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QSIMDO
02-23-2009, 06:43 PM
The cross slide lead screw on my inexpensive but amusing Asian lathe must be metric or some such as one full turn on the handle with 60 increments awards one with only .058" movement on the slide.

Call me foolish but I think that degree of accuracy should be improved upon.

I notice that precision Acme thread lead screws in the size I need aren't expensive so... Acme or just an SAE threaded rod?

Sticky part here...I confess I'm a math dolt.
(Some hobby for the numerically challenged, eh?)

What pitch should I get?
TIA!

SGW
02-23-2009, 06:51 PM
Assuming you want to work in inches, you probably want 10 tpi, left-hand Acme.
Then you can make a dial graduated either in 100 (to measure 0.100" off the radius) or 200 (to measure 0.200" off the diameter).

Evan
02-23-2009, 08:21 PM
I was going to suggest that you could make an acme lead screw but on second thought that probably isn't such a good idea...:rolleyes:

rkepler
02-23-2009, 08:28 PM
I notice that precision Acme thread lead screws in the size I need aren't expensive so... Acme or just an SAE threaded rod?

ACME, much better wear properties. SAE isn't designed for moving.


What pitch should I get?
TIA!

How much room do you have? If you're buying something (either threaded rod or a nut/tap) it'll need to fit the existing dial and cross slide. 8 or 10 tpi are traditional, but I've seen 20tpi in smaller equipment.

QSIMDO
02-23-2009, 08:50 PM
Thank you all, especially Evan for your restraint.;)

Gentlemen I'm determined to enjoy this technology and hobby regardless of the aspects which elude me.
I'll make more scrap than many but I'll appreciate any successes perhaps just a smidge more...with your help.
I've already learned more from you folks than you'll ever know just by lurking and reading here for which I am eternally grateful.
I don't mind telling you that I'm envious of you who juggle numbers like so many silk scarves, light and bright and floating in the air!
CNC and G codes, etc. a merely words to me but..to watch you folks take diffiicult to work materials and miraculously compile precision moving devices is absolutely magic to my eyes.

So...thank you for answering an admittedly tedious question.

I warn you...it won't be the last! :D

Mcgyver
02-23-2009, 09:16 PM
small technical point, but its a feedscrew not a leadscrew.

maybe rig an indicator? might be a lot less work that fitting a new screw. If you do, use acme, it can stand up to higher forces. as for what pitch, depends on your current dial (if you are reusing that); ie if its got 100 graduations, use a 10tpi screw, if its 125 graduations, use 8 tpi etc...if the number of graduations x number of teeth per inch of the thread = 1000, then each line on the dial advances things in .001". what pitch is the existing screw (how high do the numbers on the dial go)?

oldtiffie
02-23-2009, 09:41 PM
The cross slide lead screw on my inexpensive but amusing Asian lathe must be metric or some such as one full turn on the handle with 60 increments awards one with only .058" movement on the slide.

Call me foolish but I think that degree of accuracy should be improved upon.

I notice that precision Acme thread lead screws in the size I need aren't expensive so... Acme or just an SAE threaded rod?

Sticky part here...I confess I'm a math dolt.
(Some hobby for the numerically challenged, eh?)

What pitch should I get?
TIA!

If I had to guess, I say that the feed-screw is metric with a pitch of 1.5mm = 1.5mm/25.4 = 0.0591" which is very close to 0.060"

The error is about (0.060 - 0059)" = 0.001"/revolution

Or in percentage terms is (((0.060 - 0.059)/0.060)) x (100/1))%

= (0.001/0.060) x 100 = 1.67% = 0.00167

I could live with that as the "error" per "1 thou" graduation is insignificant.

Previous advice to use a dial indicator on the tool-post is very good advice for "fine" work at any time as its not how far you move the feed-screw graduated dial that is important. The important thing - for "fine" work - is how far you move the tool-post relative tho the carriage.

I'd suggest that for every-day work that using the dial graduations as they are as "1 thou" will be fine.

But the final decision is yours alone.

QSIMDO
02-24-2009, 02:25 PM
Just wanted to stop back to say I got it!
Hey, I'm a visual guy so I drew a picture and all the numbers came clear.
Thanks to all.
Especially Evan, your response spoke volumes.