Restoring Incomplete Starrett Dividers Compass

[Cleave]

Recently I stopped by my favorite antique store and picked up an incomplete pair of Starrett dividers/compass. They're the 3" size I think. It appears the spring popped off and got lost for the previous owner. While I have several other pairs of dividers this seemed like a fun machining and blacksmithing challenge.

First, I turned a pivot pin on my circa 1900 WF Barnes #6 lathe. The pin was made from a grade 8 bolt I found in the street.

This morning I took a piece of a broken handsaw blade, cut and ground it to shape, heated it up and bent it around to fit. Now the dividers work! I still have to harden and temper the spring, but may also remake the spring out of slightly thicker material as this spring is a little soft. Finally I'll finish up the handle stub that sticks up out of the spring.

 

[Cleave]

So I heat treated the spring clip - it was a 0.030" thick piece of a broken Disston hand saw.

After heat treat I tempered to a light purple, installed, and as I closed up the divider legs, SNAP! The spring broke at the middle, where the hole is.

So I remade the spring, this time from a circular saw blade, about 0.070" thick that I ground and filed to about 0.055" thick. The spring was a lot stronger. After heat treat, and temper to a light purple with a little blue, SNAP! Same problem - brittle failure where the hole is.

I am tempering in the oven at over 500 F, for 20 minutes or more.

I am thinking the spring needs to be a little bigger in diameter, and a little wider at the drilled hole, so each piece of spring experiences less strain. The .030 felt a little thin, the .055 felt too thick. A 0.035 or 0.040 would be about right. Any other ideas on how to get it right on the third try?

 

[RTM]

By 3", you are giving us height I assume. I am measuring an outside, all I have in LSS

Fully closed, 0.658 OD
Fully open 0.604

Thickness 0.040 (no pin in the top of mine)

Front to Back
Top 0.400
at legs 0.225

No idea on annealing stuff

 

[crguy]

Seems like an awful lot of time to spend on a common tool that will likely never be exactly the same.

 

[macgee]

Nice exercise project. I would try the hacksaw blade again but without the hole. Also, maybe another way of quenching?

 

[Leviton]

Cleave, I think it's wonderful that you are trying this. Tinkering is a good part of what GJ is all about. Please keep us updated on how it's going.

Thanks for saving the divider.

 

[rlitman]

...It appears the spring popped off and got lost for the previous owner...

My guess it that the spring broke. They're brittle at the best of times, and short lived when they're allowed to get rusty.

If you look at the real original ones, they taper in two dimensions. They're both thicker and wider around the handle hole. The thickness grinding is subtle, but it is there.

Oh, and I disagree with the nay-sayers above. Those are one of the nicest caliper designs ever made. I'll be damned if I'll part with mine.

As for your choice of metal, I wouldn't start with a saw blade. I would start with a scrap from a broken leaf spring like a clock spring.

 

[RTM]

If you look at the real original ones, they taper in two dimensions. They're both thicker and wider around the handle hole. The thickness grinding is subtle, but it is there.
.

Oops, I missed that dimension. Mine tapers from 0.040 to 0.028 at the legs

 

[steaks&anvils]

I know you want to restore these dividers yourself and you have done a great job so far. If you end up not being able to get the spring to work, if you know your model number, Starret sells replacement parts.

The 3in pair did not list the spring in the replacement parts, but maybe if you contact Starret and ask?

3in example:

https://www.starrett.com/metrology/product-detail/277-3

This example (for an 8in pair) is $20.

example:

https://www.starrett.com/metrology/product-detail/PT00037

 

[rlitman]

Oops, I missed that dimension. Mine tapers from 0.040 to 0.028 at the legs

I know, it's subtle, right? It's all about reducing the amount it flexes around the hole.

I know you want to restore these dividers yourself and you have done a great job so far. If you end up not being able to get the spring to work, if you know your model number, Starret sells replacement parts.

The 3in pair did not list the spring in the replacement parts, but maybe if you contact Starret and ask?

3in example:

https://www.starrett.com/metrology/product-detail/277-3

This example (for an 8in pair) is $20.

example:

https://www.starrett.com/metrology/product-detail/PT00037

Hummm, that's a pretty good idea, but it will still require some modification, since the cuts on the end are for round style dividers. Also, the OP's has the quick nut, which it seems only come in the flat style now. And they don't make anything nearly as elegant as these rectangular types.

 

[steaks&anvils]

I know, it's subtle, right? It's all about reducing the amount it flexes around the hole.



Hummm, that's a pretty good idea, but it will still require some modification, since the cuts on the end are for round style dividers. Also, the OP's has the quick nut, which it seems only come in the flat style now. And they don't make anything nearly as elegant as these rectangular types.

Yeah, I hate when manufacturers change their styles. I also didn't pay attention to the exact details of OP's pair. Maybe a email to Starret would get him something that works?

There were many different kinds and sizes of dividers etc on the website, something should be close?

 

[rlitman]

Yeah, I hate when manufacturers change their styles....

I’ll cut them a break. After all, mine say Pat. June 2, 1885.

 

[steaks&anvils]

I’ll cut them a break. After all, mine say Pat. June 2, 1885.

Nice.

This is why we all love GJ and old tools.

 

[2oolhound]

Nice work! Come to think of it I have a pair that's missing the spring around here. Humm, something to do on a rainy day. Sure beats watching tv.

How about melting or beating down an old valve spring for the metal?

 

[crguy]

Nice work! Come to think of it I have a pair that's missing the spring around here. Humm, something to do on a rainy day. Sure beats watching tv.

How about melting or beating down an old valve spring for the metal?

Surely you jest...........