Sunday, October 31, 2010

Passenger Car Window Covers (For passenger cars with sliding windows)

How often have you acquired a passenger car, complete with sliding windows with the glass all intact at the railway yard, only to discover a few days later that every glass pane in the car has been smashed by the vandals?  Or, if you have a passenger car that you want to cover the windows so that they don't get smashed, the only way to fasten the plywood covers is with screws or nails through the side of that beautifully crafted window.  
First developed by Ross Robinson back in 1992 for his GTW caboose, here's a method that requires no screws or nails into windows, frames, or beautiful mahogany wood.  If you know you are getting a passenger car ahead of time, you can fabricate the window covers in advance and install the window covers in the railway yard. 

Elements of The Window Cover
For passenger cars with sliding windows, the window rests on a steel sill and slides up into a cavity inside the wall of the passenger car.  Our window covers make use of these two facts - the steel sill and the cavity inside the wall. 
 The window cover consists of 3 pieces plus 8 carriage bolts and nyloc nuts:
  • A 1/2" plywood panel that completely covers the opening of the window frame - typically 271/2"x 34"
  • A fabricated piece of 22 gauge sheet metal fastened to the top of the plywood panel with four 1/4"x 3/4" carriage bolts and nyloc nuts
  • A fabricated piece of 22 gauge sheet metal that resembles a piece of eavestrough fastened to the bottom of the plywood panel with four 1/4"x 3/4" carriage bolts and nyloc nuts.     
For installation, with one person working on the outside and another working on the inside of the passenger car, window covers can be installed or removed in less than an hour.    Installed from the outside of the car, first, the person inside opens the window to about 9".  Second, the person outside inserts the top piece of sheet metal into the top of the window frame.  Third, they then push the bottom piece of sheet metal into the bottom of the steel window sill.  Fourth, the person inside lowers the window onto the bottom piece of sheet metal.  The window holds the window cover securely in place. 

Fabricating The Plywood Panels
By setting up an "assembly line", window covers can be readily manufactured in short order.   

The first step is to fabricate the plywood panels.  The width of the panel is the complete width of the window opening.  To this measurement, add 1" to account for the window track on each side of the window opening.  For the height of the panel, measure window opening from the top of the steel panel to the bottom of the steel window sill.  To this height, add 1" as the window cover will be about 3" away from the window.  You should end up with measurements that approximate 271/2"x 34". 
The windows in the bathrooms at each end of the passenger car may be slightly narrower at about 251/2". 

The best size of plywood is 1/2" as 1/4" plywood has no "structure" to it and will quickly warp, and 3/4" plywood results in a very heavy window cover.  1/2" poplar plywood that is used as sheathing or underlay on new houses is about the right density. 

From a 4'x 8'x 1/2" sheet of plywood, you can get three 1-piece window covers (Panel A, Panel B, Panel, C) plus 3 pieces that you can assemble for one extra window cover (Panel D-1, Panel D-2, Panel D-3), and two pieces of scrap as shown in the cutting diagram below. 
These 3 extra pieces (Panel D-1, Panel D-2, Panel D-3) will have to be joined together to produce the 4th panel.  An air-powered brad-nailer will quickly do the trick. 
On a passenger car with 44 windows, 11 sheets of plywood will produce 44 window covers.
You may be tempted to try and truck 11 sheets of 1/2" plywood away from your local lumber centre on the top of the car, in the back of the pickup truck, or minivan.  
Please don't.  Lumber centres like Home Depot, Rona, Home Hardware, etc have specialized saw service departments.  Wheel the sheets of plywood over to the saw service and have them cut up the plywood for you.  This will save you lots of time. 

It can be quite a job trying to get 11 sheets of plywood cut, painted, sheet metal pieces installed all at once.  Break up the job so that you purchase and cut 3 - 5 sheets of plywood at a time.  Once you have completed the window covers for these 3 - 5 sheets of plywood, return to your local lumber centre and purchase another 3 - 5 sheets of plywood.  

The cost of a 1/2" sheet of exterior sheathing plywood will cost about $20 - $25 per sheet - about $5 - $6 per window cover.  If you aren't in a rush to finish your window covers, lurk around the cull bin of your building material store.  Or, take a look at that stack of plywood to see if there are any sheets that have been damaged by the forklift or the metal straps.  Over a period of weeks, you may be able to find small sheets of plywood culls, or plywood with the corners smashed.  If you are sharp with your negotiations, you may be able to pick up damaged sheets for $10 or half sheets of culls for about $5.  This can reduce the cost of plywood to about $2 per window cover.

Painting The Plywood Panels
With the rain, snow, and sun, unpainted plywood will deteriorate, de-laminate, and look ugly in very short order.  Two coats of exterior gloss alkyd or urethane rust paint will protect the plywood panels and make your passenger car look better than what it currently is.  Paint the plywood panels the same colour as your passenger car so that the window cover becomes an integral part of the car. 

Painting the plywood panels is the type of work that can be done in your basement workshop at home over several evenings.  It doesn't require special equipment or space. 

Don't paint the plywood window covers white with black outlines of passengers sitting inside the car.  Plywood panels painted white or any other light colour is an invitation for vandals to have a go at the car.  

We used Home Hardware rust paint because it is a high gloss exterior urethane paint that can be tinted into our favourite railway colour.  We applied the paint with a 4" roller.  Make sure the plies of the plywood are well sealed with paint.  Paint one side of each panel and let it dry. Turn the panel over and paint the other side.  Repeat this process until two coats of paint have been applied to each panel.  Make sure you paint the edges so that the plies of the plywood are well sealed.  

In some cases, the middle plies along the edges of the plywood may be hollow, creating a space between the plies that is weak.  Mix up some auto body filler and stuff the body filler into this space.  Sand and apply paint.  Do this kind of work outside as it smells up the place and gets very dusty when sanding.  An angle grinder with a "flap-disk" will do the sanding real quick.

Painting the plywood panels is a job that can be done in the basement - either at your museum or at home.  When painting a large number of panels at once, you will quickly run out of space to hang the wet panels.  Before you start painting the panels, drill two 1/8" or slightly larger holes in each corner about 1" in from the top edges.  Hang the plywood panels on a "wire chain" from the joists in the basement.

Making "Chain Hangers"
I purchased several pieces of "wire chain" - each length about 24" long.  I cut the link at each end as shown below and then unravelled and bent the link at each end so that I had a 90 degree hook at each end.  These hooks fit into the 1/8" holes drilled into the top edges of the plywood panel.  As I finish painting  one side of the panel, I slip the hooks into the holes and hang the panel from nails I hammered into the floor joists in the basement. The chains and holes can also be used to store the window covers when they need to be removed from the passenger car windows for the season.
Here's one panel that I've just finished painting with the hooks inserted, ready for hanging to dry.
For those of you who are Canadian National Railways fans, here's the colour formula for CN #11 Green at Home Hardware - in the gallon size and in the quart size.  

CNR #11 Green
Home Hardware (Beauti-Tone) Exterior/Alkyd/ Rust Coat
Gallon Formula:
Base # 64-03 Clear Base, 1 gallon size
Colourant B: 4 ounces + 26 shots
Colourant C: 4 ounces + 14 shots
Colourant G: 16 shots
Colourant Z: 8 shots

Quart Formula:
Base # 64-03 Clear Base, 1 quart size
Colourant B: 1 ounce + 6 shots + 1 half shot
Colourant C: 1 ounce + 3 shots + 1 half shot
Colourant G: 4 shots
Colourant Z: 2 shots

Fabricating The Sheet Metal Pieces
There are two ways to fabricate the sheet metal pieces.  Purchase some 22 gauge sheet metal and fabricate the pieces yourself.  OR Take these diagrams to a sheet metal shop and ask them to fabricate the sheet metal pieces.  I chose to visit the sheet metal shop.  To save some money, I drilled the holes and snipped the corners, saving about $2 per window.  My cost on sheet metal for 44 windows on one passenger car was $250 for one 44-window passenger car - about $6 a pair.

If you fabricate the pieces, you will have to purchase the sheet metal, cut it into strips, drill and bend the sheet metal on a sheet metal brake.  This will take time, experience, and expertise. Which route you take will depend on the money available to you (this will be a good fund-raising project!), the expertise in your organization, the workshop and tools available to you.

Here are the diagrams for fabricating the pieces of sheet metal.
Top Sheet Metal Piece
The top piece is a piece of sheet metal 24" wide by 4" long with a 3/16" bend in the middle.  The top corners of the sheet metal piece are clipped so that the edges are flat (pound the corners with a hammer after clipping the corners with your tin snips). 

And here's what the top piece looks like with the holes drilled and the top corners clipped.

Bottom Sheet Metal Piece
If you decide to fabricate the sheet metal pieces yourself, the bottom piece requires a bit more thought and work.  First drill the holes before you start bending the sheet metal.  It will save you time and effort.   
Here's a photo of the bottom piece.  
For locating the drill holes, here's a graphic of the 2" side of the bottom piece.  
And here's a photo of the back of the bottom piece with the holes drilled.  
Drilling The Holes In The Sheet Metal - Some Hints
If you have the sheet metal shop fabricate the pieces for you, you can still save money if you drill the holes yourself.  At $0.50 per hole, you will save about $4.00 per set.
Mark off the holes at the distances shown in the above diagrams.  Centre punch the location of each hole.  Drill the holes out with a 9/64" drill bit.  We drilled the holes a bit oversized to allow a margin of error in drilling the holes in the plywood.  

In drilling the bottom sheet metal piece, fabricate a piece of wood with the same cross-sectional shape as the sheet metal.  Insert this into the "eavestrough" before you centre-punch and drill the holes. 

Caution - When drilling holes in sheet metal, make sure the sheet metal is securely fastened to your drill table.  The sheet metal piece may spin and cause injuries when the drill breaks through the underside of the sheet metal. 

Most of the fabricating work, drilling holes, bending corners, etc can be done on an "assembly line" basis in a couple of evenings.  Because the sheet metal shop fabricated my pieces, my work was limited to snipping off the corners and drilling the 8 holes.  

Installing The Top Sheet Metal Piece To The Plywood Panel
Plywood has a tendency to curl.  You will want to take advantage of this curl when installing the sheet metal pieces.  The sheet metal pieces are installed as shown in the diagram below - on top of the curl.  When you install the window cover, you will force the curve to "unfurl", making a nice tension-tight fit. 
  You will need the following tools/ supplies to install the sheet metal pieces to the plywood:
  • Straight Edge
  • Ruler
  • "Fine" tip permanent marker (for marking on the sheet metal pieces)
  • Pencil
  • Two pieces of scrap wood
  • Two C-clamps
  • Two T-squares (optional) (for locating bottom sheet metal piece on plywood)
  • 1/4" drill bit (a Forstener bit will reduce spintering)
  • Eight 1/4"x 3/4" carriage bolts for each panel
  • Eight 1/4" nyloc nuts for each panel
  • 1/2" box wrench
  • Hammer (for tapping the square collar of the carriage bolt into plywood)

The top sheet metal piece is fastened to the plywood panel in two steps.  First, the middle two carriage bolts are installed.  Then the outer carriage bolts are installed. 
  • Place the top piece of sheet metal on the top of the plywood panel so that the sheet metal is centred with equal space of plywood on each side of the sheet metal.  The bend in the sheet metal should be right on the top edge of the plywood panel
  • Place the piece of scrap wood under the plywood, install and tighten a clamp so that the sheet metal piece is held in place on the plywood panel.  Repeat the process at the other end. 
  • From the top of the sheet metal, drill the two middle holes in the plywood
  • Insert the carriage bolt from the bottom
  • Install and tighten the two nuts. You may have to tap the head of the bolt with the hammer to seat the square collar of the carriage bolt into the plywood. 
Insert photo of installing middle carriage bolts.
  • Remove the clamps and pieces of scrap wood from the ends
  • From the top of the sheet metal, drill the two end holes in the plywood
  • Insert the carriage bolt from the bottom
  • Install and tighten the two nuts.
Installing The Bottom Sheet Metal Piece To The Plywood Panel
Installing the bottom sheet metal piece to the plywood panel is a bit trickier than installing the top piece.  In addition to a space of about 3" from the window sash to the outer edges of the window frame, the steel window sill is stepped about 11/2" from the bottom edge of the window.  This means that the bottom of the plywood panel will be about 1½" lower than the bottom of the window sash that sits in the bottom sheet metal piece.  We have to account for this 1½" drop from horizontal when installing the bottom sheet metal piece to the plywood.  
  • Slide the rule of one of the T-squares out to about 6".  Use this T-square to draw a line 6" from the bottom edge of the plywood.  This marks where the top of the bottom sheet metal piece will be fastened to the plywood. 
  • The sheet metal pieces are 24" wide and the plywood panel is 271/2" wide - a difference of 31/2".   Slide the rule of the other T-square out to 13/4" (half of 31/2").  This will mark off the edges on the plywood where the sheet metal pieces are to be installed - ie 13/4" from each edge of the plywood cover. 
  • Place the bottom sheet metal piece on top of the plywood, lining the piece with the three lines drawn on the plywood in the previous two steps.  
  • Clamp the plywood and sheet metal together along the outer edges of the plywood.  
  • (Insert photo)
  • Turn the plywood over and drill the middle two holes in the plywood.  This can get a bit tricky as there's no way for drilling a pilot hole.  It's matter of lining up the hole visually and mentally with a little bit of ESP!  
  • Insert the carriage bolt, screw on the nyloc nut and tighten.  You may have to tap the head of the bolt with the hammer to seat the square collar of the carriage bolt into the plywood.
  •  (Insert photo.) 
  • Remove the clamps and drill the holes for the two outer holes.  Install the carriage bolts and nyloc nuts.  
  • (Insert photo.)

Voila!  You have just fabricated a passenger car window cover.  One done and only 43 more to go!
(Insert photo.)

Paint The Exterior Of The Sheet Metal
If you install  the completed window cover, you will see that the shininess of the sheet metal can "attract attention".  In order to tone it down, paint the sheet metal with two coats of paint.  This will also allow you to touch up any other spots on the window cover that may have been "tarnished" in the fabrication process (eg missed drill holes, etc).  

Here's a couple of photos of a complete set of window covers - one side of the coach.  
and a close-up of a couple of the covers.  
 If you do undertake this project, we'd be interested in seeing the results.   

Saturday, October 16, 2010

Cutting Steel With The Plasma Cutter - Like Cutting Through a Piece of Cheese!!

The sparks were flying this morning, fer shur, at Science & Tech!! Got to cut 1" thick steel with the plasma cutter. Love that machine!! Cuts thru steel like it was cutting thru cheese!!  
The plywood being held down with the C-clamps is the pattern for half a pair of "rail tongs" - instead of lifting salad, they'll be used to lift rails (as in railway track). Cut the pattern 5/8" smaller than the final shape. Hold the plasma cutter vertically against the pattern. Squeeze the button and run the cutter around the edge of the pattern. Sparks and liquid steel blow out on the underside. Voila - half a pair of rail tongs!!!

The job started several weeks ago when Ross R asked me to cut out some patterns in 3/4" plywood from full-size drawings.  Using good old-fashioned carbon paper, I transferred the pattern onto the plywood and then cut out the full-size pattern on the band-saw.  

Interior Cuts (Pattern Is Our Final Product) - Reduce Pattern Size By 5/8"
The head of the plasma cutter is 1" wide and will make a 1/4" wide cut in the steel.  We therefore have to account for this in making our wooden pattern.  We can have two different types of cutting situations.  In the first situation, the final piece of cut steel will be underneath our wooden pattern.  I call this an "interior cut" because the cut steel will be inside our pattern.  

On an "interior cut" where the steel is underneath our plywood, we have to reduce the size of the plywood pattern by 5/8".  The plasma cutter will then be cutting the steel right along the edge of the final cut.   The centre of the plasma cutter has to be 5/8" away from the edge of the pattern.  

So, using my T-square and a pencil, I traced a 5/8" pattern around the edge of the full-size plywood pattern.  Again, using the band saw, I reduced the full-size pattern to what you see in the photo below.  
 Most of our cuts will be interior cuts where the cut steel will be underneath our wooden pattern.  

The plasma cutter produces a very rough and ragged edge.  If the edge needs to be ground smooth, the pattern will only be reduced by 9/16", rather than 5/8".  This additional 1/16" will leave sufficient steel for a smooth grind. 

Exterior Cuts (Pattern Is NOT Our Final Product) - Reduce Pattern Size By 1/2"

In the second cutting situation, it may not be possible to have the cut steel underneath our pattern.  For example, our pattern may be too small for the plasma cutter to easily go around the pattern.  In this case, the pattern is outside the final cut steel and NOT on top of the final cut steel the pattern.  

On an "exterior cut"we have to reduce the pattern by 1/2".     

Plasma Cutter Settings
We've been looking at the business end of the plasma cutter.  Let's take a look at the end that makes it all possible.  Here's a "back view" of the plasma cutter.  
A large extension cord is wound up on the left side.  The extension cord is only used if the work is far away from a wall plug.  The regular cord, the ground wire, and the cutter gun are wound up on the back of the plasma cutter as shown in the following order:
  1. the regular cord
  2. the ground wire
  3. the cutter gun
 Let's now take a look at the front end of the plasma cutter.  This is where me make the adjustments (if any) to control our cut.  
Let's take a closer look at the control panel.  
To set up the controls, and with the plasma cutter plugged into the proper wall plug:

  1. Press the green "On" button
  2. Wait until both "Ready" lights are lit ("Ready" and "DC Power")
  3. Set the amps to 100%.  (If you are cutting sheet metal, see instructions below)
  4. Normally, we would be doing "stop-and-start" cutting as we move our position around the pattern and steel.  Set the "Continuous" switch to "Off"
  5. Flick the "Air Pressure" switch to "Test" and make sure it shows 75 psi.  If required, adjust the pressure to 75 psi.  flick the "Air Pressure" switch back to "Run"

To cut steel, 
  1. Connect the ground wire clamp to the steel to be cut
  2. Hold the plasma cutter gun on the steel but with the electrode off of the steel
  3. Squeeze and hold the trigger button
  4. Air will immediately come out under pressure
  5. In one or two seconds, a blue arc will emerge from the gun.  
  6. Slowly move the gun into the steel and against the pattern.  
  7. Move the gun slowly, always making sure that the liquid steel is blowing down.  
  8. Always "drag" the gun towards you.  Never away from you or sideways.  This will result in smoother cuts.  
  9. Take your time!
  10. Move your body placement so that you are always "dragging" the gun towards you.  
  11. Stop and start your cutting as required.  
  12. Release the trigger button before you lift the gun from the steel!!
To be continued.

Monday, September 6, 2010

Patterns Finished & Ready For Casting

In a previous blog we demonstrated the bits and pieces of wood that make up the speeder sweep castings.  Once we have the bits and pieces all brad-nailed and glued together, we create fillets (curves) between the ribs and the deck using automotive car body putty.  It's a simple matter of squeezing the putty out of the tube and wiping it on the wood, smoothing it out with our finger to create a nice curve.  A rubber glove helps to keep the putty off of our fingers.  Several coats of body putty, with light sandings in between coats, and soon have nice curved fillets.

The foundryman is not a pattern maker so he needs to know what part of the pattern is the final casting and what part is sand core.  In order to help him out, we give the patterns several coats of black and yellow paint - black outlining the final casting and yellow outlining the sand cores.  In this way, when he pulls the pattern out of the mold, he knows which sand core goes into which cavity.  The yellow outline of the pattern corresponds to a specific sand core.

Here's the top view of our pattern.
A head-on view.  In addition to seeing the profile of the two sand cores that we'll be using, you can readily see that our pattern is in two parts. 
You can also see where our parting line will be between the drag and the cope of our flask.  What may not be readily evident is a potential undercut on the left side of the photo.  When we put the top half of the pattern onto the base, we'll have to get rid of that undercut by building the drag part of our sand mold up before we fit the cope onto the drag.   
A side view.  Again you can see the top half of the pattern and the bottom half when you look at the wing. 
Another side view but with the pattern rotated 180 degrees.  You can readily see that our pattern is in two parts. 
And a bottom view with the top part of the wing detached from the base.  This photo clearly shows the ribs of our casting. 
Here's an end view with the two sand cores that will fit into the mold cavity created by the yellow parts of the pattern.  Note how the outline of the sand cores corresponds to the yellow outline of the pattern.

Our next step is to make sure that we can make a mold that will accept the sand cores and be ready for the molten aluminum.  In effect, this will be a trial run of our mold making to make sure that everything fits together (we hope!).

Thursday, August 26, 2010

A Morning At Alumaloy Castings (1990) Limited - How To Make A "Negative" Casting

The Company - Alumaloy Castings (1990) Limited, 424 Birchmound Rd, Toronto, ON, Canada, M1K 7M6
The Challenge - Using a "patio stone" rock as a pattern, make a "negative" of the rock.  Then use that "negative" pattern to make a mold to make an aluminum casting.  The aluminum casting will then be used by the landscape customer to make concrete "patio stone" rocks.
The Craftsmen - Marco and Paul, two of the principals of Alumaloy and their employees. 
The Circumstances - A landscape customer wants to make concrete "patio stone" rocks out of a slurry mix of concrete.  They sculped the "patio stone" pattern out of the slurry mix.  They now need to make a "negative" mold out of this "patio stone" in order to to produce dozens of stones that are similar in shape, size, and look.  Using the "patio stone" master for their pattern, they tried making the negative out of plaster of Paris without success.  The landscape customer has now come to Alumaloy for a solution.  And I've been fortunate to have just arrived on the scene with a challenge of my own (a much smaller one) to offer Alumaloy - but that's another story for another day.

Making The "Negative" Pattern
In a previous blog, I've discussed using sodium silicate and silica sand to make sand cores.  Alumaloy uses the stuff to make their one-of-a-kind sand cores all the time.  There's always a barrel or two of the stuff mixed up and ready to use at a moment's notice.

Paul put the patio stone face down on a mold board inside the cope-half of a flask that had lots of draft on the sides.  He then stuffed some mixed sand along the edges to get rid of the undercuts (there were a few on the back of the stone) and oversprayed the mixed sand with some brown lacquer.  This would help him to identify where the edges of the stone were.  He added some 1/8" steel rod as reinforcing steel and some hooks so that they could lift the negative pattern out of the green sand mold.  He also drilled some holes into the sides of the flask to insert some steel rods to tap the patio stone when it came time to remove the stone from the sand mix.
He filled the flask with the sodium-silicate-and-sand mix, tamping the sand tightly around the patio stone.  When the flask was full, he struck off the excess sand.  Next, he made about a dozen holes in the sand using a 1/8" steel rod.  These holes would be used to get carbon dioxide (CO2) into the sand mix.  The CO2 would create a chemical reaction with the sand mix to harden the sand into a solid block of sand.

I wouldn't have believed it if I hadn't seen it with my own eyes!  Using an air hose attached to the cylinder of CO2, he gave each hole a two-second (it could have been a 4 or 5 second) shot of CO2.  He repeated this again.  In less than 5 minutes, he had a solid chunk of sand which had the exact pattern (ridges, indentations, grain, etc) of the face of the patio stone.

Problem was he now had a 150 lb block of sand that he had to turn over to remove the original patio stone pattern from the block of sand.  Calling on Marco (who was always on hand offering advice and going for things to keep things moving along), they both managed to turn the flask and block over so that the patio stone was exposed.  A bit of digging and jabbing here and there and the patio stone came out.  They next sprayed the sand casting with lacquer to seal the block of sand.  As seen in the photo below, they now had the "negative" pattern that they needed to make the green sand mold and an aluminum casting.  You can clearly see the fine detail of the rock that has been impressed into the "negative" sand pattern. 
Placing the negative pattern on a pallet truck, they wheeled the 150 lb pattern into the mold-making part of the shop, lifted the heavy pattern face-up onto a mold board and then placed the drag-half of a large 30"x 36" flask over the pattern.  This provided just enough clearance around all sides of the pattern. 
 Marco then gave the pattern a good dusting with talc parting powder to make sure the pattern would easily release from the green sand when the pattern was lifted out of the drag-half of the mold.  He next riddled a fine layer of green sand over all of the pattern, ramming the sand with his fists as he covered the pattern.  If he had used an air-ram at this point, he would have cracked the pattern.
He continued layering in green sand, making sure it was rammed tightly into the drag.  At this stage, he started to use the air-ram to make sure the drag was well packed.  
When the drag was full, he struck off the excess green sand and called on Paul and another worker to turn the drag over.  The whole affair - pattern, sand, drag and mold board - weighed about 400 pounds.
With the drag and pattern now face-up, he gave the pattern and the top of the drag-surface of the mold a good dusting of talc parting powder to make sure the cope-half of the flask would readily lift off of the drag-half.
He next placed the cope-half of the flask on top of the drag and riddled a layer of green sand on top of the pattern.  Again, using his fists so as not to crack the pattern, he rammed the green sand all around the pattern.
 He next set some wedge-shaped chunks of steel on top of the pattern to create risers.  As explained in a previous blog, the risers allow the molten aluminum to come up into the voids created by the wedges and allows the gases to escape as the aluminum cools.  If the voids created by the risers are large enough, the voids also serve as a reservoirs of molten aluminum that keep the mold filled as the aluminum cools and shrinks.

He also added a sprue cutter to form a sprue that would be used to pour molten aluminum into the mold. 
In order to create a sufficiently large reservoir of molten aluminum, he removed the chunks of steel used to create the risers and replaced them with large ceramic-like cylinders (the material is similar to that found in some professional back-yard foundries and kilns).  He continued to add and ram more sand until the top of the cope was full.  He next struck off any excess green sand and used a tamper to smooth the top of the surface. 
 Marco and another worker then lifted the cope off the drag while Paul rolled the drag out of the way.  The cope alone weighed more than 200 lbs so it was strictly a "lift the cope", "roll the drag out of the way" (thank god for those rollers, eh!), and "set the cope on its side".  A minimum amount of moves in a minimum amount of time.  Any miscalculations and the cope would have ended up on the floor as a big pile of green sand. 
 They next lifted that 150 lb pattern out of the drag.  It was definitely not an easy job as it took three of them to do it.  Here's what the drag-half of the mold looks like.  You can see the pattern in the mold that carries all of the fine detail of the original "patio stone" rock.   
But first Paul and Marco worked on the cope-half of the pattern cleaning and blowing off the loose sand and fixing any imperfections that might have been on the cope-half of the pattern.  After cleaning out the holes for the sprue and risers, they cut gates into the mold so that the molten aluminum would flow evenly and continuously into the mold. 
Marco and Paul next turned their attention to the drag-half of the pattern, blowing off the loose sand and cleaning up any imperfections.  They had to cut into the sides of the green-sand mold to remove the pattern but these were easily fixed up with handfuls of green sand and smoothing with a trowel.

They next placed the cope back on top of the drag.  They clamped the four corners of the flask so that the cope wouldn't "float" off the drag when the molten aluminum was poured into the mold.  The mold was now complete and ready for the aluminum pour.
The key thing in an aluminum (or any) pour is to keep a steady "flow" of molten aluminum pouring into the mold - particularly with a mold this large.  It took two "pots" and 6 "ladles" of molten aluminum to fill the mold and to leave a good supply of molten aluminum in the risers. 
Marco then "babysat" the mold for the next 45 minutes periodically pouring a ladle or two of molten aluminum into the top of the riser tubes as the aluminum in the mold cooled, drawing down the reservoir of molten aluminum in the riser tubes.
After a couple of hours, the casting had cooled sufficiently to knock it out of the green sand mold.

The flask was upended and the casting pulled out of the green sand.  The green sand was steaming hot.  Don't touch!  It will badly burn your fingers to a crisp before you can say "Ouch!!".

After letting the casting cool overnight, it was moved over to the clean-up part of the shop where Paul wire-brushed the remaining green sand off the casting.
Once the green sand had been brushed off, you could clearly see the detail of the rock in the aluminum casting.  The sodium silicate and silica sand had clearly captured the detail of the "patio stone" rock.
Next, the sprues and risers (now solid) were cut off and any sharp edges on the sides of the casting ground down.
The casting was next transported over to the weigh scales and weighed. 
110 pounds of solid aluminum, ready to receive the concrete slurry to make "patio stone" rocks.
A job well done!  And I had a first-hand look at how aluminum castings are made.

Monday, May 3, 2010

Sand Cores Using Sodium Silicate and Carbon Dioxide (CO2)

Sand, when mixed with the correct ratio of sodium silicate, rammed into a core box, and then exposed to carbon dioxide (CO2), will result in a very hard and durable sand core.  Never having used sodium silicate (and never having made sand cores before!), this was an excellent lesson in learning what works and what doesn't. 

PQ Corporation is the largest manufacturer of sodium silicates, one of the most widely used chemicals in the world.  Their brochure on sodium silicate has this table which describes the various strengths of sodium silicate available. 
N-Grade Sodium Silicate
The key measure of sodium silicate is the weight ratio of its two major components - Silica DiOxide (SiO2)to Sodium Oxide (Na2O) (the column titled "Wt. Ratio SIO2 / NA2O").  The most commonly available sodium silicate has a weight ratio of 3.22 parts of Silica DiOxide to 1 part of Sodium Oxide with a solids content (active ingredients) of 37.6% (8.90+28.7=37.6 from Table 2 above).  The rest (62.4%) is water.  This is sold as "N" grade sodium silicate and is NOT the best choice for making sand cores as it doesn't provide very good strength to the sand core.  The sand core will slowly disintegrate when handled.  This type of sodium silicate has the viscosity of a cheap liquid dishwashing soap. Table 2 above describes it as a syrupy liquid. 

I made the mistake of using the 3.22 N-grade sodium silicate (it may even have been a weaker solution) with very poor results.  Even after 24 hours and constant exposure to CO2, the sand cores wouldn't hold together. For those that did stick together, I would end up with loose sand grains in my hand whenever I handled them. 

RU-Grade Sodium Silicate
The best type of sodium silicate for making sand cores has a weight ratio of 2.40 parts of Silica DiOxide to 1 part of Sodium Oxide with a solids content (active ingredients) of 47.05% (13.85+33.2=47.05 from Table 2 above) - a 25% increase in active ingredients over the N-Grade stuff!!  This is typically sold as "RU" grade sodium silicate and has the viscosity of concentrated liquid laundry detergent - it pours very slowly.  Table 2 describes it as a heavy syrup. 

I got some 2.4 RU-grade sodium silicate from CM and today mixed up a batch of sand to make some sand cores.

Mixing The Sand And Sodium Silicate
I first got all of my supplies, cups, bags, and stir sticks together and laid them all out on a sheet of plastic to make the cleanup easier.  (The McDonald's cup is my supply of sand - easier to pour from a small cup than from a 25 kg bag, eh!?)  I then put a smaller sheet of plastic down on top of the larger sheet so that I could easily recover any spilled sand.

To make the sand cores, I first filled the core box with dry 90m silica sand and poured it into a Ziploc bag.  I added about 10% more dry sand as it will compact more when the sodium silicate is added to the mix.
Using my Canadian Tire "Star-Frit" scale, I weighed the baggie at 376 grams.
I put an empty plastic cup (clean and dry!) on the scale and zeroed it out.
I next decanted 38 grams (10%) of 2.4 RU-grade sodium silicate from my large supply bottle into the cup.
I poured the liquid into the bag of sand and rolled the sand, sodium silicate, and bag between my hands until the sand and sodium silicate were well and uniformly mixed.  The mixture felt only slightly damp but would clump together when squeezed. 

Stuffing The Core Box
To make sure the CO2 would penetrate the sand core, I placed 1/4" steel rods into the middle of the core box so that I would have holes through the middle of the sand core.  I spooned a small amount of sand mix into the core box and rammed the sand mix around the sides of the box and the steel rod.
More sand mix, more ramming until the core box was filled to the top.  I struck the sand mix level with the top of the core box and lightly patted the sand mix so that it was firmly compacted across the top.  With a twist, I removed the steel rods from the middle of the sand core leaving nice 1/4" holes through the middle of the sand core.

Using The CO2 Gas
I wouldn't have believed it if I hadn't seen it in person but....  it only takes a few seconds of CO2 gas to turn the loose sand into a hard sand block!!  The secret is in how the CO2 is applied to the sodium-silicate-sand mix.

On the right-side of the photo above, you can see a block of wood with a couple of holes in it.  And, in the photo below, you can see a plastic container with a hole in the top.  Using my blow gun attached to the CO2 cylinder, and pressing down on the top of the plastic container, I slowly gave a 2-second shot of CO2 into the container.  This immediately hardened the surface of the sand core.
Removing the plastic container, I then placed the wooden block on the top of the core box, aligning the hole in the wooden block with the hole(s) in the sand core.  I slowly gave each hole a 2-second shot of CO2.

I then undid the screws of the core box.  Voila, the sand core easily separated from the sides of the wooden core box.  In less than 20 seconds from the time of applying the CO2 to starting to undo the screws, I had a solid sand core!  Whoodathunkit, eh!!??
I then repeated the CO2 process for my second sand core.  The solidified sand core easily slid from the core box.

Ratio of Sodium Silicate To Sand (By Weight!!) Is Very Important!!
The whole secret in using sodium silicate is in the sand mix and the application of the CO2.

In my first try at using sodium silicate, I was short about 3 tablespoons of sand mix.  I hastily mixed up a small batch that had about 30% sodium silicate.  Bad news!!  It wouldn't hold its strength even when repeatedly exposed to the CO2.  The sand core was still as soft as when I had rammed it into the core box.  I presume the extra liquid prevented the CO2 from penetrating the sand core.  So, whether you use a 6% ratio or a 10% ratio, the relative ratio (by weight) of sodium silicate to sand is very important.  It doesn't take a lot of sodium silicate. 

How You Apply The CO2 Is Very Important!!
While CO2 is heavier than air, my first attempts at using sodium silicate weren't that good.  I placed the wet sand cores into a plastic bag and applied the CO2.  On my trip to Alumaloy Castings, I saw how they applied the CO2 to the sand - a small cup-like device attached to their CO2 hose, and a piece of wood with a hole in it held on top of the sand core and aligned with the holes in the sand core.  A couple of 2-3 second shots of CO2 and the sand core was as solid as a rock.

It was obvious I needed to apply the CO2 in a more "aggressive" fashion.  So I modified my process using a 1-litre plastic container with a hole drilled in the top.  I was then able to drive the CO2 right into the exposed surfaces of the sand cores.
To get the CO2 into the holes created by the 1/4" steel rods, I simply drilled a couple of holes into a piece of 1/4" plywood so that I could drive the CO2 right through the middle of the sand cores.
Bigger sand cores?  Simply use a bigger plastic container.  These sand cores were almost instantly as hard as a brick with very smooth surfaces and sharp edges. 

In any event, I'm very pleased with the results.  Now we go into full-scale production with the sand cores using sodium silicate and CO2.