Leadscrew

[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