Anybody Have an Optical Comparator?

[wlw-19958]

Hi There,


I'm having some difficulties is getting taps and dies for making
replacement screws for No. 3 revolvers and some of the problem
is I don't have accurate drawings or representations of the thread
profiles. I had a die made for the .104"x 46tpi screw thread but
but it isn't quite right. They will work but the threads are looser
than I would like them to be.

What I need is to use an Optical Comparator (also called a Profile
Projector). I don't own one and I have no experience using one
but it is the right tool for the job. I would buy a used one but
I don't know enough to ask the right questions and to make sure
the machine I find will have the necessary attachments and will
be in good usable condition. Plus, a little hands on instruction
would go a long ways.

So, I am looking for suggestions.


I've attached a pic showing the thread the die cuts and below
it is an original screw with the threads I want to reproduce.



Cheers!
Webb

 

[rct269]

I only sort of know what an optical comparator is, so no help there.

What I do have is a document titled Thread List, Smith & Wesson ------and dated 5-20-'09, which appears to give the specifications for every threaded item in ten different models---presumably those in production at the time. (I've never checked that.) It shows diameter, pitch, and tapping size. It shows those for a "44 D.A." (which is not the Triple Lock, because that's shown as "44 H.E.")--------------and I'm given to presume that's a "No. 3" revolver. Yes? No?

Any help?

As an aside, the models shown are 32 SAF, 38 SAF, 32 D.A., 38 D.A., 44 D.A., Model M, Model I, Model K, 44 H.E., and 38 PER.

Ralph Tremaine

 

[wlw-19958]

Hi There,

I have a copy of that May 20th, 1909 "Thread List" but it lists only
a couple of screws that may pertain to the No. 3 model and the
information is in variance to other publications (like David Chicoine's
book on Gunsmithing Guns of the Old West).

An Optical Comparator is a metrology device used to measure small
objects accurately by projecting their image onto a screen (called
a target) usually in profile so that measurements of a part can be
made. Usually this image is enlarged (10X, 20X, etc) to make it
easier to make accurate measurements of the part.

You see, these lists of screws lack vital information such as the
major and minor diameters of the screw and the shape of the
Crest and root of the thread.

These threads were developed before the 1918 National Screw Thread
Commission was enacted to standardize screws and bolts threads.

The last time I had custom taps made, I was able to provide samples
of the screw the tap was to thread holes for. This time I didn't want
to send them a sample screw for fear it would be lost (I nearly
lost my sample screw the last time). The last time I was making
new jaws for the steady and follower rests used by South Bend
Lathes. Which were 3/8" X 10tpi Left-Handed.

Cheers!
Webb

 

[rct269]

HMMMMMMMMMMMMMMMM?!!

I had supposed the "Thread List" came from S&W. Yes/No?

Given the variance you note, do you know which is correct?

RCT

 

[jjrr]

Don't claim to be an expert but the bottom screw in the photo posted looks like an acme thread. There are lots of proprietary screws that are very difficult to duplicate in my experience.

 

[mmaher94087]

Webb, the bottom screw thread appears very close to the old Whitworth thread. I don't remember what the thread angle is, sorry.

 

[wlw-19958]

Hi There,

HMMMMMMMMMMMMMMMM?!!

I had supposed the "Thread List" came from S&W. Yes/No?

Given the variance you note, do you know which is correct?

RCT

I assume the list was made by S&W but the information is overly
terse. It would be enough to take screws and sort them but not
enough information is contained to accurately duplicate the thread.

You see, if the thread form conformed to say, the American National
Screw Thread, all that would be needed to duplicate the thread
would be the outside diameter and the thread pitch. All other
dimensions would be determine using established formulas for
the thread form.

The thread on the original screw doesn't conform to this formula.
I didn't realize this until I received the threading die. I thought
I would be able to make adjustments (being an adjustable split
die) to compensate for variance but I was wrong.

The flat on the crest (major diameter) of the original screw is
much wider than the bottom of the thread root (minor diameter).
I'm not even sure the thread conforms to a 60° prismatic form.
It isn't an Acme thread because an Acme thread has the same
width on the top of the thread as the bottom plus the angle is
29° and not 60°. Acme was designed to self center the screw
in the nut but still have good linear thrust properties (like a square
thread).

Using the Optical Comparator, the profile is displayed on a ground
glass screen (in silhouette) that usually has a scale on it for
making measurements. If the image is projected at 10X, the
the measurements are taken and divided by 10 (or just move
the decimal point one position to the left) to achieve accurate
results. This information is recorded and can be given to the
tool & die maker to make an accurate tap and/or die that will
duplicate the desired thread.

I should mention that when making a tap or die, a clearance is
usually specified. You see, if a tap threads a hole and a die makes
a fastener of exactly the same size, there is going to be an inter-
ference between the two and will not readily thread together by
hand. So, established clearances are part of the standards used
in making taps and dies. For external threads, these are 1A,
2A and 3A where 3A is a tight fit, 2A is the "normal" clearance
for most fasteners which is loose enough for quick assembly but
has adequate thread overlap for good strength and 1A is for
high clearance where allowances are needed for expansion,
plating, or rapid machine assembly. Internal threads has the
same system but they are 1B, 2B and 3B.

When I ordered the die, I specified a 2A fit (which is typical) and
the die was made to conform to the American National Screw
Thread because I didn't have the correct thread form infor-
mation and I thought it would be close enough (silly me).
But I am not satisfied with the results and before I order a-
nother die, I want to specify a more correct thread form.

So, I need some time on a comparator to make the measurements
for the screw the die will be able to reproduced the screw ac-
curately.


Cheers!
Webb

 

[Frailer]

Good news:used optical comparators are surprisingly inexpensive.

Bad news:they are very bulky and heavy. One would have to have an excess of shop space to justify buying one.

I might reach out to a local community college (the school I work with has two of them) or an old-school machine shop.

 

[iby]

Is there a 3D printer for metal?

 

[nedlate]

Would you be able to make a wax casting and split it to get your information to the die maker?
Ed

 

[wlw-19958]

Hi There,

Good news:used optical comparators are surprisingly inexpensive.

Bad news:they are very bulky and heavy. One would have to have an excess of shop space to justify buying one.

I might reach out to a local community college (the school I work with has two of them) or an old-school machine shop.

Well, a proper Optical Comparator with an X/Y adjustable stage
and a set of objectives can run 14 to 15 thousand dollars new.
I would have thought with today's digital technology that a modern
version would be made cheaper but manufacturers load them up
with software features that keep the prices high. I've been looking
for a good used one but most of the deals I see are too far away
or look like they have been neglected for decades and rusty and
covered with an inch of dust. I wouldn't know how to properly
check one out and make sure the reflecting prisms were still
aligned and could make accurate projections. Most of the old
manufacturers are no longer around to get replacement parts
or if they are, they no longer support the older models. Nikon
still makes some but they are a top notch optical company and
their prices reflect their pride.

Around here on the mid Atlantic coast, vocational schools were
closed down 50 years ago and the equipment sold or scrapped.
That was the time to look and get good deals.

I have been thinking of trying a digital microscope and building
a stage but it should be back lit so the object will be displayed
in profile. I'm just surprised there isn't something available in
the budget class.


Cheers!
Webb

 

[wlw-19958]

Hi There,

Would you be able to make a wax casting and split it to get your information to the die maker?
Ed

Wax would be too soft and would deform in heat (like in the back
of a mail truck). Plus, it couldn't be projected to take measure-
ments.


Cheers!
Webb

 

[nedlate]

Hi There,





Wax would be too soft and would deform in heat (like in the back
of a mail truck). Plus, it couldn't be projected to take measure-
ments.


Cheers!
Webb

I was thinking of a tool makers microscope for measurements.
Unless you had someone local to help I can see the problem.
Ed

 

[wlw-19958]

Hi There,

Webb, the bottom screw thread appears very close to the old Whitworth thread. I don't remember what the thread angle is, sorry.

That could be. I know the Springfield Armory used Whitworth
thread form on the screws for the Trapdoor rifle/carbine. The
Whitworth thread form used a 55° angle prism form with a rounded
root and crest of equal size. The ISO thread form uses a modified
American thread form with a flat crest and a rounded root. This
makes a thread that is technically stronger (having no sharp corners
in the root of the thread). The 60° prism form is an equilateral
triangle which simplified the math and has been adopted by ISO.


Cheers!
Webb

 

[ameridaddy]

Webb, the bottom screw thread appears very close to the old Whitworth thread. I don't remember what the thread angle is, sorry.

Whitworth BSW is 55 deg. and IIRC, same pitch as NC. There are lots of charts and thread form info on the net.

 

[wlw-19958]

Hi There,

I was thinking of a tool makers microscope for measurements.
Unless you had someone local to help I can see the problem.
Ed

I have been thinking in that direction. I would like it if the tool-
maker's microscope hand a camera attachment and a reticle with
the necessary measurement scale. Then I could take images
of the screw and there would be a scale to make the compar-
isons and establish dimensions. I saw one made by Nikon but
it didn't have any camera or digital imaging capability and I
would like to just take a pic and send it to the tool and die maker.
This is something I have been pursuing but I haven't
found one with the features I want without spending around a
thousand dollars.


Cheers!
Webb

 

[nedlate]

MITUTOYO, Stereo, Monocular, Toolmaker Microscope - 54GF77|176-821A - Grainger

You mean like this!
If a person had an extra $8,500 laying around!
Ed

 

[wlw-19958]

Hi There,


I just picked-up a used bench model optical comparator today.
It is a small one with a small screen (only 6 inches) but I think
it will suffice. Here are a couple of pics (unfortunately, the file
manager turned the pics sideways).


Cheers!
Webb

 

[kraynky]

I had a small handheld optical comparator some time back that I used to measure the thickness of roof coatings, back when I was a consultant in the commercial roofing industry.

Haven't seen it in a few years, but I'm sure it's still here somewhere. It wasn't a high-end tool. It had lenses I could swap out for different levels of thickness, and I just looked through it at the side of a cut out sample and compared coating thickness to the measurements seen in the comparator.

Measurements were in mil thickness on it for my needs. I think it was less than $50 bucks way back then.

I'm sure there are some cheapo "China made" ones available now for less money.

 

[SweetMK]

This guy does it (measure grooves that are like the threads)
and he did it back in 2017,,,
using his phone!!

Cost,, 000 dollars,, which is pretty good,,

He does it step, by step, in the video,,,

[ame]https://www.youtube.com/watch?v=0NxULMN3EQU[/ame]