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Posted by Mark Rand on August 23, 2008, 6:51 pm
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On Sat, 23 Aug 2008 13:44:26 -0700 (PDT), John S
>
>> As to using the leadscrew to drive a carriage or cross feed, I just
>> think it may be too much of a compromise - will the leadscrew still work
>> properly as a leadscrew? Will the pin or whatever which rotates stick?
>> will it cause the carriage to move?
>>
>> I don't know the answer to either question, but I have doubts.
>>
>> -- Peter Fairbrother
>
>In which case ask Myfords, Boxford, South Bend, Emco and a host more
>companies who did and in some cases still do employ this situation of
>a combined lead and feed screw.
>
>John S.
I would suggest that:-
1) It's a nasty compromise that reduces the size needed for the apron and the
cost of the lathe :-(
2) It results in self cleaning half nuts and keyway :-)
Take your choice.
Mark Rand
RTFM
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Posted by on August 23, 2008, 7:35 pm
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>
> I would suggest that:-
>
> 1) It's a nasty compromise that reduces the size needed for the apron and the
> cost of the lathe :-(
Not sure I totally agree with your suggestion here Mark, it wasn't
"nasty" enough to stop Schaublin using "the robust, buttress-thread,
4mm pitch leadscrew, for both screwcutting and the sliding feed", and
I don't think many describe them as compromised down to a price - at
least not a price I could afford :-)) To be fair though they didn't
bother with power cross feed at all and the drive system for the
leadscrew was far from simple, so as you say you pay your money and
choose your compromise.
>
> 2) It results in self cleaning half nuts and keyway :-)
>
> Take your choice.
>
> Mark Rand
> RTFM- Hide quoted text -
>
> - Show quoted text -
Helps with the sale of replacement half nuts as well, at least it does
if the lathe is good enough in other ways to last a reasonable amount
of time.
Regards
Keith
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Posted by on August 23, 2008, 8:03 pm
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On 24 Aug, 00:35, jontom_...@hotmail.com wrote:
>
>
>
> > I would suggest that:-
>
> > 1) It's a nasty compromise that reduces the size needed for the apron and the
> > cost of the lathe :-(
>
> Not sure I totally agree with your suggestion here Mark, it wasn't
> "nasty" enough to stop Schaublin using "the robust, buttress-thread,
> 4mm pitch leadscrew, for both screwcutting and the sliding feed", and
> I don't think many describe them as compromised down to a price - at
> least not a price I could afford :-)) To be fair though they didn't
> bother with power cross feed at all and the drive system for the
> leadscrew was far from simple, so as you say you pay your money and
> choose your compromise.
>
>
>
> > 2) It results in self cleaning half nuts and keyway :-)
>
> > Take your choice.
>
> > Mark Rand
> > RTFM- Hide quoted text -
>
> > - Show quoted text -
>
> Helps with the sale of replacement half nuts as well, at least it does
> if the lathe is good enough in other ways to last a reasonable amount
> of time.
>
> Regards
>
> Keith
Sorry to reply to my own post but on reflection after another glass of
white, I need to clarify my comments which were merely intended as a
"humerous reposte" to Mark's comment.
I don't want anyone thinking that Schaublin actually used anything as
vulgar as a "slotted leadscrew"; although the leadscrew does in fact
do both tasks it is "an exceptionally large (40 mm diameter and 4 mm
pitch), hardened and ground, running in angular-contact bearings and
clasped by long nuts provided with positive oiling" (see Tony's Lathes
page). On the later 125/135 and later machines they do also provide
power cross feed but use a shaft that runs down the back of the
machine driven by bevel gears. They still do though use the leadscrew
to provide both threadding and sliding feed, so that part of my hunour
is correct.
Regards
Keith.
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Posted by on August 23, 2008, 6:16 pm
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>
> Suppose I want to face 4mm off a 50mm dia surface in something hard.
>
> Remembering that mine is a small lathe and can only take 0.25mm cuts in
> the hard stuff, that's 16 cuts. At 1mm per turn each cut will need about
> 30 turns, 60 if only cutting one way.
>
> That's 960 turns of the handle, and it gets tiresome real quick - damhikt=
.
>
> As to using the leadscrew to drive a carriage =A0or cross feed, I just
> think it may be too much of a compromise - will the leadscrew still work
> properly as a leadscrew? Will the pin or whatever which rotates stick?
> will it cause the carriage to move?
>
> I don't know the answer to either question, but I have doubts.
>
> -- Peter Fairbrother- Hide quoted text -
>
> - Show quoted text -
Peter
I have to say that, stopping the machine, reversing the direction of
the feed shaft, re-starting and re-engaging the feed 16 times is also
tiresome. I guess if I was going to do that job very often I would
have to consider if the compromise of using too small a lathe was
worthwhile. Although I have PCF on all my lathes, when facing
something where I want a good surface finish right to the centre, I am
more inclined to feed by hand. I can at least try to vary the feed
rate to match the surface speed. With practice and using the "hand
over hand" method one can produce a very good facing cut. With that in
mind, I agree with an earlier comment that PCF is somewhat overvalued
in small lathes. Lets be honest, it's a hobby (well it is for me) and
if we don't enjoy twiddling hand wheels we possibly need to look at
CNC anyway as we won't need to turn any hand wheels at all then.
This system for providing power feeds is well established, much used
and reliable. I have a couple of PCF Myfords and a Boxford which all
use a single leadscrew to provide power feeds, they all work
faultlessly and have done so for many years. All also work very well
as a "leadscrew" and produce accurate threads if I have taken the
trouble to correctly set up the drive train. Again, these days many
thread accuracy issues are down to compromises in trying to cut as
many imperial and metric threads as possible with a couple of change
gears (smaller are cheaper so 21 rather than 127) or with a simple
limited range gearbox.
In fact, my BH600 which has both a leadscrew and a power feed shaft
does tend to stick sometimes when I try and disengage it. I guess it
is all down to appropriate design and manufacture. In my experience,
the issue with sticking when disengaging feeds is much more prevalent
on feed screws with very fine threads, something which is often used
these days on the smaller, cheaper lathes. Trying to disengage the
feed on my old Naerok 920 when approaching a shoulder at speed was
guaranteed to provide an increased heart rate. I would also point out
that if facing, particularly with a hard material, good practice would
have the saddle locked anyway; supposing the facility is available of
course.
Regards
Keith
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Posted by Chris Edwards on August 24, 2008, 6:21 am
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wrote:
>I have never understood the fascination for power cross feed on a lathe
>that only has 3 inches of movement anyway.
>When you compare the number of small machines built to the ones with
>power cross feed it must be a very low percent but it's never stopped
>anyone from making parts yet.
>
>It seem only of importance to armchair machinists and nit pickers who
>wouldn't even buy one anyway.
>
>P Riedie.
I can only imagine you're a troll with such a specious comment
--
Chris Edwards (in deepest Dorset) "....there *must* be an easier way!"
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