Tuesday, April 20, 2021

Casting The Turntable Platter - Sweep Pattern Experiment

The next step for the turntable project was to make the platter.  This was the second of the two cast aluminum components to make, and unlike the Technics which is die cast, this too would be sand casted.  However, this time I chose a different molding method called "sweep casting" which is applicable to radially symmetric parts like this.  This was my first time trying this, and I had relatively favorable results.

The sweep pattern is basically a cross section of the part being casted, which is swept around a central axis to remove pre-rammed sand layer by layer.  Think of it like those center pivot irrigation systems, except instead of watering crops, we're removing sand to create a mold cavity.

This has the advantage of saving time since I didn't have to make an entire wooden replica of the turntable platter.  I began by transposing the final dimensions of the platter, plus shrinkage allowance, into a flat piece of plate.  Pardon the atrocious blueprints

Here is the sweep pattern completed, outline added for clarity.  I made the pivot mechanism from a piece of tubing, and a piece of round stock with a flat milled on it to align the plate with the radial axis.  The portion to the left of the dashed red line is not actually part of the final casting.  This extension merely provides a flat surface to gauge where the pattern is supposed to bottom out on the flask/sand layer.  Also note, because of this, the platter is not going to be as thick as the entire pattern height.  The actual pattern portion, to the right of the dashed line, includes the platter thickness plus the offset from the portion to the left.

Here I am using the pattern to make the first half of the mold.  I started by ramming sand into the empty flask with the piece of tubing sticking through the middle.  I learned this method takes skill and patience, since you can't just take off all of the sand in one pass.

Pouring the metal.  Here utilizing the new two-person pouring shank upgrade and extended crucible.  As I was pouring, I noticed the mold was taking a lot more metal than expected.  I then noticed a gap between the flask halves was leaking a large amount of aluminum on the ground!  I kept pouring anyways in hopes that the mold would actually fill.

I thought this attempt was a total failure until we shook out the casting and it was actually there!  The problem of the excessive flashing was because I accidentally swept the pattern too deep creating a gap on the parting line.  I guess this is what the foundry textbook meant by "sweep patterns require skill".

Surprisingly, this casting was still usable, but it posed a few problems that had to be worked around.  Firstly, the sprue, or pouring channel, didn't fill because of the metal leaking.  I was intending to use this protruding shaft for clamping it in the lathe.  Instead I had to machine a small counterbore and screw on a round piece to clamp from that side.  Secondly, the poor metal filling combined with shrinkage caused the top of the platter to be very thin.  Machining the aforementioned counterbore greatly exaggerated this problem since it made the middle section about 1/8" thin.  The resulting section thinness led to intense unrigidity and chatter whilst machining the outer portions of the platter.

For the first lathe setup, I chucked on the stub on the bottom of the platter.

Here I've machined the counterbore.  If I'd noticed earlier how thin the section was, I would've not gone as deep.

I then machined a round piece of steel to fit into this counterbore.

I mounted the steel piece with 5/16" holes threaded directly into the casting.  I then performed the second lathe setup by chucking on the steel piece.  On this side I started by machining the V belt diameter.  I didn't machine the actual belt groove at this time fearing the chatter result in an overly rough surface which could destroy the belt prematurely.

Next, I turned the round stub in the middle to have a smooth and concentric surface for chucking on.

I then turned the casting around again and bored a 3/4" hole through the middle.  This is where the spindle will mount.

After that, I went to work facing and turning the remaining surfaces smooth.... or at least as smooth as I could get.

Here's a close up of that chatter.

The next order of business was the spindle.  My original plan was to have the spindle mount in the bottom of the platter with a taper, like the Technics does.  I'd machined this version of the spindle years ago, but decided the bearing was too small and wimpy for the type of scratching & turntablism I do.  I also figured since I'm using belt drive, there will be radial force on the platter that could dislodge it from a taper mount.  I designed a new spindle to instead mount with a set screw in the platter, and be supported by two even bigger bearings in the base.

One problem, how am I going to drill and tap a set screw hole in the side of the stub on the bottom of the platter?  Even if I had a drill long enough (probably at least eight inches), I would've had to drill through the side of the V belt groove.  I couldn't use a right-angle hand drill either because it was too close to the bottom surface of the platter.  What I decided to do was mill a slot in the side of the stub, and dowel in a threaded insert machined from steel.  A really idiotic method in my eyes, but it seemed like my only recourse at that point.

Here I have inserted the threaded piece.  I made two of these since they were two halves of the same round piece, and in case I end up making the second turntable.

With this piece inserted, I could finally insert the spindle to chuck on and machine the platter surfaces with the most concentricity.  But first, I had to deal with the problem of the nonridgidity.  I was recommended to just fill in the bottom of the platter with Bondo.  Genius.  This was my first time using Bondo, and being unaware of the fumes this stuff emits, I went ahead and performed this step in my bedroom of all places.  Needless to say, it smelled like a paint factory in there for weeks, it was horrible.  Shown in the photo, I made a little cardboard shield to prevent the Bondo from blocking access to the set screw hole.  This ensured enough clearance to fit a hex wrench in there.

In spite of the terrible smell, the Bondo actually did the job.  I then took it to the lathe to complete the final operations: facing the top surfaces smooth, and completing the V-belt groove.

Next Step - Tonearm Linkage Components

Previous Step - Machining The Turntable Base Casting

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