Crosshead and Slides

Revised: 1/6/2013          Subject to Revisions

Crosshead Guides

The crosshead is a fitting on the end of the drive piston shaft that is travels parallel to the piston rod held between guide slide bars.  The guide slide bars in turn are mounted between the rear cylinder cover and the front expansion link support frame.  The guides must of course be a close fit to the crosshead and parallel to the piston rod and each other.

First Set of Crosshead Guide Slide Bars as Printed

Design details were developed using the Alibre Pro 3D CAD software using drawing information from the USRA Heavy Mikado plans from the Internet.  Each had pilot holes in place for the front and rear mounting points.  As printed they came out well as shown above.  They were done in white ABS to agree with the prototype photograph being used as a guide.  The upper part of each guide bar in the above photo is the outside of the part on the locomotive model.

Cross Slide Guide Bars After Removal of Support Material Before Clean up

The guide bars were easily removed but as expected had some support material residue to be removed.  Removal was done using the rotary sanding wheel.  The top two bars show the typical amount of support strips that remained.  The bottom bars show the outside away from supports.

Crosshead Guide Bars After Clean Up

The guide bars cleaned up easily without any problems.  After clean up they were drilled at the front end on the left of the photo for 0-80 screws and tapped at right for 00-90 screws.  Only one 0-80 screw is used to mount the front but four 00-90 screws were used on each bar in a manner similar to the prototype.  This was done because the size of the front expansion lever bracket support provides only a very small mounting region.

Engineer Side Crosshead Slide Bars Installed

Fireman Side Crosshead Slide Bars Installed

Once installed the guide bars were found to be anything but parallel to one another.  After examining the issue the author determined that new bars with adjusted position 0-80 screw mounting holes would fix the problem.  Also, the guide bars were too wide side to side with insufficient clearance between the inside of the bar and front side rod mounting screw.  The author elected to narrow the bars to provide clearance for the crosshead inside to the coupling rod mounting screws on both sides of the locomotive.   Because the distances and position were slightly different between the two sides of the locomotive because of tolerance build-ups, the author designed custom bars for both top and bottom and each side of the locomotive.

One Set of Custom Crosshead Slide Bars as Printed

The new design slide bars were marked by designing indent lettering to keep them identified as to which side of the locomotive they were for.  The above photo shows the markings highlighted in blue.  These also printed very well and in virtually the same fashion as before.  They differ only a modest amount.

Modified Engineer Side Guide Bars Installed

Modified Fireman Side Guide Bars Installed

After clean-up the new modified guide bars were installed as before.  This time the corrections resulted in parallel bars which also were parallel to the cylinder and the rails below.  The narrowed bars also provided enough clearance for the crosshead to front coupling side rod mounting screws.

Crossheads

The crossheads run back and forth between the slides keeping side load pressures from flexing the piston rod.  The side loads are caused by the connecting rod attached to the third driver.  The crosshead is actually a sandwich that contains the end of the connecting rod on a pin and attaches to the piston rod.  In the model the design will consist of one side with the piston rod and the other side with the pin for the connecting rod.  The two parts will be held together with 00-90 screws with the inside half being threades and the outside half having clearance holes for the screws.

Set of Crossheads As Printed on 3D Printer

The crosshead set of four parts was printed together as shown in the picture above.  One outer section has already been separated from the raft and supports.  Each part has flanges to keep the crosshead held on the slides.  The inner section has the piston rod.  The parts took nearly five hours to print.  The results were good.  Small dummy bolt heads are located between the connecting screws on the outer sections.  There are four screws, two just below the upper slide flange and two more just above the lower slide flange.  The outer section has two pins, one insdie for the connecting rod and another outside for the union link that connects to the combination lever.  The connecting rod will be held in place by the sandwich halves, the union link with a small screw and washer.

 

Crossheads After Removal From Raft and Supports

The majority of residual support material (green) was on the interior of the sandwich halves.  The picture above shows one set before (upper) and another set after (below) cleanup.  Most removal was done with a model knife because of the small size.  A file was also used to smooth the flange inside surfaces that would run on the slides. 

Crosshead Sets Rigged for Hole Drilling

The crossheads were built with the noles defined in the CAD drawings.  Small partial holes and circular printed rings provided the locations for the holes.  They required drilling and tapping.  First the parts were held tightly together and drilled with the very small #65 drill (0.035" diameter).  The halves were then separated and the outer portions drilled through with #55 drill to provide a clearance for the 00-90 screws.  The Dremel drill press was used with the tool.

Crossheads Assembled on The Crosshead Slides

Close Up of Assembled Crosshead on Slide

The assembled crossheads fit too tightly on the slides and some further enlargement of the space for the slides was made using a file.  The inside and outside halves are bolted together with the four small black 00-90 screws as shown in the photos.  The screws used feature a Torx type head which is a star shaped opening to fit a Torx driver.  A T2 star driver was used to drive the screws in place.  This works suprisingly well for the very tiny screws.  Between each pair of holes horizontally are two dummy screw heads (without the star opening).  The crossheads are printed in white ABS above and will be painted later. 

Expansion Link and Brackets

Revised:  11/23/12          Subject to Revisions

The expansion link brackets and their mounts had been designed in the earliest stages of learning the Alibre Pro 3D CAD program more than a year ago.  Now that the cylinder assembly has been installed it is time to address the brackets and the expansion link itself.  On the model the links are about 2" long and pivot between two horizontal supports.  The expansion link brackets are in turn mounted between two fairly wide horizontal frames that mount to the frame and extend out beyond the drivers and drive rods.    The forward support also mounts the rear of the crosshead guides.    The front support is quite complex, thin and fairly large being some 5" wide and nearly 4" high and only a little of 1/8" thick.  This part would be a challenge to build on the 3D printer.

Crosshead Guide and Expansion Link Bracket Front Support As Printed

The author revised the design to fabricate the crosshead guide and expansion link bracket support in three parts that would later be glued together to form the overall part.  The above photo shows the three part sub-assembly as printed.  The top of the part as printed is actually the back side of the part.  The bar shown in the middle will join the two ends at left and right.

Crosshead Guide and Expansion Link Bracket Support Parts Before Cleaning

The above photo shows the support material residue on the right hand section which is removed on the left and joining bar at left and top in the photo above.   Two angle brackets with angled shoes will mount the overall assembly to the locomotive frame eventually.  The author realized that since this and the rear support must join several parts with some accuracy, drilling holes in the frame will await fit checking and fabrication of several other parts that mount on the support.  Parts that mount on the support frames will be attached with small 00-90 screws rather than glue so that that portion of the locomotive that support the valve and crosshead can be adjusted as needed.  The curved sections are boiler supports.  This is a busy part that must support many things: crosshead guides that must align with the cylinder piston and main connecting rod, expansion link which must align with the piston valve via the radius bar and fly crank through the fly-crank rod.

Rear Expansion Bracket Support rail as printed

The rear support for the expansion link support brackets is mounted across the frame and is nearly the same width as the crosshead guide support shown above.  This support also supports the boiler along the curved sections that protrude above the bar. 

Set of Frame Mounted Supports

The above set of frame mounted supports each provide support for various portions of the boiler as well as other parts of the locomotive mechanisms.  The top support is between the front and intermediate driver, the second is between the intermediate driver and main driver and finally the lower is between the main driver and rear driver.  Each mounts to the frame  with brackets that lie along the top of the frame and support the boiler above on the curved section.  The top and middle supports provide attachment for the expansion link brackets between them while the top support attaches the rear of the crosshead guides.

Expansion Link Bracket Set as Printed

The expansion link is a curved cam lever that is supported about 1/3rd of the way back of the first support frame.  It pivots to provide proper drive to a rod that in turn moves the piston valve.  The pivot is the hole in each bracket.  The expansion link is supported by two pivots, one on either side which are formed as a hole in the brackets that lie on each side of the link.

Expansion Link Brackets After Clean-Up

Like all printed parts the expansion link brackets have green ABS support material residue that is removed using the Dremel tool and model knife.  The large hole is the expansion link pivot.  Not readily seen are four smaller holes, two on each end that will be tapped for 00-90 mounting screws.

Expansion Brackets Being Tapped for 00-90 Screws

The four small pilot holes were hand tapped for the 00-90 screws as shown in the above photo.

Expansion Link Fit Checked in Brackets

The expansion link is a cam shaped to drive the radius rod that connects eventually to the piston valve.  The radius rod passes through the middle of the expansion link and rides in the curved slots on either side. 

Assembly of Inside Expansion Link Brackets to Front and Rear Supports

Top View of Expansion Link Inner Brackets, Front and Rear Support Assembly

The expansion link inner brackets were then assembled with the front and rear supports as shown above.  The brackets are held using small 00-90 screws.  They are so small they are nearly invisible in the photo. 

Closer View of Expansion Link Bracket Attached to Rear Support With 00-90 Screws

Fireman Side Expansion Link Installed in Brackets



The fireman side expansion link was installed between the bracket pair as shown in the above photo.  The link rotates freely around the white pivot that extends through the brackets.  The parts are held together with screws so that it can be progressively assembled on the locomotive and disassembled later if needed.

Expansion Link Bracket and Supports Fit Checked on Locomotive

The above photo shows the expansion link bracket and support assembly placed in position on the locomotive frame.  The expansion link is well outside the path of the rods and does not interfere.  An extension below the bracket on the front support will eventually support the crosshead guides.  So far this portion looks to fit well. 

White Side Rods

Revised:  11/23/12          Subject to Revisions

Pictures of various steam engines including Mikados often have what appear to be white or bright metal rods including side, main and valve rods.  Many also feature white sidewalls near the rims of drivers, pilot, trailing and tender truck wheels.  The author elected to replicate the black side rods in white for now.  They may be painted a silver light grey at a later time.  The same build files were used and the 2nd extruder material changed from black to white ABS.

White Intermediate Side Rods As Printed

Rear Side Rods as Printed

White Side Rods on Locomotive Model

The new white side rods work essentially identical to the black ones installed previously.  They needed the same amount of clean-up and fit they same.  The photo above also shows a white expansion link mounted on the link bracket.  The author intends to make all the side rods and cross slide rails in white to emulate those seen in the photos. 

Cylinder Block Covers

Revised:  11/12/2012          Subject to Revisions

Front Covers

Covers on front and back of the cylinder and valve bores complete the cylinder block assembly.  The first covers built were the front covers.  These do not have an opening for a rod.  Photographs of front covers on various Mikado locomotives indicate that they are plain.  More than likely the photographs depict insulated or isolated covers as no bolts are observed. 

Cylinder and Valve Front Cover Sets

Front covers are plain with the exception of a protrusion in the center.  In photographs protrusions on the larger cylinder covers appears to like a hockey puck in shape.  The covers also have rounded outer edges.  The valve covers have a short rod like protrusion.  These features were estimated based on photographs as the author did not have those features shown on USRA plan views. 

Inside and Outside of Front Covers

The outside of the covers do not have any remaining support material.  The inside was down on the raft and had support material that needed removal before installation. 

Inside View of Front Covers and Outside of Covers Placed on Cylinder Block

The Dremel sanding wheel was used to remove the bulk of the residual support material.  Very little touch-up was needed otherwise.  Some sanding of the interior of the cylinder block was done to provide a suitable fit for the front covers.

Front Covers Installed on Cylinder Block

The front covers were then glued in place on the cylinder block.  Super glue was used as was done for all prior gluing operations. 

View of Front of Cylinder Block with Covers

The front covers look very close to those seen in photographs of prototype locomotives.

Rear Covers

Isometric CAD View of Rear Valve Cover Design 

 

Drawings and photographs of the rear valve covers were converted to CAD design as shown above.  The rear valve cover has a modified bell-like housing to support the valve slide mechanism a short distance from the valve cover.  The author elected to simplify the housing a bit and eliminate the use of the slide which would be too small for an effective working model.  Instead a guide hole is provided to support the valve rod at the linkage end with dummy supports to surround the linkages.

Rear Valve Covers After Removal From 3D Printer

Printing went well with the rear valve covers.  Even the relatively small dummy bolts turned out fairly well.  The support material was used mainly to support the upper portion of the bell housing. 

Rear Valve Covers After Removal of Support Material

Support material was removed fairly easily with a needle nose pliers and model knife.  The valve rod holes were drilled out to make them clean for the eventual free movement of the valve rod.

Rear Covers Installed on Cylinder Block

The photos above and below show the rear valve covers installed on the cylinder block.  The valve rod will be operable (but not the valve) to illustrate the motion of the valve and linkage mechanisms on the model.

Close-up View of Rear Valve Cover

The rear valve cover is fairly small and is readily built on the 3D printer.  A small complex part such as this illustrates well the advantage of a 3D printer to make complex models.

CAD Isometric of Rear Cylinder Cover

The rear cylinder cover illustrated above will have two threaded blocks to attach the front of the cross slides to the cover.  The cross slides support the front slider of the main drive rod to prevent bending or distortion of the cylinder rod.  The rear of the slides are held on a support attached to the frame.

Rear Cylinder Covers As Removed From Printer

The rear cylinder covers have more details than those on the front.  The rear has a ring of dummy bolts plus the attachment blocks for the slides and of course the cylinder rod hole.   The printing came out very well with good detail.

Outside and Inside View of Rear Cylinder Covers

As for the rear valve covers support material inside the cover was removed to enable installation on the end of the cylinders.  This was done with the sanding wheel. 

Rear Cylinder Covers During Fit Check on Cylinder Block

Some grinding of the inside of the cylinders was done to facilitate a fit of the covers.  The region around the cylinder rod hole was cleaned out using a model knife.  After installation the rod hole will be drilled to final diameter.

Tapping Slide Mounting Blocks For 0-80 Screws

The pilot holes built into the slide mounting blocks were hand trapped for 0-80 screws.

Rear Cylinder Covers Installed on Cylinder Block

Rear Cylinder Covers With 0-80 Screws Inserted

View of Cylinder Block With Rear Covers in Place

The installation of front and rear covers essentially completes the cylinder block assembly. 

Half-Cylinder Block

Revised: 10/11/2012      Subject To Revisions

Designing The Large Half-Cylinder Part For 3D Printing

The cylinder assembly is quite large, too large to be made at one printing.  The prototype plans provide an overall width dimension of 123 and 1/2 inches.  That translates to 5.146" for the model.  Large parts of this kind of dimension would warp or have a raft to support release which would distort at least a portion of the part.  The author decided to make the cylinder section in two halves.

Plan Section Views With Cylinders Highlighted

The plan section view above shows two half-sections showing the cylinders.  The width and height are such that it is believed that the 3D printer will do a better job with only half of the cylinder section at a time. 

Side View of Cylinder

The cylinder main block is also symmetric front to back as can be seen in the side view below. The view shows the internal structure beneath the insulation lagging.  That detail will not be shown on the model, only the exterior of the cover lagging over the insulation.  The above view also shows detail of the interior of the steam feed pipe coming into the valve section.  The author will only show a straight hole in order to reduce the amount of ABS material needed to make the part. 

Other details are revised to provide a means to mount the cylinder with screws.  Since the two halves are identical, the author provided a feature to help align the two halves for gluing.  The cylinder assembly will then be bolted to the frame.  Included is also a feature to provide a means to bolt the front of the boiler assembly in place with access to a screw down through the chimney.

Below are a series of views of the finished part 3D CAD model taken from the Alibre design software.

Front View of Half-Cylinder Block

 

The half-cylinder block has hollow cylinder and valve bores although they will be empty on the model.  This was done to reduce the amount of material and therefore the amount of time to make the part.  At the top of the view the steam pipe is also hollow.  There is a hole from the boiler region through the steam pipe into the valve bore. 

Top Center Front View of Cylinder Half

The interior of the half-cylinder bock between the cylinder and valve bores is hollow as well, again to save material.  This is similar to prototype practice where the cylinder block is a large sand casting with passages for steam to enter and exhaust.  The bores are machined to exact dimension on the prototype and the front and back are machined to accept the bore covers.  The author will make the covers separately and glue them in place.

The region above the frame just inside the cylinder bore is made thicker to be tapped for mounting bolts that will penetrate vertically up from the bottom of the frame to secure the cylinder block in place.  There is a boss located at the center with a half round groove at the top of the center region.  When the two halves are glued together a full boss will be created which will then be tapped for a bolt used to secure the front of the boiler.  That bolt will be accessed down through the exhaust stack of the locomotive.

Also on the half-cylinder block has a rib at the rear edge of the opening down the center line that protrudes over the center line.  Two halves will then be mated with those ribs entering the opposite half as they come together making it easier to achieve alignment.

Bottom Rear View of Half-Cylinder Block

As seen from the bottom rear the various hollowed out sections are obvious.  The cylinder block will rest on the locomotive frame along the flat horizontal wall that is closest to the inside of the cylinder.  The cylinder block will be drilled in place on the frame then removed and tapped.  The frame holes will be enlarged by drilling with a bit size that allows the bolts to slide through.   The rectangular boss with the groove protruding down from the top center of the opening at the center line is half of a bolt boss that will be drilled to size and tapped through the groove region.   The boss will be used to provide a threaded section to accept the bolt holding the front of the boiler in place.

Outside Rear (or Front) View

The half-cylinder is fully symmetric so that two can be joined at the center to form the full cylinder block.  Covers for the cylinder and valve openings will be made separately and glued in place.  Those at the rear will have holes for the cylinder and valve shafts.  The front and rear covers will differ at the center to include those holes.  The rear cylinder covers will also have supports for the slides.

Top Side View of the Half-Cylinder Block

The view above shows the outside of the cylinder block.  The outside has some of the lagging detail.  The lagging covers the cylinder exterior to provide insulation to reduce heat loss to the air outside the cylinders and valves.  The steam pipe has air insulation inside on the prototype.  The author's model shows a row of bolts down the front and rear vertical ribs indicating that the cylinder halves were bolted together.  This may or may not have occurred on prototypes depending on how they elected to make the large cylinder castings.

Statistics of the Alibre 3D CAD design include:  24 features (15 extrusions, 3 mirrors, 6 fillets), 560 edges, 208 faces and 369 vertices. 

Future postings will show results of attempts to build actual 3D printed ABS parts.  The author intends to use this model to build the part oriented as shown in the last view with the boiler saddle up and frame mounting region down.  That way the parts should have good definition along the outside and top of the printed part and less residue of support material to clean off in areas where the model surfaces can be viewed the most.

Building the Parts in the 3D Printer

The first step after designing the part in the Alibre 3D CAD software is to output the result in STL format.  That file in turn is the input to the BFB 3000 conversion program Axon.  The Axon software translates the STL file into a BFB file that is used to build the part on the printer.  File transfer between the Alibre 3D CAD program and the Axon program is through windows on the computer.  File transfer between the Axon program and the BFB 3000 printer is via a computer stick.

Axon Display of Part and Support Material

 The Axon software displays various outputs including the one above that depicts the part in red and support material in blue.  The Axon software indicated that this build would be in 538 layers.  Since the part and support material use separate print heads the overall build cycle will be quite long in duration with the need to switch heads on each layer.  Head switching takes a couple of minutes to warm up the next head on each change.

Just Finished Part in Printer

The printing took 17 hrs 13 minutes.  No evidence of part separation or serious distortion was observed.  The view above shows the part on the build platform of the BFB 3000 printer.  Below are the reels of ABS build material. 

End View of Part in Printer

The end view of the part above shows the green ABS support material used as a framework for subsequent layers of black ABS part layers.  Support is needed for sections that have no part material to provide support.   The bottom most layer of green ABS is the raft which is deposited directly on the build platform.  A tool provided by 3D systems with the printer is a sort of thin metal blade used to slide under the part and separate it from the platform. 

Side View of Part in Printer

The part black layers came out fairly smooth and mostly free of support material as expected. 

Part and Support Assembly After Removal from Printer

The photo above shows a top front view of the part and support material after being removed from the printer.

Part Front View with Ruler

The ruler in the above photo shows the approximate height of the part.  A build of this vertical dimension required 538 printing layers to complete.  Each layer required a number of minutes to print after the particular head warmed up to printing temperature. 

Top Outside View

Top Inside View

 

The top inside view above shows the surface of the inside support layers of Green ABS and the rather thick section of support below the center of the part.  Most of the green ABS support material will be removed to reveal the finished part.  The author chose to leave much of the inside support material in place within the part where it will not show.  Material inside the valve and piston bores will be removed.  There was no support material inside the steam pipe.

Part Front Outside View After Support Material Removal

Pliers were used to peel away most of the support material which came away easily.  Some residue strips remained to be removed with model knife or sanding wheel.

 

 

Inside Rear Top View

Support material inside the hollow center of the part will not be removed fully, just enough for the alignment flange to fit in from the mating part half.

Residue Stripes After Support Material Removal

Residue stripes of support material remain attached after pliers removal from the part.  They will be removed using either a model knife or sanding wheel later.

Interior View of Part Center Section

The part center section interior is filled with support material which will not be removed more than shown in the above photo since it will not be visible once the two halves are joined.

Top Rear View of Part After Support Removal

Bottom Inside Rear View of Part After Support Stripe Removal

The bottom region of the part had quite a few residue stripes which were removed mainly with the sanding wheel.  Small rough spots were trimmed with the model knife.

 

Front Outside View of Finished Part

Rear Inside View of Finished Part

Outside View

Front View

Top Front View

Bottom Rear Lower View

The successful build of the above part has triggered a build of a second identical part.  The two parts will be glued together to form the cylinder section of the locomotive.  After assembly it will be drilled and tapped for mounting to the frame and boiler firebox mounting.  Separate builds of front and back valve and piston bore covers will be made and glued in place.

Second Half-Block Beside First Part

Both Cylinder Block Halves Held By Robber Bands

The two halves were joined with CA superglue and held together with rubber bands while the glue cured.

Fit Check of Temporarily Assembled Cylinder

Closer Rear Engineer Side View of Cylinder Block on Frame

Front View of Cylinder Block on Frame

The next steps will be to drill mounting holes through the frame into the cylinder block.  The holes in the frame will be enlarged to clear the bolts and those in the cylinder block tapped to thread them.

Locomotive Frame Inverted on Drill Press For Cylinder Screw Holes

Frame Inverted in Drill Press With Cylinder in Location

 

To drill the frame the locomotive frame was inverted over support blocks and arranged on the drill press.  The photo above shows the arrangement just after the four holes had been drilled through the frame and into the cylinder assembly.  The hole spacing differs on each side of the frame to facilitate correct orientation during assembly.

View of Bottom of Cylinder Block After Hole Drill and Tapping

 

The four cylinder mounting holes were tapped to accept the mouting screws.  The 1-72 screws 1/2" long on hand are a bit short, but will just grab a couple of threads.  Slightly longer screws will be ordered so more threads will be engaged.