[time-nuts] AN/URQ13 reference AT cut crystal?

[Bob kb8tq]

Hi

> On Feb 18, 2021, at 9:17 AM, Lux, Jim <jim at luxfamily.com> wrote:
> 
> On 2/18/21 3:53 AM, Poul-Henning Kamp wrote:
>> --------
>> Bob kb8tq writes:
>> 
>>> Turning an “idea” into a production capable part involves making many
>>> batches of test samples. Think in the thousands of batches and hundreds
>>> of parts in each batch. You have a “search” process at the blank chopping
>>> level. You also have a search at the resonator fabrication level. Getting the
>>> chopping part right is only a small part of the whole process….
>> I realize this used to be a manual process, but today I would expect
>> that you could automate a lot, of not most of it, if you wanted to ?
> 
> 
> That was my first thought, and then I thought through all the steps. I think it would be challenging to automate (and that brings up your question below about "is it worth it?")
> 
> First, are you starting with natural or grown quartz? ( While grown quartz is used for most crystals, isn't there some performance benefit from natural crystals?)

Finding natural mono crystalline quartz is a challenge. Finding big pieces of it is 
more of a challenge. (sand is easier to find that big chunks). 

Finding un-twinned crystals ( it comes in left and right hand orientations) is impossible. You
are left working out a strategy on a per chunk basis to avoid twinning and inclusions.
Once you are done, it’s slow growth quartz so it may be better in some respects.

> 
> I assume there is some sort of process to create "bars" of quartz from the raw boules or crystals.

Quartz is grown from a seed. You dump raw quartz bits along with a chemical 
soup into a pressure vessel. ( think in terms of a 16” naval gun barrel ….). You 
seal it up, pressurize it and heat it up. At high enough temperatures the silicon
dioxide feeder material dissolves into solution. You try *not* to dissolve your seed
blanks at the same time :)

The temperatures of the vessel ( = there are multiple zones) are varied over time.
The pressure could also be varied as well. As you drop the temperature the crystals
grow on the seed bars. The faster they grow, the more likely there are to be minor 
defects in the resulting crystal. 

The seed gets cut out of the bar before it really goes anywhere. 

> 
> I believe you'd need to do some analysis to determine the crystal axes and then the bars go into a series of saw steps - those might be automateable, in terms of sawing angles - is the sawing with a wire, with a diamond blade, or these days, perhaps abrasive water jet? 

For volume production, a multi pass wire saw or a blade pack saw are pretty common. 
For one off “lab” stuff a diamond saw might be used. In between the “grow it” and 
“cut it” stages there is an x-ray process and some method of mounting the material 
in a holder. 


> That would give you a bunch of slabs with the cut with the right angles - I'm sure the machine they use today probably dumps them in a hopper, and I've watched enough food packaging shows to know that you could get those fairly large slabs onto a conveyor.

For an OCXO crystal, the “slab” is going to be < 25mm square, generally much less. 

> 
> Then you'd have to cut your desired crystal shapes out of the slab (whether round, rectangular, or some other shape) - another sawing or grinding step, I assume.

The most common shape for a precision part is a round blank. You stack the angle corrected
squares together and put them on a centerless grinder. … oh, angle correction … Well you 
can’t cut blanks accurately enough with a sawing operation, you have to correct the angle 
after you have lapped them flat … oh, yes, you have to take the parts that come off of the
saw over to a lapping machine and get the sides parallel, generally to a fraction of a light 
band. 

> 
> And then mount in a holder automatically.

Before you put it in a holder, you need to contour the blank. There might be another angle
correct stage in there. Once you go through contour, you get to polish and then to chemical
etch (to remove the debris). After that you baseplate the parts. Without that, you have no
way to get an electrode on the “bottom” ( think TO-5 / TO-8 / HC-40) of the blank. 

After baseplate the blank goes into the mount. If it’s a 4 point mount, that’s a manual 
process. Next up is epoxy and baking to cure the epoxy. After that finish plate and seal. 

For an OCXO part, you do as many of those steps (past baseplate) without breaking vacuum
as you possibly can. You also do a bit of cleaning and bake before the final seal. 

> 
> All the individual steps are sort of "mass production" but I think today, there's significant (manual) setup time for the machine between steps (kind of like making tiny screws on a Swiss Screw Machine - some time for setup, then feed in bar stock and tiny screws or spacers come out of the machine)
> 
> https://www.youtube.com/watch?v=mhwUHgWzzKs
> 
> 
> 
> 
>> 
>> It would still be a lot of work, and very expensive, but like
>> biochemist trying out hundred of thousand compounds from their
>> "libraries", robots really lower the cost.
> 
> Lot of work, I think, underestimates the magnitude of the task. It would be interesting to compare the processes used for creating high performance crystals (e.g. for a USO, where they start 1000 blanks to get a dozen or so oscillators) and those used for mass production of crystals for things like kitchen timers and microcontrollers.

The real contrast is to watch crystals. You photo fabricate multiple crystals from
one blank. You also may not do much of a seal on them.  Indeed there are 
crystals that are not destined for a watch that get processed that way as well. 

Fun !!!!!

Bob

>   I have heard that for USOs, there's a couple people who have the "knack" for installing the crystal in the holder in a way that minimizes the stresses, etc.   That is *really hard* to automate.
> 
> This is, of course, where SiLabs has a thing - they make hundreds (if not thousands) of MEMs oscillators at once with lithography, so the piece parts are very inexpensive - but they're performance limited by the material.
> 
> 
>> 
>> The real question must therefore be, if anybody reasonably expects
>> there to be any superior "new" cuts to find in the first place ?
>> 
>> What properties would you program a quartz-crystal-prototyping robot to search for ?
>> 
>> Which parameter(s) of current crystal-cuts are "their weak point" ?
>> 
> 
> 
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