Saturday, April 10, 2021

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.

This, of course, forms the big radius on the front of the turntable.  Similarly, I machined the smaller corner radii on the lathe.  Though this was extremely easier.

I began construction of the pattern by cutting a template plate (basically, the outline of the turntable milled into plywood about 1/4" deep) on the CNC router. This would help align the various pieces that make up the pattern.

With the round parts of the contour completed, I filled the remaining gaps with flat pieces.  I screwed blocks onto the template plate to make aligning them easier.  This pattern has a 3° draft, so all the side pieces had to be cut at a slight angle.  In fact, one of the biggest reasons I cut the 16" diameter piece in the lathe was to add this taper angle, which actually switches directions halfway inside because of the way the parting line goes through the platform supporting the platter (I later realized I could've just done this with a belt sander, but that wouldn't have been as fun or "precise").

I added this notch to the big round piece so the top plate could screw on for extra support.

Here with the top plate.  Notice the overlap.

I then cut out this piece which would become the platter support platform.

Here the platform has been inserted, and I'd begun screwing and gluing the pattern pieces together.  I screwed spacer blocks underneath the platform to keep it from moving.  I also drilled 1" holes where the mounting bosses would be attached.

I drilled dowel holes through the side of the big round piece to support the platform.  I didn't have any actual dowel pins, so I just used some wood (bamboo?) chopsticks I found in the trash.  The platform needed as much support as possible so it wouldn't collapse when ramming the sand mold.

Here are those mounting bosses previously mentioned. These too where machined on the lathe with the same 3° draft angle.  The large one was intended for the platter bearing, whilst the two smaller ones for the tonearm mounting boss.

 Here most all the screwing and gluing has been completed.

At this point I could remove the pattern from the template plate.

For added reinforcement, I routed a notch in the bottom where I could connect the pieces with blocks screwed on.

Here with the blocks keyed in.

Lastly, the meticulous tasks of adding the fillet radii, filling in all the gaps, and sanding everything smooth.  For the fillets, I used the old trick (or so I'd been told) of sawdust mixed with glue.  This didn't exactly work the best, since it left a lot of voids after drying, to which I kept adding more and more layers.  In retrospect, I might've been better off just using hot glue.  In school foundry lab, we melted wax to make the fillets with which looked and felt a lot smoother by comparison.

Here's how it looked after drying.  Eww!

After a long time, I did manage to get the pattern smooth and fill in all the gaps.

Last step was to paint it, any color, I didn't care.  This was less cosmetic, and more to prevent sand from sticking when making the mold.

Alright, the pattern is done and ready for the foundry! or so I thought.....

Next step - Casting The Turntable Base

Return to Turntable Project Directory

No comments:

Post a Comment

Hi folks, please only leave comments relative to the blog post. All spam will be removed and spammers will be blocked.

Note: Only a member of this blog may post a comment.