Which Wavy? How Much Gravy?

  I am looking at buying a set of wavy parallels; mostly they exist so you can put thin parts in the vise which are thinner than a single (1/8 or so) conventional parallel, while still enjoying the benefit of a parallel, ie; aligning the part with the vise ways, and thus (presumably) with the mill's table surface.
Like standard parallels, they come in sets containing various heights.

these are sane curves for wavy parallels

 They are not terribly expensive, depending on whose word you are willing to trust, and how much you want to spend.  They _should_ be easy to make.  Cut out with laser or waterjet, stamp shape with simple tool, heat-treat, stack a bunch of them together in a vise or on a magplate, and surface grind to dimension.  Do a hundred (of each size) at a time.

 So when shopping, I immediately noticed something: most of the less expensive sets have an "accuracy" stated as ±.0002 (over the length of the tool) which is a typical accuracy for both kinds of parallel... BUT most of the cheap sets aren't six inches long.  They are ±.0002 over FOUR inches, not six.  I've even seen a few sets that were three inches long.  I hope it's obvious that they are less accurate than parallels with the same spec over six inches because I'm still in a lot of pain and I don't feel like typing out the explanation. 

 Caveat Emptor. Buying name brand does not necessarily gain you anything; the ones I've looked at are expensive because name brand, but the promised accuracy is the same.

Aside: the acid test for these things - that is, to see if they are any damn good - is all about their spring temper, testing that they can be compressed and will spring back to their original full width.  It goes like this:

1. pick one out, lay it flat on something reasonably flattish
2. measure the wave height - a scale is sufficient, no need to bust out the surface plate and height gauge :) - and write it down
3. stick it in the mill vise and smash it flat.  Ignore it until the next day
4. take it out and measure the wave height again
5. if there is no difference, or only a very tiny (< 1/16) difference, they have good temper and will probably hold up.  If it loses a bunch of wave height, its temper is too soft - if it cracks, its temper is too hard - in either case, return the set to the vendor as defective.

 The other critical feature - how parallel are they really, requires a surface plate, height gauge, and a tenths indicator.

 By the by, the sideways wave shape of these things should be quite shallow, like a shallow sine wave.  Yet I've seen some wild (deep) curves in a few of the cheap ones, even in the Grizzly set, which I suspect will become problematic some day.

Have a look at how deep the curves are
vs. the ones in the photo up top

  It just seems to me that if wild curves were good and workable, all the name brands would have them, since it lets them support wider parts, even parts you would normally need two standard parallels for.  But they don't, which makes me suspicious of the few Chinese-made versions that are deeply curved.  I would avoid those, without real evidence, on general principles and the above reasoning. :)
(and I'll be happy to be proven wrong in this case)

 And another thing: they give you pairs of wavy parallels, but unlike regular parallels, you should never need two wavy parallels.  I asume it's because there are two parallels in regular kits - where you actually need two.  If I ever find a use case that is handily solved with two wavy parallels, you can bet I will brag, I mean blog, about it here.  Otherwise, they're just spairs.

 ~≈{👁}≈~