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cutting spiral onto a rod



 
 
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  #11  
Old May 9th 05, 08:04 PM
Peter W.. Rowe,
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On Mon, 09 May 2005 11:21:48 -0700, in ?? Abrasha wrote:


A Taig LATHE and a Sherline MILL are two very different tools!


True enough. I was confusing what you were discussing, as I thought it was you who were
suggesting that one needed the fully equipped mill, not a lathe. But I'd have made the
same comment substitutine my Unimat lathe, which is also vastly underpowered. Cutting
deep contours with it is indeed a pain in the ass, which I assume it shares with the
Taig. But you CAN do it. What you cannot do is use a shaped cutting too that forms the
entire shape of the groove at the same time. That would of course be the usual method
with things like threads, and many other types of contoured cuts like radius forming, or
the like. Can't generally do that with this small lathes



And no, you cannot cut a helical groove on your Sherline mill, unless you have
one with an indexing head (dividing head), which is coupled to the automatic
power feed of the table. This is only possible on the Sherline, if it is
equipped with stepper motors. Which means a CNC interface. And even if you
have all of this, how are you going to hold the 15 mm silver rod. You have to
have an indexing head on one end and a tail stock on the other end. Given that
the table of the Sherline mill in only 13" long, that won't leave you much
length and room to maneuver for the rod.

nor is my little old unimat lathe.


Your little old unimat lathe is good only for the most basic of tasks. It not
useful for the task that the original poster asked about. Cutting a helix on a
10-15 mm diameter silver rod.


Didn't say you could. I was responding only to what i'd thought you'd said, which was
that the low power and flimsy drive belt of the Taig, and others of these small
machines, was the stopper. I fully agree with you that the real stopper is the needed
coupled index head and longitudinal feed, and that a mill so equipped is by far the best
way to do it.. But for a faster helix, at what point does one stop calling it a
"thread" and start calling it just a helix? Often, the distincition is the shape of the
groove, which is generally precisly defined for a thread. Some few lathes set up for
thread cutting may have the ability to cut a low enough pitch thread, as to qualify for
the type of shape the poster desires. That lathe I mentioned using in grad school for
that time I did a similar helical form was cutting the thread at 2 tpi, on a rod with a
diameter of about 3/4 of an inch, if I recall. And it did not resemble a normal machine
thread because the cutting tool I was using was not the usual 60 degree thread cutting
tool, but somewhat similar to a standard cuttoff tool, with the side relief greatly
increased to accomodate the helix. The result was a straight walled, flat bottomed
groove, pretty much just as the poster is requesting.

However, I'll easily admit that this radical a tpi setting is not the norm for most
lathes, even pricey ones. Nor is the nifty taper cutting attachment that lathe also was
equipped with, or the headstock thrust bearings that made it usable as a decent metal
spinning lathe as well. And it's price was WAY out of the bounds of the poster's
request. When I did that piece, the choice of the lathe was simply because that tool
was able to do it. The bridgeport mill we also had there could not do it, simply
because we didn't have any sort of index head or rotary table that could be tied to the
table feed to generate the helix.


"The enlightened machinist will not erroneously use the word spiral or helix as
is almost universally done even by some writers. To correct such error is
almost hopeless and perhaps nothing can be done about it. However:


I humbly thank you for correcting our linguistic error. Brought a grin to my face,
recalling a few other times I've heard widely misused terms in the english language
corrected by those who, ironically, learned english as a second language. Sometimes,
that way to learn a language results in someone with a better use of proper grammar than
we native abusers thereof... Fortunatly, I suspect that most readers of this thread did
not misunderstand the intent of the post due to the misuse of the words.. At least i
hope so.


When you are cutting a helix, the feed rate is determined by the pitch of the
spiral. You cannot just set your feed rate "slow".


Again, no doubt my own incorrect terminology. I'm not referring to the transverse feed
rate, which is indeed set by the machine, and determines the pitch of the thread or
helix. I was referring to the rate at which one manually feeds the cross slide into the
work to advance the cutting tool farther with each additional pass. If you take a very
small "bite each pass, removing less metal, there is less stress on the motor, and less
bending force on the workpiece, so long as you also limit the width of the cut as well
as the depth. This is, frankly, a major bother and potential cause of innacuracy, since
intead of letting the geometry of the grove be generated by the shape of the cutting
tool, as is normally done in thread cutting, one might need to use a narrower included
angle on the tool, or otherwise take small bites adjusting not just the cross slide feed
into the work to increase the depth of the cut, but also it's transverse position, so
the full width of the groove is arrived at with multiple narrower and smaller bites. A
properly sized and powered lathe, of course, does not normally require such tom foolery,
though in this case, a narrow rod of softer material might still require it to avoid
bending the rod during cutting.


Besides, both the Taig and the Sherline do not have lead screws, so what feed
rate are you talking about? Unless you have a lead screw and change gears, you
cannot set a feed rate. Your feedrate in only manual on both machines. Not
very useful when you are cutting a helix in any set up.


Agreed, though my unimat does have a lead screw. Only for finish cuts, not adjustable
much for threading, so equally useless for this application. Unimat does make a thread
cutting adjustment, and odd kludge of an affair that uses a thread form and follower to
generate the transverse tool feed. I dont' own one of those, so don't know how well it
works, but it too, likely does not generate the degree of pitch this poster needs.



1: You didn't cut a spiral, but a helix.


yeah, yeah. thanks again :-) :-)


2: You cut a certain very special kind of helix, i.e. a screw thread, which is
not what the poster is after I think.


Well, no. See above. While the lathe may have thought it was cutting a thread, the use
of a differently shaped cutting tool created a flat bottomed, straight sided thread,
which is more or less what the poster is trying for. What may not have matched is the
pitch of my thread to his desire, but mine was unusually low (2 tpi, if I recall)

Screw threads are cut on lathes equipped with leadscrews. Helices are cut on
milling machines, in a manner I have now described in several posts.


Agreed, though the two forms are essentially similar geometry. At what point does a
thread pitch become so low as to be called only a helix? Certainly the typical shape of
a twist drill is not something one cuts on a lathe, but the division where one becomes
the better tool is mostly based on what your machinery is capable of. And you are
correct that the mill is no doubt the better tool for this job.

Just a thought, though. I recall seeing wood workers setups used with a router,
intended for doing the helical forms for things like twisted shape table legs or
railings, or whatever. A form much like a coarse twisted rope. Those jigs use a
premade form with the needed helix. Moving the router along the workpiece causes a form
much like a half nut on a threading lathe to follow the preform, and the preform is
linked to the workpiece, so the preform rotates and the workpiece follows along,
resulting in a similar helical cut on the workpiece.

It might take some ingenuity, but I'm wondering if our poster could construct a similar,
smaller scale cutting jig to cut his helical grooves. I see lots of mechanical problems
in getting a precise enough device to work well in metal, but the basic geometry of how
those router jigs work might be applicable here if the fellow is inventive enough. The
principal is not that different from what the thread cutting attachment for the Unimat
does, with a form and follower defining the cutting tools path

Peter
Ads
  #12  
Old May 9th 05, 08:06 PM
Peter W.. Rowe,
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On Mon, 09 May 2005 11:21:52 -0700, in ¸õ Abrasha wrote:

Carl 1 Lucky Texan wrote:
Like Peter said, plus, if the piece were not too large, perhaps it could
be done in wax and then cast?

Carl



To cut a helix in wax you would need the exact same setup as for metal.

Abrasha
http://www.abrasha.com


But if the need for precision is less, one could reasonably consider whether one might
simply hand cut the form in wax. Would take some very careful layout, and careful wax
carving, but it's possible. No doubt not as accurate as a machine generated and cut
helix, but perhaps that degree of accuracy is not needed.

Peter
  #13  
Old May 9th 05, 08:11 PM
Andrew Werby
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"Abrasha" wrote in message
...
Andrew Werby wrote:
"norwick" wrote in message
...

I need to cut a spiral onto a silver rod.
The sides of the cut can be at 90 degrees/and/or inclined. I envisage
this to be done by a lathe (milling machine?).
Diameter of rod could be 10-15mm,
spacing between edge of the next groove ca 3mm,
groove width & depth ca 2mm (ideally there should be some scope for
visual considerations).
Non electric system would be acceptable (or very slow speed lathe),
machine size should not be more than 1m, weight under 50kg.
Cost should not be more than $1000-1500, but this assumes that it can
be used for other applications, eg cutting tool steel etc.



[You could do this on a miniature CNC milling machine (like the Taig
mills I
sell) equipped with a rotary table, chuck, and tailstock. You'd need to
program the toolpath to advance the tool along the X (long) axis while
rotating the A axis (the rotary table) so that the grooves were properly
spaced. A ball-end tool would give you a round-bottomed groove; a regular
endmill would give a flat-bottomed groove. The weight and size of this
machine would be about what you're asking for above, but the price would
be
higher - about $3k, with the accessories you'd need. These machines can
be
used for many other things, but for best results with tool steel anneal
it
first, and use carbide cutters.]


The motor that typically cames with the Taig does not have enough power to
cut a
2 mm deep groove in a 15 mm diameter silver rod. The cutting force would
stop
the spindle dead in it's track, or the rubber band (the drive belt that
comes
with the Taig is no more than a big rubber band) would just slip.

Abrasha
http://www.abrasha.com


[Want to bet? Taig recently started providing a bigger motor with their
mills than previously. The 1/4 hp Franklin motor now standard would have no
problem cutting a 2mm groove in a piece of silver - or a piece of steel, for
that matter, in one pass, if one went at an appropriate speed using a 2mm
2-flute cutter. It could also make multiple faster cuts at shallower depths.
The Gates drive belt, while it may superficially resemble a "big rubber
band" is actually quite resistant to slipping, when tensioned correctly. The
pulley settings can be changed to provide more torque, or more speed, as
needed. Have you ever actually used one of these mills?]

Andrew Werby
www.computersculpture.com






  #14  
Old May 10th 05, 01:50 AM
Carl 1 Lucky Texan
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Possibly not the same power requirement, hobby lathe might be OK.
I admit, he seems to need to do longer pieces than might be practical to
cast though. And certainly they wouldn't have comparable
finish/precision, etc.
Just a thought.

Carl

Abrasha wrote:

Carl 1 Lucky Texan wrote:

Like Peter said, plus, if the piece were not too large, perhaps it could
be done in wax and then cast?

Carl




To cut a helix in wax you would need the exact same setup as for metal.

Abrasha
http://www.abrasha.com


--
to reply, change ( .not) to ( .net)

  #15  
Old May 10th 05, 04:52 AM
Bob
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Peter writes:

"Just a thought, though. I recall seeing wood workers setups used with
a router,
intended for doing the helical forms for things like twisted shape
table legs or
railings, or whatever. A form much like a coarse twisted rope. Those
jigs use a
premade form with the needed helix. Moving the router along the
workpiece causes a form
much like a half nut on a threading lathe to follow the preform, and
the preform is
linked to the workpiece, so the preform rotates and the workpiece
follows along,
resulting in a similar helical cut on the workpiece. "

Peter -- That is a reasonable approach to doing it on these small,
underpowered lathes -- using active tooling, such as a dremel motor
mounted in a toolholder on the cross-slide. A ball-nose end mill cuts
nice flutes. Then, as Abrasha points out, to cut really slow pitches,
you need either a full cnc setup, or one of these:
http://www.avatartools.com/

The Frog, in threading mode, syncs the lead screw advance to the lathe
spindle using a hall-effect sensor, and is only limited in pitch by
spindle speed and maximum slew rate of the stepper motor on the lathe
leadscrew. As the pitch gets coarser, you reach a point where the
cutter can't slew fast enough to keep up with the spindle. But with
the variable speed motor on a Sherline lathe, you could cut some very
coarse pitches. I have this setup on my Toyo (Sakai) lathe, and have
cut helical flutes with it in wax and soft metals. A follower rest on
the toolpost helps control workpiece deflection.

You are limited to a single helix, however. I haven't found a way to
cut multiple starting point threads with the Frog.

That's my machine, btw, at
http://www.avatartools.com/Merchant2...ory_Code=Sakai

I've since added a third stepper on the mill Z-axis.

Regards,

Bob


  #16  
Old May 10th 05, 04:52 AM
Abrasha
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Peter W.. Rowe, wrote:

I was referring to the rate at which one manually feeds the cross slide into the
work to advance the cutting tool farther with each additional pass.



That's called the "depth of cut"

Abrasha
http://www.abrasha.com

  #17  
Old May 10th 05, 04:53 AM
Peter W.. Rowe,
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On Mon, 09 May 2005 20:52:12 -0700, in ¸õ Abrasha wrote:

Peter W.. Rowe, wrote:

I was referring to the rate at which one manually feeds the cross slide into the
work to advance the cutting tool farther with each additional pass.



That's called the "depth of cut"

Abrasha
http://www.abrasha.com



thanks again, for the correction.

once, I seem to recall knowing all those correct terms. wonder what happened...

(grin)

Peter
  #18  
Old May 10th 05, 07:41 AM
Abrasha
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Peter W.. Rowe, wrote:
On Mon, 09 May 2005 20:52:12 -0700, in ¸õ Abrasha wrote:


Peter W.. Rowe, wrote:


I was referring to the rate at which one manually feeds the cross slide into the
work to advance the cutting tool farther with each additional pass.


That's called the "depth of cut"

Abrasha
http://www.abrasha.com




thanks again, for the correction.

once, I seem to recall knowing all those correct terms. wonder what happened...

(grin)

Peter



You probably stopped using the lathe and mill, and your need to know disappeared.

I use my lathe and mill almost daily.

Abrasha
http://www.abrasha.com

  #19  
Old May 10th 05, 07:41 AM
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norwick wrote:

I need to cut a spiral onto a silver rod.
The sides of the cut can be at 90 degrees/and/or inclined. I envisage
...
Any suggestions?


In one of the Foxfire books there is an article about how to rifle a gun
barrel 'by hand'. It would be some work to make the rig, but you could
modify their design, turn the cutter "inside-out" and make it work for
what you need.

The tool itself would require no electricity to use, but it wouldn't be
fast.

--
Carl West http://carl.west.home.comcast.net

change the 'DOT' to '.' to email me


"Clutter"? This is an object-rich environment.

  #20  
Old May 10th 05, 07:43 AM
Abrasha
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Andrew Werby wrote:


[Want to bet?


No. However your reply seems to indicate your need to be right by trying to
bluff me into a bet.

So I challenge you to produce the helix, the original poster asked about, on
your mighty Taig mill. I say, it can't be done. Go ahead, prove me wrong.
You've got the Taig. If you succeed, (which I know you won't), you have a
strong selling point. If I am not mistaken, you are after all trying to sell
these things, which seems to make you a bit biased. I find that rather amusing.

Don't get me wrong. The Taig is a very valid and verstile tool and a useful one
at that. And it cannot do what the original poster asked. You can try to bluff
me all you want, it can't be done. You can't do it.

Taig recently started providing a bigger motor with their
mills than previously. The 1/4 hp Franklin motor now standard would have no
problem cutting a 2mm groove in a piece of silver - or a piece of steel, for
that matter, in one pass, if one went at an appropriate speed using a 2mm
2-flute cutter.


We weren't talking about a groove. We were talking about a helix! That's what
the original poster wanted. Something very different.

However, you claim that the Taig, does a 2 mm depth of cut cut in steel, on a 15
mm diameter (original poster's measurement) in one pass? Prove it!


SNIP

Have you ever actually used one of these mills?]


No, I have no need to. I have a real mill, with a 3/4 HP, 3 phase motor.
Actually, it is a rather wimpy mill as far as true power is concerned. I'm
ready for something more powerful, like a Bridgeport.

But a Taig, gimme a break, why would I ever want to use one of those when I have
a Clausing 8520?

Now go and make that helix, you claim you can make on it. I say, you can't do it!

Abrasha
http://www.abrasha.com

 




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