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Pete Bergstrom
 
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Default Leadscrew design questions for mill retrofit

Last winter I bought a small milling machine at an auction. It was set up as
a production mill with a hydraulic drive on the X drive; as far as I can
tell it never had a leadscrew on this axis. The hydraulic pump looks pretty
beat up, and I don't want to screw around with it, so I am now going to
convert the mill to all leadscrew drive. I've got a few questions on doing
this efficiently (meaning with as few missteps as possible).

The mill is a Schaffner F7, made in Switzerland in the mid-70s. The table is
about 7x30" and is supported on a knee. I built a RPC for it (largely using
the design at http://www.frugalmachinist.com/rpc.html, but modifying it to
have a power-fail safety circuit), and both vertical and horizontal spindles
(ISO30 taper toolholders) work very nicely. Once I get this task done, I'll
put up a project picture show in the Dropbox. Later on, I plan to add a DRO
using the Shumatech project parts.

I've taken the hydraulics off, and there is pretty good support for my plan:
the table has ~1/2" thick aluminum endcaps that will be perfect for mounting
the ends of the leadscrew. There is a 7/8+" hole through the casting
supporting the table, so I can use that for the leadscrew. On one side of
this casting is a good spot for attaching the thrust nut and the end caps
should serve to anchor the leadscrew side to side.

I've been looking at the MSC catalog and they have a 3/4" x 5 precision
ground leadscrew (.003"/ft) and parts for a decent price (~$150-200 for
leadscrew, thrust nuts and flanges plus two handles). I'm tending toward the
5 tpi screw so I get 0.200 advance per turn; easier both for keeping track
of my milling and also for making an adjustable dial later on. Although the
mill is constructed with metric fasteners, the Y and Z axis dials are in
inches, so I'll stay with that.

My questions:

1. Do you know of magazine articles or books that cover this kind of
project? I haven't turned up anything at Village Press, and haven't come
across any other projects like this.

2. I plan to use bearings on at least one end of the leadscrew. Should I
attach the leadscrew on both ends of the table or just one? I plan to put a
handle at each end, but I'm not sure if it's wise to fasten it to each end
given the diversity of materials (cast iron table, aluminum endcaps, alloy
steel leadscrew) and the thermal expansion differences.

3. What sort of bearings should I use at the ends of the leadscrews? Thrust
would be simplest, and if I have to do both thrust and radial, I may want to
do it in multiple steps since I need to get the mill working enough to be
able to bore out the endcaps for radial.

4. How can I minimize backlash? I've been thinking that a pair of the thrust
nuts with some Belleville washers between them will create enough tension to
do this, but I've never done something like this before. I assume that I'll
have to keep the thrust nuts aligned for this preload to work. Any
suggestions? Reference books covering this?

5. Any better ideas? I'm not ready for ballscrews and automation, but will
likely tackle that in a few years. This mill is far more rigid than any of
the mill drills, and is still in great shape, so I don't consider buying
something else a good path.

6. What's been your experience with the 3 foot lengths of precision ground
leadscrew and shipping or handling damage? I might be able to just squeak by
with a 3 foot length, but I've been thinking that the 6 foot length is safer
since I can take the choicest section from the middle.

Thanks,
Pete Bergstrom
(in St. Paul, MN)


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Anthony
 
Posts: n/a
Default Leadscrew design questions for mill retrofit

"Pete Bergstrom" wrote in
:



My suggestion:
Use INA combination thrust/radial bearings (both bearings in one package),
and pre-load the screw. I would probably exchange the alum endcaps for
steel of the same design, the set-up will be more rigid, the alum will
deform under the bearings over time or heavy load, leading to increased
backlash.
How about a split-nut set-up for backlash compensation? You can either have
two half-nuts with an adjuster, or to full nuts with one drilled for set-
screws to press against the first to push them apart. You could do this
with one nut, provided the length it comes in agrees to that method.




--
Anthony

You can't 'idiot proof' anything....every time you try, they just make
better idiots.

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  #3   Report Post  
Eric R Snow
 
Posts: n/a
Default Leadscrew design questions for mill retrofit

On Wed, 4 Aug 2004 16:54:14 -0500, "Pete Bergstrom"
wrote:

Last winter I bought a small milling machine at an auction. It was set up as
a production mill with a hydraulic drive on the X drive; as far as I can
tell it never had a leadscrew on this axis. The hydraulic pump looks pretty
beat up, and I don't want to screw around with it, so I am now going to
convert the mill to all leadscrew drive. I've got a few questions on doing
this efficiently (meaning with as few missteps as possible).

The mill is a Schaffner F7, made in Switzerland in the mid-70s. The table is
about 7x30" and is supported on a knee. I built a RPC for it (largely using
the design at http://www.frugalmachinist.com/rpc.html, but modifying it to
have a power-fail safety circuit), and both vertical and horizontal spindles
(ISO30 taper toolholders) work very nicely. Once I get this task done, I'll
put up a project picture show in the Dropbox. Later on, I plan to add a DRO
using the Shumatech project parts.

I've taken the hydraulics off, and there is pretty good support for my plan:
the table has ~1/2" thick aluminum endcaps that will be perfect for mounting
the ends of the leadscrew. There is a 7/8+" hole through the casting
supporting the table, so I can use that for the leadscrew. On one side of
this casting is a good spot for attaching the thrust nut and the end caps
should serve to anchor the leadscrew side to side.

I've been looking at the MSC catalog and they have a 3/4" x 5 precision
ground leadscrew (.003"/ft) and parts for a decent price (~$150-200 for
leadscrew, thrust nuts and flanges plus two handles). I'm tending toward the
5 tpi screw so I get 0.200 advance per turn; easier both for keeping track
of my milling and also for making an adjustable dial later on. Although the
mill is constructed with metric fasteners, the Y and Z axis dials are in
inches, so I'll stay with that.

My questions:

1. Do you know of magazine articles or books that cover this kind of
project? I haven't turned up anything at Village Press, and haven't come
across any other projects like this.

2. I plan to use bearings on at least one end of the leadscrew. Should I
attach the leadscrew on both ends of the table or just one? I plan to put a
handle at each end, but I'm not sure if it's wise to fasten it to each end
given the diversity of materials (cast iron table, aluminum endcaps, alloy
steel leadscrew) and the thermal expansion differences.

3. What sort of bearings should I use at the ends of the leadscrews? Thrust
would be simplest, and if I have to do both thrust and radial, I may want to
do it in multiple steps since I need to get the mill working enough to be
able to bore out the endcaps for radial.

4. How can I minimize backlash? I've been thinking that a pair of the thrust
nuts with some Belleville washers between them will create enough tension to
do this, but I've never done something like this before. I assume that I'll
have to keep the thrust nuts aligned for this preload to work. Any
suggestions? Reference books covering this?

5. Any better ideas? I'm not ready for ballscrews and automation, but will
likely tackle that in a few years. This mill is far more rigid than any of
the mill drills, and is still in great shape, so I don't consider buying
something else a good path.

6. What's been your experience with the 3 foot lengths of precision ground
leadscrew and shipping or handling damage? I might be able to just squeak by
with a 3 foot length, but I've been thinking that the 6 foot length is safer
since I can take the choicest section from the middle.

Thanks,
Pete Bergstrom
(in St. Paul, MN)

Pete,
As Anthony said, replacing the aluminum pieces with steel would be
best. Bronze nuts for the steel leadscrew will work best. You should
be able to buy split nuts. These have a screw that pulls the two
halves of the nut together to minimize backlash. Also available, but
harder to adjust in the location you must put the nut in, are nuts
which screw together. The thread that holds them together is a much
finer pitch that the 5 TPI for the leadscrew. Screwing them in or out
changes the backlash. If you are really going to put CNC on this mill
later consider doing it now. The CNC should use ballscrews. They have
much less friction. This lack of friction makes them unsuitable for a
manual machine because they self feed easier. So when climb milling
the table lock must be set to drag enough to prevent the cutter from
dragging the work into it and breaking things. As far as shipping
damage goes, if it arrives damaged send it back. And why should the
choisest part of a new screw be in the middle? Are you planning to
measure the lead error? If so, I guess you might find a three foot
area in the six foot length that is best. BTW, I've bought acme screws
several times in the last 25 years and they have always been much
better than the maximum error allowed. But I've always bought USA
made. I have seen leadscrews from Taiwan that were terrible. I made a
little stepper driven X-Y stage to put on the Bridgeport for milling
small complex curved parts and the screws it came with were visibly
out of tolerance. The thread looked like the die was wobbling while
cutting the threads.
ERS
  #4   Report Post  
Richard J Kinch
 
Posts: n/a
Default Leadscrew design questions for mill retrofit

Pete Bergstrom writes:

2. I plan to use bearings on at least one end of the leadscrew. Should
I attach the leadscrew on both ends of the table or just one? I plan
to put a handle at each end, but I'm not sure if it's wise to fasten
it to each end given the diversity of materials (cast iron table,
aluminum endcaps, alloy steel leadscrew) and the thermal expansion
differences.

3. What sort of bearings should I use at the ends of the leadscrews?
Thrust would be simplest, and if I have to do both thrust and radial,
I may want to do it in multiple steps since I need to get the mill
working enough to be able to bore out the endcaps for radial.


The Bridgeport design uses a pair of precision angular contact bearings
on one end and (to maintain alignment) an inexpensive radial bearing on
the other.

4. How can I minimize backlash? I've been thinking that a pair of the
thrust nuts with some Belleville washers between them will create
enough tension to do this, but I've never done something like this
before. I assume that I'll have to keep the thrust nuts aligned for
this preload to work. Any suggestions? Reference books covering this?

5. Any better ideas? I'm not ready for ballscrews and automation, but
will likely tackle that in a few years.


If you're really going to undertake this kind of custom work, you might
as well do it with ballscrews. The parts cost will not be that much
more, and the effort about the same (even simpler in some respects), and
the results will be worth it.

Lotsa surplus ballscrews on eBay.
  #5   Report Post  
jim rozen
 
Posts: n/a
Default Leadscrew design questions for mill retrofit

In article , Pete Bergstrom
says...

2. I plan to use bearings on at least one end of the leadscrew. Should I
attach the leadscrew on both ends of the table or just one? I plan to put a
handle at each end, but I'm not sure if it's wise to fasten it to each end
given the diversity of materials (cast iron table, aluminum endcaps, alloy
steel leadscrew) and the thermal expansion differences.


What hardinge does in this case is to fix the handle/dial end of
the leadscrew with an effective thrust bearing, at the dial end.
The far end of the screw is simply supported in a radial bearing,
but the clearances are such that the turned down end of the
screw is allowed to float axially inside the bearing.

3. What sort of bearings should I use at the ends of the leadscrews? Thrust
would be simplest, and if I have to do both thrust and radial, I may want to
do it in multiple steps since I need to get the mill working enough to be
able to bore out the endcaps for radial.


You can approximate a thrust bearing by loading two radial bearings
up against each other.

4. How can I minimize backlash? I've been thinking that a pair of the thrust
nuts with some Belleville washers between them will create enough tension to
do this, but I've never done something like this before. I assume that I'll
have to keep the thrust nuts aligned for this preload to work. Any
suggestions? Reference books covering this?


There are *two* kinds of backlash you need to consider. The first
is the sort that happens when you push/pull on the handle itself,
and the imperfect thrust bearings allow the entire leadscrew
to shift axially with respect to the slide. If you are using
radials loaded against each other, or true thrust bearings, this
is trivial to get down to around a thousanth or less.

As others have suggested the bronze nut you choose for the
leadscrew will have some natural lash which you can reduce
by making it a split nut of some sort. I myself happen to be
partial to hardinge's two-part nuts that thread together.
I would do it that way.

5. Any better ideas? I'm not ready for ballscrews and automation,


Others have mentioned using ball screws. If you are planning
on running this as a manual machine, do NOT use ballscrews. The
lack of friction in those means the table will walk away from
whatever cut you are taking, or hog in if you are climb milling.
Unless you put a lock on the table and on the shaft, it will
not work well.

Jim

--
==================================================
please reply to:
JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com
==================================================



  #6   Report Post  
DoN. Nichols
 
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Default Leadscrew design questions for mill retrofit

In article ,
Richard J Kinch wrote:
Pete Bergstrom writes:


[ ... ]

5. Any better ideas? I'm not ready for ballscrews and automation, but
will likely tackle that in a few years.


If you're really going to undertake this kind of custom work, you might
as well do it with ballscrews. The parts cost will not be that much
more, and the effort about the same (even simpler in some respects), and
the results will be worth it.

Lotsa surplus ballscrews on eBay.


If you *do* put ballscrews into a manual mill, be sure to also
provide a brake for the handwheel so the work can't drive the ballscrew.
(It normally can, if you don't have either a stepper or a servo motor
loading the handwheel to prevent rotation. A climb milling cut on an
axis with ballscrews can get *very* exciting *very* suddenly. :-)

Good Luck,
DoN.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---
  #7   Report Post  
Pete Bergstrom
 
Posts: n/a
Default Leadscrew design questions for mill retrofit


"jim rozen" wrote in message
...
In article , Pete Bergstrom
says...

2. I plan to use bearings on at least one end of the leadscrew. Should I
attach the leadscrew on both ends of the table or just one? I plan to put

a
handle at each end, but I'm not sure if it's wise to fasten it to each

end
given the diversity of materials (cast iron table, aluminum endcaps,

alloy
steel leadscrew) and the thermal expansion differences.


What hardinge does in this case is to fix the handle/dial end of
the leadscrew with an effective thrust bearing, at the dial end.
The far end of the screw is simply supported in a radial bearing,
but the clearances are such that the turned down end of the
screw is allowed to float axially inside the bearing.


Excellent idea and it seems very simple to do.

3. What sort of bearings should I use at the ends of the leadscrews?

Thrust
would be simplest, and if I have to do both thrust and radial, I may want

to
do it in multiple steps since I need to get the mill working enough to be
able to bore out the endcaps for radial.


You can approximate a thrust bearing by loading two radial bearings
up against each other.


I'm primarily attracted to thrust bearings because of space issues, but
simplicity is important too. I don't know a lot about bearings, but my
understanding is that a good thrust bearing will have some control over
radial movement while primarily working to control axial movement. It seems
like that's exactly what I need.

4. How can I minimize backlash? I've been thinking that a pair of the

thrust
nuts with some Belleville washers between them will create enough tension

to
do this, but I've never done something like this before. I assume that

I'll
have to keep the thrust nuts aligned for this preload to work. Any
suggestions? Reference books covering this?


There are *two* kinds of backlash you need to consider. The first
is the sort that happens when you push/pull on the handle itself,
and the imperfect thrust bearings allow the entire leadscrew
to shift axially with respect to the slide. If you are using
radials loaded against each other, or true thrust bearings, this
is trivial to get down to around a thousanth or less.


I'm not sure what you mean by imperfect versus true thrust bearings. Can you
say more? I meant the bronze nut and flange (like MSC has on page 3890
online) when I said thrust nut, since that is what I'd attach to the knee
casting. No bearing involved there. The ends of the Acme rod would be
supported by a radial bearing at one table end as you described above and
thrust bearings at the other end.

As others have suggested the bronze nut you choose for the
leadscrew will have some natural lash which you can reduce
by making it a split nut of some sort. I myself happen to be
partial to hardinge's two-part nuts that thread together.
I would do it that way.


Are there pictures or drawings of this arrangement available somewhere? For
example, if I buy CDs from lathes.co.uk, would it be on there? A manual from
Hardinge? I'll probably jump in and get it working with one bronze nut and
simple end bearings first, and then refine it with some of the suggestions
I've been given.

5. Any better ideas? I'm not ready for ballscrews and automation,


Others have mentioned using ball screws. If you are planning
on running this as a manual machine, do NOT use ballscrews. The
lack of friction in those means the table will walk away from
whatever cut you are taking, or hog in if you are climb milling.
Unless you put a lock on the table and on the shaft, it will
not work well.


Oh, and I really do like climb milling results, based on the couple of
projects I've milled to date.

As far as I can see, I don't think I have a lock on the X axis for the table
(there is a lock handle that might work for either X or Y, but it looks to
have been misused since it swings freely and I can't tell which axis it
applies to), so I'll eventually have to put one on. Using ball screws would
move this up in urgency, but I'm hoping that putting a lock on the shaft
will get things going for this initial step.

Thanks!!
Pete



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Pete Bergstrom
 
Posts: n/a
Default Leadscrew design questions for mill retrofit


"Anthony" wrote in message
...
"Pete Bergstrom" wrote in
:
My suggestion:
Use INA combination thrust/radial bearings (both bearings in one package),
and pre-load the screw. I would probably exchange the alum endcaps for
steel of the same design, the set-up will be more rigid, the alum will
deform under the bearings over time or heavy load, leading to increased
backlash.


I think I've found these now in MSC's catalog. They look like they'll do
very nicely!

How about a split-nut set-up for backlash compensation? You can either

have
two half-nuts with an adjuster, or to full nuts with one drilled for set-
screws to press against the first to push them apart. You could do this
with one nut, provided the length it comes in agrees to that method.


Thanks!!
Pete


  #9   Report Post  
Pete Bergstrom
 
Posts: n/a
Default Leadscrew design questions for mill retrofit


"Eric R Snow" wrote in message
...

As Anthony said, replacing the aluminum pieces with steel would be
best. Bronze nuts for the steel leadscrew will work best. You should
be able to buy split nuts. These have a screw that pulls the two
halves of the nut together to minimize backlash. Also available, but
harder to adjust in the location you must put the nut in, are nuts
which screw together. The thread that holds them together is a much
finer pitch that the 5 TPI for the leadscrew. Screwing them in or out
changes the backlash.


Ah, that kind of nut sounds like a really good way to go. Do you have any
leads on where to find these? I haven't seen them in MSC or McMaster.

If you are really going to put CNC on this mill
later consider doing it now. The CNC should use ballscrews. They have
much less friction. This lack of friction makes them unsuitable for a
manual machine because they self feed easier. So when climb milling
the table lock must be set to drag enough to prevent the cutter from
dragging the work into it and breaking things.


That's kind of a way out, and I'd rather have a working mill than an ongoing
project right now. Thanks for the pointers.

As far as shipping
damage goes, if it arrives damaged send it back. And why should the
choisest part of a new screw be in the middle? Are you planning to
measure the lead error?


Nope, just have come to expect damage in handling, even with something
labeled precision. Your points are good.

If so, I guess you might find a three foot
area in the six foot length that is best. BTW, I've bought acme screws
several times in the last 25 years and they have always been much
better than the maximum error allowed. But I've always bought USA
made. I have seen leadscrews from Taiwan that were terrible. I made a
little stepper driven X-Y stage to put on the Bridgeport for milling
small complex curved parts and the screws it came with were visibly
out of tolerance. The thread looked like the die was wobbling while
cutting the threads.


I've been trying to keep to American-made materials in projects; with this
one it's a no-brainer.

Thanks!!
Pete



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