Friday, April 23, 2021

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.


Since we're talking about the locknut, let's begin with that piece.  I knurled a piece of steel in the lathe, then drilled, bored and tapped it with the same 1"-20 tap I used to tap the turntable base.  This tap required a lot of torque, so I actually cut most of the thread with an I.D. threading tool prior.  (If you're wondering why I'm using, or where I even got a 1"-20 tap, being a nonstandard size, it was from my old workplace when it closed down, and for the longest time it was the largest tap I had, so I've come to use it for applications like this where a large fine thread is desirable.  See also: New Foundry Oil Burner)

Next, I made some grooves where the locknuts would be parted off, and added the outside chamfers prior to parting.

I then took the parted locknuts and chamfered the inner diameter.  I also ran the tap through them again to clean up the threads.


The bushing was made from a similar diameter piece of steel.  I turned and threaded it in the lathe, and drilled and reamed the shaft hole to 3/8" +.001"  This would ensure a smooth rotating fit with the shaft.

I used the locknut as a thread gauge to ensure the threads were cut deep enough.

I then knurled the head of the bushing buy setting it up in a scrap piece I'd also tapped 1"-20.


The shaft was made from a convenient scrap piece that was already precision ground to 3/8" diameter and with a set screw flat on it.  The only thing I needed to do was drill a 3/16" hole through it for the tonearm wires to pass through.

Shaft Collar

Nothing special, just a round piece with the same 3/8" hole reamed in it as the bushing.  I then drilled and tapped a set screw hole in the side (not shown).

Counterweight Stub & Counterweight

The counterweight stub was made from a piece of 1/2"-13 threaded rod with a portion turned down to insert into the rear end of the tonearm, which was a piece of 1/2" tubing.

The counterweight was a knurled piece of 1" round stock with a 1/2"-13 hole threaded in it.  I later realized this wasn't enough weight, so I made a larger one from 1-1/4" stock.  I also drilled and tapped a set screw hole in the side for locking it down at the desired weight setting and prevent it from unscrewing during use.

Tonearm Pivot

Now we're getting to the real action.  The tonearm pivot was made from 1" round stock.  I began by turning the diameter smooth, and facing the first end.  I drilled and tapped a 1/4"-20 hole to accept the 5/16" shoulder bolt.  I also added a nice big corner round prior to parting it off.

I then faced, drilled, tapped, and rounded the second side.  Here I was checking the length dimension which needed to be very accurately 1.25".  I wanted a precision fit between the pivot and swivel to minimize the slop between the two components.

I then took it to the mill and machined three #10-24 set screw holes.  The middle one is for securing the tonearm and counterweight stub.  The outer two are for securing the shoulder bolts.  I know it seems improper to secure a set screw onto the threads of a shoulder bolt, but that's the way I designed it for simplicity.

Next, I rotated it 90° and machined the 1/2" hole for the tonearm.  I used a pilot drill and then plunged it with a 1/2" endmill to achieve a better finish cutting through the cross hole.

Tonearm Swivel

This part was made from a scrap piece of steel. The first step was to open up the middle to create that tuning fork shape. I began by drilling the two corner holes, and then some other holes to aid in removing the material. Funny enough, the scrap piece already had that big hole in the middle. I was glad I finally found a project that I could utilize it for, since that portion was going to be removed.

Next, plunge milling a slot.

After that, I could saw cut the middle chunk out.

Next, I set it up in the mill vertically to finish mill the inside.

I used a good end mill to achieve a smooth precision fit with the tonearm pivot.

Not satisfied with the blocky look, I put it in the vise on an angle to mill a taper on the outside.  This step was merely cosmetic.

Next, I drilled and reamed the 5/16" shoulder bolt holes.

Lastly, I machined the shaft hole, reaming it for 3/8".

Test fitting the shaft.

Tapping the set screw hole for the shaft.

The tonearm itself will be a separate post due to the complexity of the headshell connector.

Next Step - Tonearm and Headshell Connector

Previous Step - Turntable Platter

Return to turntable project directory

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