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I'm attempting to machine some ukulele bridges and the fixture I built isn't giving me reliable enough results. The part isn't staying down hard to the fixture when I'm machining the shape to the top surface. Drilling, slotting and profile aren't a problem. Have tried double sided tape too with no success.

I'm looking for some ideas that will be an improvement on what I came up with.

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Allen R. McFarlen
Barron River Guitars & Ukuleles
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Cairns, Australia

I assume you're using vacuum? First thing would be to make sure you're measuring the amount of vacuum to make sure you're getting a good seal.

We have problems with pickguard fixtures on narrow sections that vacuum doesn't hold reliably. We ended up using screws in those areas to keep things solid. This improved the finish and eliminated chatter on the edges.

No, I've not tried setting up a vacuum fixture for this. The footprint of the bridge is 23mm x 100mm and is only 6.5mm thick when finished. I didn't think that vacuum would do the trick on something this small.

Holding them in a jig with stationary fence on 3 sides to fit the size and a movable one to clamp it.

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Allen R. McFarlen
Barron River Guitars & Ukuleles
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Cairns, Australia

I use the Stew Mac double sided tape to machine bridges. I can't imagine it budging as I need to wick acetone in to remove parts. Being a pro, maybe you are using pretty aggressive machining feeds to meet production needs.
Mike

That could be it. I'm not babying it for sure.

I've got some of the StewMac tape. I'll give that a go. I was using double sided tape from the local hardware store that has worked fine for other jobs up to now.

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Allen R. McFarlen
Barron River Guitars & Ukuleles
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Cairns, Australia

You might try the double sided carpet tape available at hardware/home stores. It's really strong. I've never had any trouble with that stuff releasing and often use it for small pyramid bridges.

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

That is what I was using Andy.

The wings on my bridge design are 1.5mm at the ends and if they are off by the tinniest amount in depth it makes the part unsalvageable. I may have to rethink the design for another one.

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Allen R. McFarlen
Barron River Guitars & Ukuleles
Facebook
Cairns, Australia

There are two kinds of double sided carpet tape that they sell at the hardware/home stores too. One kind is thin plastic double sided. It's not too strong. The other kind is a woven fiberglass (or something like it) heavy-duty tape that really holds. I've machined many small parts with tight tolerances without any problem in machining. Where I HAVE had problems at times is getting the part un-stuck from the tape without breaking the part....

Dave

I've gotten a few different kinds of carpet tape, and the variance is pretty huge. The best/worst stuff I ever got had the thinnest imaginable web (plastic), it had to be less than 0.002" thick, and it held like you wouldn't believe. But it was a process about 40 minutes long with a number of very thin wedges to get a -fretboard- off of my machine's table without breaking it. You'd have to take that carpet off with a flamethrower...

You can hold down almost anything with vacuum (down to truss rod cover size) if you make sure it can't move laterally with pins and you're careful with your cutting forces. Holding down a bridge isn't a big deal if you use pins, and having face-on-face contact with your fixture makes it a lot more predictable on thickness than using adhesive. It's all about controlling where the cutter's pushing it and how hard- cut lengthwise if at all possible rather than across your part, and leaving an onion skin you can zip off with a razor can give you more clamping area. Also hollow out as much area as possible under your part, that helps a lot with pressure gradients.

You can also hold a blank down with a couple drops of CA on a flat surface and knock it off with a hammer. Surprisingly delicate stuff, even, if you place your glue right and give it the proper release thwack.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

I use the vacuum fixture shown below.
1/8 o-ring cord stock just inside the perimeter of the bridge and vacuum distribution grooves put in randomly, free hand with a router.
works well.
If I was to use MDF for a fixture like this I'd seal it outside of the o-ring groove.

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Jim Watts
http://jameswattsguitars.com

Thanks guys. Good tips that I'll try out.

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Allen R. McFarlen
Barron River Guitars & Ukuleles
Facebook
Cairns, Australia

Ditto what Bob said.

Think...pins for lateral constraint...and vacuum for vertical constraint....and you can apply that pretty much universally for parts that will set flat on a tool surface.

If the geometry allows....you can sink a profile into the tool surface itself and then use that along with vacuum for lateral constraint....having cut that profile into the part on the first operation.

For tiny parts I'll make custom vice jaws but for bridges...it's pins and vacuum.

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I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

For my CNC (careful, nimble cutting) bridge fixture, I use two 3/16 pins without tape or vacuum, and it stays put very well. Need a chisel to get it off! Although it is just for saddle route.

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Pat

I have mine mounted on another quick change vacuum base type of thing that I use for other small things. Works well for me.

Where did you get that plastic barbed fitting? I've been looking for a cheaper alternative to the brass ones for quite some time now.

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

Can't speak for Fleck, but I've been using the plastic McMaster Quick Turn Tube Couplings for both vacuum and air fittings for years--Steve Lindsey used these on the Airgravers I got from him:

http://www.mcmaster.com/#quick-turn-tube-fittings/=11qefnp

Dave

Jim where are the vacuum holes or slots. I don't see any.

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Beautiful and unusual tone woods at a reasonable price.
http://www.rctonewoods.com/RCT_Store
The Zootman
1109 Military Rd.
Kenmore, NY 14217
(716) 874-1498