Turntable Final Assembly

Now that all the main components of the turntable are done, let's put it together!  I had a few hangups in assembly because, well, my blueprints were a bunch of scrap pieces of paper in a folder collected over the course of 3-4 years, and stuff didn't line up.  Anyways, in no particular order, here it is.

First, I needed the driving belt.  I bought two of these because I was originally planning to make two turntables (though after realizing how much hard work it was, I'm settling for one).  This was a standard 3/8" V-belt and, as noted on the label, they are 38" long.  These belts were actually too wide for the pulley that came stocked on the motors, but I couldn't find anything smaller.

Next, the plug outlet box.  As much as I love my Technics 1200s, the one feature they don't have is removable RCA and power cables.  This was one feature I missed about my first cheap set of Stanton turntables, so I decided to replicate it.  This box was to be made from some aluminum sheet metal.  The piece was actually left over from my old bike basket mount, which explains the unnecessary preexisting holes.  I drilled new holes for the connectors, mounting holes, and rivets prior to bending.

Turntable Headshell Connector

One of the biggest challenges in this turntable build was attempting to recreate the headshell connector.  This part allows for quick and easy interchangeability of needle cartridges between turntables.  Otherwise, I would have to unscrew and disconnect the cartridge cables every time I wanted to remove it.  Wanting this turntable to be compatible with my other ones, which accept standard headshells, I figured I had no choice but to make this connector.

The challenge here was the amount of small and complex parts needing to be made; a threaded locking mechanism, a slotted body, and insulated piece to house four spring-loaded brass contact pins, and the pins themselves.  Then on top of this, figuring out how it will connect to the tonearm and wiring it.

My version did a fair job replicating it, but it took a lot of hard work, several failed attempts and a few design flaws.  Part of the reason for my shortcomings was the fact that I didn't want to disassemble my Technics to see how it worked.  I just did the best I could from the measurements I could get on the outside.

Here's a diagram of my headshell connector.  The biggest difference between mine and the Technics is a snap ring holding on the locknut.  This is because my tonearm is a larger, and having to match this larger diameter on the connector, I couldn't slide the locknut on through the back.  On the Technics, the locknut is secured by a shoulder on front of the connector body.

Turntable Tonearm Linkage

Now that the turntable base and platter have been completed, the next step was to machine the tonearm components.  This assembly consisted of several parts which, though I tried to simplify it, ended up being one of the most complex and time consuming parts of the project.

I decided early on that I wasn't going to attempt recreating this gimbal mechanism the Technics has.  My version also omits several features for simplicity.  Namely, the anti-skating function and tonearm lift.  I was originally going to make the clip for securing the tone arm, but never got around to it.  I did however include a height adjustment for leveling the tonearm with the platter, but a more crude version.  I also made mine with a straight tonearm instead of the S-shaped.

My version:

Here's a diagram of the tonearm linkage and how it connects into the turntable.  Note, I ended up not using the locknut in the final build (above photo) because the tonearm would've been too high.

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

Machining The Turntable Base Casting

Now that the turntable base has been cast, it's time to machine the features to make it functional.  I started by setting it up on the milling machine upside down with spacer blocks underneath, and clamping through the middle.  In this setup I faced the bottom contour.

Here is a closeup of the finish, now the base will sit flat for the next setups.

Casting The Turntable Base

As much work as building the pattern for the turntable base was, this was only half the battle when it came to making it.  Casting the actual base posed a whole different set of challenges.  The first of which was the need to construct a flask big enough to make the sand mold.  Standard turntable size is bigger than anything I'd casted, about 14.5"x18", so the flask had to be at least this big with extra room for sand all around.  I decided to fabricate this monstrosity from scrap pieces of steel plate.  I bought a large piece of rectangular tubing and cut it up to create the rounded corners.  For the parting surfaces, I originally welded a strip of 1/4" steel on the edges, but due to problems of the steel deflecting and not aligning, I resorted to screwing small pieces of angle iron onto the strips to form a good flat edge.  On top of all this trouble, the flask ended up being really really heavy.  I was originally going to weld ribs all around the outside perimeter to prevent the walls from bulging out when ramming in the sand, but I either ran out of material, or figured it was already way too heavy.  Even with no sand inside, it almost needs two people to lift it.

Here is the pattern in the flask ready for molding.  Ideally, there would be a bigger gap between the flask and pattern walls so the sand would have more support, but I wasn't interested in making this thing even heavier.

Building The Turntable Base Pattern

I made the turntable base from cast aluminum.  Unlike the Technics 1200 which has a die cast body, I sand casted mine.  This is because die casting is suited for mass production and requires a permanent mold, which is hard work (if you don't believe me, see the soap dish mold project).  Actual die casting also requires really expensive equipment to hydraulically clamp the mold shut and inject the liquid metal under very high pressure. Clearly I wasn't going to go through all that trouble and expense just to make one or two turntables, thus sand casting was the natural choice.  In order to cast it like this, I had to construct a pattern for making the sand mold.

First let's discuss the design of the turntable base.  I chose this weird half-rectangular, half-circular shape because it seemed easier to mold than if it were a rectangle.  It also required less material.  I was determined to cast the base in one piece, and without the use of any sand cores (because for some reason, my foundry technology level hasn't reached that point yet).  The base also had to have this big hole in the middle for a drive belt to go through.  I chose belt drive since I didn't feel confident enough in my electrical skills to attempt a brushless direct drive motor like the Technics.

Now let's get into the pattern.  I started this project years ago.  You may remember a video of me cutting a 16" diameter piece of wood on my lathe, well this is what it was for.