"Matthew Zenkar" wrote in message
news | Please pardon me if this is not the correct forum for this message.
|
| I am working on a design that I think is best described as a precision
| positioning assembly for optics. I contacted a professional gear
| designer, and I was told by this person that the design would be
| improved if I used an acme thread for the threaded rod that is in the
| assembly.
|
| My initial design called for a 3/8"-40 thread as this is the finest
| standard thread size I could find to fit a 3/8" diameter rod. However,
| looking in Machinery's Handbook, I found that the finest standard acme
| thread is 1/4-16. I also figure that I would be able to use a 20 pitch
| with another minor adjustment in the assembly.
|
| Is it common practice to have custom threading made for those cases
| where standard threads do not meet the needs of a project? For my
| case, I would like to have acme threaded parts where the thread size
| is 3/8 or 1/2-20, more preferably, 3/8 or 1/2-40.
|
| Also, if it is not heard of to have acme threads custom acme threads
| made in those particular sizes, what about having what Machinery's
| Handbook calls "unified screw threads" cut to a non-standard size, say
| 1/2-40 - is this done for cases where standard threads do not meet the
| needs of a project?
|
| Also, does anyone have any recommendations as to where I might get
| this done (preferably by a professional shop)?
|
| Thanks in advance for your response.
|
| Best Regards,
| Matthew

Given the same amount of tolerance, the finer the thread pitch the more
precise the movement.
The diameter of the rod is really not a critical factor.

"Matthew Zenkar" wrote in message
news
Please pardon me if this is not the correct forum for this message.

I am working on a design that I think is best described as a precision
positioning assembly for optics. I contacted a professional gear
designer, and I was told by this person that the design would be
improved if I used an acme thread for the threaded rod that is in the
assembly.

My initial design called for a 3/8"-40 thread as this is the finest
standard thread size I could find to fit a 3/8" diameter rod. However,
looking in Machinery's Handbook, I found that the finest standard acme
thread is 1/4-16. I also figure that I would be able to use a 20 pitch
with another minor adjustment in the assembly.

Is it common practice to have custom threading made for those cases
where standard threads do not meet the needs of a project? For my
case, I would like to have acme threaded parts where the thread size
is 3/8 or 1/2-20, more preferably, 3/8 or 1/2-40.

Also, if it is not heard of to have acme threads custom acme threads
made in those particular sizes, what about having what Machinery's
Handbook calls "unified screw threads" cut to a non-standard size, say
1/2-40 - is this done for cases where standard threads do not meet the
needs of a project?

Also, does anyone have any recommendations as to where I might get
this done (preferably by a professional shop)?

Thanks in advance for your response.

Best Regards,
Matthew

Acme threads are generally restricted to rather coarse pitches-----so you're
highly unlikely to be successful in your quest to use 1/2"-40.

Standard 60 degree threads (the unified thread series) are generally
selected for your application, and are available in pretty much any
pitch/diameter combination you desire. *******, or non-standard, taps are
readily available from supply houses, but should you elect to go with
something that is considered unusual, or non-standard, you can have taps
made for a reasonable price. You should be able to buy a 1/2"-40 tap with
no trouble.

The nature of your device may dictate the need for a ground thread.
Threads can be generated by various means, from cutting with a die, a die
head, rolling, single point threading on a lathe, or grinding. I may
have even left out some other method. It might be helpful to know more
about your thread requirement before suggesting a system that would be in
keeping with your requirements. Are you looking for one, or many? How
long must the thread be? What material? Considering you are using the
thread for adjusting, or locating, you may even wish to consider threads
without lash, or clearance.

Harold

Please pardon me if this is not the correct forum for this message.

I am working on a design that I think is best described as a precision
positioning assembly for optics. I contacted a professional gear
designer, and I was told by this person that the design would be
improved if I used an acme thread for the threaded rod that is in the
assembly.

My initial design called for a 3/8"-40 thread as this is the finest
standard thread size I could find to fit a 3/8" diameter rod. However,
looking in Machinery's Handbook, I found that the finest standard acme
thread is 1/4-16. I also figure that I would be able to use a 20 pitch
with another minor adjustment in the assembly.

Is it common practice to have custom threading made for those cases
where standard threads do not meet the needs of a project? For my
case, I would like to have acme threaded parts where the thread size
is 3/8 or 1/2-20, more preferably, 3/8 or 1/2-40.

Also, if it is not heard of to have acme threads custom acme threads
made in those particular sizes, what about having what Machinery's
Handbook calls "unified screw threads" cut to a non-standard size, say
1/2-40 - is this done for cases where standard threads do not meet the
needs of a project?

Also, does anyone have any recommendations as to where I might get
this done (preferably by a professional shop)?

Thanks in advance for your response.

Best Regards,
Matthew

Matthew Zenkar wrote:
I am working on a design that I think is best described as a precision
positioning assembly for optics. I contacted a professional gear
designer, and I was told by this person that the design would be
improved if I used an acme thread for the threaded rod that is in the
assembly.

My initial design called for a 3/8"-40 thread as this is the finest
standard thread size I could find to fit a 3/8" diameter rod. However,
looking in Machinery's Handbook, I found that the finest standard acme
thread is 1/4-16. I also figure that I would be able to use a 20 pitch
with another minor adjustment in the assembly.
....

You haven't told us why the "professional gear designer" believes
an acme thread would be better. Whether that is so depends, of
course, on what your device has to do. Acme threads are used in
several places on lathes, for positioning things accurately and
solidly. You might be able to buy an assembly to do what you want;
eg search for compound slide on ebay to see items like
http://cgi.ebay.com/ws/eBayISAPI.dll...tem=7609388048
http://cgi.ebay.com/ws/eBayISAPI.dll...tem=8923903725 and
http://cgi.ebay.com/ws/eBayISAPI.dll...tem=7607673342
(The first item has a 1/4-20 thread, not acme)

If the optics are lightweight, you might be better off adding a
DRO to your device and keeping the 3/8"-40 thread. See examples
http://cgi.ebay.com/ws/eBayISAPI.dll...tem=7606647083
http://cgi.ebay.com/ws/eBayISAPI.dll...tem=7606532048 and
http://cgi.ebay.com/ws/eBayISAPI.dll...tem=7606892197
and other items in same categories.
-jiw

Matthew Zenkar writes:

Is it common practice to have custom threading made for those cases
where standard threads do not meet the needs of a project?

Absolutely. Instrumentation is full of it. A common example is filter
threads on cameras.

You seem to have settled a lot of design questions prematurely, as if you
hadn't considered aspects like backlash and thread pitch inaccuracy.

Also, does anyone have any recommendations as to where I might get
this done (preferably by a professional shop)?

I design and built a lot of finely threaded parts for optical applications.

http://www.truetex.com/

On Wed, 12 Apr 2006 23:27:31 -0500, Matthew Zenkar
wrote:

Please pardon me if this is not the correct forum for this message.

I am working on a design that I think is best described as a precision
positioning assembly for optics. I contacted a professional gear
designer, and I was told by this person that the design would be
improved if I used an acme thread for the threaded rod that is in the
assembly.

My initial design called for a 3/8"-40 thread as this is the finest
standard thread size I could find to fit a 3/8" diameter rod. However,
looking in Machinery's Handbook, I found that the finest standard acme
thread is 1/4-16. I also figure that I would be able to use a 20 pitch
with another minor adjustment in the assembly.

Is it common practice to have custom threading made for those cases
where standard threads do not meet the needs of a project? For my
case, I would like to have acme threaded parts where the thread size
is 3/8 or 1/2-20, more preferably, 3/8 or 1/2-40.

Also, if it is not heard of to have acme threads custom acme threads
made in those particular sizes, what about having what Machinery's
Handbook calls "unified screw threads" cut to a non-standard size, say
1/2-40 - is this done for cases where standard threads do not meet the
needs of a project?

Acme threads have better load-bearing properties than V threads due to
greater cross-sectional area, but it appears that your application is
more about positioning than about load. I think micrometers have
V-threads, not Acme threads.

"Don Foreman" wrote in message
...
On Wed, 12 Apr 2006 23:27:31 -0500, Matthew Zenkar
wrote:

Please pardon me if this is not the correct forum for this message.

I am working on a design that I think is best described as a precision
positioning assembly for optics. I contacted a professional gear
designer, and I was told by this person that the design would be
improved if I used an acme thread for the threaded rod that is in the
assembly.

My initial design called for a 3/8"-40 thread as this is the finest
standard thread size I could find to fit a 3/8" diameter rod. However,
looking in Machinery's Handbook, I found that the finest standard acme
thread is 1/4-16. I also figure that I would be able to use a 20 pitch
with another minor adjustment in the assembly.

Is it common practice to have custom threading made for those cases
where standard threads do not meet the needs of a project? For my
case, I would like to have acme threaded parts where the thread size
is 3/8 or 1/2-20, more preferably, 3/8 or 1/2-40.

Also, if it is not heard of to have acme threads custom acme threads
made in those particular sizes, what about having what Machinery's
Handbook calls "unified screw threads" cut to a non-standard size, say
1/2-40 - is this done for cases where standard threads do not meet the
needs of a project?

Acme threads have better load-bearing properties than V threads due to
greater cross-sectional area, but it appears that your application is
more about positioning than about load. I think micrometers have
V-threads, not Acme threads.

Correct micrometers have V-threads and the threads for decimal micrometer
will be 40tpi and the tumbler has 25 equal divisions which gives you a
..001 resalution, i purchased a 1/2 X 40tpi die from msc a few years back
just to clean up micrometer threads when they get gummed up from use
in a coolent rich enviroment. I also have some 1/2 X 80tpi taps and dies
left over from a prototype i built back in the early 90's that came from
msc.

MSC or Field Tool can supply those special thread taps and dies.

Best Regards
Tom.

Acme threads have better load-bearing properties than V threads due to
greater cross-sectional area, but it appears that your application is
more about positioning than about load. I think micrometers have
V-threads, not Acme threads.

Haven't seen your design, so this may not work. Years ago, I had to me able
to adjust a position within 0.005". We started with a micrometer, machined
off the C frame to a mount and then bonded the plunger to the device that
had to move. Cheap quick and accurate.

Karl

I have some precision positioni assemblies that were, oddly enough
procured from a screen printing table.
These tables were used in the manuf. of printed circuit boards. The
very fine threaded rod threads thru a nylon ball that is captured
between two aluminum plates with concave divits. This allows the
assembly to swivel and still be precise.
A trained operator can make minor adjustments as small as ..001" maybe
less as temp stabilizes.
would like to sell these for $30 plus shipping

One place to start with this sort of thing is a design textbook on
kinematics, you might find your design changing. Acme threads are used
for heavy-duty load moving, generally not for precision work. For
short-distance precision manual movements of 1-2", micrometer heads are
readily available. A lot depends on what you define "precision" as and
how fine you want your adjustments to be. Are they to be powered? You
can get fine metric threaded rod from a number of sources although if
you insist on large diameter and fine threads, you're going to have to
look for somebody to thread up something custom. Or see if you can
find some surplus optical table X-Y positioners and use those screws
and nuts or maybe the whole assembly. Edmund Optics has a lot of stuff
like that, just got their new catalog.

Stan

In article , Matthew Zenkar says...

I am working on a design that I think is best described as a precision
positioning assembly for optics.

Do not re-invent the wheel. There are a plethora of companies out there
(Thorlabs, Ealing, Newport, etc) that manfacture all kinds of optical
tranlators, in varying sorts of load capacities.

If you read those catalogs you will get some idea of how this is best
done.

Unless you specify a load rating and required precision for your
particular application, any suggestions about what kind of threads
to use will be sketchy at best.

Jim


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

"jim rozen" wrote in message
...

Do not re-invent the wheel. There are a plethora of companies out there
(Thorlabs, Ealing, Newport, etc) that manfacture all kinds of optical
tranlators, in varying sorts of load capacities.

And certainly cheaper than paying someone $65+/hour to build an unproven
design. I know New Focus used to sell their precision fine-pitch screws with
matching brass nuts, thumbwheels and ball-bearing points as a package. Not
sure how expensive, but definitely cheaper than a job shop.

Regards,

Robin

"Robin S." wrote in message
.. .

"jim rozen" wrote in message
...

Do not re-invent the wheel. There are a plethora of companies out there
(Thorlabs, Ealing, Newport, etc) that manfacture all kinds of optical
tranlators, in varying sorts of load capacities.

And certainly cheaper than paying someone $65+/hour to build an unproven
design. I know New Focus used to sell their precision fine-pitch screws
with matching brass nuts, thumbwheels and ball-bearing points as a
package. Not sure how expensive, but definitely cheaper than a job shop.

Regards,

Robin

$65.00 an hour ?? I charge $200 an hour and make lots of mistakes.

Bob Swinney

....
$65.00 an hour ?? I charge $200 an hour and make lots of mistakes.
....

Ahha. That makes you a consultant, not a machinist. Grin

Karl

In article , Robert Swinney says...

$65.00 an hour ?? I charge $200 an hour and make lots of mistakes.

I raise your 200, make lots of mistakes, *and* break stuff too.

Plus I require large quantities of coffee and donuts too.

:^)

Jim


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

Get a New Focus catalog for some design ideas if you want to go that route.

A micrometer head can be purchased for a lot less than it would cost to
have one made.

Fred

jim rozen" wrote in message
...
In article , Robert Swinney says...

$65.00 an hour ?? I charge $200 an hour and make lots of mistakes.

I raise your 200, make lots of mistakes, *and* break stuff too.

Plus I require large quantities of coffee and donuts too.

:^)

Jim


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

Yeah but Jim, you gotta public job! *Poor* craftsmen have to forego some
of the perks, don't you know. 200 bux an hour is cheap labor compared to
the defense contractor workers that only make $50 an hour but each project
takes inordinately long because of the ridiculously tight, usu. unnecessary,
tight tolerances.

Bob (ducking incoming) Swinney

On Sat, 15 Apr 2006 18:55:25 -0500, Matthew Zenkar wrote:

Many thanks for all the replies.

I get the feeling that I'm somewhat out of my league here. I do not have a
mechanical background, and I have been teaching myself what I need to know
for my project as I go along. I'm doing the best that I can under the
circumstances, and the help is appreciated.


Have you considered a differantial screw? Basicly a left hand pitch
working with a right hand pitch. By selecting the differance you can come
up with some very fine feeds and with a couple of tricks one can reduce
backlash to near zero.

Scott

Many thanks for all the replies.

I get the feeling that I'm somewhat out of my league here. I do not
have a mechanical background, and I have been teaching myself what I
need to know for my project as I go along. I'm doing the best that I
can under the circumstances, and the help is appreciated.


On Wed, 12 Apr 2006 20:53:10 -0700, "Harold and Susan Vordos"
wrote:


"Matthew Zenkar" wrote in message
news
Please pardon me if this is not the correct forum for this message.

I am working on a design that I think is best described as a precision
positioning assembly for optics. I contacted a professional gear
designer, and I was told by this person that the design would be
improved if I used an acme thread for the threaded rod that is in the
assembly.
snip

Thanks in advance for your response.

Best Regards,
Matthew

Acme threads are generally restricted to rather coarse pitches-----so you're
highly unlikely to be successful in your quest to use 1/2"-40.

Thanks. That is valuable information to have.

Standard 60 degree threads (the unified thread series) are generally
selected for your application, and are available in pretty much any
pitch/diameter combination you desire. *******, or non-standard, taps are
readily available from supply houses, but should you elect to go with
something that is considered unusual, or non-standard, you can have taps
made for a reasonable price. You should be able to buy a 1/2"-40 tap with
no trouble.
I had no idea that "non-standard" threads of many different sizes were
available. As I see it, 1/2-80 or even 1/2-100 thread would bring me
much closer to my goal in the assembly.

First, let me say this, I am not working on an off-the-shelf optical
mount. The components that New Focus makes give me an idea of what is
possible, however, the stock components do not appear suitable for my
application.

Assemblies of the type that I am working on exist in the commercial
marketplace, however, they are extremely poor in terms of precision
positioning and, once the precision adjustment is achieved, they are
poor at locking the adjustment in place. Once the adjustment is locked
in place, it is highly unlikely that a user would reposition the
assembly. There are cases where this might happen, however, the
majority of use cases would not require this. (On that note, to the
person who replied with the DRO suggestion, that option might come in
handy for these few use cases. Thank you for that suggestion.)

Also, let me say this. I have been working on this for a long time. I
have some pretty good 3-d modeling and FEA tools, so I have been
through quite a few configurations and extensive analysis of the
assembly. While I realize that the 3-d and FEA tools may not perfectly
reflect the real-world device, I have a good idea of how to accomplish
my goal, and I have a solid background in physics and math. I'm open
to suggestions. However, I may be reluctant to deviate much from my
current course especially if it is going to require a significant
redesign of the assembly.


The nature of your device may dictate the need for a ground thread.

The primary goal is to achieve precision positioning.

Threads can be generated by various means, from cutting with a die, a die
head, rolling, single point threading on a lathe, or grinding. I may
have even left out some other method. It might be helpful to know more
about your thread requirement before suggesting a system that would be in
keeping with your requirements. Are you looking for one, or many? How
long must the thread be? What material? Considering you are using the
thread for adjusting, or locating, you may even wish to consider threads
without lash, or clearance.

For now, I am looking to have perhaps three or four each of
internal/external threaded pieces to build three or four prototypes of
the assembly. After that, I do not know if I will need more. I am
hoping to go to market with it.

Currently, here is what I am looking at -

1/2-80 or 1/2-100 thread as follows: (I'd prefer the 1/2-100, however,
I don't know what can be done. I'm hoping for feedback as to what is
possible - even if that means someone out there has to tell me that
I'm insane - ;-) )

Assuming this is possible given the dimensions of the material, the
external thread will be six-inches long starting from one end of a
seven-inch piece of 304 stainless tubing, 0.5" OD, 0.37" ID

The internal thread will be 1/2-inch long in a four-inch disk of the
same material.

As to lash (what I am familiar with is backlash - or if one were to
reverse the direction of the screw while adjusting it, how far one
would have to turn the screw before movement in the opposite direction
would occur), again, I don't know what is possible and I do not know
how to specify this. However, if it were possible to make threads
without lash without incurring excessive expense, that would be ideal.
(The current products have significant lash and this is a drawback,
however, I don't see it as being the major problem with the current
products.) I suppose that I really need to know how difficult this is
to do and have some idea of how expensive it is to minimize lash.

I don't really understand the term clearance. What I think it means is
the distance between the external and internal threads. I would
greatly appreciate an explanation of it, and how it would affect the
thread. (That said, it seems to me that having minimal or zero
clearance between external and internal thread that are mating would
make for more precise movement than with threads that have significant
clearance. Would this not also make the threads more difficult to
adjust, i.e, would the friction between the mating threads be higher
such that the adjustment would be more difficult?)

As to the load that the assembly will carry, I expect that it will may
have to carry 250 pounds, however, the majority of cases fall in the
less than 130 pounds range.

Thanks once again for all the replies. The help is appreciated.

Best Regards,
Matthew


Harold

In article , Matthew Zenkar says...

Many thanks for all the replies.

I get the feeling that I'm somewhat out of my league here. I do not
have a mechanical background, and I have been teaching myself what I
need to know for my project as I go along. I'm doing the best that I
can under the circumstances, and the help is appreciated.

By all means then, obtain and read a catalog from an optical house
supplier. Like the McMaster Carr catalog, they provide excellent
tutorials. You can learn a great deal about nearly any optical
fundamentals that way, including the basics of translator design
and the specification of exactly what detailed requirements you
will need.

Jim


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

"Matthew Zenkar" wrote in message
...
snip-----

Assemblies of the type that I am working on exist in the commercial
marketplace, however, they are extremely poor in terms of precision
positioning and, once the precision adjustment is achieved, they are
poor at locking the adjustment in place. Once the adjustment is locked
in place, it is highly unlikely that a user would reposition the
assembly. There are cases where this might happen, however, the
majority of use cases would not require this. (On that note, to the
person who replied with the DRO suggestion, that option might come in
handy for these few use cases. Thank you for that suggestion.)

In order to achieve a level of precision that doesn't change when you lock
the assembly, the thread is but a part of the requirement. It would be
important that mating surfaces are properly fitted so the locking mechanism
doesn't cause movement, or at least not enough to be a bother. It's all a
matter of how important a few thou, or even a few tenths, are to the
successful operation of your device. As the level of precision increases,
you can expect the price to go up exponentially.

However, I may be reluctant to deviate much from my
current course especially if it is going to require a significant
redesign of the assembly.

You may have to revisit your choice of materials. Stainless running on
stainless is a recipe for disaster, particularly when you must have little,
or no clearance. Stainless galls very easily--often without obvious
provocation. Heat treated 400 series (or the use of the precipitation
hardening varieties) would likely exempt you from that scenario, but would
require a ground thread-----which is likely to be mandatory anyway,
considering your statement:

The primary goal is to achieve precision positioning.


For now, I am looking to have perhaps three or four each of
internal/external threaded pieces to build three or four prototypes of
the assembly. After that, I do not know if I will need more. I am
hoping to go to market with it.

Currently, here is what I am looking at -

1/2-80 or 1/2-100 thread as follows: (I'd prefer the 1/2-100, however,
I don't know what can be done. I'm hoping for feedback as to what is
possible - even if that means someone out there has to tell me that
I'm insane - ;-) )

While the use of the extra fine thread may be in your best interest regards
the level of precision you might achieve, you'll come to realize that
working with such fine threads has its own problems. One of them is how
easily they can be damaged, or how easily the smallest piece of
contamination would cause the thread to seize. Also, the level of
precision required would eliminate the chance you could make the parts with
any degree of success on anything but a grinder, assuming the fits you
require are close so you don't get (back) lash. As threads get finer and
finer, the tolerance of all the features gets smaller and smaller.

I'd encourage you to examine a micrometer for measuring in inches. They
are built with a 40 pitch thread, so each revolution of the spindle is only
..025". You should look carefully at such a thread and come to terms with
how fine it is before making a decision to go to a finer one, particularly
in 304 stainless steel, which is very soft and easily damaged.


Assuming this is possible given the dimensions of the material, the
external thread will be six-inches long starting from one end of a
seven-inch piece of 304 stainless tubing, 0.5" OD, 0.37" ID

The internal thread will be 1/2-inch long in a four-inch disk of the
same material.

See above statement about material selection.


As to lash (what I am familiar with is backlash - or if one were to
reverse the direction of the screw while adjusting it, how far one
would have to turn the screw before movement in the opposite direction
would occur), again, I don't know what is possible and I do not know
how to specify this. However, if it were possible to make threads
without lash without incurring excessive expense, that would be ideal.
(The current products have significant lash and this is a drawback,
however, I don't see it as being the major problem with the current
products.) I suppose that I really need to know how difficult this is
to do and have some idea of how expensive it is to minimize lash.

One of the things you might consider incorporating in your design is a
Delrin AF nut. If you can see a way to incorporate the idea in your
design, you can run an interference thread fit on the stainless. No
backlash, no galling, and no lubrication required.

I don't really understand the term clearance. What I think it means is
the distance between the external and internal threads. I would
greatly appreciate an explanation of it, and how it would affect the
thread. (That said, it seems to me that having minimal or zero
clearance between external and internal thread that are mating would
make for more precise movement than with threads that have significant
clearance. Would this not also make the threads more difficult to
adjust, i.e, would the friction between the mating threads be higher
such that the adjustment would be more difficult?)

All threads (not tapered pipe threads) from class 3 and down have
dimensioning such that parts are guaranteed to fit, even when the male
thread is on top tolerance and the female thread is on bottom tolerance.
The clearance is generated not only on the major and minor diameters, but on
the pitch diameters, so you end up with some slop in order for the parts to
fit together. The higher the class, the tighter the fit, but there's
always clearance. You can get around that by using an adjustable split
nut, or two nuts opposing one another that can be adjusted, or, as I
suggested above, by using an interference fit and dissimilar materials.
It would be unusual design to expect threads to not have lash-----which
would be lost fairly quickly in the real world once the device was put to
use. Wear is always a problem.


As to the load that the assembly will carry, I expect that it will may
have to carry 250 pounds, however, the majority of cases fall in the
less than 130 pounds range.

Can't help you there----I'm not an engineer, just a retired toolmaker.

Harold

In article , says...

"Matthew Zenkar" wrote in message
...
snip-----

Assemblies of the type that I am working on exist in the commercial
marketplace, however, they are extremely poor in terms of precision
positioning and, once the precision adjustment is achieved, they are
poor at locking the adjustment in place. Once the adjustment is locked
in place, it is highly unlikely that a user would reposition the
assembly. There are cases where this might happen, however, the
majority of use cases would not require this. (On that note, to the
person who replied with the DRO suggestion, that option might come in
handy for these few use cases. Thank you for that suggestion.)

In order to achieve a level of precision that doesn't change when you lock
the assembly, the thread is but a part of the requirement. It would be
important that mating surfaces are properly fitted so the locking mechanism
doesn't cause movement, or at least not enough to be a bother. It's all a
matter of how important a few thou, or even a few tenths, are to the
successful operation of your device. As the level of precision increases,
you can expect the price to go up exponentially.

Take this to heart. It's much easier to position something to within
..0002 than to lock it there. Several times I've had to design and build
both manual and automatic mechanisms to do this and it's never as easy
as you might think.




I don't really understand the term clearance. What I think it means is
the distance between the external and internal threads. I would
greatly appreciate an explanation of it, and how it would affect the
thread. (That said, it seems to me that having minimal or zero
clearance between external and internal thread that are mating would
make for more precise movement than with threads that have significant
clearance. Would this not also make the threads more difficult to
adjust, i.e, would the friction between the mating threads be higher
such that the adjustment would be more difficult?)

Read, not skim, the section "Screw Thread Systems" in Machinery's
Handbook. It's many pages of dry material, but worth the effort if you
need to understand screw threads.

Ned Simmons

jim rozen wrote:
In article , Matthew Zenkar says...

Many thanks for all the replies.

I get the feeling that I'm somewhat out of my league here. I do not
have a mechanical background, and I have been teaching myself what I
need to know for my project as I go along. I'm doing the best that I
can under the circumstances, and the help is appreciated.


By all means then, obtain and read a catalog from an optical house
supplier. Like the McMaster Carr catalog, they provide excellent
tutorials. You can learn a great deal about nearly any optical
fundamentals that way, including the basics of translator design
and the specification of exactly what detailed requirements you
will need.

Jim




Edmond Scientific specializes in optics. I bought my first 6 inch
reflecting telescope mirror kit from them many years ago.(1957) They
are located in New Jersey.

John