Casting an Antique Key

A friend recently asked me to cast an antique-style "white metal" key that he broke. It goes to a secretary desk and the desk is important to him.

It is a tiny casting and that will present new challenges for me as I am used to casting parts weighing 1000 times as much as the key. And that is the reason I agreed to do it. Finding a replacement online for less than a dollar is probably quite possible. I sensed though, that it was important to him to go the casting route.

My general plan is to use the glue-repaired key as a pattern that I will cast in iron. I may also end up casting it in bronze or brass, but will first try iron. I have 3-D printed tiny flasks and a follow board mold as well.

Here is the glued key. It had be fractured at the center groove in the shank of the key. I glued it at that break using cyano glue.


Here is the key fixtured for gluing with Plastolina modeling clay. To get visually acceptable results it was necessary to align the two pieces along the long axis and to have the "fin" of the key in the same plane as the finger loop end of the key. I thought about various ways to fixture this somewhat irregular part before tumbling to the modeling clay. Using the clay, it was pretty easy to get things properly oriented by eye. Once aligned I just applied a drop of medium viscosity cyano glue at the joint. It is holding well enough to act as a pattern anyway.


End view to make sure the fin and loop are clocked correctly


Once the pattern was together, I needed to make the follow board. I plan to use epoxy-bound sand as it holds detail very well and is easy to compress into a mold, and makes a very strong mold.

Here is the key half embedded into RTV-2.

End view of same.


The shallow box is of PETG coated with Inhibit-X. The dimensions of the box match that of two flask halves
and has registration holes matching those in the flask set.

When I pulled the key out of the cured silicone I was surprised and for a moment puzzled to see that there was a soupy defect in the silicone right where the break had been. Of course! The cyano glue inhibited cure where it was holding the two pieces together. I should have thought of that. But, as is typical, I was so focused on aligning the pieces and designing the boxes and the process of molding, that I overlooked that important detail.

Here is what the silicone looked like when I peeled it out of the box. Note the hole in it at the location of the glued joint. Time to do it again. That little piece of square toothpick was used as a prop under the loop prior to pouring the silicone. It prevented the key from rolling away from a horizontal orientation.


So, I decided to just see if Inhibit-X was able to inhibit the inhibition. I painted the key with it and repoured the silicone. If I get the same defect (entirely possible), I'll paint the joint with epoxy as it generally does not interfere with silicone cure.

Looking ahead, I intend to gate the piece on the loop end and hope that a shrink does not occur where the fin is. I am hoping the key will freeze beginning at the fin end and smoothly work its way back to the loop. The loop is thin enough that it should collapse like a plaque does and not form a shrink. Brave words. We shall see whether they work out in actual practice.

Comments/cautions welcome.

Denis

 

White metal alloys like 11R, lead free solder and pewter are high tin content and kind of bendy and soft, would zinc (790 F melting) or zamac of some flavour be better strength-wise?.

 

Kinda funny how little things sometimes take as much time and effort as big ones.

Pete

 

I think it will fill just fine.
Here's a few bottle openers I've gated into the loop.
All filled with no shrink even though the mass was largest at the far end.

Tiny gate by mistake. I did fifty of these.

 

I'm first going to cast these in iron. I think old time keys were iron some of the time. I otherwise might try bronze or brass. The design of the key is pretty scant and the circumferential score lines are less than inspired. They weakened the shaft substantially. It is only .125" thick between the lines.

So true!

That is encouraging and a cool design that your boating buddies loved I am sure. Original with you, Bonz?

THe Inhibit-X did its job on the second trial of RTV-2 casting of the follow board.
The silicone cure just fine at the joint and reproduced every detail faithfully.


Here are the flasks that go with the board. They have 2 degrees of internal draft. So, I think that will allow the epoxy-bound mold to pop out.
I will mark them so that I do not (often at least!) make the dumbass mistake of reversing the draft.


Denis

 

I am very much thinking along the same lines as you with respect to small parts and cooling. But, I also have observed small vent holes I make in larger molds that are only a 60 thou in diameter that fill quite nicely. I will ensure a good head of pressure.

I will fill from the loop end. The nose was my first thought for filling, but I am concerned about possible shrink defects in that cylindrical part. Cylinders have gotten me lots of time in the past because of the low surface/volume ratios. So, I am filling from the loop end where the "flat" surfaces can accordion together hoping the nose will freeze first and then the freeze will continue toward the loop. If ther are problems filling the nose, I intend to extend the mold there to allow hot metal to run through unobstructed---in a sense lengthening the key at the nose and increasing its diameter to say 1/4" I'll snip that off later.

THis is an interesting challenge for me.

Denis

 

I did pack some sand in the flasks today and had pretty good luck.

The drag with follow board packed with 3% 5-minute epoxy + sand.



Follow board removed---like it is coped down all around.


Close up of same.


Key pattern popped out.


Key back in place and graphite applied as parting.


Registration divots carved in the drag.


Cope and drag separated.


Here is a video of opening the flasks:


Now I need to carve in the gate, runner, and pouring basin and sprue. I'll use my Foredom and a burr to do that.
I plan to "bury" the reassembled mold in green sand for support and ease of handling.

Most likely it will be early next week when I try this a a couple other key molds along with some other casting. These will be done with the "extra" iron from a much larger pour. The ridges of sand on the cope side of the mold are extremely delicate. So, I will reassmble the cope and drag while still contained in the plastic flasks and then slip off the flasks.

Denis

 

Today I molded a second key mold similar to the first. I scraped out a gate entering the loop end and drilled a sprue and vent in the cope. I found that spinning a 1/16" bit and a 9/64" (sprue) in my finger worked better than using a drill motor to spin them. The holes were cleaner done by hand though both are OK.

The last thing I want on these delicate molds is for the cope to fret on the drag with handling as that will cause loss of sand at the joint. I want that to be as clean as possible. So, using the registration bumps and corresponding divots, I reassembled cope on drag and then used medium thickness cyano glue to fuse cope to drag along the outer seams. This provided a solid bond between the two and prevents any relative motion. To prevent accidental contamination with sand I also glued paper over the pouring basin and will just leave that in place expecting the iron to burn through it instantly.

On the first mold I glued a wrap of paper over the sprue and on the second I just used a patch of paper. You can see the shiny bead of glue along the edge of the held mold. I put beads on all four sides. And near my thumb you can just see the vent hole.


Denis

 

One way to drill vent holes through resin bound sand is the use a bicycle spoke with one end hammered flat and sharpened into a diamond tip similar to a spade drill for wood. It doesn't need to be too sharp to break off the sand grains and it'll drill extended distances.


Edit: it can be wobbly to start (in a battery drill) , so you may need to steady it with one hand to begin with.

 

Sounds like a great tip. I'll try it next time I make an epoxy mold---soon. Doing some maintanence on my muller and hoist today.

Denis

 

Today the weather was somewhat cooperative as the rain chances decreased to 10% for a few hours this afternoon.

So I cast the key mlds as well as 54 pounds of other more routine stuff. The two keys weighed a total of 14.5 grams. Both molds filled and the results were pretty good.



To cast those tiny epoxy molds, I buried them in green sand and kept sand out of the molds by keeping the paper, previously installed, in place and also used paste-board discs over the epoxy mold pouring basins to aid in keeping the sand where it belonged.


I just poured the iron on top of the paper and paste board and it must have burned through instantly. There was no evident smoke or sound related to the paper.


Here is the underside of the combined green sand and epoxy pouring basin(s). The pouring basin weighed 4.5 pounds compared to the key weight of 7.5 gms each. So the ratio of pouring basin to casting must be nearly a record for undesirability at 140:1 ! With a little care much smaller pouring basins could have been used. But, I was mostly interested in just brute forcing this rather than trying to finesse it.

Another view of the pouring basins.


So, overall I am really pleased that the keys filled fully. I am going to see how the same process using sodium silicate as a binder works in a future pour. I'd like to get a bit better finish.

Denis

 

They compare favorably with the original key.

 

Here is a vid of the pour. The keys are the smallest (second) flask poured.


Denis

 

I like your little trolley tongs. Did you make that yourself?

 

thank you!

Yes, that is my design and fab. There is a thread on them from a few yrs ago.

http://forums.thehomefoundry.org/index.php?threads/hoist-and-releasing-tong-build.199/

Denis

 

Denis, did the new keys get fettled and do they work in the lock? They turned out nice.

 

I have not really fettled them yet. I am going to try to make a couple more that will be a bit cleaner and will use a slightly different pour method.

But, the lock it goes into is pretty crude. So, I’m pretty confident it will turn it. I imagine a little wiggling of a screwdriver would open this sort of lock.

Denis