Author Topic: Should I or shouldn't I ?  (Read 4541 times)

Offline tekfab

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Should I or shouldn't I ?
« on: December 20, 2013, 06:51:05 AM »
Decisions decisions, I'm about to make a new cross slide leadscrew and nut for my trusty old lathe and have hit a really trivial decision.  The existing screw is 8tpi and therfore .125" per rev but I could make the new one 10tpi and therefore a much easier mathematical .100" per revolution. Okay I'd need to make a new 0- 100 graduated dial but that's the easy bit.
Am I missing something important ? or am I doomed to a life of indecision .
Mike

Offline awemawson

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Re: Should I or shouldn't I ?
« Reply #1 on: December 20, 2013, 08:49:28 AM »
I'd go for 10 tpi personally. But also I'd buy the acme rod as it's pretty cheap, then fettle the ends to suit.

Andrew
Andrew Mawson
East Sussex

Offline flutedchamber

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Re: Should I or shouldn't I ?
« Reply #2 on: December 20, 2013, 10:07:29 AM »
You should check the accuracy of the lead screw.  For the cross slide most lathes are in the vicinity of an error of .001 per foot of lead screw. 

Look up "precision lead screw" or "precision acme screw".  Most offered are in the .004 to .009 range, far to much inaccuracy for a cross slide.

Offline Pete.

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Re: Should I or shouldn't I ?
« Reply #3 on: December 20, 2013, 11:12:53 AM »
Your facing feed rates will be reduced by 20% but that's no real biggie.

Offline Meldonmech

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Re: Should I or shouldn't I ?
« Reply #4 on: December 21, 2013, 04:23:56 AM »

    Modifying the lathe may devalue it, if at any time you want to sell it.

                                                                                                                Good luck whatever you decide   Cheers  David

Offline awemawson

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Re: Should I or shouldn't I ?
« Reply #5 on: December 21, 2013, 03:19:01 PM »

    Modifying the lathe may devalue it, if at any time you want to sell it.

                                                                                                                Good luck whatever you decide   Cheers  David

If he keeps the old bits he's not got a problem from that p.o.v.

Andrew
Andrew Mawson
East Sussex

Offline tekfab

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Re: Should I or shouldn't I ?
« Reply #6 on: December 26, 2013, 06:39:40 AM »
Thanks for the various inputs,  I've decided to stick with the 8tpi .
I could now do with a arithmetic check please. From what I have read and referenced for Acme standard thread 8tpi depth of thread is 0.0725" am I correct ? I will be using a purchased form tool for the exterior thread but grinding up a tool for the internal and as I've never ground an acme tool before any helpful tips will be much appreciated.

Mike

Offline rotorhead

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Re: Should I or shouldn't I ?
« Reply #7 on: January 02, 2014, 07:05:32 PM »
Hi Folks,

Happy New Year to All.

Funny this thread has come up, as I've had a similar conundrum regarding modifying the cross slide leadscrew on my Son's Bantam 800.

To start with the details, thread is 9/16"dia x 5 x 2 start lh acme, i.e. 200 thou divs on dial.

E-bayed for bits, they price ranged from ridiculous to insane, scrubbed that.

The nut was worn as was the leadscrew (reason for replacing http://madmodder.net/Smileys/default/doh-45.gif).

To cut a very long story short, we bought a 5/8" x 10 lh acme tap, some internal and external acme 10 tips, and left and right toolholders, also a 1/2" x 10 rh acme tap for a topslide nut replacement.

I also had to grind a 3/16" woodruff cutter for the splined section, just doing that incurred making a grinding jigs for the Clarkson.

I used EN16T 5/8" bar for the cross slide screw.

I should have added that in going this route I've halved the dial readout, so what is dialled is what's taken off, far better for my Son anyway, although I'm not having a pop at anyone, this is a tool to be used, not an artefact.

Pics below tell the tale....

Chris
Ulceby, North Lincolnshire.

Offline Baron

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Re: Should I or shouldn't I ?
« Reply #8 on: January 04, 2014, 09:06:18 AM »
Very interesting.  I had need for a leadscrew to drive a quadrant a while back.  I re-purposed the worm and nut from a broken B&D work table.

Best Regards:
                     Baron

Offline bertie_bassett

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Re: Should I or shouldn't I ?
« Reply #9 on: January 05, 2014, 03:33:24 PM »
rather then cutting your own internal threads, would it be any easier to mold a acetal/delrin 'nut' around the actual shaft?, should give pretty good accuracy from what iv seen/heard
a competent engineer uses the tools and knowledge available, to get a challenging job done.

 An incompetent "engineer" tells his boss that the existing equipment "can't do the job" and to get another machine