Next Steps

I spray painted the lid lifting lever assembly a blue color that almost matches the blue of the mobile base.

Tomorrow I want to mount the lever assembly to the bottom of the furnace body.  I do not want to have to disassemble the whole furnace to do this.  

I am planning to take the lid off and lay a square piece of plywood over the top of the heating chamber.  I will tightly strap the plywood around the entire furnace body with ratchet straps.  Then I will lift the furnace, turn it upside-down, and set it on the work table.  

Now the bottom of the furnace body is accessible.  I will drill holes into the bottom and install four, 3/8-inch rivnuts to drive the lifting lever mounting screws into.

With the lever assembly attached, I will turn the furnace back over and set it back on the floor to test how much lift I get from the lever.

If there is not enough vertical travel, I may have to cut a small triangular segment out of the lever and bend the short end down a few degrees.  We will see soon.

Another task that I need to complete soon - I will have to use the trim router and cut some slots in the ceiling brick for the angle support rivnuts to rest in.  This will create some dust, but the task should only take a few minutes.

Slowly, but surely, I am making progress on completing this electric resistance shop furnace.

Welding and Grinding

This morning I got the welder and welding table out.  I clamped the lid lift lever to the table and attached the grounding cable to it.  I welded the angle iron to the 1/2-inch tube.  Next I welded the square piece of stock to the short end and I welded the rectangular piece of stock, which will be the foot pedal, to the long end.

My welds look atrocious, but I got the angle grinder out and smoothed them out and cleaned up the spatter.  Next I put the rotating wire brush into the drill and brushed all of the mill scale off of the stock and cleaned it all up with acetone on a rag.

I took the assembly outside onto the grass and laid it out on newspaper.  I sprayed a good primer coat on all surfaces and left it to dry.

Back inside I cleaned up all of the swarf from the floor and began working on the adjustment bolt that will screw into the end cap of the inside pipe.  I marked the center and drilled a pilot hole through the cap.

Next I inserted the 29/64-inch tap drill into the drill press, oiled everything well, and drilled the hole through.

Then I took the cap and vise off of the drill press and set it on the table.  I put the 1/2-inch NF tap into a tap handle and slowly, carefully, cut the threads for the adjustment bolt.

With that completed I put the cap onto the pipe using the bench vise and a pipe wrench.  Then I cleaned the outside surfaces of the inside pipe and the outside pipe with an acetone soaked rag.

Weekend Lid Work

If you look closely at this picture you will see that I got a lot done this weekend.

First I made clips to hold the brickwork in the lid.  The first iteration of this idea involved cutting tabs on one side of perforated angle and attempting to bend it into a circle to fit the lid.  This did not work out.  I had to discard the material and start over with a new piece of perforated angle.

What I ended up doing was cutting the angle into pieces 4 inches long.  Then I drilled out the circular holes on the ends so that a 5/16-inch rivnut could be inserted and crimped.

These are a little longer than ideal and I will have to use the trim router to cut slots on the perimeter of the brick to receive the rivnuts.

Next I cut steel angle and drilled holes for 3/8-inch mounting bolts and a hole through the other flange for a 1/2-inch pivot bolt.  The angle and the steel tube will be welded together.  A piece of 1/2-inch steel tube is fit over the pivot bolt.  The plan is to mount the two angles to the bottom of the steel drum with 3/8-inch rivnuts and 3/8-inch, 1-inch-long bolts.

I cut a piece of 1x1 angle and drilled a hole just large enough for the 1/2-inch steel tube to pass through.  This will be the lid lifting operating lever.  I cut pieces of 3-inch wide, 1/8-inch plate to fashion a foot pedal and a resistance plate.  These will be welded to the operating lever.  The effort arm is about 21 inches long and the resistance arm is about 6 inches long.  So, the mechanical advantage is about 3.

Right now the travel at the end of the resistance arm is about 1-1/4 inch, and I am not sure if that is enough lift to give the lid clearance, but I am planning on bending the resistance arm down about 5° and that should increase the lift.

Brickwork

The furnace lid is going to be a composite construction using firebrick, ceramic fiber insulation board, and ceramic wool insulation.

I began by cementing firebrick together with high temperature mortar to make a hard ceiling for the heating chamber.  

Once the mortar was dry and the assembly could be moved around without clamps on it, I drilled two parallel 3/8-inch diameter holes all the way through the brick and I counter bored the ends 1-inch diameter about 1-1/2 inches deep.

I drew an 18-inch diameter circle on the back of the brick assembly and began cutting the excess firebrick away with a regular wood saw.  With most of the excess brick gone, the ceiling was brought to a circular shape using a rasp plane.

Into the 3/8-inch through holes I inserted 1/4-20 UNC All-Thread rod.  I put 1-inch washers on each end and tightened the assembly with nuts.  These reinforcing rods are there to hold the brickwork up if the mortar joints fail in the future.

Finally I patched up gaps in the mortar joints and filled in voids on the brick face with more of the 3000° mortar.  Next I will smooth the inside-facing side of the brick with sandpaper and apply a coat of ITC100 to help the hot face reflect infrared.

I will probably work on the lid lifting mechanism over the next few days.  This part of the project has unique challenges - geometry to work out, some parts to be welded, and finding the best way to attach the mechanism to the steel furnace shell.  

Projects like this one are good problem-solving exercises.

Lid Lifting Mechanism Plan

1" pipe inside 1-1/4" pipe

To assist with lifting the lid up off of the furnace so that the operator (usually me) has access to the interior for charging material, inspecting the charge, or removing the crucible full of melted metal, a mechanism with a foot pedal will lift the lid about an inch, then the lid will swivel forward or backward out of the way.

A few other furnace designs have used parallelogram systems, cam lifters, and hinges.  The idea that most builders seem to use consists of a pipe-inside-of-a-pipe mechanism.  The inside pipe is lifted by levers operated by a foot pedal, and it moves up and down.  The lower outside pipe is attached to the furnace shell and simply guides and supports in the inside pipe.  The upper outside pipe is attached to the lid and it is lifted by the inside pipe and it is free to rotate on the inside pipe.

 

I purchased 1-1/4" pipe for the outside pipe.  I got a 1-1/4" pipe cap for the top.  The inside pipe is 1" steel pipe and I got a 1" pipe cap for the bottom.  A hole will be drilled and tapped through the bottom pipe cap to hold the adjusting screw.

An example of the lift device.

Because of where the thermocouple box is located under the furnace, the lifter pedal will have to be behind the control box and the pipe-in-a-pipe mechanism will be on the opposite side of the furnace.  This will work acceptably.  I wanted to be sure that when the lid swivels away that it does not come to rest above the control box.  The brick in the lid will be radiating a lot of heat.

The foot pedal will be welded to some 1/2"x1/2" steel square tube.  This will be welded to a short pipe that has a bolt through it to pivot on.  From this pipe two more parallel levers are welded at a 5° angle to a steel pad that will lift the adjusting bolt and the inside pipe.  The pivot bolt will pass through two blocks of steel that will either be screwed to the mobile base, or into the bottom of the furnace shell itself.

One last thing that has to be taken care of - a limit switch will be installed so that when the lid is lifted and swiveled out of position, power will be cut to the heating elements for safety.  I have to decide where to mount the switch and how it will be actuated.  To wire this, I will probably have to disassemble the whole furnace and pull the wire to and from the switch.

Start Lid Construction

Yesterday I began construction of the furnace lid.  The lid is going to be assembled inside the top part of the steel drum.  The heaviest part of the lid will be the refractory firebrick in the furnace ceiling.

I started by selecting 8, 2600 °C refractory firebrick and assembling them on the table.  Two of the brick were cut in half to fill in the corners.

The faces of the brick that have mortar joints were painted with mortar thinned in water.  This was done to reduce the amount of mortar and water absorbed by the bricks.

I mortared the brick joints and clamped bricks together to ensure good close contact and thin tight joints.  I made the lid in two halves and then joined them together into one assembly this morning.

I am going to drill two 3/8-inch holes horizontally, all the way through the brick and 1" counterbore these holes about 1.5" deep to hold all-thread rod, 1" washers, and nuts.  These will provide additional compression and support for the lid in the event that the high temperature mortar fails.

Tomorrow I will draw the 18" diameter circle on the bricks and begin sawing and rasping it into a perfect circle.

The brick circle will rest on ceramic fiberboard, and this will be held up by perforated angle iron attached to the furnace lid shell by 5/16" rivnuts and screws.

Behind the brick will be another circle of ceramic fiberboard, several inches of ceramic insulating wool, and another circle of ceramic fiberboard at the top of the lid.

The lid is an important part of the furnace.  The lid holds heat inside the furnace and helps the furnace heat up quickly.  Once the lid is finished, I can begin testing again.  I will test to discover the heat-up rate and cool-down rate.  I will discover how hot the outside surfaces become after hours of operation.

Insulation

Using a utility knife, I cut ceramic wool insulation for the furnace.  I purchased a roll of 2300 °F insulating blanket from amazon.com for about $100.  

 

I cut a length off, cut that in half width-wise, and folded it in half.  I cut this into segments about 16 inches long, and carefully wedged these into the space between the firebrick heating chamber and the steel furnace shell.

This came about half way up, so I cut another length off and repeated this process for the upper half of the annular space.  I had to work the wool around all of the screws that protrude into the space.  I packed the wool down so that it came level with the bottom edge of the raised circular area at the top of the firebrick.

To keep the wool sealed inside the furnace shell, I cut a circular piece of refractory fiber insulation board and in that I cut out a circular hole that matches the raised circular area on the top of the firebrick.  I carefully pressed this into place between the firebrick heating chamber and the steel furnace shell.

If you do this yourself, please work safely and wear gloves and respiratory protection.  I wore a filter mask and nitrile gloves while cutting and handling this material.  The ceramic wool is almost like working with fiberglass insulation - it makes your skin itch.  When handled small particles break off and if airborne these can get deep into the lungs and cause damage.  

The next step will be working on the furnace lid/cover.  I have selected some 2600 °F refractory firebrick for the lid, and I will use the leftover ceramic insulating blanket in the lid.  A well-insulated lid is crucial to furnace performance and economical operation.

Troubleshooting and Success

This morning I went down to the garage and pulled the front off of the control box to begin troubleshooting the problems from yesterday's testing.

The first thing I changed was to move the leads that the Elements On lamp connects to.  These were connected to the Power-In terminal on one SSR and to the Power-Out terminal on the other SSR.  This is why it was lit continuously.  I moved the leads to the same terminals that the heating elements connect to.  Now the lamp should only light when the heating elements have power.

Next, I had to figure out why the heating elements were not getting power.  I looked at the screw terminal diagram on the side of the PID controller and I noticed that there was an empty screw terminal between the two lines to the SSR.  I unscrewed one of the connectors and moved it over one screw and tightened it back up.

I plugged the controls in and turned it on and . . . . NOTHING.  The elements were still not heating.  This time I opened the control box and left the front panel hanging while I started it up.  The SSRs have a small red LED that indicates when they are energized.  I wanted to see if they were even trying to close.

Then I noticed that the control (or input) side of the SSRs are marked (+) positive and (-) negative.  The control side of the SSR is a DC circuit.  I wondered if the screw terminals on the PID controller had polarity too.  Sure enough they do!

I unscrewed both SSR control terminals and switched their positions.  I tightened these down again and closed the works up.  

This time everything worked as intended!

I entered a setpoint of 300 °C and let it begin heating up with two bricks over the top of the heating chamber.  The temperature steadily rose and as it approached 300 ° the heating elements were turned off, but there was an overshoot of 68 °C.  Then the temperature slowly came back down to 300 °C where the controller maintained it within 1° or 2°.

Next I loaded a crucible with clean aluminum scrap and set it in the heating chamber.  I changed the setpoint to 700 °C and covered the heating chamber up.  It took about an hour, and this time the temperature rose up to 700 °C and it did not overshoot.  The aluminum that I charged into the crucible melted!  I scraped off the dross and poured two small ingots from my first melt.

While this test was running, using the infrared non-contact thermometer, I checked the shell temperature, the heating chamber firebrick temps, control box temperatures (especially the temps at the heat sinks on the back), and everything was about what was expected.  The firebricks covering the heating chamber were about 250 °C, the back of the heating chamber bricks were about 160 °C, the furnace shell was about 36 °C to 40 °C, and the heat sinks on the back of the control box did not rise above 40 °C.

I expect that the shell temperature will come down once I have installed the ceramic wool insulation.  This will cut down on convection and radiation from the heating chamber to the steel shell.

Tomorrow I will begin working on the lid assembly.

Wiring

Today, using the step drill, I bored a hole in the furnace shell and attached a 90° elbow to the back. Then I pulled the wires through that.  Next, I attached aluminum armored conduit to the control box and threaded a piece of wire through it.  I taped the line wires and the thermocouple wires to it and pulled all wires back through the conduit and out of the front of the control box.

I took the control box off of the furnace shell and drilled a hole in the back of the control box for the electric power cord and its strain relief.  When I put the control box back, I inserted some drilled out plastic poker chips on the mounting screws under the screw head and washer.  These are intended to reduce thermal bridging and keep the control box cool.  You can see these in the photo of the control box internals.

Finally, while sitting down on a milk crate, I made all of the connections inside the control box.  This was harder than you would think.  If I had to do this again, I would make the wires that connect components on the front of the control panel to components in the back of the box much longer.

I was afraid of having a lot of extra wire inside the control box.  As you can see from the picture, I still have a lot of extra wire inside the control box.

After closing the front of the box up, I plugged in the power cord and switched the power on.  The PID controller came up and the Elements On lamp was lit continuously.  However the elements were not heating.  Using a torch, I checked to be sure that the thermocouple was working properly, and it was.  So, it was a partial success.

Since I was tired at the end of the day, I decided to wait until morning to begin the troubleshooting.