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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?
128x128halcro

Showing 2 responses by axelwahl

axelwahl
788 posts
 
D. wow!!
Now we have to give 10/10 for your concise and non-dogmatic answer, this is sharp, I like it. I also think it'd very hard to argue your point at all like this :-)

I think your answer covered each angle, including the possible trouble(s) caused by variable mounting distance challenged arms...

Since Henry doesn't have any SME's to contend with, I'd agree to get that mounting distance to as close as close can be.
I know with oblong holes you can 'twist' the cart (zenith) to get, even with an out of tolerance distance, to two null-points with tangential 0 error.
But if the arm designer had a best "OFFSET-ANGLE" for his arm in mind, you are best served to get the mounting distance A1 OK.
A.
axelwahl
788 posts
 
Larryi,
y.s.:
>>> That is because the null points are placed along an arc corresponding to the precise pivot to stylus distance specified for your arm and the ***precise distance between the pivot and the spindle***.<<<

That's the ***'kicker'***! If your spindle to pivot is OUT, that arc is NOT the same arc either, yes?

I have done alignments that way by e.g. using a Linn protractor for a Pro-Ject 9c arm ---- because it was the same OVERHANG! Yeah, but not the same spindle to pivot distance! If that is the case always one thing does not match --- and that is why I (can you believe it?!) agree completely with Dertonarm on this point.
Also, I should think fudging with the off-set angle (as will be the result)and not according to intended arm design is not recommended in my book :(
A.
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