1950s - 1980's drop-stack players, and large 45 center holes

[blacknwhite]

PURPOSES OF THIS POST:

(1) To present the RIAA dimensional spec for 7-inch Large-Hole vinyl record singles, specifically the part of the spec related to CENTER HOLE EDGE THICKNESS; and

(2) to present a reason why It's A Good Thing For Plants To continue to remain Compliant With That PArt Of The Spec IF POSSIBLE, if selling records to the USA market.

It's understood (per an earlier brief exchange I had with Mossy) that there can be conflicts of interests, such as record buyers demanding thicker pressings, and, depending on availability of press molds, the possible inability to make a thicker 7-inch playing surface while maintaining the same thinner At-Spec 45 rpm large-center-hole edge thickness.

I am presenting here, for anyone who's interested, my argument on the side of Maintaining The RIAA Dimensional Spec For Center Hole Edge Thickness Whenever Possible. THE MAJORITY of 7-inch large-hole 45's on the market STILL DO, from what I've seen as a vinyl buyer.

I'm sure many would wonder, "why would anyone care" -- thus, this post.

==================================

It occurred to me, something that people outside the USA probably don't know about - and by now, probably lots of people in the USA don't know about either:

We all probably know that the 7-inch 45 rpm record format was invented in US by RCA Victor in 1949, with a large center hole, approx. 1.5-inch-diameter.

What many folks may not know, atleast folks outside of USA: RCA introduced, along with the 45 rpm large-hole format, the first 45 rpm turntables, which were ALL Drop-Stack Changers: None of this First Wave of 45 rpm turntables were manual single-play turntables; they were ALL automatic, all made by RCA:



These used a larger center spindle with 2 pairs of knife blades, which were spaced apart the perfect thickness for their design of the 45 rpm record dimensions. One after another, the knife blades would alternate coming in & going out, droping each record one by one onto the turntable for play.



The records had to be pressed with the correct center hole thickness and correct contour in the label area, to work properly on these types of changers.

This "45" format, and these RCA players, were Promoted Very Hard by RCA in the US, even to the point that RCA gave away a number of these automatic "45" players for FREE. RCA was desparate to get their new 45 rpm format and players into everyone's home. It worked, and this Large-Hole 7-inch 45 rpm format "TOOK ROOT" in the US. Of course, all other home record-player manufacturers jumped in to produce similar drop-stack players, most of which could play BOTH the normal small-hole format 33 rpm & 78 rpm records, AND the Large-Hole RCA 45 rpm format.

. . . . . .

In the USA, in the 1950s up through the very early 1980's (when they went out of style), there were a TON of home players made to be Automatic Drop-Stack players, with large-hole "slicer" automatic spindle attachments for stacking large-hole 45 rpm records.

Other countries (wisely, I would say) ignored the whole foolishness of using a different larger-sized center hole for 7-inch 45 rpm singles, and stuck with the same standard smaller center hole for 7-inch discs. But Not in the USA. Here, the large-hole 45 became Standard, and became part of the RIAA's Dimensional Standards for US 45 rpm production, published October 16, 1963: This was an agreement between record pressers and these US automatic drop-stack changers, to ensure that the dimensions of the discs would be Within Certain Tolerances, and that the makers of the machines designed their spindles to work with the same thicknesses. That way, all brands of Records made in the US, would work properly with all manufacturers brands of these large-hole autmatic drop-stack changers popular during the 1950s-1970s in USA:

RIAA spec - see Bottom Right of diagram in this link: 45 rpm large center hole thickness specification of 0.030 (+/- 0.006) inches:
http://www.aardvarkmastering.com/riaa-4.gif

The DEFINITE MAJORITY of large-hole 45's on today's market (I buy a lot) are still compliant with that spec.

The purpose of this post is to share with everyone worldwide, a reason Why It's A Good Thing For Plants To CONTINUE to REMAIN Compliant With That Spec WHEN POSSIBLE, if selling records to the USA market: because those old players were so well-built, a lot of them still work, and a lot are restored as a major hobby, so many people still use them. (See below for examples.)

I've restored vintage players for over 20 years. Although the RIAA spec dictates a 7-inch 45 rpm large-hole center hole thickness of 0.030 (+/- 0.006) inches, all my restored stacker players (different makes & models) seem tolerant of thicker discs until about 0.040 inches, then they start malfunctioning. Some of players have "spring-loaded" slicer blades, so if a record has a center hole thickness of 0.040 inch or more, the disc just doesn't drop. On some players, such a disc drops halfway, and then could fall onto the tone arm below during play. Other players, which AREN't spring-loaded, the knives try to force their way out, and either blast out a chip of vinyl from the center hole of the record, or the entire turntable stops rotating as the whole mech seizes up, or some of the metal working parts of the turntable mechanism get damaged.

So, that's the reason why I suggest It's A Good Thing For Plants To CONTINUE to REMAIN Compliant With That Spec WHEN POSSIBLE, if selling records to the USA market.

Following are examples of people Still Using US-made 1950s - early 1980's players with this type of drop-stack, blade-slicer, large-hole 45 rpm spindles:

These are all pics posted by Other People On The Web, Not Me, to make the point that I'm not "the only one who cares", it's a record-buying community who restores & uses these players. The large-hole 45 rpm drop-stack home player is an Iconic Part Of US History, and still popular among the "vintage / retro" crowds, and these folks Do Buy Records:

. . . . . . . . . . . . . . . . . .

Anytime you see these kinds of spindles, that's the kind that require RIAA-specified 7-inch large-center-hole edge thickness of of 0.030 (+/- 0.006) inches (or, At Least, Less Than 0.040 inches thickness, from what I've found with my different brands of stackers):

. . . . . . . . . .

One can go to youtube and search "45 record changer" and similar searches, and see people playing records on Lots of these.... EACH OF THESE VIDEOS IS FROM A DIFFERENT YOUTUBE USER - Check the video userIDs, not a single repeat in this list: These videos are All From Different People, All Over The Place, and it's just a sampling, there's Tons More on youtube:

http://www.youtube.com/watch?v=8J70R0t9PA0

http://www.youtube.com/watch?v=Z8AGDrhSNNA

http://www.youtube.com/watch?v=f5TzPM2MSOo

http://www.youtube.com/watch?v=9alxB7lvd2Q

http://www.youtube.com/watch?v=X8inPcgwXi8

http://www.youtube.com/watch?v=IrBKH8l1BEU

http://www.youtube.com/watch?v=LRyraKvEg98

http://www.youtube.com/watch?v=VCa-GN6njOs

http://www.youtube.com/watch?v=Cangs9_UU_o

http://www.youtube.com/watch?v=ao2v0xuVB2o

http://www.youtube.com/watch?v=MdreTVQXEhM

http://www.youtube.com/watch?v=7mXgVMGivkA

http://www.youtube.com/watch?v=4XqbukNo_yM

http://www.youtube.com/watch?v=X9fYe7eR7Wo

http://www.youtube.com/watch?v=xUIu03s3oNY

http://www.youtube.com/watch?v=89TkE6LZA7I

http://www.youtube.com/watch?v=B3o6Xrf5RFc

http://www.youtube.com/watch?v=C3DxmMTw460

http://www.youtube.com/watch?v=R_lO6KJOfI4

http://www.youtube.com/watch?v=eMnRiQ01GbU

http://www.youtube.com/watch?v=a9b0JpHlq8E

http://www.youtube.com/watch?v=zOn9dSLafSs

http://www.youtube.com/watch?v=jjBQtEEaTng

http://www.youtube.com/watch?v=jL8ZvlAfNUM

http://www.youtube.com/watch?v=0hPDaMZUE14

http://www.youtube.com/watch?v=31WkJe9GWYk

http://www.youtube.com/watch?v=NZfY6VSVPXA

http://www.youtube.com/watch?v=YmIVfh0wqvg

http://www.youtube.com/watch?v=Rngux-BEs3w

http://www.youtube.com/watch?v=agsHU5abKik

http://www.youtube.com/watch?v=x1JyRyvvR38


...etc, etc.

China is even pumping out these "repro" changers under the "Crosley Stack-O-Matic" brand name:

.

The blade-slicer automatic large-hole 45 rpm spindle record players, which require RIAA-spec thickness on 45 center hole edges, are iconic, and these players still have a following today.

. . . . . . .

IN SUMMARY:
Above is the reason why I suggest It's A Good Thing For Plants To CONTINUE to REMAIN Compliant With That Large-Hole 45rpm Spec WHEN POSSIBLE, if selling records to the USA market.

Thanks,

- Bob

[maniman]

Yes Yes , absolutely!!! ,wow a lot of info ... here in Spain are a relatively large community of oldies turntable fans (specially the Dual fans) called "El Gramoforo" (is a combi of Gramophone and forum) , I discovered via Jordiphon from here the trolls , is all in Spanish lang , the record changers is one of the most hot area of that forum and I guess many many people in "El Gramoforo" needs to read that post , to make an idea how uncertainly is the future of that stuff that the audiophiles and collecters love as sons. (hehe Regards to the Dual fans) , but.... (all have buts we are in Spanien , cows , sangria , and all that sh*ts ) , its sad , but many many people in the "Gramoforo" cant read in english , if have some time , I will translate the Post and post it in that other forum that im talking about.

http://elgramoforo.esforos.com/

Best Regards
Mani

[andybee]

now, I understand



thanks a lot for this post!

With DIN Specs, a 7inch must have 42gramm total weight,
I don´t know with or without big hole....

[andybee]

I just checked:

0.004 inches are approx. 0.1 mm, which is VERY thin...

I think, most 7inches have approx. 1mm thickness in the
near of the middlehole...

[andybee]

oh men, please take care with the decimal place

sooooo:

thickness near the hole (after the http://www.aardvarkmastering.com/riaa-4.gif specs) are:

0.03 inch = 0.76mm (tolerance +/- 0.006 inch = +/- 0.152mm)

[blacknwhite]

THANK YOU andybee !

I went back and changed every mis-quoted occurance of
"0.003 (+/- 0.006) inches" in my post, to the correct quote of
"0.030 (+/- 0.006) inches".

"Remember the Hubble..."
http://www.jstor.org/pss/4165153

- Bob

[andybee]

so, a question:

how important is the "thick" area between the hole and the
recording area?

there are actually some different moulds in the presses,
I don´t know, which one is the best for the jukeboxes...

I like to buy one of these funky RCA 7inch changers...
Where is the best place? ebay.com ?

Thanks again for this post

[haxixin]

Hi

did you see my email about our release?

[greybeard]

Hi blacknwhite

thank you very much for this exposé. Your mapping of the variety existing is much appreciated. I support your plea for adherence to the industry standard for these records; this is absolutely essential. Imagine that they changed the dimensions of "D"-cells. Or dropped the 600 Ohm telephone standard.

The RCA system is very well described in technical terms by Carson, Burt, and Reiskind, in an issue of the RCA Review of 1949. I believe it has been made available here, but I can't find it today. The corresponding patent is by Benjamin Carson alone, it is US 2,634,135.

It is absolutely essential, not just to adhere to the greater thickness under the label. but also that there is again an area of thinner material inside the part with the greater thickness, just surrounding the large hole. This is to make room for the knives.

Again, thanks for the good documentation!

Cheers, Greybeard

[Steve E.]

A lot of my old 7" singles have designs that deal with this by having the center area taper down gradually as they near the hole.

[blacknwhite]

It is absolutely essential, not just to adhere to the greater thickness under the label. but also that there is again an area of thinner material inside the part with the greater thickness, just surrounding the large hole. This is to make room for the knives.

Yes - Thanks Graybeard.

there are actually some different moulds in the presses,
I don´t know, which one is the best for the jukeboxes...

I've worked on a few jukeboxes too and own an AMI & two Rock-Olas, and helped a friend restore a Seeburg.... so NOT a jukebox expert, but have worked on a few...

From What I've Seen: Jukeboxes don't seem to care about anything but Center Hole Diameter, and Outer Edge Diameter.

Jukes seem to be very forgiving machines, unless center hole is too small, or outer diameter way off. This makes sense, because they are supposed to be Industrial Strength, reliable. A juke expert might have more details.

I personally wouldn't consider this a "real jukebox", so this is NOT a serious concern... But just to be complete: There is one "novelty" juke that did care about center hole thickness, but it is very rare. It did have a real locked coin box and coin mechanism. It was the Williams (pinball-machine manufacturer) Music Mite, and it used the same kinds of blade slicer spindle as the RCA, but again, so rare, Not a real "Jukebox Concern":

.

Showing the mechanism, on youtube:
http://www.youtube.com/watch?v=RW-AYwAHx64

Andybee: Here is a Fascinating German-made jukebox... and I *think* it may use knife-slicers too, like the RCA 45 player, but I don't know... maybe you have a friend of a friend who knows?... I am curious... the NSM Fanfare, from 1950s/1960s: But, in general, that's the only Full Size jukebox I know which Might care about exact thickness - and again, youtube owners who post videos say they're Very Rare, so, Not a real "Jukebox Concern", but still interesting:

.

Showing the mechanism:
http://www.youtube.com/watch?v=B6JoMNxGCbo&feature=related

And here, playing reggae: Probably one of the standard-thickness UK/European re-issues, not a thick original Jamaican pressing...
http://www.youtube.com/watch?v=A4B1ZqXa-N0

BUT, even if most jukes don't care, there are obviously going to be more drop-stack changers in US for home use than there are jukeboxes, so, the RIAA spec for 45's is still useful for US market.

I like to buy one of these funky RCA 7inch changers...
Where is the best place? ebay.com ?

Thanks again for this post

Yes, probably eBay is easiest. Glad you liked the post; thanks for your correction.

- Bob

[markrob]

Hi,

Great thread! Thanks for putting that out there. A good book on these types of players is "The Fabulous Victrola 45" by Phil Vourtsis

http://www.amazon.com/Fabulous-Victrola-45-Phil-Vourtsis/dp/0764316370

http://www.schifferbooks.com/newschiffer/book_template.php?isbn=0764316370

It has many re-prints of RCA documentation, schematics, exploded views, ads, and a nice section on restoration. The big restoration issues with these are the rubber parts (the compound idler is a biggie) and the fact that early units used crystal pickups and need to be replaced. I have a nice RCA 45EY4 from 1951 that has a push-pull amplifier. Sounds great.

The main point to the 45 as engineered by RCA was the fact that the changer mech and record profile were designed as a complete system. If the specs are followed, the results are a very reliable system. Not always the case for the small hole records/changers.

Mark

[mossboss]

No one makes out of spec records Full stop.
These machines are a very niche area of the vinyl market which provides absolutely minimum sales for a pressing plant
These are great machines no doubt about that
At the end of the day someone has to pay the rent and it is not the owners of this great collection that Bob has kindly posted up
The previous thread has been removed where I did post a lengthy reply and Andy B also expressed his view with a single word What?????
regardless it is hardly an area that warrants any special consideration in view of the cost associated in running a plant
It is a case that when clients demand a specific product that may or may not satisfy the requirements of these devices the plant owner will have to meet that demand or else there will no plant so as to press any record.
The demands are certainly within the specifications apart from a slight increase in thickness which would not bother any of the swing arm stackers but may cause issues with the rather intricate mechanism of the gib type non over the stack type of arm stackers
The above is my view of course having enough plants around the place it may be slightly twisted but here it is any way

[Serif]

Thanks for the presser's perspective, MossBoss.

45 is better fidelity at the expense of diminished playing time. I tend to think you should have to pay for that luxury with responsible, if laborious, cueing and handling, rather than make a stack of your discs fall one on top of the other and be held down across the top discs' grooves by the metal arm. A small hole in the record center is like the Cine hole in the analog tape reel (which is the default for Studer), and, for discs, that means it can be played on the Technics spindle without an adapter (not that they don't have an adapter, here...). We also have a two-knife stacking adapter on the console tri-o-matic, however, it never gets used (...is clamped to the underside of the tt lid).

Even though I am in favor of, say, disc juke box compatibility, and I love the vintage pictures, shown, here, I'm not keen on helping anyone drop stack change his good vinyl. I should think it's a good idea not to encourage stacked disc playback, for the safety of the grooves, even if it's easier on the ergonomics and is a tighter verblig during listening sessions. Maybe it's not as bad as it sounds. But even if it did no damage to the stacked grooves through impacting contact or risk of stick-slip scuffage, the fact that it raises the floor of contact of the stylus can't be good, sonically, at least theoretically, can it? We have striven to match a target rake angle with the cutterhead and now you tell me the variable target is moving as the night goes on? (;

My father e-mailed to approve of my photos of what he called my "cutter thingee." He said that it reminded him of a series of 78 rpm discs of Joseph Stalin he had once listened to, which were mastered to something like 20 sides. Not sure how they were cued, but there're 19 sides of speech, in Russian, and 1 side of thunderous and approving applause!




- Bray Kerpanol

"The nice thing about mastering is, you really only have to scratch the surface to get good results."

[greybeard]

Hi,
Serif, my friend, you said: "I'm not keen on helping anyone drop stack change his good vinyl. I should think it's a good idea not to encourage stacked disc playback, for the safety of the grooves, even if it's easier on the ergonomics ...."

However, that is precisely the beauty of the RCA invention. The labels are sitting on thicker goods, so that unless you take a bulldog clip to the edges, there is no way that the grooves (well the land, actually!) can touch in a pile. And RCA further found out how large the label must be to avoid slippance. Please help people get this fabulous functionality by working to the original RCA measurements.

Now, you are completely right when you say: "We have striven to match a target rake angle with the cutterhead and now you tell me the variable target is moving as the night goes on?". This is the general problem of all fall-down record changers, and RCA did not solve that. But they did provide the best of the type there was - for 45s. And the invention was made at a time when spherical stylii were the order of the day, and spherical stylii are like a ball bearing - it will adjust to any vertical tracking angle.

Keep up the good spirit (but never on shellac).

[mossboss]

Ha Greybeard -
Always of course with the finer points which most of the time I take for granted
Sure thing serif the 45 x7" format is Still my favorite format followed by the 10" most likely the best compromise
We really want to be quite clear about a few things There are stackers and stackers as manufacturers at the time circumvented well thought out RCA patents so as to get in the market
Any one could make 7" record but not the machine which was covered by
A ( stack ) of them
At worst as I have pointed out the 7x45 today may be slightly thicker in the centre with the next item out of spec the play area being longer
This was required by some Juke boxes so as to trigger the End of play mechanism associated with them
To be precise the lock groove had to be right down close to the label which of course provided a longer play time
The centre hole was confirmed by Bob to be precise as well as other dimensions that matter
The issue as I see it which is what prompted my stance is this
Bob posted photos of shabby records where the label was peeling of after it had been punched out to the larger diameter
As a way of explanation all 7"s are pressed with the standard hole of 9/32" they are than punched out in the machine or elsewhere ie in a small press whatever
So the issue was not that these where out of spec just BADLY made that's all
I can elaborate on a whole host of reasons why this could happen but there would be no point as I am sure the culprit would not be reading this
At the end of the day there was badly made records in the past and there will be badly made records now and in the future
Warning pressing guys about out of spec records does not cut it
Quality records would have been a better heading IMHO
Any way we all got a fabulous collection of photos that most likely would not have been posted otherwise So there is a huge plus over this kerfufle or a storm in a tea cup
Cheers

[Serif]

The labels are sitting on thicker goods, so that unless you take a bulldog clip to the edges, there is no way that the grooves (well the land, actually!) can touch in a pile. And RCA further found out how large the label must be to avoid slippance. Please help people get this fabulous functionality by working to the original RCA measurements.

Now that's some close tolerance. And the only 7" 45's I own exhibit slip when stacked.

GreyBeard, you are correct in that, whilst the very center area, closest to the big hole, is, indeed, thinner plastic than that of the groove land area, it's nevertheless true that the area between the very center area and the dead wax is thicker plastic than that of the groove land area!

However, in spite of this accommodation, I observed the following interaction with a Gordy/Motown issue of T. U. T.'s "Smiling Faces" and a Rolling Stones/Atlantic issue of "Everything's Turning to Gold." (both, large hole 7" 45's from the US): When I place the pickup stylus on the lead in area (conventional outside-in cuts) of a stacked 7" that has dropped onto the tt, the stacked lands that are directly under the stylus (flip side of top disc and top side of bottom disc) do touch each other, even though they stop touching each time/as those sectors rotate away from the stylus. Also, by the time the tone arm has swung (arc-like) about half way (maybe a little less) across the recorded radius, then the raised center areas of these 7" discs are properly doing their thing and keep the disc surfaces from touching. maybe these pressings are not perfectly spec'd for drop stacking, or maybe the spec is wanting when it comes to the keeping those surfaces apart during the first 20 rotations... And this tone arm is not adjustable - has flip-syli which have a jewel on front and on back of shank. Weight's too heavy for microgroove and stacked 45's, but great for shellac, with that 3 mil sapphire tip.

I'd tend to shy away from a practice which requires manual stacking of records or any discs. It's an invitation for abrasion, even if, on paper, the stacked rills are not quite kissing. This is like how closely the tape rides across the heads of the A80 if no lifter or outside roller threading method is used to cue reels. It doesn't quite touch, but it's so close that there's no tolerance left for reality.

Also, none of my 70's era 45's slide down the Voice of Music large hole drop stack adapter, fwiw. Maybe Mossy's saying it's a little late for compliance to something which is niche and even somewhat hack-like, w/a/d/r. of course.

Now, you are completely right when you say: "We have striven to match a target rake angle with the cutterhead and now you tell me the variable target is moving as the night goes on?". This is the general problem of all fall-down record changers, and RCA did not solve that. But they did provide the best of the type there was - for 45s. And the invention was made at a time when spherical stylii were the order of the day, and spherical stylii are like a ball bearing - it will adjust to any vertical tracking angle.

I was actually wrong. What I meant to say was that the vertical tracking angle would be different for each stacking shuffle. The rake angle came to mind, but I was imagining raking the neighbors lawn instead of my own!

From B. B. Bauer, et al., in Transient Response and Intermodulation Studies in Phonograph Reproduction (1962), "When the pickup tracking angle is not equal to the cutter head angle, for either lateral or vertical modes, phase modulation takes place... in the square wave."

My old timer still plays both 33 and 78 steadily, but in drop stack mode does experience wow and slippage. Bad records - for that purpose, yes (and bad setup). But the setup works fine and the discs do, too, when used in conventional applications.

Ach so, I have only fine line Ortofon on the Technics, but think that the Astatic flip-styli on the Tri-O-Matic might just be spherical.

Keep up the good spirit (but never on shellac).

Bottoms up to Top-Down.


- Eddy Berliner

[mossboss]

Hey all
It has always been a moot point with me when discussion takes place in so far as tracking angle of a phonograph Record on any turn table
When a record is cut it is cut in a straight line as we all know
There has been a myriad of designs so as to overcome this inherent drawback from cutting to playback
The linear style of arm on turntables produced some years ago provided the ideal scenario some designs where not that great due to variable pitch presented to them however some where excellent
Unfortunately they all required a specific stylus which was often expensive available only from the makers of the specific T/T
In my view everything else was but a compromise getting very close to ideal but never quite there
I still have an Ideal as well as a B & O which do provide a very fine result with hardly any issues associated with swing arm devices
Not that we have a shortage of those around the place either
It still surprises me that discussion the subject still takes place which is fine But to talk about that on stackers?
I think there are enough issues relating to tracking playing a single record let alone a stack of them
Further it was not unusual for record manufacturers to have serations around the label area so as to prevent slippage when stacked
It is not a big deal to do
It is a simple heated die which is pressed lightly against ithe lacquer melting the area where they are required
That is of course prior to silvering plating etc
It was done when 7" records where pressed by the 1,000"s for juke box and
stacker use
It can be done today as well without any great expense however in the last
however many years no one has requested such or we had complains about record slippage
A further point is the fact that the label paper used in days past was a very porous type which provided a fair amount of resistance to slippage
This type of paper is still available today However it is not used on auto machines as the vacuum operated loading up system has a tendency to suck more than one lable which results in rejects
No one wants that so this type of paper is used for manual operated presses only
Records pressed on manual machines will not slip mainly due to that type of paper
Checzk republic sevens do not slip as they still use the porous paper
These are useless on an auto presses, as we have had them more than once we now refuse to use labels printed there since double labeling was the end result ruining the records
OK that's all at this point
The sun is up the sea is calling He he

[greybeard]

We are getting into matters, which are only relevant for extreme HiFi, and you cannot get that from a stacker anyway. However, first, the rake or vertical cutting angle defines the direction of the wrinkles in the groove (wasn’t that a nice way to put it?). If you have a line contact stylus (Shibata, Pramanik, van den Hul, etc., but the effect is there to some degree even with “ordinary” elliptical stylii), you can only get a good tracing of the groove wrinkles, if the “line” (which is the edge of the stylus) is aligned to the wrinkles, that is, if the instant vertical tracing angle (not tracking, we are coming to that in a moment) is the same as the rake. Furthermore, if you want to have what you trace in the left flank of the groove to be on the same time instant as that which you trace on the right flank of the groove, you need the “lines” to be where the cutting stylus edge was when it cut.

That can only be obtained with a tangential arm, but with an offset radial arm there are only two radii (radiuses to some) on a whole record side where you are getting it. That is not enough, in my opinion, although the errors are quite small. And you need to be certain that the cutting stylus has not been rotated in its holder. Obviously certain Presto and other swing-arm cutters do not need a tangential arm, but they need an arm of the precise length of the swing-arm. And from the non-suction era, sometimes the cutting stylus was indeed rotated, to throw the swarf (chips) to one side for collection by a central brush wheel integral with the clamping nut.

Now, irrespective of the above nice physical angles defining the orientation of the cutting face with its two cutting edges, the movement of the stylus may be out of the plane of the cutting face. This is the angle of modulation, which results in the need to adhere to a vertical tracking angle that corresponds to the angle of modulation. Otherwise you get a distortion that is generated by the projection of the vertical tracing angle onto the plane of modulation (which is what Serif referred to in the paper by Benjamin Bauer from 1962 and further developed in the IEEE Transactions on Audio in 1963, also in a discussion in Audio Magazine, including one very persuasive one by E. Roerbaek Madsen, the chief designer at B&O at the time).

Now, if you do not align the “lines” as in “line contact” with the vertical cutting angle, you get a distortion that in principle is like azimuth distortion in magnetic and optical recording, although it is more likely that you damage the grooves. And if you do not get the proper synchronization between channels (the “time instant” above), there is a remedy: to use what CEDAR erroneously used to call an Azimuth Corrector, but which is in reality an inter-channel synchronizer. Once you have run your signal through that, it really does not matter any more that you are “only” using a radial arm. Your signal will be as correct as possible, and for mono your vertical sum is minimum.

The spherical stylus is a compromise; it is universally applicable, but it is not ideal.

Whew, that was a lot, but think about the movements of the cutting face and that we have to get that back when we reproduce a cut groove.