment of the smokebox front and the rear ring attaches to the next boiler cylinder, the sandbox section. 

Because of the parts size, it will be printed with the cylinder standing with the front up.  The boss for the attachment screw is tapered at 45 degree angles to allow the fabrication without support materials.  The smokestack opening will require support from the base, the back of the cylinder in this case.  No other supports are required to fabricate this part in this manner.  This part has already been through the Axon software to create the build code for the 3D printer.  The smokebox cylinder will take over 16 hours to print and requires some 369 layers of printing to complete the 3.75" height of the cylinder sitting upright.

Smokestack

The smokestack is slightly tapered and has a flanged ring that curves to match the radius of the smokebox boiler cylinder.  The author shortened the bottom portion below the flange as interior details of the smokebox are not being modeled.  The bottom portion passes through the hole in the cylinder.  The curved flange portion will be glued to the top of the cylinder. 

Smokebox Front

The smokebox front is a complicated design with many details.  The outer ring of bolts are fake details to model the bolt ring on the prototype,  Five of those are replaced with 00-90 screws to hold the front to the cylinder.  The inner door is a non-working model with the clamps and door hinges as details.  The small flat section at the bottom of the inner door is for the headlight bracket. 

The three parts are shown mated together in the first illustration above.  On the 3D CAD model the assembly is done to check alignment and fit of the various parts.  Based on that this section should go together needing only cleanup after printing along with drilling and tapping the holes for the screws.  The hold down screw hole on the inside bottom is not tapped but provides slide fit clearance for the 4-40 screw.  The small holes in the front cover are slide clearance fit for the 00-90 screws while the holes in the main cylinder for those screws will be drilled and tapped for the 00-90 screws.

Smokebox Section Build

The build on the 3D printer was done in three sub-sections, boiler tube section, smoke stack and smokebox front.  Each was built using the 3D CAD designs shown previously. 

Boiler Tube Section

Completed Smokebox Boiler Tube Section in 3D Printer

Smokebox Boiler Section as Printed

Because the smokebox boiler section is built in 360+ layers each having a fair amount of material, it takes a long time to print; 16h 50min.  As can be seen in the above photos the layers are distinctly visible.  Each layer is very close to the same size, but since the material is actually a round thread the boundaries show when looking at the printed item.  A mounting boss was built in as shown inside the section.  A round hole defines the location of the smoke stack.   If you look close at the full size photo a single ring of rivet details are located at the front and a double row at the back of the tube. 

Smokebox Boiler Tube Section During Fit Check With Cylinder Saddle

Good Fit of the Two Mating Curved Surfaces

The smokebox boiler tube section was temporarily mounted on the cylinder saddle for a fit check which came out very well. 

Smoke Stack As Printed in 3D Printer

Bottom of Smoke Stack After Removing Bulk of Support Material

The next part printed was the smoke stack.  This part required 3h 58min to complete.  The part had good shape definition.  Some support material was on the top of the flange fairing between the stack and boiler.  Residual support material on both top and bottom were removed using the Dremel tool with a small round cutter.  Most of the top residual was removed using a model knife.

Smoke Stack Fit on Smokebox Boiler Section During Fit Check

Smokebox With Stack in Position on Locomotive Cylinder

Photo of Smokebox With Ruler

The smoke stack fit well on the boiler after removing residual support material.  The double curved fillet between the stack and boiler was difficult to design, but the finished part fits very well.  The smoke stack will later be glued in place.

Smokebox Boiler Section With Attachment Screw Installed

The cylinder assembly was tapped for a 4-40 hold down screw.  The small hole in the boiler bottom was drilled out to provide slide clearance for the screw.  The above photo shows the screw in place.

Smokebox Front in 3D Printer After Completion

Completed Smokebox Front With Support Material

The smokebox front was next in the 3D printer taking 8hr 31min to print.  A considerable amount of support material is underneath the domed front of the part.  The level of detail was fairly good with the small bolts coming out as circles.  Once again the layering of the printer is clearly visible particularly on close inspection.  The fine detail of the various shapes looks like a terraced hillside with most of the sharp corners rounded.  From a bit of a distance the part looks quite good.

Smokebox Front on Smokebox During Fit Check

Closer View of Smokebox Front

The bulk of the support material was removed leaving a thin layer around the interface flange at the edge of the smokebox front.  The part has an alignment ring on the back which fits closely inside the boiler.  The part slides in place slightly snug and would hold in place without screws.  However,  the outer ring would show the residual support material between the smokebox front and boiler.  The author decided that the best way to remove it was to use the Sherline CNC mill.

Back of Smokebox Front After Removal of Residual Support Material on Outer Flange

Closeup of Smokebox Front After Removal of Residual Support Material

The smokebox front was carefully aligned on the mill rotary table so that the cutter would follow the outer flange.  All the residual material was then milled away.  For this operation all movements of the mill were manually controlled as the rotary table would follow the shape easily.  Residual material on the middle portions of the part were left in place.

Smokebox Front and Boiler Being Drilled With Tap Holes

Smokebox Front Being Drilled With Clearance Holes

Boiler Section After Being Tapped for 00-90 Screws

The next step is to drill and tap the five holes used to hold the smokebox front in place.  This was done on the Dremel drill press using a very small #65 drill (0.035" diameter).  The boiler section was sized to provide adequate material for the screw threads.  After drilling the front and boiler together to assure hole position alignment, the front was removed and drilled with clearance #55 drill holes (0.042" diameter).  

Boiler Smokebox Front on Smokebox During Assembly

Smokebox Front with 00-90 Screws in Place

Closeup of Screw Partially Installed

Assembly was painless as the holes were all aligned.  The second photo shows the front with the attachment screws in place.  The closely match the dummy bolt ring around the rim.  The bottom photo shows a closeup of one screw and the surrounding details.  The closeup clearly shows the level of detail and the layering effect of the printing process.

Smokebox Assembled and Installed on Locomotive Model

With the exception of gluing the smoke stack in place the smokebox section of the boiler is completed less the many smaller details that will be added later.  The author found the need for a longer crosspoint screwdriver to reach the attachment bolt at the bottom inside. 

The boiler sections are projected to be glued together making one long assembly including the firebox and cab.  Another bolt or bolts at the rear portion will hold that end of the boiler to the locomotive chassis while the bolt accessed through the smoke stack hold down the front.

Valve Rod Set

Revised Feb 18, 2013          Subject to Revisions

Valve Rod Set as Designed With Alibre 3D CAD

 

 

 The above the illustration shows they valve rod set designed with the 3-D CAD software.   The author elected to print the components on edge which seems to suit the small parts better.   The orientation of the parts is the preferred way for good detail and ease of clean up.

Valve Rod Set as Printed with the 3-D Printer

 The above photo shows the valve rod set immediately after being printed.  The part definition is good.  the longer rods are about three inches long and the shortest is a bit less than three quarters of an inch.  Some of the smaller rods have a wide section to provide enough material for tapping.  The two inch long valve shafts are round with a U shaped end to hold the combination lever.  Those are at the bottom left side of the print.  Just above are the combination levers.  At the top are the long radius rods.  On the bottom right side the two shortest rods are the lifting links.  Between them and the radius rods at the right side are the union links.

The Set of Rods After Removal From the Printer Raft Before Cleaning

The rods were readily removed from the printing raft of green ABS.  Small amounts of raft residue are visible.  It was relatively easy to remove the raft residue using the Dremel tool sanding wheel and model knife.  The parts are very thin and fairly small.  Despite their smallish size, the printer did a good job.

Outside portion of the Crosshead During Tapping

The outer portions of the crosshead were removed then drilled and tapped for 00-90 screws to provide the attachment point for the union links.  The above photo shows one with the tap in place.  Tapping was done by hand with just the tap, no tap holder.  It is relatively easy to turn the small tap in the ABS plastic.  A number 65 drill was used to drill the tap hole.  That size is just 0.035" in diameter and is easily misplaced or lost if one is not careful.

Fireman Side Valve Rod Set in Place

The full set of the fireman side valve gear was assembled using a number of 00-90 screws and installed on the model.  The thin radius bar at the top passes through the center of the curved expansion link and attaches to the top of the combination lever at the left and the lifting link at the right.  The combination lever attaches to the valve stem near the top just below the connection to the radius bar and at the bottom to the union link.  The union link connects the bottom of the combination lever to the crosshead.  The lifting link at the top right will connect to the lifting arm attached to the reversing shaft after it is installed.  The lifting arms and reversing lever and shaft will be built and attached a bit later in this blog.

Engineer Side Set of Valve Rods and Levers and Crosshead Cover

The rod set consists of the short lifting link at the top of the above photo; next below it is the union link, then below it is the combination lever.  Below that is the valve stem then the long radius bar.  Finally at the bottom is the crosshead cover and an example 00-90 screw.  The various rods are drilled either for clearance of the screw or for tapping the threads. 

Assembled Engineer Side Valve Rod Set

The valve rod set is shown assembled above.  At the top left corner is the lifting link.  Attached to it the long rod going to the right from that link is the radius bar.  The radius bar connects to the top of the combination lever which goes downwards passing through the valve stem where it is also connected.  The valve stem goes to the right side in the photo.  The downward pointing combination lever connects to the union link at it's bottom end.  The union link connects back to the left to the crosshead cover.  One thing to note is that the long radius bar has two pins built in towards the left side.  Those pins will sit in the curved slot of the expansion link already installed on the locomotive model.  In the prototype and the model, the radius bar slides up or down in the expansion link depending on the position of the lifting link and lifting lever to which it attaches.

Engineer Side Valve Rod Set Installed

The above photo shows the set of valve rods installed.  In order to install the radius rod the expansion link was disassembled along with the link mount.  A number of small screws hold everything in place so that the rods may be disassembled, adjusted and if necessary replaced.

Set of Lifting Lever, Reverse Lever, Reverse Shaft and Mounts

 

 

 The set of parts composing the lifting and reverse levers, shaft and mounts are shown above.  They are printed in black ABS.  The shaft has a bend to provide clearance for the boiler as in the prototype.  The two mounts are at the top of the print.  At the center right side is the fireman side lifting lever while just below is the engineer side combination lifting and reverse lever.  The overall assembly will be attached to the top of the expansion link mounts.

Reversing Lever Assembly Mounted on Locomotive Model

The white ABS lifting link at the bottom left of the photo attaches to the fireman side lifting lever.  That lever is on the end of the reversing shaft.  The shaft passes through the mount bracket that is attached in turn to the expansion link brackets below the shaft end.  Small 00-90 screws hold the brackets in place on the expansion link brackets.  The shaft then passes across the engine frame underneath the boiler to the engineer side at the top of the photo.  Just before that end of the shaft another bracket supports the shaft and is mounted on the engineer side expansion link brackets in the same manner as on the fireman side at the bottom center of the photo,  The L shaped combination lifting and reversing lever is attached on the engineer side. At a later point in the model construction the engineer side reversing lever will attach to the reach rod that on the prototype is operated by the engineer to set the proper position for the lifting levers.

Model with Completed Valve and Reversing Gear Installed

The model features fully working motions of the various drive and valve rods along with the reversing gear.  The model now has all of the motion gear installed.  It all works fairly smoothly even though this is a static model.  This feature allows one to demonstrate the fully working motion gear for interested visitors. 

The next phase of the project will be to build the boiler assembly.  This will be done in segments as the overall boiler is much too large to print at one time.  The author contemplates printing the boiler in four round sections and the firebox.  The cab will be a separate assembly.  Details such as the smokestack, domes, walkways, grab irons and many other small details will be done separately and glued or screwed in place.  The assembled boiler will be removable with a screw down the smokestack and another somewhere in the rear attaching to the cylinder and frame assemblies. 

Eccentric Crank and Rod

Revised 2/6/2013          Subject to Revisions

Eccentric Cranks

Eccentric cranks are the third outermost item attached to the main driver crank shafts.  On the model they will provide retention of the main connecting rods which in turn retain the side coupling rods. the cranks are held in place using a 0-80 screw.  

Two pair of eccentric cranks

 In the photo above two pair of eccentric cranks as they came out of the 3-D printer are shown.   Two pair of cranks were built although only one pair will be used on the model. This was done in case there were printing problems.

Pair of eccentric cranks with 0-80 screws

 

Two 0-80 screws are threaded into the crank as shown above.  At the narrow end the eccentric rod will be attached using a short screw.  The hole is tapped for the screw.  At the large end a screw is used to compress the two sides of the crank around the crank shaft end.  On the model the crank shaft is round with two flats to keep the crank at the proper angle of 25 degrees relative to the crank shaft to center hole angle on the wheel.

Eccentric crank installed on end of main driver crank shaft

The above photo shows one crank installed on the outermost end of the crank shaft outside the main connecting and coupling rods.  One problem was observed, the accuracy of the 3-D printed shaft was very poor and the designed round with two flat shape did not come out well.  The author attempted to file the flats a bit however the overall shape of the shaft tip was highly distorted. 

Main driver crank shaft showing distorted eccentric crank mount

As a result the shaft tips were not usable.  The cranks however had nearly perfect shape.  As a result of the distorted crank shaft tips the cranks would not stay attached and would not hold angle.  They would easily fall off with any kind of pressure.  The author decided to remove the tips and attach machined metal tips in their place.

Eccentric rod set immediately after 3-D printing

Meanwhile a pair of eccentric rods were printed.  These turned out good needing relatively minor cleanup.

Set of three eccentric crank mounts in mill just after milling out screw head relief cavities

A design for eccentric crank mount shaft tips was done using two 00-90 screws to attach the tip to the driver crank shaft.  The old tips would be milled away leaving a flat surface and two holes drilled to match the newly machined tips.  The photo above shows the set up on the Sherline 2000 mill with a aluminum work piece in place.  The work piece has three tips defined each having two clearance holes for the 00-90 screws and relief cavities for the screw heads already machined.



 

Tap holes were drilled through the wood backing block

After tapping the wood backing block the three parts will be bolted to the it during outline machining

The three parts are each held to the backing block with two 00-90 screws

Parts in the mill after machining the outline shape

View of the Sherline 2000 mill used to machine the parts

The parts are very small being 0.2289" diameter with two flats 0.180" wide on the side.  The part is 0.122" thick and has two holes for the 00-90 screws and a relief cavity for the heads so they will be flush with the outside edge after mounting.

Three eccentric crank mounts while still attached to the backing board

Aluminum work piece removed showing the three repair eccentric crank mounts

Work piece after removal showing that the locations for the parts was completely cut through

Backing block with center eccentric crank mount and eccentric crank

Eccentric crank on repair mount

 After machining a fit check showed a very good snug fit of the crank on the mount with almost no play.  After tightening the crank compression screw the crank was found to be firmly in place with no tendency to move or slip off.

Main driver during removal of the defective crank shaft portion with the mill

Drilling two tapping holes for the 00-90 screws using the mill to get proper locations

Drilled holes in end of modified crank shaft

The crank shaft diameter where the tapping holes were drilled is 0.321" in diameter.  The holes are 0.035" diameter on 0.1145" centers.

Main driver with 00-90 hand tap in place in crank shaft end

Eccentric crank and repair crank mount portion of crank shaft

Crank shaft with repair part screwed in place on crank shaft end

Eccentric crank mounted on repaired main driver

Assembled eccentric crank and rod on model during final fit check

Very small details such as the eccentric crank shaft end do not turn out well and may not be usable to mate moving parts as occurred in this model.  The drivers were fabricated very early on and subsequent model building does not provide confidence that sufficient improvement might be possible .  The repair crank shaft end worked very well and could be built with very good accuracy, more than sufficient for the modelling effort.

Connecting Rods

Revised: 1/14/2013          Subject to Revision

Connecting Rods as Printed on 3D Printer

The connecting rods are larger than normal parts built in the past, about 6 inches long.  Despite  that the author chose to print them in a manner similar to the coupling rods that were built in the past, that is laying down on the side.  The parts were designed and laid out so that a set could be built side-by-side on the same support structure.  The above photo shows th result was good. The parts had good detail and came out quite straight unlike other long parts that had been attempted in the past .

Connecting Rod After Clean Up Ready to Install

 as for other parts built in the past, the bottom side and residue of the support structure.  the residue was removed using model knife, criminal tool with both the grinding wheel and the small cutter.  the above photo shows a cleaned up connecting rod temporarily being fit checked with the exterior portion of the crosshead.  The four small screws shown are used to attach the exterior to the interior portion of the crosshead still positioned  on the guides.

Connecting Rod in Position Before Crosshead Cover Attachment

 in the above photo the connecting rod is no placed on the third or main driver and the front is resting in position on the interior portion of the crosshead. the exterior portion of the crosshead and its attachment screws are located at the bottom alongside the torx screwdriver, 

Connecting Rod After Installation

 





 The above photo shows the completed installation of the connecting rod with the crosshead and third or main driver.  The connecting rod as held in place using an eccentric crank that will be installed later, however the rod will not come off the main driver shaft unless also removed from the crosshead.   The connecting rods moved freely as do the crossheads.