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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  #1   Report Post  
B.B.
 
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Default Less precise machines building more precise machines

I was reading about "Gravity Probe B" yesterday.
So I was wondering today how we arrived at the point of having such
accurate machinery when we used to not. My limited knowledge of
machining implies that the precision of a part is limited by the
precision of the machine making it, but I'm sure there are ways to get
beyond that.
I asked my teacher about high-precision bearings and he suggested
centerless grinding as a possible way. Can a centerless grinder be used
(even if tedious) to manufacture bearings more precise than those
already in the grinder? Granted, it's a theoretical question--I have no
plans to make any bearings that way.
Or just in general, what are some of the straightforward methods of
making accurate parts on less accurate machines?
For that matter, how did they scale upwards to build lathes and such
many yards long?

--
B.B. --I am not a goat! thegoat4 at airmail.net
  #2   Report Post  
Stephen Young
 
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B.B. wrote:
I was reading about "Gravity Probe B" yesterday.
So I was wondering today how we arrived at the point of having such
accurate machinery when we used to not. My limited knowledge of
machining implies that the precision of a part is limited by the
precision of the machine making it, but I'm sure there are ways to get
beyond that.
I asked my teacher about high-precision bearings and he suggested
centerless grinding as a possible way. Can a centerless grinder be used
(even if tedious) to manufacture bearings more precise than those
already in the grinder? Granted, it's a theoretical question--I have no
plans to make any bearings that way.
Or just in general, what are some of the straightforward methods of
making accurate parts on less accurate machines?
For that matter, how did they scale upwards to build lathes and such
many yards long?

Hand fit/scraping
  #3   Report Post  
Sal D'Ambra
 
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BB,

Some of the most precise surfaces in the world, mirrors for astronomical
telescopes, can be accurately shaped to on sixteenth a wavelength of light
or approximately 1/1,000,000 of an inch. The work is done by hand grinding
with simple equipment.

Sal (A lurker here)
--------------------------
Sal D'Ambra
Blue Ridge Summit, PA
"B.B." u wrote in message
news
I was reading about "Gravity Probe B" yesterday.
So I was wondering today how we arrived at the point of having such
accurate machinery when we used to not. My limited knowledge of
machining implies that the precision of a part is limited by the
precision of the machine making it, but I'm sure there are ways to get
beyond that.
I asked my teacher about high-precision bearings and he suggested
centerless grinding as a possible way. Can a centerless grinder be used
(even if tedious) to manufacture bearings more precise than those
already in the grinder? Granted, it's a theoretical question--I have no
plans to make any bearings that way.
Or just in general, what are some of the straightforward methods of
making accurate parts on less accurate machines?
For that matter, how did they scale upwards to build lathes and such
many yards long?

--
B.B. --I am not a goat! thegoat4 at airmail.net



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B.B.
 
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In article ,
"Tim Williams" wrote:

"B.B." u wrote in message
news
Or just in general, what are some of the straightforward methods of
making accurate parts on less accurate machines?
For that matter, how did they scale upwards to build lathes and such
many yards long?


Simple: machines don't make machines. You can speed up the process of
building a lathe or another mill by milling the surfaces, but if you want a
certain precision, that will only go so far. You have to manually reduce
the errors, and the rule-of-three method does this quite nicely. What's
done is you use the rule-of-three to lap three surface plates flat, then
compare these surfaces to the surfaces that need to be flat and remove
material from just that surface (aka scraping).

Tim


What about curved things like shafts? Is there a similar "by hand"
way to achieve good regular curves?

--
B.B. --I am not a goat! thegoat4 at airmail.net
  #5   Report Post  
Larry Jaques
 
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On Mon, 13 Sep 2004 23:23:15 -0400, "Sal D'Ambra"
calmly ranted:

BB,

Some of the most precise surfaces in the world, mirrors for astronomical
telescopes, can be accurately shaped to on sixteenth a wavelength of light
or approximately 1/1,000,000 of an inch. The work is done by hand grinding
with simple equipment.


Or successfully polished by machinery for several years to obtain
a mirror finish at the wrong curvature and focal length, with which
the Hubble was originally fit. The retrofit was a set of bifocals.


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Daniel A. Mitchell
 
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Stephen Young wrote:
B.B. wrote:

I was reading about "Gravity Probe B" yesterday.
So I was wondering today how we arrived at the point of having such
accurate machinery when we used to not. My limited knowledge of
machining implies that the precision of a part is limited by the
precision of the machine making it, but I'm sure there are ways to get
beyond that.
I asked my teacher about high-precision bearings and he suggested
centerless grinding as a possible way. Can a centerless grinder be
used (even if tedious) to manufacture bearings more precise than those
already in the grinder? Granted, it's a theoretical question--I have
no plans to make any bearings that way.
Or just in general, what are some of the straightforward methods of
making accurate parts on less accurate machines?
For that matter, how did they scale upwards to build lathes and
such many yards long?

Hand fit/scraping


True indeed. It's amazing the degree of precision that can be obtained
with very simple tools and techniques ... applied properly. You can do
precise work with a saw, chisel, file, and abrasives ... it just takes
knowledge, time, and great care. Consider the surface plates and optical
surfaces that were (and sometimes still are) constructed by mostly hand
labor alone. As you correctly surmise, any machine is capable of doing
work MORE precise than it's own level of precision. The difference is in
the operator's knowledge and skill.

Dan Mitchell
============

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Leon Heller
 
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"Sal D'Ambra" wrote in message
...
BB,

Some of the most precise surfaces in the world, mirrors for astronomical
telescopes, can be accurately shaped to on sixteenth a wavelength of light
or approximately 1/1,000,000 of an inch. The work is done by hand grinding
with simple equipment.


I saw somewhere that the 200" Hale telescope mirror was finished by lightly
rubbing the high spots with talcum powder on a thumb.

Leon


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B.B.
 
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Default

In article ,
"Tim Williams" wrote:

"B.B." u wrote in message
news
What about curved things like shafts? Is there a similar "by hand"
way to achieve good regular curves?


I don't know how the first shaft was made, probably lapped or scraped to the
bearings so it happens to turn round.

Once you have a shaft running parallel to the bed (easy to do with yet
another bit of trickery), you can advance a carriage along the bed and turn
straight shafts. If you just turn the end of the spindle, you can very
easily improve the precision of the headstock right there.

The fun thing about turning things, though, is that if you spin one point in
space around an axis, it traces a circle, which defines a plane. Viola, you
have a perpendicular reference!

Tim


I wonder. If you scrape two plates flat, then weld them together at
an angle to form a trough, filled it with an abrasive slurry, could you
drop a rough shaft in there and rotate it, letting it rub against both
sides, could you polish a shaft into straightness?
I figure it'll rub down the high spots just because they'll hit the
sides first. It could probably eliminate taper too since the bigger end
would have a higher edge velocity rubbing the sides than the smaller
end, and so would rub down more quickly.

--
B.B. --I am not a goat! thegoat4 at airmail.net
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Vince Mulhollon
 
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"B.B." u writes:

I wonder. If you scrape two plates flat, then weld them together at
an angle to form a trough, filled it with an abrasive slurry, could you
drop a rough shaft in there and rotate it, letting it rub against both
sides, could you polish a shaft into straightness?
I figure it'll rub down the high spots just because they'll hit the
sides first. It could probably eliminate taper too since the bigger end
would have a higher edge velocity rubbing the sides than the smaller
end, and so would rub down more quickly.


It will be as round as a twist-drilled hole. In other words it'll
be vaguely round, but will be something of a rounded polygon, probably
a rounded triangle. I forget the geometric name for the shape.

What would work better would be to place the shaft on a single plate.
That would fix random error and would be nice and round.
But eliminating systematic error is a puzzle to me at this time.
  #10   Report Post  
B.B.
 
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In article ,
Vince Mulhollon wrote:

"B.B." u writes:

I wonder. If you scrape two plates flat, then weld them together at
an angle to form a trough, filled it with an abrasive slurry, could you
drop a rough shaft in there and rotate it, letting it rub against both
sides, could you polish a shaft into straightness?
I figure it'll rub down the high spots just because they'll hit the
sides first. It could probably eliminate taper too since the bigger end
would have a higher edge velocity rubbing the sides than the smaller
end, and so would rub down more quickly.


It will be as round as a twist-drilled hole. In other words it'll
be vaguely round, but will be something of a rounded polygon, probably
a rounded triangle. I forget the geometric name for the shape.

What would work better would be to place the shaft on a single plate.
That would fix random error and would be nice and round.
But eliminating systematic error is a puzzle to me at this time.


How about keeping the angled plates, but varying the angle
occasionally to avoid a pattern forming?

--
B.B. --I am not a goat! thegoat4 at airmail.net


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Charles Erskine
 
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"B.B." u wrote in message ...
In article ,
"Tim Williams" wrote:

"B.B." u wrote in message
news
Or just in general, what are some of the straightforward methods of
making accurate parts on less accurate machines?
For that matter, how did they scale upwards to build lathes and such
many yards long?


Simple: machines don't make machines. You can speed up the process of
building a lathe or another mill by milling the surfaces, but if you want a
certain precision, that will only go so far. You have to manually reduce
the errors, and the rule-of-three method does this quite nicely. What's
done is you use the rule-of-three to lap three surface plates flat, then
compare these surfaces to the surfaces that need to be flat and remove
material from just that surface (aka scraping).

Tim


What about curved things like shafts? Is there a similar "by hand"
way to achieve good regular curves?


Given enough time, can make what you can measure.

Mount the shaft between centers and file/sand it to a uniform
diameter. This works even if the centers are not precisely aligned if
you measure it at many points along its length. You can use outside
calipers to compare the diameter at different points. If you are going
to scrape the bearing to fit the shaft, you don't care so much what
size it is as long as it is cylindrical.
  #12   Report Post  
brownnsharp
 
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Good Grief!!! Y'all should go to the library and get a book on the
history of machine tools.

People made accurately round things long before they made accurately
flat and straight. Take a part, put a dimple in each end. Support the
part with points stuck in the dimples, and spin. Cutting on the
outside of the spinning part defaults to round.

The magic trick to getting a straight-round shaft is to mate reference
straight parts to the support points stuck in the aformentioned
dimples. The device that does this is called a "lathe".

"History of Machine Tools" by Steeds is a good read.
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