We just solved an interesting mounting challenge.
A customer presented us with a Kuzma 4Point-9 mount for a Luxman PD-171A turntable.
Well, ferreting out mounting details is a strength of ours, but this one really pushed us to the limit.
Geek alert … this post is overflowing with numbers, diagrams and techno-speak. If high-school trigonometry sent you into an apoplectic fit, consider yourself warned …
A Vintage Bias
Luxman's Japanese design team has a Japanese, vintage tonearm bias - Jelcos, Ikedas, vintage Ortofons, and such (not that there's anything wrong with that). It appears as if they expected the Luxman market to favor their tonearms of choice as well.
Well, with COVID affecting manufacturing and support organizations, mounting a Kuzma to a Luxman means that you're in for some heavy lifting.
Fortunately, and unlike many turntable manufacturers, Luxman publishes fairly detailed mounting diagrams, but one key dimension isn't easily derived from their drawing.
We could have readily addressed this, if the turntable wasn't 1200 miles from our shop in Colorado. Working remotely however, required some ingenuity.
1st Attempt & COVID Throws a Spanner in the Works
We recreated Luxman's dimensioned drawing in our CAD software, in order to "find" the center point of the armboard. We were missing one key angular dimension however, and I contacted the US distributor.
I find that asking your question in the form of a proposed answer is a productive strategy for answering questions like this.
It shows that you've done your homework, communicates that you know what you're doing, and makes it easier to get a clear and accurate response.
We've had great success with this strategy, and as a result of our efforts, Kuzma now has a minor modification recorded in their database to establish compatibility with the Brinkman Bardo turntable (contact us for your Bardo mounts).
I digress …
Luxman USA is a fantastic organization, but I learned that Luxman Japan has been stressed by COVID. My US contact had little confidence in my question being translated correctly.
I get that. COVID has challenged our production capabilities here at Galibier, with turnaround time from our machinist tripling. I'm seeing long delays from my other suppliers as well.
We both agreed that the safer bet was for me to roll up my sleeves to solve the problem. COVID was not going to defeat this mount
The Key Challenge …
You'll note that the Luxman's armboard is round, and affixed to the base with 6 mounting bolts.
Visualize this: for any tonearm that is not mounted in the exact center of the armboard, rotating the armboard will alter the pivot to spindle distance.
So, the challenge lies in correctly orienting these mounting holes so the Kuzma's bore lies along the mounting axis - the line connecting the record spindle with the center of the armboard. This is the crux of the problem.
Rolling Up Our Sleeves …
I realized that I had to establish this key angle through other means, and I created two work drawings in my CAD software:
- I imported Luxman's drawing to expose its underlying vector coordinates.
- I imported a top view photo into another drawing for the same reason.
This top view photo of the turntable was very close to orthogonal, so there was minimal distortion.
With these two files, I mapped several key dimensions, as well as deriving the angular offset of the armboard (the 12.809 and 12.902 degree angles). The fact that my derivations from these two different sources differed by less than 0.1 degree gave me a high level of confidence.
I had some double-checking to do before authorizing the machining, however.
The harder I work, the more luck I seem to have ...
One thing was working in our favor.
Because the Kuzma's mounting center is only 5.63mm from the center of the armboard, rotational errors would have minimal effect on the desired 212mm pivot to spindle distance.
I was confident that my solution was easily within one degree, so I simulated two extreme error scenarios: a 10 degree counterclockwise error and a 10 degree clockwise error.
You'll note that a 10 degree error in either direction resulted in only a .09mm pivot to spindle change (that's .0035"). So, even an error so much as 1 degree would result in an "error" that's one-tenth of that (.00035 - less than four ten-thousandths of an inch).
Four ten-thousandths is a smaller number than the tolerance we specify for our armboards (.001") and I now had the confidence to pull the trigger on the machine work.
Not bad for a day's work (well .. a lot more than one day's work)!
Success!
The customer got back to me with the above photo and commented that he had a spot on Baerwaald setting for his Lyra Etna, using his trusty Feickert protractor.
This one goes in the record books.
If you read this far, then this challenging mount will capture your attention as well.