Arm geometry and mounting distance

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.

Unless you are dealing with those arms that don't have a slotted headshell, you usually have enough adjustment range so that you can attain tangency at the two null points, hence, you can get the correct geometry. So, even if the arm board is not drilled absolutely right, you can compensate by moving the cartridge in the headshell to slightly alter the pivot to stylus length.

If you can achieve tangency at the two null points on your protractor, you have achieved the geometry for which that protractor is designed. The problem with most two point protractors that have grid lines around the two null points has to do with ease of use. One has to rotate the platter each time one moves from one point to the other and find the precise rotation to get the needle to sit on the alignment point. Even being slightly off is NOT good. Most of these types of protractors don't seem to precisely define this point, and don't offer the mirrored surface that allows for precise alignment of the cantilever and your eye's sight line to do the job properly.

The Mint and Wally protractors not only make the alignment of the cantilever and your sight line easy to achieve, it also allows one to simply pivot the arm back and forth between the two null points without having to move the platter. 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.

The requirement for using this type of protractor is that the spindle to pivot distance must be accurately set because the arc inscribed in the protractor is for one precise spindle to pivot dimension. If this is achieved, once the cartridge is adjusted in the headshell for the correct overhang, the stylus will precisely follow the arc inscribed on the protractor. The null points then allow one to twist the cartridge for proper tangency at the null points. If the spindle to pivot distance is off, one may not precisely follow the arc. However, one can still achieve correct geometry by using the null points just as one would with the conventional two point protractors. You may have to slightly move the platter when moving between null points and you may also have a slightly harder time "guessing" whether you have to move the cartridge slightly forward or backwards to achieve proper overhang, but you still can get proper tangency at the two null points (just with MUCH less convenience).

The convenience factor of the Mint/Wally type protractors (or the more universal Feikert), should not be discounted. It is hard to achieve precise alignment. The more randon trial and error approach of conventional two point protractors makes it easy to succumb to a "close enough" judgment.

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.

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.

Dear Henry, you are an architect.
Let me try to illustrate the point in a language which is familiar to both of us.
As long as there is any overhang still possible with the tonearm you mount (even if the mounting distance is off according to the manufacturers specs) AND as long as you can get 2 zero point on the arc crossing the tangential line towards the spindle within the grooved area of a given record - as long as this is still the case and the mounting area of the tonearm allows for alignment of offset - you can adjust the tonearm.
It has a different effective length now, a different overhang and a different location of the 2 zero errors and most likely different (higher) maximum tangential errors.
The mounting distance does determine the geometry of a given tonearm following its designers intentions and calculations.
Thats all.
You an alter this geometry by changing the mounting distance and thereby altering all other parameters too.
Most likely it will not be a change to the better.
Cheers,
D.
It has a "new" geometry now and this geometry is most likely not perfect nor close to that.