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I use a 3" drum sander in the drill press. A 3X3 block of wood is clamped to the table to serve as the sanding bed. It has a thin fence attached to the bottom to guide the bone blank. The block is adjusted to sand a few thousandths off at a time. I use a pusher stick.....no sled.
I generally leave 60 grit paper on the sanding drum because it will remove material quickly. I sand the saddle with the drum to within 0.003" to 0.005" of the finished thickness, then scrape and sand by hand to fit the saddle to the slot.

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John

I just run them straight through my performax. I push them through with a dowel.

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Old growth, shmold growth!

I still squat down to eye level with the platen and slide the bone on the the belt sander. I've never used any type of holder, as I am constantly pulling it back to measure with calipers and sight by eye. I also find issues with inconsistent heat building on one side when sanding a single surface for too long to cause warping which can affect inconsistency along it's length (this may not be an issue on a drum with the more narrow point of contact?). For this reason I'm constantly feeling and flipping the blank every few seconds to keep heat more evenly distributed, even in rough thicknessing before I really start measuring closely. Then on top of all this, there's the tacit, tangible factor of leaning a bit more pressure under this finger or that to focus a change in areas I want.

All these things combined make the board holder much less appealing to me personally, as I feel it is cumbersome, slows me down, and introduces a barrier between my senses (both visual and touch) and the bone which I find critical to quick direct control over where or how much material I want to remove.

So for me it's my fingers, eyes, the bone, calipers, a belt sander, and then when I get within a thousandth or two of target I finish it up on a mini surface plate fit with sandpaper on by bench. Of course it takes a lot of practice and a lot of screw ups to get a good feel for controlling precise thickness this way, but since that's how I learned I've become quite comfortable with it, and can achieve quicker and better accuracy than I personally can with other methods.

Might sound silly, but after fitting thousands of nut and saddle blanks this way it's almost become a meditative practice, where I don't even think about it so much as it just happens.

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Eschew obfuscation, espouse elucidation.

In the spirit of May 4th, I use the force

fwiw; tabletop milling machine for me. insignificant heat, no sanding, very little dust, no guessing, no fussing, no fudging, no holders, no forms, no inserts, no jigs, no fixtures other then a vise and some parallel bars. done once and move on. simple and final -although not very romantic apparently, but who cares? the price you set for the job or instrument caps your profit so everything else that doesn't add value or efficiency is waste.

parallelism, perpendicularity, and flatness less then .0005 per the entire length -easy. put in a corner rounding end mill and you can have your end radii too.

Nope.....not even close.
You will laugh when I describe how I adjust my block on the drill press. I use a large C-clamp, attached a couple of inches past where the sanding drum is closest to it. The clamp is only partially tightened during adjustments. This allows me to tap the left end with the heel of my hand, bumping it closer to the sanding drum. If I overdo it, I simply loosen the clamp and pull it out a little.
With a little practice, it is fairly easy to consistently sand a blank within 0.002" to 0.003" of the target thickness.
Heat buildup can cause the blank to bow outwards on the sanded side, but that can be addressed by flipping it over a few times duiring the thicknessing process.

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John

We do them in batches 3/32" 1/8" 3/16" -- take 10 or so place the length-wise on a piece of Masonite (any board would work) apply a bead of hot melt glue on both ends. Then run them through our little 10-20 Jet. When they all have a sanded flat, peel them off the Masonite, flip over apply more hot glue. Check the thickness with a caliper using the depth end. You certainly could do one at time and even use a reference block to check thickness by feel. The shop smells weird the rest of the day when sizing bone.

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Ken Cierp

http://www.kennethmichaelguitars.com/

I use an open ended thickness sander table made for 6x48 belt sander.
Stick finished side to 18" piece of masking tape and drag it through.
zero calipers on the tape and your good to go.
very little heat on the belt sander. very quick.
real screw micrometers are great for final fit.