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Arm geometry and mounting distance

OK I know there have been dozens of posts on this subject but a few things are still a little unclear to me?
There is a clearly defined MOUNTING DISTANCE for every arm which dictates the pivot to spindle centre dimension.
There is a clearly defined EFFECTIVE LENGTH for every arm which dictates the stylus to pivot point dimension and thus the OVERHANG.

If we can accurately set the mounting distance correctly to the nearest +/- 0.2mm and then the overhang to +/-0.1mm, surely this must be as accurate as we can get to achieving the designed Baerwald or Lofgren geometry as long as the cartridge is aligned tangentially at the two relative 'null' points?

The problem actuality I believe, is achieving this degree of accuracy with the MOUNTING DISTANCE?

If your turntable was predrilled for the arm and mounted in the factory, you would imagine that the tolerances could be close to those specified but if a dealer drilled and mounted your arm or you yourself did so, I doubt that it would come within coo-ee of those sorts of tolerances?
Unless you have a machined metal template such as the Feickert Jig Feickert Jig to 'lock-in' the spindle centre, together with a machined and calibrated 'beam' to accurately span the distance to the arm pivot centre, it would be a fluke to achieve anything like the accuracy required.

Now I find little mention of the Feickert jig in all the discussions on tonearm geometry yet I find many references to the 'Wallytractor' (which I have), and also the 'Mint Arc Protractor'.

My question is this:-
If your tonearm MOUNTING DISTANCE is out by 5mm (1/4"), can you accurately align the arc using the Wally or Mint and thus all will be well?

I know that when I use the Feickert Jig and then run it over the WallyTractor I achieve perfect alignment whilst if I try to use the Wally first, it's impossible to achieve perfection?
halcro

Showing 1 response by dougdeacon

dougdeacon
3,338 posts
 
My question is this:-
If your tonearm MOUNTING DISTANCE is out by 5mm (1/4"), can you accurately align the arc using the Wally or Mint and thus all will be well?
No. Two independent factors must be correct for a stylus to trace the arc on such a protractor:

1. Placing an arc protractor on a TT spindle positions the (invisible) center point of the protractor's arc at one precise distance from the spindle. Spin the protractor where you will, this dimension can never change.

The tonearm must be mounted with its pivot point at exactly the same distance. Otherwise you'd be trying to trace the SAME arc using DIFFERENT center points, which is obviously impossible.

2. Once you've achieved #1, the stylus must trace an arc of the same radius as the arc on the protractor. This is easiest to adjust if your headshell has slots.

***
As to fine-tuning that tonearm mounting distance, several factors can help and a Feickert-like device may or may not be necessary:

1. A pivoting armboard simplifies gross adjustment. In such cases the armboard drilling need not be perfect.

2. Holes in the tonearm mounting plate which are a little larger than the diameter of the mounting screws allow fine adjustments. Again, drilling perfection isn't required.

3. The mounting distance headshell jig supplied with a Graham should be as accurate as a Feickert. A TriPlanar's pivot point is easy to see and measure. I can get as close as a Feickert by laying a ruler across the top of my TT spindle and pivot. It sits perfectly level while I tweak the position before tightening the mounting screws.

As usual, there are several ways of skinning this cat. The Feickert is certainly a good one, but it's not the only. I've not needed it with my tonearms but on others I might.
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