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I Forge Iron

Restoring Incomplete Starrett Dividers Compass

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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.





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Great work! Nice save on an old tool.  Impressive addition of the WF Barnes lathe comment by the way (Velocipede in the shop here)

Bookbinding buddy of mine recently tried to piece together odd ball parts of dividers like this and ran into odd little differences in different years of manufacture.




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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?

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How about just contacting Starrett and seeing if they have replacement springs available. Those are technically called Yankee spring calipers, just incase you do not know and do contact them they will know what style caliper you are referring to. 

I always liked Starrett's measuring instruments. Seems easier to read than Brown and Sharpe or Mitutoyo, at least for me. 

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2 minutes ago, BillyBones said:

How about just contacting Starrett and seeing if they have replacement springs available.

That may work but would defeat the purpose of trying to figure this out myself... I have other dividers - that's not the problem either.

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24 minutes ago, Irondragon ForgeClay Works said:

Springs like that (mostly for muzzleloader locks), I usually temper at a higher temp. I heat to non-magnetic and quench in oil, then float them in my molten lead pot (700° F),  till the lead no longer sticks to them and cool in the air. Very seldom do any of mine fail.

I wondered about pushing the temper temperature higher. I don't know just how hot the oven will go on broil.

I'm not set up for a lead bath - would you recommend tempering with a propane torch and watching the colors? I've done that before but want to keep the temperature as even as possible.

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Could you start another spring with the thicker circular saw blade and leave the center area thick and taper both ends down to .03 or so with a gradual slope? Perhaps make the circle a little larger? That would make the tapped hole supported with thicker material and being longer would spread out the movement of the spring. As far as heat treatment I have read (no personal experience) that for gun springs, use a small container just big enough to hold the spring with oil just deeper than the spring width. ( think shoe polish can)Heat the spring and drop it into the container. Then to draw the temper, light the oil on fire and let it burn the oil away. Hardening and tempering all in one basic operation! For what it’s worth gun springs are usually tempered to a deep blue. 

hope this is of some use, Bill D. 

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I am thinking to take that very approach. Its the same theory as bow making (I've successfully made one 10 lb kids bow from walnut), where you taper the spring to get the same stress throughout the spring, so no one section gets overstressed causing a weak point.

I've never tried tempering in the manner you described, not sure if I'll try that or just try to evenly do it with a propane torch.

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  • 2 weeks later...

Yesterday I made up a third spring, this time out of a dull sawzall blade (0.035" thick).

I did a test heat treat and temper on a scrap of this blade.

I made the spring about 1/8" longer, and a little wider in the middle.

I quenched in a small tin of oil, then tempered in the oven as hot as it would run. It came out a very even pale blue.

And, it works! Full range of motion, great feel. I also machined the little post on the top. Now they can sit in my toolbox and keep the other dividers company.

Thanks all for your remarks.



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Gang solved problem, VERY cool! :D

I was thinking about suggesting you leave the spring wider where it was breaking but I'd be speculating from limited hands on spring making experience. 

Looking good, way to go gang! You guys have no idea how good this makes me feel.

Frosty The Lucky.

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