From embossing to cutting: but... problems!

[pentlandsound]

Hi Trolls,

I'm revisiting my record-lathe project after several years' break. When I set it aside, I had a system that embossed reasonable-sounding records on PC, but (as ever) there were a number of things that had to be improved on:
- low recorded signal;
- shallow grooves;
- mono only;
- incorrect speed because of low torque in turntable used for recording.

The first step is to produce a better recording head, my original one having been banged together with hand tools, using bits of aluminium angle, nylon rod and sections of beer can. Three-D printing is much more affordable than it was even a few years ago, and as I had student access to Autodesk Inventor, I drew up some part and assembly files for a head based upon a pair of 'surface transducers' which I note that some developers have used. The ones I have are 50 mm in diameter and weigh about 250 g each.

The prototype is printed in PETG, recommended by the man who actually did the printing, as it is stronger than PLA if not as neat when printed. It cost about twelve pounds plus postage. The original (and very massive) metal 'spiders' that were originally attached to the transducers have been made away with, and replaced with 4-prong cones. The cones have cut-outs to keep mass low, and also to reduce the noise produced by the transducers while making a record. The torque tube and V-spring are one part, held in place by two screws and anchored to the back of the head by a thin steel wire. The cones connect to the stylus-holder part of the tube using a pair of short carbon rods, and are anchored to it by set-screws.

The stylus hole in the holder leads to two stylus 'channels': one at lagging 30 deg. for embossing, with a set-screw at the side; and one vertical for cutting, with the set-screw at the front.

The whole head weighs over half a kilo, and is attached to a pivoted arm with a counterweight on the opposite end (the counterweight is a hanging cylindrical container filled with ballast). The dashpot - just visible in the photo behind the coloured wires - was sold as intended for use in a toy car.


For embossing, I find that a stylus force of about 125 g gives good results on a Myshank blank, as far as yielding a playable groove. However, the high frequency response from this new head is not good (and not as good as my old head, which used the much smaller CUI drivers). I suspect the total moving mass of about 7.6 g - coils, cones, torque tube, set screws and stylus - may still be too great; from another thread, one contributor noted that a plastic torque tube is likely to cause HF problems.

For cutting - this is where I have run into trouble. New blanks seem just about unobtainable here in UK, and I have been reduced to scouring eBay for cheap, un-sought-after single-sided used acetate discs so that I can experiment using the blank side.

I have tried angling the head so that the stylus contacts the disc at various angles around the vertical, and increasing the stylus force gradually from about 10 g; but as soon as the stylus contacts the disc under any circumstances it produces a very audible 'tearing' sound. Chip is generated, and the groove is playable, but it has a dull, grey appearance and the recorded signal is seriously affected by loud hiss. (On the plus side, the recording does sound 'spacious', if not brilliant stereo, and the signal level is far louder than that of an embossed groove.)

I am using steel styli, as my budget will not yet run to a jewel. One is a 1950s NOS needle, made by "Truetone"; the other is a modern production purchased on eBay. Questions I can't find answered on LT so far are these: is it even possible to cut acetate/lacquer discs fairly noiselessly using an unheated steel stylus? Might the age of the blanks (25 to 50 years) be a factor? Do these discs have a shelf life? ... Or is there some other glaring thing that I have simply got wrong?

Thanks,
David

[audiosteam]

From my experience, lacquer blanks have a shelf life. The coating reacts to heat and humidity, after 25 or even 50!! I'd say they're dead.. except if they were stored in laboratory conditions?
As a stylus for cutting, I'd use sapphire and ruby for softer materials, diamond for harder records.
Stylus heating is definitely helping.. sometimes just heating the record with a bulb. Heating may help you extending your stylii lifecycles too.

The new transducers you're using seem too big- and heavy, I'd go for something lighter.

Check out the latest threads on homemade heads, specially the Knop lathe one

[pentlandsound]

Many thanks for your reply audiosteam. I wondered if the blanks might be past their best. The way forward is probably going to be Myshank's blank discs and a diamond stylus, one day...

Oh and I'm mightily impressed with KNOP's lathe project! Very well done, truly inspirational!

As I mentioned, I found that those weighty surface transducers weren't as good as the CUI/Digikey tweeters that I had used in my earlier build. I have several pairs of the CUIs left, and so I'm designing my new cutter head around these. This time I've taken the little plastic saucers off.


They are, I think, slightly bigger than the ones used by KNOP (40 mm square against 32 mm(?), with a coil of 26 mm diameter). As my free licence for Autodesk Inventor has run out, I'm trying to get to grips with a product called FreeCAD, and putting my initial designs together as I go.

For radial cutter movement, I'm going to use some CNC bits and pieces that I acquired some years ago but never used then. My old lathe moved the whole turntable on a kind of 'sled' under a stationary head, the construction being of aluminium angle and M6 threaded rods. Looking at it now, after several years packed away, it does look somewhat 'under-engineered', a bit flimsy. The very solid (and monumentally heavy) CNC stuff might be overkill to move a small cutter head a few centimetres, but given that I hope to do 'proper' cutting, in the fullness of time, using a 'proper' (expensive) stylus, I want to minimise as far as possible the risk of anything bending, breaking or falling to bits.

Another problem I've wrestled with is that of insufficient torque provided by the recording turntable. The 'high torque' attributed to some DJ decks apparently relates to starting torque only - that is, how rapidly the platter can be brought up to speed from stationary - and not to how the deck will perform under a sustained heavy load. My idler-drive Lenco 75 works more-or-less for embossing, but the speed is not entirely constant under a 125 g force and it can't handle larger diameters than 7".

The plan is to rebuild the Lenco, replacing its 15 W shaded pole motor with something a bit more powerful. I found this 200 W BLDC motor on eBay for £25 - a real bargain. It delivers 0.4 Nm at 3,600 rpm. (I can't slow it down at all with my fingers.)

It would probably just about fit in the place occupied by the present motor, but I suspect that a better way to go might be to use belts, placing the motor outside the platter, rather than rely on the idler. The platter has a lip on its lower edge, concealed by the chassis, that could be used to accommodate a belt: this would mean doing away with the Lenco's chassis and constructing my own.


This is the kind of thing I have in mind, to reduce the maximum speed from 3,600 rpm at the motor shaft to just less than 100 rpm at the platter, with corresponding multiplication of torque:

The motor driver has an 0 - 5 volt input for speed control, which it might be possible to use in conjunction with a rotary encoder and an Arduino board to allow closed-loop control.

A great many things to think about... as someone on this board once noted, a disc recording system is the combination of so many physical principles working perfectly together...!

[symatic]

for the hiss noise you mentioned, it might be the metal stylus? i got awful results with a 5C head and metal stylus, but now I have Sapphire embossing stylus the difference is already huge, and thats just from my first few test cuts

[Xertz]

This looks like a nice project you've got going on here. One thing, I would not recommend a DIY turntable for cutting as you can get a Numark TTX turntable for $150. I don't own one but it seems like a nice turntable and has more torque than Technics SL1200 (!).