Platter machining

Are there machinists here that work on aluminum platters?

 

Ok I’ll bite. Why?

 

I'm curious too... maybe a DIY project? I've never heard of a turntable platter needing to be trued.

 

It’s been difficult to get my tone arm/headshell level. It’s a Pioneer PL 530. The way it’s built, the motor and tonearm base are mounted on the same chassis. I’m getting a 1.5 mm cork mat, that should help a lot. If I need to drop another mm, milling 1mm off the bottom of the stem that sits on the motor spindle is the only solution.

 

Your tonearm and headshell don’t absolutely have to be level. Besides that, you’ve already thought of the simplest solution - a thicker platter mat. Platter mats are available in every thickness graduation from less than 0.5mm up to 8mm thick. Mats of different thicknesses are used all the time to adjust vertical tracking angle.

 

Well, you could always get a different height cartridge unless you already are using a short one. Headshells aren't all the same either. And the cartridge could be shimmed in the front to get the correct angle too.

 

Quite often one has to fit a spacer
Under cartridge and raise REAR OF
Tonearm to achieve tonearm to be level.
If tonearm cannot be raised/ lowered
Then I know what I would do.

 

Agreed, and mat thickness choice has always been my first choice when correction is needed. I have a 1200G that I use three different mats from 3mm to 5.5mm to account for different setups. The 1200 platform can be elevation challenged with a good number of cartridges / headshells I've amassed.

I've never been a big fan of cartridge shims / spacers.

 

Thin mats will only help so much on this platter. It has a 11.5” diameter recess with an outer ridge sitting 1.5 higher, so with the new 1/16” mat I just bought, it just barely clears that outer ridge, IOW, anything thinner and the record will end up sitting on that outer ridge of the platter instead of on the mat.

My carts are vintage Pickerings, they do not have flat top surfaces that are shin-able. They have two raised ears on either side that hold the screws.

 

Maybe the safest thing to mill is that raised outer edge. Then I’m not messing with where the platter stem rests on the motor spindle. With that outer rim gone, then I can use a .5mm mat.

The mat that came with it was 4mm thick and the tone arm and headshell angle were raised up quite a bit.

 

You could still add a spacer/shim between the Pickering ears and the headshell...

 

Is a .5mm thick enough to keep noise under control? What are they made out of at that thinness? The 1.5mm one I ordered is cork.

 

That would increase the tonearm angle even more. The platter sits too high in relation to the tonearm.

 

I thought so. I was just pointing out that a shim *could* be added that way if it became advantageous.

 

Check the Origin Live site for the details here:

Turntable Platter Mat Upgrade - 30 Day Money Back Guarantee

It’s very good at damping ringing platters and makes a ruler flat bed for LPs.

 

1mm thick. Looks good. Thanks. If I wanted to go with that one, I’d have to mill off that outer rim.

 

Lots of great suggestions, I appreciate all the guidance. It still looks like I need machining. I’ll have to google around locally then.

 

. . . and this one is even thinner and works well:

TEAC - Washi Turntable Mat

- never modify a platter to accommodate a cartridge. It doesn't matter who the machinist is - the chance of unbalancing the platter is too high.

- I assume that your tonearm does not have a VTA/height adjustment on or in the base and that's why you're considering mats and milling.

- VTA can be a critical adjustment with some cartridges, but I don't recall that old Pickering cartridge being particularly affected one way or another.

 

I can’t go below 1.5mm mat thickness because it would be below that outer rim on the the platter and then the record would be sitting on that outer rim instead of flat on a mat.

Looks like that TEAC mat is unavailable.

No, the tonearm on that model has no height adjustment. The relationship between it and the platter spindle on the motor is fixed.

 

Then you need a special thickness that Herbie's Audio Lab may be able to accommodate:

Way Excellent II Turntable Mat

Contact them about a mat that either fits precisely into the recessed platter area inside the rim, or for a mat that includes a lip that extends the mat surface diameter to cover the top of the raised lip. If that last bit defeats the purpose of the effort because the VTA will end up being too tail-down for your liking, then the first part might do if Herbie's can produce one of its mats in the exact right thickness. If not, then I guess it's on with the milling and we'll all cross our fingers and chant to the pagan good luck idols for you.

The biggest concern I've encountered with modifying an existing platter is the chance of splitting or cracking it under the stress of milling or grinding at any speed. Then again, I've only modified four platters during 48 years of horsing around with mono and two-channel audio. Of those four, two of them ended up worse. The last platter I touched (or rather, handed off for modification), ended up unbalanced and cracked, this from a highly qualified and careful shop. The platter metal (an unidentified alloy) couldn't take the stress, so it misbehaved while the inside of the lower rim was being grooved to accept an inner perimeter weight (to create a greater flywheel effect). The first two platters I ever touched were drilled out to create seven equidistant holes in each one, into which I mounted seven heavy slugs for increased damping and increased flywheel effect. Those two worked beautifully - great improvements.

As time went by over the years, I learned more and more about how much fussing and experimenting that turntable designers get into when developing a TT. They typically spend quite a bit of time to get the platter just right with a given motor, drive/transmission system, plinth, tonearm base, and so on. Because most products are built to stay within a production cost limit in order to be sold at a specific retail price point, the final TT (at least when it has been produced by a good company with a recognizably good track record) ends up being a well-balanced design (mechanically and tonally) at the money. I've found that changing one thing mechanically about a stock TT (unless it's a tonearm-matched cartridge upgrade from either a factory-installed one or from the one chosen originally by the owner) usually results in a downgrade. Others differ on that, and I respect their ears and their assessments if they feel that they've made an improvement. I just 'worry' that removing metal from a known-good and known-balanced platter will lighten it enough to cause a different ringing frequency that causes audible problems in turn, slightly unbalance it (unless the machine shop is *really* careful), or inadvertently damage it due to the stress of cutting or grinding. An unbalanced platter is a wow-producer, and also places unnecessary stress on the spindle bearing.

 

How about using a flat file to make the rim flush? It may be more work, but doing it this way would likely prevent the platter from being inadvertently damaged. Filing it down would also be slower than machining it, however slow and steady means less chance for damage to occur (especially heat related damage), would allow you to stop immediately as soon as you detect anything going wrong, and would help in fine-tuning the balance of the platter when you're done.

Question for the more experienced people: Could the OP use a lawn mower blade balancer to static balance the platter, or would they need access to a dynamic balancing rig of some sort?

 

I answered my own question, and the answer appears to be, "No."

Here's some interesting reading...

Design Philosophy For Turntables: A Paper From Pierre Lurne Of Audiomeca

 

It would not be difficult for a machinist or tool maker to remove the outer rim if that’s what you decide to do. Look for a machine shop in your area.
I would caution you to stress to them to use care when clamping the platter down. I can see someone cranking down on bolts thru the 2 holes and possibly cracking the platter. Castings only have some much give in them.

 

Modifying the platter is a really bad idea. Either use a shim, a different mat, a different height headshell, or find a different TT + cart combo. In the past I used shims at the headshell.

 

I don't think I could file the stem that perfectly flat. Seems like a good idea, but not for me. I'll forget about machining.

I have the 1.5mm cork mat coming, it sits just inside and a hair above that outer rim, so I'm hoping the records don't touch that rim.

Based on some visual tests with that clear plexi lined guide block, the thinner mat will certainly drop the arm and the headshell to what looks pretty close to level with a standard record. Seems like that's what most people shoot for.

I mean really, 150 to 200 is quite a range of vinyl, and with the range of cutting masters, and LP's with slight uneven-ness, there really is no way to have a perfectly consistent stylus rake angle all the time. Seems like most people prefer a tail-down arm, meaning the headshell end is operating in a range of a hair to a few hairs above level depending on the LP. Does that sound about right?

The way it is now with the current thick rubber mat, the tonearm is very tail-down.