Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work.

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Ron Thompson
 
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Archive is at Google.
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Or, if you prefer:

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ff&group=rec.crafts.metalworking

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USA

http://www.plansandprojects.com

Where did everyone go? Oh, yeah.
http://groups.yahoo.com/group/castinghobby/
Y'all come, ya hear?
*******
"Zipper" wrote in message
news:NJ0Wa.24152$Ho3.4351@sccrnsc03...
I was curious if anyone had any advice for a lathe for a complete newbie.

I
have done a little MIG welding and I'm really enjoying metal working and
would like to continue. I've thought about getting a lathe, but I really
don't know that much about it. From looking around, I'm not really sure

how
reasonable getting a lathe is since I don't have a lot of money, but I
thought I'd ask. I'm trying not to pay more the a $600 for a lathe, and I
realize this won't even buy me a decent one. Any help would be

appreciated.
Sorry if this question has been asked, but this newsgroup doesn't seem to
archive.





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John B.
 
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"Zipper" wrote in message news:NJ0Wa.24152$Ho3.4351@sccrnsc03...
I was curious if anyone had any advice for a lathe for a complete newbie. I
have done a little MIG welding and I'm really enjoying metal working and
would like to continue. I've thought about getting a lathe, but I really
don't know that much about it. From looking around, I'm not really sure how
reasonable getting a lathe is since I don't have a lot of money, but I
thought I'd ask. I'm trying not to pay more the a $600 for a lathe, and I
realize this won't even buy me a decent one. Any help would be appreciated.
Sorry if this question has been asked, but this newsgroup doesn't seem to
archive.


For that amount of money you are pretty much limited to Sherline,
Taig, or the 7 x 10 lathe Harbor Freight sells.
You could fall into the deal of a life time on some used American
iron, only used to turn a marshmallow that one time - it happens all
the time! (just never happens to me).
While both the Sherline and Taig are great little machines, the HF 7 x
10 is a heavier machine with more features for about the same money
(under $400)and is pretty hard to beat. Lots of info/support on the
web. And still leaves you money for some tooling.
I started with one, and then traded it for a quick change tool set for
my next lathe, a 9 inch South Bend (I'm still kicking myself).
You'll have to clean off all the Chinese ear wax and do a little
tweeking, but if you decide to stick with machining you can always
sell it, or trade it for some tooling. - just make sure you get what
it's worth - damn it!
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Lennie the Lurker
 
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(jitney) wrote in message om...


-Seems to me that there is a lot of slop and backlash in that Harbor
Freight machine. Can it be tightened up by the doit yourselfer?-Jitney


It doesn't matter who you buy the machine from, there's a lot of slop
and backlash in all of the imports. Most places, it's just a matter
of adjusting it out, others, where it's less critical, it may take
some metal removal and replacement. The 7 X 12 from Homier is a
little better "bang for the buck", it's somewhat cheaper, and with a
much longer bed. But like any other import machine, you have to do
what the factory didn't. None of the gib and backlash adjustment are
right, and the burrs from the machining are still there. On my
Homier, after taking off the burrs and adjusting the backlash and
gibs, the carriage handwheel has a nasty feel to it. All it's going
to take is to bore the hole in the apron out to accept a bushing, but
it's not bad enough that I'm going to do it right now. But, until you
remove the burrs and make the adjustments, it's going to feel like a
turd. Like I've said many times, it ain't no Hardinge, you didn't pay
for a Hardinge, and you didn't get one. They can be made to work very
well, but it isn't going to be done in ten minutes. It also doesn't
take any great amounts of genius to know that if you run your fingers
over an edge and it cuts you, that edge needs to be knocked back with
a file or stone. (Ok, I'm stretching it, take the damn burrs off.)
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MichaelMandavil
 
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Subject: lathes for a newbie
From: Gary Coffman
Date: 8/1/2003 1:53 AM Central Daylight Time
Message-id:

On 31 Jul 2003 17:28:05 -0700,
(jitney) wrote:
-Seems to me that there is a lot of slop and backlash in that Harbor
Freight machine. Can it be tightened up by the doit yourselfer?-Jitney


Sure, but you must have some backlash, otherwise it'll bind. That's
true for any machine (except those using ballscrews).

Backlash isn't really a problem, even if it is half a turn, as long as you
remember to always back up and approach the work from the same
direction, so backlash is taken up before you reach the desired setting.
This is standard procedure even on high buck machines.

What is more of a concern is if the leadscrew pitch isn't uniform from
one end to the other, or if it is badly worn at some point along its length.
That's much harder to work around.

There are similar issues with gibs. You can adjust them to minimize
slop, but if wear (or construction accuracy) is not even along the full
travel, you can have binding if you reduce the slop too much.

Gary


Hello, Gary,

I had the same question that Jitney did, and I assumed that the answer would
probably be as you have given it, though I appreciate Jitney and you clearing
this up, for I had already decided to pose the question myself. And now I have
become curious about those ballscrews. I guess I'll do a little research on
that.

Michael


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MichaelMandavil
 
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You DO NOT want to use ballscrews on a manual machine. With normal
screws, there is enough friction and mechanical advantage to prevent the
machine from kicking back when cutting. But with ballscrews there isn't.
You need a motor drive holding them. That's why ballscrews are only used
on CNC machines. (It is also why trying to convert a CNC machine to
manual can be a big expensive problem.)

Gary


Thanks for the additional information about ballscrews. I guess I can forget
about ballscrews until I convert my lathe to CNC then, which won't be for
awhile.

Michael
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Jack Erbes
 
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On Fri, 01 Aug 2003 14:27:30 -0400, Gary Coffman
wrote:

You DO NOT want to use ballscrews on a manual machine. With normal
screws, there is enough friction and mechanical advantage to prevent the
machine from kicking back when cutting. But with ballscrews there isn't.
You need a motor drive holding them. That's why ballscrews are only used
on CNC machines. (It is also why trying to convert a CNC machine to
manual can be a big expensive problem.)


I don't think this is right, what is meant by "kicking back"?

I don't think there is any mechanical reason to not put ball screws on
a manual machine. And I seem to recall that it was done on some older
high precision manual machines.

There should not be any problems with ball screws when cutting with a
conventional feed. For climb cutting, the hand turning the feed
crank, the weight of the table, cross and compound slides, and
properly adjusted gibs, will be enough to keep the cutting forces for
overcoming all the other friction and actually turning the screw.

If it does not, uncontrolled movement would be limited to the small
amount of freeplay in the ballscrew and that eliminates the danger of
pulling a chip heavy enough to break a cutter.

I can't imagine sustained cutting forces being used on a manual
machine converted to ball screws that were heavy enough to overcome
all the other friction and drag and drive the hand feed through a ball
screw.

If that could happen, the operator would feel the lighter or lack of
feeding force on the handwheel. They should realize that they are not
feeding the material but instead chasing a series of continuous self
feeds. That would be a wake up call for me, time to clamp the ways
tighter or lighten the cut.



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Gary Coffman
 
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On Sat, 02 Aug 2003 17:46:34 -0400, Jack Erbes wrote:
On Fri, 01 Aug 2003 14:27:30 -0400, Gary Coffman
wrote:

You DO NOT want to use ballscrews on a manual machine. With normal
screws, there is enough friction and mechanical advantage to prevent the
machine from kicking back when cutting. But with ballscrews there isn't.
You need a motor drive holding them. That's why ballscrews are only used
on CNC machines. (It is also why trying to convert a CNC machine to
manual can be a big expensive problem.)


I don't think this is right, what is meant by "kicking back"?


A ballscrew has very little friction, so it won't stay where you dialed
it against forces acting on the table. In other words, it will allow the
table to drive the screw. An ordinary Acme leadscrew has plenty of
friction, and won't do this unless cutter forces are extreme.

There should not be any problems with ball screws when cutting with a
conventional feed. For climb cutting, the hand turning the feed
crank, the weight of the table, cross and compound slides, and
properly adjusted gibs, will be enough to keep the cutting forces for
overcoming all the other friction and actually turning the screw.

If it does not, uncontrolled movement would be limited to the small
amount of freeplay in the ballscrew and that eliminates the danger of
pulling a chip heavy enough to break a cutter.


There should be near *zero* lash with a ballscrew. That's not the
issue. The problem is that there is so little friction that the table can
drive the screw, regardless of whether you are up or down milling.

I can't imagine sustained cutting forces being used on a manual
machine converted to ball screws that were heavy enough to overcome
all the other friction and drag and drive the hand feed through a ball
screw.


A ballscrew will work in reverse with only a few *ounces* of pressure.
Cutting forces are almost always higher than that.

If that could happen, the operator would feel the lighter or lack of
feeding force on the handwheel. They should realize that they are not
feeding the material but instead chasing a series of continuous self
feeds. That would be a wake up call for me, time to clamp the ways
tighter or lighten the cut.


Well, yeah, you could tighten up the gibs to create more drag, but you'd
have to tighten them up *a lot* to compensate for the lack of normal
leadscrew friction. Remember that the mechanical advantage of the
screw multiplies the effects of screw friction.

It is also worth noting that ballscrews typically are half or less the pitch
of conventional Acme screws, in other words, where you'd normally
expect an 8 TPI Acme screw, the equivalent ballscrew would be 4 TPI.
So there's less of a multiplier for the little friction that the ballscrew does
have.

If you disconnect the drives on a CNC machine with ballscrews, you
can grab the table and shove it back and forth, while the ballscrew
spins madly. Try that with a table that has conventional screws and
it is doubtful you could budge it.

Gary

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Jack Erbes
 
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On Sat, 02 Aug 2003 19:28:37 -0400, Gary Coffman
wrote:

I don't think this is right, what is meant by "kicking back"?


A ballscrew has very little friction, so it won't stay where you dialed
it against forces acting on the table. In other words, it will allow the
table to drive the screw. An ordinary Acme leadscrew has plenty of
friction, and won't do this unless cutter forces are extreme.


You seem to be discounting the weight and friction of all the other
pieces. This is not my first rodeo.

If it does not, uncontrolled movement would be limited to the small
amount of freeplay in the ballscrew and that eliminates the danger of
pulling a chip heavy enough to break a cutter.


There should be near *zero* lash with a ballscrew. That's not the
issue. The problem is that there is so little friction that the table can
drive the screw, regardless of whether you are up or down milling.


You say "near zero", I say small amount, we're taking about the same
thing. And again, you seem to be discounting the weight and friction
of all the other pieces. I have never seen a situation where fitting
ball screws to a manual machine would result in all the weight and
moving mass being transferred to the ball screws and all other
friction eliminated.

A ballscrew will work in reverse with only a few *ounces* of pressure.
Cutting forces are almost always higher than that.


I say again, you seem to be discounting the weight and friction of all
the other pieces. This is a converted manual machine. And the
operator still has his hand on the crank.

If that could happen, the operator would feel the lighter or lack of
feeding force on the handwheel. They should realize that they are not
feeding the material but instead chasing a series of continuous self
feeds. That would be a wake up call for me, time to clamp the ways
tighter or lighten the cut.


Well, yeah, you could tighten up the gibs to create more drag, but you'd
have to tighten them up *a lot* to compensate for the lack of normal
leadscrew friction. Remember that the mechanical advantage of the
screw multiplies the effects of screw friction.


I think you are overstating the amount of the friction that is derived
from the leadscrew. Imagine a properly lubricated hardened and ground
leadscrew running in a well fitted bronze nut. Losing some of the
friction is not necessarily going to be a bad thing on older machines
converted to ballscrews. Don't ignore the presence of the other
sources of friction that typically exist in older machines.

It is also worth noting that ballscrews typically are half or less the pitch
of conventional Acme screws, in other words, where you'd normally
expect an 8 TPI Acme screw, the equivalent ballscrew would be 4 TPI.
So there's less of a multiplier for the little friction that the ballscrew does
have.


How about the increased helix angle in the coarser thread? Have you
factored that in? And then there is the question of how many angels
can dance on the head of a pin.

The only machine specifics mentioned in this discussion so far has
been $600 lathes and a 7 x 10 minilathe. Those kinds of machines are
good candidates for ballscrew conversions and there are ballscrews
with finer pitches that can be used on them.

And I think we are talking in a venue where people are tinkering with
ideas, learning things, trying to make things work, and simply not
constrained by the rules and dictums that exist in the full sized CNC
machinery world.

If you disconnect the drives on a CNC machine with ballscrews, you
can grab the table and shove it back and forth, while the ballscrew
spins madly. Try that with a table that has conventional screws and
it is doubtful you could budge it.


Ah! Now I get it. You were thinking of CNC machines with ballscrews
already fitted. Aren't there any other sources of friction in those?
Are there no resting forces whatsoever?

You said "You DO NOT want to use ballscrews on a manual machine." I
disagree. There is no good reason to not do that within the framework
of the discussion here.



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Gunner
 
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Well, yeah, you could tighten up the gibs to create more drag, but you'd
have to tighten them up *a lot* to compensate for the lack of normal
leadscrew friction. Remember that the mechanical advantage of the
screw multiplies the effects of screw friction.

It is also worth noting that ballscrews typically are half or less the pitch
of conventional Acme screws, in other words, where you'd normally
expect an 8 TPI Acme screw, the equivalent ballscrew would be 4 TPI.
So there's less of a multiplier for the little friction that the ballscrew does
have.

If you disconnect the drives on a CNC machine with ballscrews, you
can grab the table and shove it back and forth, while the ballscrew
spins madly. Try that with a table that has conventional screws and
it is doubtful you could budge it.

Gary


Very true, and something I do everyday, diagnosing machines.

Ive given slightly worn ballscrews away, (.0007 backlash), to at
least one fellow, (.200 per rev) and he reported he had to put a lock
screw in the gib, to keep it from moving when he converted his
crossfeed to use the ballscrew. He said it works very well,very
accurately, but has to lock the gib everytime he makes a cut, and is
not sure it was worth the effort to do the conversion simply because
of that.

Gunner

"What do you call someone in possesion of all the facts? Paranoid.-William Burroughs
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Gunner
 
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On Sun, 03 Aug 2003 10:11:45 -0400, Jack Erbes
wrote:


If you disconnect the drives on a CNC machine with ballscrews, you
can grab the table and shove it back and forth, while the ballscrew
spins madly. Try that with a table that has conventional screws and
it is doubtful you could budge it.


Ah! Now I get it. You were thinking of CNC machines with ballscrews
already fitted. Aren't there any other sources of friction in those?
Are there no resting forces whatsoever?


Very very few resting forces. Remove the motor, and they have only
inertia for the most part. They are engineered to be as friction free
as possible, with the use (generally) of linear ways/cars or box
ways/bearings/gibs, with the lube film being accounted for.

See my earlier post on a gent whom did a crossfeed conversion.

Gunner

"What do you call someone in possesion of all the facts? Paranoid.-William Burroughs
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jim rozen
 
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In article , Jack says...

I have some ball screw
slides at work, and when the stepping motor is de-energized,
it takes very, very little force to move the slide and spin
the ballscrew.


On manual small lathes or mills?


In this case they were crossed slides for some motion
control experiments. If the rail was up-ended a bit, the
slide would zip to the end, just from gravity.

The small stepping motors generated so much linear force
that one had to be really, really careful not to get
an appendage caught between the slide and the end stop.

You guys are going to have to be more careful. You may be stifling
genius here. These guys may grow up to be your bosses. :)


Heh. Just 'cause he's the boss, that don't make it right!
Sometimes the hard part of dealing with the boss, is being
tactful when explaining that some part of his plan is
technically wrong. Incorrect. Won't actually work.
Which is actually one reason why I like my present one.
He's right most of the time so I don't have to do that
dance, hardly ever.

Jim

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