The Evolution of the Crucible Dividers

Posted on 05 October 2017

Customers have asked why we call our dividers “improved pattern” dividers. In toolmaking lingo, “improved pattern” means the designer took a long-established tool and found a way to make it function better. That’s exactly what I think Raney did with our dividers, and the following is a small part of the tool’s story.

It begins when my mom lived in New Mexico and ran a stall in an antique mall as a side business. Sometimes she’d visit the neighboring stalls and would pick up tools she thought I’d find interesting. She has a good eye. She used to collect wooden measuring tools, and some of those have been profoundly influential on my work.

Anyway, one Christmas she sent me a bundle of dividers she had picked up from other stalls and in the pile were the pair shown at the top of this entry. They were a bit large for furniture joinery, but I was struck by their sleekness and the way they felt like a seamless piece of smooth metal when closed. I thought their hinge looked a little small compared to the legs, but I otherwise liked them.

I loaned them to Raney along with a heap of other dividers I had been studying. He was drawn to the same sleek pair of Mom Dividers. That’s when the wheels started turning in his head.

Homemade but Clunky
One of the other dividers I had sent to Raney was similar in form but had a screw adjuster. While I liked the screw adjuster – it reminded me of my Starretts – it was too slow to be practical at the bench. Also, the way the dividers changed their shape from flat at the hinge to the roundish legs looked awkward to me.

But what I really liked about these dividers was their size. They were perfect for furniture joinery (laying out dovetails and tenons) and big enough to lay out cubbyholes in the gallery of a secretary. This is when Raney began to fire up the machine tools.


The Klingon Pair
I like tools that are stripped to their bare essence. Raney sympathizes, but he also likes a little ornamentation. This version was an effort to take the original Mom Dividers and add a nice curve that actually might be functional.

With this set, the idea was that the little Klingon fangs on the side would help you press the tips into the work. In practice, I didn’t think they added much. Raney was quick to ditch these.

This also is when we were still using a peened pin to keep the hinge in tension. It’s a common feature on blacksmith-made dividers. But it requires maintenance.

Can it Be Simpler?
One of my faults/gifts as a designer is to try to cut away anything unnecessary and find the beautiful skeleton below. Sometimes I go too far. As part of this exercise, I asked Raney to make a series of dividers with different tolerances and different numbers of leaves in the hinge.

We had good luck with three leaves, but I asked him to take it down to two. Would it make any difference?

It did. These two-leaf dividers were easy to make but they also loosed up too quickly. At this point Raney went deep into his lab and I didn’t hear from him for weeks, maybe two months.

The Working Prototype
When he emerged, he had synthesized all my comments and then taken a giant intellectual leap forward on his own. He designed the sex bolt that allows you to set the tension. He added the chamfers that I adore. And he created a tapered and curved shape that gives machinists fits to recreate, even on advanced CNC mills.

But we love it. The tools are incredibly difficult and expensive to make. The handwork to get the finish just right is grueling. But, as I’ve said before, they’re the best pair of dividers I’ve ever owned.

— Christopher Schwarz

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  • Damien: October 06, 2017

    I have two dividers, one with my great-grandfathers initials on it and one from a later generation that is effectively improved. The head is also thickened with a sex nut and is by this rounder making it easier to step. The overlap is reduced in width to a neck and improves the grip to the head. Like here a deep internal bevel makes opening easier. The deep bevel is then moved to the exterior of the legs down to its triangular tip. I guess the triangular profile is not only for aesthetics but also to reduce weight while keeping most of the stiffness. It’s a success as it is my dancing divider with a good head and not too much weight in the legs.

  • tsstahl: October 06, 2017

    Any thought about offering different sized dividers?

  • Michael D Nicley: October 06, 2017

    I own a pair of Crucible dividers and I love them. The only question I have is: why use a sex nut that requires a special bit to tighten? The two-dots aesthetic is sexier, but it would be a lot easier to adjust if a simple #8 flathead screwdrive could do the work.

    Keep up the great work,

  • raney: October 06, 2017

    Bill – every example of Peter’s dividers I’ve seen were 3-leaf designs, so I suspect we might have a nomenclature problem here. On our blacksmith dividers (between Chris and I we have samples from Peter Ross dividers, Seth Gould, and quite a few historical ones) one leg has two leaves, and the other leg has a single leaf that slots between them. We term that a 3-leaf design.

    Two-leaf designs on shop dividers are rare in my experience – and universally crap for workshop use because the tension between legs is much lower, and they lose settings too easily.

    Where they’re very common is in drafting/drawing instruments – where absolute tension is much less critical because they’re used in a precise, controlled environment at the drawing board.

    And Matt – that application (2-leaf drawing instruments) is also where your solution is common in the best tools. Until the second half of the 19th c., either an ancillary (softer) metal disc was used, or beeswax in the sort of pocket you describe. Later, I’ve seen more friable material in either a rubberized or heavy-oil based medium.

    The commonality here, though, is that we prefer a much higher level of tension in the shop, where it’s common to set dividers and want them to STAY at that setting longer. Drafting requires less overall tension, but prefers a broader range of linear motion for rapid, controllable adjustments.

    Finally – David: yes, I’ve ‘messed’ with Belleville’s a fair bit. I’d characterize them as 90% spring with a bit of washer functionality. As such, I find they’re used mainly to spread out the linear portion of axial loading (read as: a broad range of tension settings) but that isn’t something we found necessary. We DO use a wave washer – which is 90% washer and a bit of spring function – to distribute the axial load better for smoothness, and to spread the linearity very very slightly.

    So here’s another nomenclature issue we run into: we call these ‘adjustable’, but really what we’re talking about has more to do with correcting wear than needing a broad range of tension settings.

    The 3-leaf, simple, blacksmith designs Bill is (I think) talking about are, in many ways, my favorite as well. BUT – correcting for wear in those (and sooner or later that does happen) is a somewhat more intimidating process of re-peining and then cleaning up the joint. That’s totally fine for me – I pein in my sleep after 10 years of platemaking – but we found a lot of resistance to it among woodworkers.

    A simple bolt with a high-side-of-normal thread count (32 or 36 tpi) gave us all the adjsutability we find remotely necessary, and can be done by anyone with zero training or ‘confidence’. So in the end, that’s what we settled on as the best design for semi-mass production.

    One final point – Matt, you rightly point out that ‘tools milled to crazy specs’ are hardly a good goal in and of themselves. But that’s not at all how I would characterize these dividers. The match tolerances between legs matter for aesthetic reasons, but the only tolerance I’d consider remotely ‘tight’ is the fit tolerance between the pivot axle and the holes in the legs. If that fit gets beyond .0005", there is a bit of slip between the legs at all tensions as the slop in that joint is taken up. It’s not all that problematic, really (and I’ve seen it in many – if not most – blacksmith dividers) – but we still wanted to eliminate it. So we did.

    The ‘problems’ of making these has very little to do with tolerances, really – it’’s the shape of the legs themselves, combined with the sort of numbers of these we’re producing. Those lead inevitably to certain means of production, which don’t prefer the sort of long/thin/curved shapes we want.

    If a larger company made these – you’d almost inevitably see the lower, curved portion of the legs chopped off. Then a turned, regular-cross-section hardened steel ‘pin’ would be screwed or peined in to those truncated legs. Why? Because it’s cheaper and easier to make.

    In looking at these sorts of tools, I find there is a bit of a polarity of approaches, which I’d describe as the range from: a) folks on the theoretical side who will suggest alternatives that are creatively engineered, and very elegant — but tend to be hard to actually make in the real world in the scales we are working it — and b) people who make one-off items themselves who have much simpler solutions that require more experience and skill to make consistently (and I have some experience in that world).
    But in the end, if you’re making things, you settle on compromises and choices in the middle ground between those two poles – choices that you think best suit the goals. That’s what we have done, and so far we’re quite happy with those choices.

  • david la touche: October 05, 2017

    Did you try Bellville washers as a tensioning device? Very adjustable.

  • david la touche: October 05, 2017

    Did you try a bellville washer?

  • Matt_Rob: October 05, 2017

    Some times the simplest tools are over thought such as vice jaws and the material used for friction to keep work in place. Vice jaws as far as I can tell they work best with a friable material such as a material softer that what is wanted to be held in compression. Such is most friction fits such as the clutch in your old VW or the brakes on your car or truck they are supposed to be worn in the act of friction as to not build excessive heat and perish. Such could be used in a high end device that chooses to be held in place as desired by applied pressure. A simple block of compressible friction material inside a milled pocket between the legs of the dividers could be used to apply as much or as little resistance as wanted. Tools milled to crazy specs is wicked cool but unnecessary to be usable if logic and not machine practices that have come before are applied. Anarchy to look forward and not behind?

  • Bill Anderson: October 05, 2017

    A good example of the reality that you can not improve on a great design! I have 3 pairs of Peter Ross dividers that I use for all sorts of layouts. These are pressure fit, two leaves, no screws, etc. and are direct descendants of a design that is hundreds of years old. So far, after years of use, no maintenance needed. They stay tight and do not slip in use. Plus completely hand forged and shaped. Can’t remember the price, but cheap in any respect versus functionality.

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