How can I measure an internal taper inside a Rivett wax (shellac?) chuck?

[part-timer]

I use a Rivett watchmaker's lathe almost daily, and I have a few collet-shaped chucks with an internal 8-48 thread. Besides the internal thread, there is also a very small area near the tip of the chuck's internal perimeter that is tapered. I'm assuming the function of the tapered area is to register with a matching tapered area of some wax (or shellac) chucks. My question is how can I accurately measure the angle of the taper? I'm attaching close-up pictures that I hope will help.

My question arises because I made a set of brass pieces (also pictured here) to fit in the thread and hopefully match the chucks. The 8-48 thread fits well, but determining a matching taper angle is proving to be a challenge. I cut the taper of the brass pieces at 30 degrees with my Rivett sliderest. I made marks on the brass tapered areas with a Sharpie pen, and when I screwed down each of the three brass pieces into one of the chucks, I saw a circular ring (also pictured here) in the marks that seemed to indicate an edge rubbing the mark, instead of an overall smudge. It seems clear to me the chuck's registering ring is NOT 30 degrees. But what IS the angle? How to measure it? There's only one way my little pea-brain can think of, but it's convoluted and fits my habit of making mountains out of molehills - so I thought I'd try here first.

I'm thinking that even if my angle is off a few degrees, the circular parts that mangage to touch each other should be enough to assure concentricity. But I guess I'd rather pursue perfection rather than "good enough".

On to the pictures... here are the chucks in question:

The tapered area:

The three brass pieces I've already made with the 30-degree shoulder:

Close-up, with red arrows pointing to where the chuck rubbed my Sharpie marks:

So... how can I measure the angle of the tapered area in my second photo? Again, I have an idea that I think would work, but I'm not a machinist or a mechanical engineer - so maybe there's a simple way that I don't know about.

 

[the 3rd dwarve]

measure the length of the taper = c

measure the height of the taper = a

angle at A a/c = sineA

angle at B 90 -A

D`

 

[L. Vanice]

One way to get a better look at an internal taper is to make a cast, producing an exact external taper for measurement. A classic casting medium used by machinists is melted sulfur. Overheating sulfur produces harmful fumes. I have used a bismuth alloy called Cerrobend in the past, which melts at 158 deg. F. You can find it on eBay. Again, you do not want to overheat it. Note that this alloy will melt at such a low temperature that it will not harm the steel collet or wood or paper used to plug the threaded portion of the collet.

Or you could just make a series of brass pins with various angles on the end until you get one that you think fits.

Larry

 

[part-timer]

One way to get a better look at an internal taper is to make a cast, producing an exact external taper for measurement. A classic casting medium used by machinists is melted sulfur. Overheating sulfur produces harmful fumes. I have used a bismuth alloy called Cerrobend in the past, which melts at 158 deg. F. You can find it on eBay. Again, you do not want to overheat it. Note that this alloy will melt at such a low temperature that it will not harm the steel collet or wood or paper used to plug the threaded portion of the collet.

Or you could just make a series of brass pins with various angles on the end until you get one that you think fits.

Larry

Thanks, Larry. That's something I was wondering about. Being the resident rodeo clown in this room full of PhD's, I doubt I'll be messing around with harmful vapors. I remember seeing something at Hobby Lobby that involved making a plastic cast, but I may have been dreaming. In any case, I might need to resort to creating various angles, and trying them with a tiny dab of Prussian Blue.
I was hoping for an easy magic trick that would just get me in the ballpark! Once I get the angle dialed in, I plan to make these out of 1/4" 12L14 steel.
PS... check your messages.

 

[dave-b]

Even if you accurately measure the angle, it will be no use to you. You will only get "perfection" by trial-and-error The angle set-up on your lathe just isn't able to be set to fractions of a degree. That angle looks ok for practical use.

 

[Jerry Kieffer]

I use a Rivett watchmaker's lathe almost daily, and I have a few collet-shaped chucks with an internal 8-48 thread. Besides the internal thread, there is also a very small area near the tip of the chuck's internal perimeter that is tapered. I'm assuming the function of the tapered area is to register with a matching tapered area of some wax (or shellac) chucks. My question is how can I accurately measure the angle of the taper? I'm attaching close-up pictures that I hope will help.

My question arises because I made a set of brass pieces (also pictured here) to fit in the thread and hopefully match the chucks. The 8-48 thread fits well, but determining a matching taper angle is proving to be a challenge. I cut the taper of the brass pieces at 30 degrees with my Rivett sliderest. I made marks on the brass tapered areas with a Sharpie pen, and when I screwed down each of the three brass pieces into one of the chucks, I saw a circular ring (also pictured here) in the marks that seemed to indicate an edge rubbing the mark, instead of an overall smudge. It seems clear to me the chuck's registering ring is NOT 30 degrees. But what IS the angle? How to measure it? There's only one way my little pea-brain can think of, but it's convoluted and fits my habit of making mountains out of molehills - so I thought I'd try here first.

I'm thinking that even if my angle is off a few degrees, the circular parts that mangage to touch each other should be enough to assure concentricity. But I guess I'd rather pursue perfection rather than "good enough".

On to the pictures... here are the chucks in question:
View attachment 754635

The tapered area:
View attachment 754636

The three brass pieces I've already made with the 30-degree shoulder:
View attachment 754637

Close-up, with red arrows pointing to where the chuck rubbed my Sharpie marks:
View attachment 754638

So... how can I measure the angle of the tapered area in my second photo? Again, I have an idea that I think would work, but I'm not a machinist or a mechanical engineer - so maybe there's a simple way that I don't know about.

There are simple, quick and accurate methods of duplicating tapers such as this utilizing machine tools.

Unfortunately, if you are limited to a watchmakers lathe, they have very limited machining capabilities. However, if you get lucky, you may be able to utilize the following method using a cross slide.

For procedures such as this, I start with a boring bar mounted in the tool post per first photo.

Then the black arrow boring bar cutting surface in the second photo is positioned against the red arrow surface and locked in position.

Third photo, the bar is positioned so that when a slide is retracted back toward you, it will shave the full width of the taper fourth photo.

The OD tapers can be done in the same manner with capable equipment and tooling fifth photo.

Jerry Kieffer

 

[part-timer]

There are simple, quick and accurate methods of duplicating tapers such as this utilizing machine tools.

Unfortunately, if you are limited to a watchmakers lathe, they have very limited machining capabilities. However, if you get lucky, you may be able to utilize the following method using a cross slide.

For procedures such as this, I start with a boring bar mounted in the tool post per first photo.

Then the black arrow boring bar cutting surface in the second photo is positioned against the red arrow surface and locked in position.

Third photo, the bar is positioned so that when a slide is retracted back toward you, it will shave the full width of the taper fourth photo.

The OD tapers can be done in the same manner with capable equipment and tooling fifth photo.

Jerry Kieffer

View attachment 754732 View attachment 754733 View attachment 754736 View attachment 754737 View attachment 754738

Jerry, I was hoping you'd see this. Your method is brilliant, very likely I would not have thought of it. I have a couple of micro boring bars, and as you know the Rivett sliderest has very accurate angle markings.
As I mentioned earlier, I have an idea that might give me a rough idea of the "registering angle". I'm going to give that a whirl first, and then I'll use the boring bar method.
Many thanks, Jerry!

 

[Jerry Kieffer]

Jerry, I was hoping you'd see this. Your method is brilliant, very likely I would not have thought of it. I have a couple of micro boring bars, and as you know the Rivett sliderest has very accurate angle markings.
As I mentioned earlier, I have an idea that might give me a rough idea of the "registering angle". I'm going to give that a whirl first, and then I'll use the boring bar method.
Many thanks, Jerry!

Part timer

Thank you for the kind words.

The fact that you have taught yourself or are teaching yourself your own creative methods and have the confidence to perform them is inspiring.
It will pay huge dividends in the future.

Jerry Kieffer

 

[wefalck]

Perhaps, one could also step back one step and ask, whether the centring is really necessary.

I am not so familiar with the use of wax-chucks, but my understanding is that you screw the brass part into the holder, heat it, apply the shellac, put the part on and then bring the whole assembly to the lathe, where you center the part with the tailstock, while the shellac is still soft. So effectively, the brass part does not need to run perfectly true.

However, it has to run flat. For this the annular register yould be enough, when you true-up the surface on the lathe.

As a matter of fact, if I had to make such part, I would probably true-up all the outer surfaces, when the brass is set into the holder to which it will belong.

 

[the 3rd dwarve]

You can find that angle in less than three minutes with simple math.

Measure the largest diameter of the cone =LD

Measure the smallest diameter of the cone =SD

Subtract SD from LD = a

Measure the length of the cone = c

Divide c into a = a/c = sine A

Go here and find the angle A to the nearest degree.

Sine Tables Chart of the angle 0° to 90°

Sine Tables Chart of the angle 0° to 90° for students.

www.mymathtables.com

If you need to be more accurate you can extrapolate between degrees to seconds.

Page 74 of the Machinery's hand book has a great explanation of the solution of triangles.



Alternatively you can chuck a dial indicator on your cross slide and just zero it out on the cone face to set the cross slide angle.

D`

 

[Jerry Kieffer]

Wax chucks typically have have a taper for accurate repeatability when removing and replacing the chucks with parts mounted on them per attached photo.

Larger ID and OD tapers are easily calculated per the explanation in post #10.

However, ID tapers in holes around 5mm (.200") where the tapers are about 1mm -- 1.2mm (.050") long such as in the photo, can be a major issue to accurately measure even with quality indicators and luck.

Short cuts can of course be taken, but it becomes a habit for others to deal with and ones reputation.

Jerry Kieffer

 

[part-timer]

Perhaps, one could also step back one step and ask, whether the centring is really necessary.

I am not so familiar with the use of wax-chucks, but my understanding is that you screw the brass part into the holder, heat it, apply the shellac, put the part on and then bring the whole assembly to the lathe, where you center the part with the tailstock, while the shellac is still soft. So effectively, the brass part does not need to run perfectly true.

However, it has to run flat. For this the annular register yould be enough, when you true-up the surface on the lathe.

As a matter of fact, if I had to make such part, I would probably true-up all the outer surfaces, when the brass is set into the holder to which it will belong.

Centering isn't really what I'm asking about. I was asking about determining the angle of the tapered registering ring shown clearly in the second photo of my original post.
EDIT: on review, I realized that you meant to type "center ring" instead of "centring". I read it as "centering".
But yes, it's important to me. That's because I don't know what the angle is, and I want to know. Therefore I'm going to figure it out one way or another. Just for sake of solving mysteries... that's how I would put it. It might be irrelevant or necessary to someone else. That's fine with me.

You can find that angle in less than three minutes with simple math.

Measure the largest diameter of the cone =LD

Measure the smallest diameter of the cone =SD

Subtract SD from LD = a

Measure the length of the cone = c

Divide c into a = a/c = sine A

Go here and find the angle A to the nearest degree.

Sine Tables Chart of the angle 0° to 90°

Sine Tables Chart of the angle 0° to 90° for students.

www.mymathtables.com

If you need to be more accurate you can extrapolate between degrees to seconds.

Page 74 of the Machinery's hand book has a great explanation of the solution of triangles.



Alternatively you can chuck a dial indicator on your cross slide and just zero it out on the cone face to set the cross slide angle.

D`

View attachment 754877

Please refer to the photos I supplied. I'm familiar with figuring triangles etc., but doing so with a chuck as small as mine with the instrumentation I have is beyond my scope. Thanks anyway.

 

[part-timer]

Part timer

Thank you for the kind words.

The fact that you have taught yourself or are teaching yourself your own creative methods and have the confidence to perform them is inspiring.
It will pay huge dividends in the future.

Jerry Kieffer

Thank you for your remarks. I'll be trying to implement "my idea" for (hopefully) measuring the internal registering angle of my threaded "shellac chucks" or "wax chucks". Rivett made their non-split chucks in both 8-48 thread and Rivett's proprietary taper. As you may or may not know, Rivett's proprietary taper angle (used in all their tailstocks and other uses) was never formally revealed to the public and has lain buried under many tons of trash in a landfill somewhere. A recent discussion on the Rivett 608 group (Log In) turned up documentation from 1949 that indicates it was "two degrees and 55 minutes". Message me if you'd like to see the documentation I'm referring to.

 

[the 3rd dwarve]

I would use gauge pins to make the three measurements. I would need to use a loupe now because nothing works like it used to.

My set was manufactured to a 0.0002" tolerance so that should be accurate enough although I haven't had them calibrated in a while. I don't find myself working to thee decimal places too much anymore.

If you don't have a set of gauge pins you can turn the two pins you need for the large and small cone diameters from any small round stock you have and measure the diameters. Then just turn the diameter of one to match the length of the cone or you can file a dog leg length gauge from a piece of flat stock.

D`

 

[part-timer]

Yesterday I finally got a few hours of shop time, so I tried this crazy idea I've had floating in my head. All it entails is setting a protractor in front of my lathe's spindle at center height (in this case, 53mm) and carefully placing a magnetized sewing needle in the wax chuck's opening in such a way that the "non-pointed" end is contacting the internal taper shown in the second photo from post #1 above.

I selected non-magnetic items to support the protractor - a lead weight for about 50mm of the height, and bits of brass to bring up the final height to 53mm. For the protractor, I just found a vector image (actually, an .svg file) online and printed it onto a piece of 4"x6" photo paper, and then I tacked it down to my top brass piece with double-sided Scotch tape. Once I had all that cobbled together, I fussed and finaggled the positioning of the protractor so that I could see it was centered & aligned with the lathe bed (not easy!).

Then I picked up my magnetized sewing needle with brass tweezers, and placed the back end slightly inside the wax chuck's opening, and against the internal taper on one side or the other.

Then came the hard part - nudging the needle (with a small piece of sharpened pegwood) until I could tell it was actually contacting the whole length of the taper - that process involved repeatedly disturbing the needle until I could tell it was contacting the bevel. Eventually I got to the point I felt I could reliably duplicate that process with both sides of the bevel because I could see the side of the needle "jump" to contact the bevel, and the needle kept pointing to the exact same degree marking. And...I have four separate wax chucks! I repeated the whole process until I could see I was getting the same needle reading every time, and that helped me to reassure myself that I was getting the closest to a true reading as I was able to get, given the crude methodology.

The biggest surprise was to discover that between my four threaded wax chucks, there were two different internal bevel angles! On two of my chucks, my contraption said the internal bevel angle was 30 degrees. But my contraption said my other two chucks had 34 degree angles! That discovery prompted me to use some Prussian blue to visually check the contact between my chucks and the threaded brass pieces I had already made, with their (presumably) 30 degree shoulder angle. I took pictures of every step of my day except for the Prussian blue test. Anyway, the two 30 degree chucks showed a different contact pattern than the the two 34 degree chucks.

Next post, pictures of my "rodeo clown grade" internal bevel measuring setup.

 

[part-timer]

My Rivett model 2B watchmaker's lathe with one of my 8-48 threaded wax chucks:

Lead weight in front of the spindle:

Two pieces of brass on top of the lead weight:

Last piece of brass in front of spindle, with protractor image taped to it:

First needle reading I could get, only 7 more to go:

Closeup:

At this point, I stopped and tore off the protractor image & made a slightly smaller one so the needle could actually intersect with the degree markings. I didn't bother with pictures of that setup, but you get the idea.

Anyway, now I have two starting points to experiment with my sliderest. Hopefully I can find a shoulder angle that will yield a solid contact between chuck & homemade piece. I realize it's a stupid waste of time, but I don't care. I enjoy it.
Doug

 

[gmorse]

Hi Doug,

This could distort your readings, since the centre of the protractor scale should really be the point where the two sides of the taper converge, inside the chuck and not at its nose.

Regards,

Graham

 

[part-timer]

Hi Doug,

This could distort your readings, since the centre of the protractor scale should really be the point where the two sides of the taper converge, inside the chuck and not at its nose.

Regards,

Graham

You're absolutely right...I thought about that - there are several factors that create distortion of my readings, and that was maybe the 2nd or 3rd that occurred to me later on! But, it give me a starting point if I want to refine the process and serves as a proof-of-concept.

 

[the 3rd dwarve]

Once you have decided on the angle you want you can fabricate a brass lap that you can mount in the tailstock. That will give you consistency across all of your chucks and minimize axial runout.

D`

 

[part-timer]

Once you have decided on the angle you want you can fabricate a brass lap that you can mount in the tailstock. That will give you consistency across all of your chucks and minimize axial runout.

D`

That's intriguing... I'm not sure what you mean. Can you elaborate? I think of a lap as a surface to charge with diamond grit, to be used for polishing...?? I'm probably way off here!!!

 

[the 3rd dwarve]

No, you are right on. This cone could be lapped in, lapping compound comes in different grits if you need to remove stock or finish polish. If you mount the lap in the tailstock so the center line is on the same line as the center line of the lath that should eliminate any axial runout.

Here's a kit that illustrates my thought and might be a good start point for you.

Muzzle Lapping Kit - Silicone Carbide Lapping Compound

Make sure to cover your ways just in case.

D`

 

[part-timer]

No, you are right on. This cone could be lapped in, lapping compound comes in different grits if you need to remove stock or finish polish. If you mount the lap in the tailstock so the center line is on the same line as the center line of the lath that should eliminate any axial runout.

Here's a kit that illustrates my thought and might be a good start point for you.

Muzzle Lapping Kit - Silicone Carbide Lapping Compound

Make sure to cover your ways just in case.

D`

AHHH!!! Okay, just one short glance at that website gave me the whole idea! Thanks for that!
That must be how the little taper at the end of pistol muzzles gets touched up, no? Funny how a picture is worth a thousand words!

 

[Wimberleytech]

you can extrapolate between degrees to seconds

"interpolate" or use a calculator. Calculator wins the day.