How can you accurately measure mainspring dimensions when the margin of error in the measurement is too significant?

[GregG]

Sorry for the confusing title, allow me to explain.

If you're measuring something that is 50 cm across, it doesn't matter if your measurement is +/- 0.5 mm.

But I'm trying to measure the thickness/height of a mainspring to order a replacement.  I use my digital calipers, which measures out to the hundredth of a millimeter.  If I put the mainspring between the jaws and use a moderate amount of force to clamp it together, I get a reading.  But if I open the jaws and close back down on it, the reading might be different.  If my force in closing the jaws is too little, the reading will still be different.  Basically there's no repeatability.

And like I said, if the object is large, then this isn't an issue.  But if I'm trying to measure a 0.1mm thick mainspring, and my variation is +/- 0.05mm, that is an enormous variation.  My goto for mainspring information is Ranfft, but that website doesn't always have the movement I'm looking for.

What are your methods for accurately measuring the mainspring dimensions?

[jdm]

13 minutes ago, GregG said:

If I put the mainspring between the jaws and use a moderate amount of force to clamp it together, I get a reading.  But if I open the jaws and close back down on it, the reading might be different.  If my force in closing the jaws is too little, the reading will still be different.  Basically there's no repeatability.

There is no force control device like on a micrometer (which is a must have for serious watchmaking) but with a bit of practice one can easily repeat with 2 hundredths or so.

 

13 minutes ago, GregG said:

But if I'm trying to measure a 0.1mm thick mainspring, and my variation is +/- 0.05mm, that is an enormous variation.

Yes sometime there is some doubt but cross referencing the document below or using Cousins UK search one can see what the closes available size is, and it will work fine.

13 minutes ago, GregG said:

 My goto for mainspring information is Ranfft, but that website doesn't always have the movement I'm looking for.

Download the GR catalog from Cousins UK, most likely it will list code and size for the mov.t you have.

[JohnR725]

27 minutes ago, GregG said:

But I'm trying to measure the thickness/height of a mainspring to order a replacement.  I use my digital calipers, which measures out to the hundredth of a millimeter.

I can see somebody beat me to the answer?

 

5 minutes ago, jdm said:

There is no force control device like on a micrometer

 

at work we have digital calipers and you're right measuring the thickness of a mainspring not quite a waste of time but not very accurate either. If you really want the best use a micrometer. I have a link below the talks about the differences it doesn't mention the problem were having. Caliper ideally would like to measure something that's flat without a curvature. If you doing a mainspring it's very easy to angle  yet in the jaws and that will give you a variation. Squeezing the jaws together of course gives you a variation. A micrometer can close down and clamp on the mainspring and it's an absolute. If you want the best measurements get a micrometer.

But so you're off my little bit who cares? Mainsprings typically do not, in an infinite number of thicknesses it depends on the watch. Then if you get in the pocket watch size or vintage you get other complications like the original blued steel versus the modern steel is there a difference in strength of the same thickness. For American pocket watches once had a whole bunch of sizes and now you're lucky to get just one thickness because are not going manufacture five different thicknesses they're only going give you one if you're lucky. So their caliper will get you close the micrometer would be better.

 

https://www.fullyinstrumented.com/micrometer-vs-caliper/

 

 

 

[Nucejoe]

 No use to concern yourself with significance of figures beyound already manufactured sizes. 

[VWatchie]

Here's a post that suggests a nifty way to measure the thickness of a mainspring.

 

[GregG]

13 hours ago, jdm said:

There is no force control device like on a micrometer (which is a must have for serious watchmaking) but with a bit of practice one can easily repeat with 2 hundredths or so.

 

Yes sometime there is some doubt but cross referencing the document below or using Cousins UK search one can see what the closes available size is, and it will work fine.

Download the GR catalog from Cousins UK, most likely it will list code and size for the mov.t you have.

Thank you everyone for the wonderful answers.  It may be because I'm on mobile, but you said "the document below", but I don't see anything. I've also checked ranfft and the cousins gr catalogs and cannot find information on the bulova 3AK mainspring. The gr has other "3xx" movements but not the 3AK.

[praezis]

If you don't own a micrometer and your calibre is mentioned nowhere, there is still a variant to find the right strength:
Measure the inner diameter of the barrel D and calculate strength and length.
Strength is 1/80 to 1/90 of D. For fully jewelled movements rather 1/90.

You cannot be sure that the old spring was of correct dimensions.

Frank

[jdm]

3 hours ago, GregG said:

 "the document below", but I don't see anything. 

That is, the GR catalog "mentioned below".

16 hours ago, JohnR725 said:

Caliper ideally would like to measure something that's flat without a curvature.

And ever more do micrometers with rounds tips, so either get the specially shape tip version which cost a lot  more, or clamp down defeating the torque knob,which kinds of goes against repeatability.
So one use the tapered end of the caliper jaws, for that and many other occasions.

[JohnR725]

3 hours ago, jdm said:

And ever more do micrometers with rounds tips, so either get the specially shape tip version which cost a lot  more, or clamp down defeating the torque knob,which kinds of goes against repeatability.
So one use the tapered end of the caliper jaws, for that and many other occasions.

I didn't mean to torque the micrometer down with a pair of vise grips that's not what I was saying. You can gently close them what I'm saying is we are holding a mainspring in your hand it's really hard if not impossible to hold it parallel to the jaw's of the caliper. So I find you can the get varying numbers just because it's a wiggling around it's not like a large flat piece of something worth a lot easier to hold. In size you usually find a flat spot on the mainspring.

Then yesterday when is playing with my beer calipers of course the other problem is just squeeze them touch them a look at them wrong or open and close them and every single time you'll be off by a tiny bit they just aren't super super accurate even if they haven't nifty digital display with lots of numbers. Of course maybe if you had a precision one unlike the cheap things we haven't worked.

6 hours ago, GregG said:

I've also checked ranfft and the cousins gr catalogs and cannot find information on the bulova 3AK mainspring

one of the things helpful for finding parts is movement number you gave us the movement number would've been helpful to give it sooner and a picture would've been really nice because we have a mystery here kind of

so we get the first link will a parts list normally would get enough information or availability apart we could get to the mainspring but parts list sucks but it does give us this MOVEMENT BASE MODEL FF 59-21 ULTRA FLEX (N8) ELASTOR (N1-7). Just to confirm it would be nice to have a picture the movement so I googled it for you and confirmed what I suspect. There appear to be no listings really of this yes there's listing at the link below there is the mainspring doesn't appear to list. If I look at bestfit online they don't even have this movement it doesn't seem to exist but what about the base caliber maybe it exists.

Really weird looking in the mainspring catalog it doesn't seem to exist either an unknown watch except googling gives us pictures which I must wipe off eBay in the absence of yours. also I have a clue as to what the problem is Bulova puts date codes on stuff at least past the 50s so n4 which is on the movement I'm currently looking at would be 74 I believe that is why it's not in any of the references it's relatively modern. This is a problem of having mainspring catalog that was printed probably before this bestfit book definitely printed before this there would be a lot of literature.

so if my research is right here watch should be at the third link. They might've even left the OEM number under the balance wheel which will confirm this also but you can look at the picture that hopefully remember to attach.

So if my research is right the fourth link tells you the mainspring is very common and lots of people have it probably because it looks like it fits a lot of watches. Even if you can't measure the thickness because that's pretty darn thin the width and the length should correspond to the mainspring found at that link if they do then I would assume the thickness is correct and find one of those are probably on eBay.

http://cgi.julesborel.com/cgi-bin/matcgi2?ref=BUL_3AK

http://cgi.julesborel.com/cgi-bin/matcgi2?ref=Y_kDiiotWYp

http://cgi.julesborel.com/cgi-bin/matcgi2?ref=FF_59-21

http://cgi.julesborel.com/cgi-bin/matcgi2?ref=qWIXHk