Leadscrew

[pavlo]

Why do you need such a tight pitch? I would have thought that a standard lead screw turned slowly would be just as good.

I will follow your build with interest because my son wants me to help him build one.

[fusionkid]

Well the cutter carriage needs to travel at 2mm min and it needs to travel smoothly i.e moving all the time, so surely you need a very fine pitch lead screw to acheive this?
I'm not 100% about this but that's the sense i make of it anyway, if thats not how it works what's the purpose of the 0.050" lead leadscrew that i saw for sale?
And i ask again can anyone give me any leads as to info on leadscrews? books, websites whatever i just need to know more before i can make informed decisions.
Many Thanks

[BillTodd]

Well the cutter carriage needs to travel at 2mm min and it needs to travel smoothly i.e moving all the time, so surely you need a very fine pitch lead screw to acheive this?

No, you just need to turn the screw slowly ;)

For your application all you need is to ensure any noise (vibration) produced by the screw is not transmitted to the cutter head (making a silent slide way for the cutter is the greatest problem you will have)

The lead screw itself need not be too special - I'd use a polished stainless steel rolled threaded rod with a PTFE nut for a home-brewed machine

I would use a servo motor and a simple speed controller with tacho feedback (a positional servo controller is unnecessary and would add noise to you machine)

I would use one or perhaps two stages of plain rubber belt speed reduction between the motor and lead-screw (stepper motors, gears and notch belts are noisy)

Are you going to use vari-pitch groove cutting? (i.e. making the groove spacing wider as (before!) the signal level increases)


Bill

[fusionkid]

Thanks for your response Bill it's appreciated.
Eventually i will use vari pitch groove spacing so this is a consideration in my design, although this will not be employed until much later since the signal monitoring and algorithm required is going to be one of the more difficult features to design. So until that time the variable groove will only be used for lead in/out grooves.
Yes servo motor with analogue feedback is the way to go. What are the pro's and cons' of the PTFE nut that you suggest? As far as i know commercial machines are metal nuts.
Usually a feedscrew arrangement would use a "half nut" indeed commercial recording lathes do but i'm thinking this is not essential, because of the limited travel needed (about 175mm) I could possibly do away with this, and possibly make the build easier? (fewer moving parts)
After all why not just have an auto return carriage when the cut is finished?
Positional repeatability is important for my design. Back to the leadscrew surely despite what you say the shorter the lead of the screw the better for my application?
Otherwise why make such a range of leads? And whats the purpose of a 0.050" lead screw? or why would you want that over a longer lead with a slower drive? etc

[BillTodd]

What are the pro's and cons' of the PTFE nut that you suggest?

PTFE will eventually breakdown but it is quick and easy to use.

As far as i know commercial machines are metal nuts.

If you were making a commercial unit then it would need to last longer - bronze half nut would outlast any plastic.

I don't think you'll need a half-nut (although it would not be difficult to make)

Back to the leadscrew surely despite what you say the shorter the lead of the screw the better for my application?
Otherwise why make such a range of leads? And whats the purpose of a 0.050" lead screw? or why would you want that over a longer lead with a slower drive? etc

50 thou" pitch is a simple 20 TPI that can be cut on almost any screw cutting lathe.

The reason for the proliferation of leads is to match the screw to the motor, drive and required speed. For a lathe that's only creeping along a fine pitch would be my choice.

BTW if you have a manual or other details of a VMS 70 then pinch as much of the design as you can (why re-invent the wheel :))

since the signal monitoring and algorithm required is going to be one of the more difficult features to design

I don't think vari-pitch is particularly complicated. It just needs a delay line (or a Studer with preview heads) to give the lathe time to open the groove before a loud passage arrives.

I used to make up U-matic digital master tapes with, if memory serves, a 300mS mono preview on the analogue tracks, this was back in the days before we had digital delay lines for the cutter preview.

[BillTodd]

Well a VMS70 is a precision engineered unit,

I know, I've spent many a happy hour in the cutting rooms at Whitfield Street (CBS Studios in those days)

I wasn't suggesting you copy a Neumann , just pinch the ideas e.g. the size and pitch of the lead screw & the slide-way design etc. etc.


What type of cutter head are you planning to use? (and who on earth makes them these days???)

It would be cheaper to spend £10 000 on a used unit

Is that what they fetch (I'd heard that Whitfield Street's ones were dumped in a skip!)

I have a question, what type of bearing are you going to use for the turntable. It will need to be supported over the whole surface and have extremely low noise levels,

Noise and vibration are certainly a problem, but the cutting forces are not large. I've heard of successful DIY lathes made from the older Technics DD decks, so I'm sure fusionkid will be able to make something work :) He's talking about a machine for DJs (not DMM), so surface noise will be no worse than the crap recorded :naughty:

BTW did you know the earliest lathes (for 78s) were powered by a string and a falling weight ?

Bill

[fusionkid]

I would use one or perhaps two stages of plain rubber belt speed reduction between the motor and lead-screw (stepper motors, gears and notch belts are noisy)
Bill

So if i was to use an 0.050" lead leadscrew thats 1.27mm per rev so my 2mm feedrate will require 1.575 revs of the leadscrew per minute. (correct me if i'm wrong!)
What would be a sensible gearing ratio? I'm thinking a single stage might be better, to reduce losses/ slip ?? What kind of speed would a servo motor prefer to run at whilst giving me some leeway? This is all new to me, does anyone have links regarding servo motors or tacho generators?
If i was to have 2 or more stages of speed reduction surely i would be better to have the tacho generator driven by the leadscrew itself? Is this ever done?
Many Thanks

[BillTodd]

If i was to have 2 or more stages of speed reduction surely i would be better to have the tacho generator driven by the leadscrew itself? Is this ever done?

It is possible (I have made a speed controller with a feedback from the a 90:1 gearbox) but, because of backlash in the gearbox, or flex in the belts in your case, motor speed can be hard to stabilise (this didn't really matter in the above example).

It is far better to control the motor speed with a close coupled tacho. There are many different type of tacho feedback; some have DC generator coupled to the motor shaft, most PC fans have a magnetic sensor that's part of the brush-less motor driver but, most modern servo motors will have an optical encoder wheel.

There are plenty of tacho feedback motor speed controller chips and designs around.

Old cassette machine motors used to use a centrifugal switch that modulated the windings directly, which looked crude, but worked incredibly well.

You might be able to get away with a simple back emf speed (sometimes referred to negative resistance generator) since your motor load will not vary very much. All you need is a stability (low wow and flutter) , the actual speed is not critical

to be continued...

[i2i]

How about a standard screw (for quality) and a worm drive to give you a smooth reduction for the servo

[fusionkid]

Thanks again for your input Bill.
Well a VMS70 is a precision engineered unit, extremely heavy duty and cost £478 000 in 1978 and Neumann say they never made any money out of recording lathes' !! The level of engineering required to copy that design is beyond me, certainly a job for pro's. It would be cheaper to spend £10 000 on a used unit. I'm not reinventing the wheel at all, i'm simply making a new machine based on old technology. I intend to try and use off the shelf guide rails and lead screw married with a professional turntable. My new machine in a way has a new use, to be used by amateurs and DJ's etc, it needs to be built to a cost too so just nicking a pro design is not really going to give me what i need. The Neumann machine is more or less a "money is no object" super high engineer'd product designed for big buisiness.