Why use a super accurate cartridge protractor

That's easy with a mirrored protractor like the MintLp.
There are (thicker) perpendicular lines on the arc at the two null points.

Using a magnifier - and with the stylus resting precisely on the marked null point - align so the cantilever is parallel in-between the two guidlines when the latter are aligned with their mirrored reflection (thus accounting for parallax error).

Hi Tonywinsc

As Tobes says, you would have to use a mirrored protractor for the reasons he states.

For the SME 309, it should be LofgrenA/Baerwald DIN.

But importantly, it should be a TWO point protractor, not an arc, as you will have to make adjustments at the arm base rather than the headshell.

As an aside, this actually is interesting because it raises the possibility of using the SME base for measuring the variation in cantilever/cartridge body alignment. as the alteration in overhang/mounting distance for a given effective length means a specific change in offset angle. It also offers a practical way to measure typical accuracies by comparing the difference in base position for repeated setups by different users without altering the cartridge position.

The accuracy that the stylus can be positioned independently on an arc can be measured, and the variation from that position in the alignments can give a number for the offset accuracy...

Anyway, back to work...

08-23-11: John_gordon
But importantly, it should be a TWO point protractor, not an arc, as you will have to make adjustments at the arm base rather than the headshell.

Yes, I was forgetting that the SME has no adjustability to maintain a fixed pivot to stylus distance (for cartridges that vary with their mount to stylus distance).
A strange decision IMO....I'm dubious about the rigidity argument. Other manufacturers seem to get excellent results with slotted headshells.

The Mint would work, but only if ordered for effective length when used with a specific cartridge (and other cartridges that conform to the same spec).

Hi Tobes,

if you want to know more re SME geometry, see this link for my take on why SME doesn't need this facility.
SME Geometry
There are some serious misconceptions and misunderstandings regarding their choice of geometry, which have led to erroneous comments from many quarters, including arm and cartridge designers who should know better.

Given the vagueness in cartridge specs regarding stylus to mounting hole dimensions, I would be loath to assume in practice that the actual arc of the arm would be as calculated, based on published data for the arm and the cartridge, and consequently it would not necessarily match exactly that of the Mint, which is pointless if its arc is as accurate as claimed.

In any case, with the SME, as with slotted headshells, there is still the possibility of twisting the cartridge by a small amount (or more, by using smaller diameter screws) to align the cantilever.
regards,

Thanks John, very informative explanation.

One wonders why SME couldn't provide something similar(?).

At least SME provide some detailed specifications with their arms (it's strange that some other designers don't make these details readily available).

However, as you say in your article, SME's specification of pivot to stylus (to a hundredth of a mm) is silly and misleading given the arms design.

Cheers - Paul