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Hey guys, the more I'm reading about vacuum fixturing, the more I think that's going to be the way to go so, I've got a couple of questions:

Do MDF fixtures work OK or do you have to use some sort of plastic?

When creating the seal, is the fixture made such that the part is sitting on the gasket or does the vacuum pull it down against the top of the fixture?

For a part that's roughly 4" wide by 27" long, would vacuum hold the part down well enough or would I need locating pins in addition to vacuum?

TIA,

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

Andy, MDF can work but it needs to be sealed as the vacuum will leak through MDF, CA, epoxy maybe even shellac should work. I use baltic birch myself. Phenolic boards such as Richlite are great for this appliction, as is aluminum of course.
I use an 1/8 neoprene cord stock for my seals and cut a groove .125 X .085 deep and the part sucks down against the fixture. If the part does not bottom out on the fixture the vacuum pressure can deform thin parts such as fingerboards. With the part down against the fixture the part will definately stay put, no need for additional pins.
Hope this helps.

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Jim Watts
http://jameswattsguitars.com

You can over come the leakage in mdf with a high enough volume pump. This is what we do as an alternative to sealing. Just lazy I guess.

I've been trying Baltic birch lately. It's softer than mdf, tapped holes don't feel as solid and it splinters so a crisp edge is harder to achieve.

I just got a 24 x 24 x 3/4" sheet of phenolic in from Mcmaster. Havent used it yet. Anyone know of less expensive source? This seems like the next best choice after M6 aluminum.

Sheldon, Check out Richlite, they make a product just for this.
http://www.richlite.com/richlite/industry.asp
I don't know the prices as been years since I've bought from them and I was also buying multiple sheets at a time. It might be worth inquiring though.

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Jim Watts
http://jameswattsguitars.com

MDF can be alright but you need a lot of pump volume. I use aluminum or plastic for all of mine, and I would recommend biting the bullet on a permanent fixture. Why deal with warp and leakage forever for the sake of $20 worth of material, right? OTOH, it's probably not a bad idea to prototype in cheaper material until you're certain of your fixture designs. I have a fixture graveyard upstairs in my shop with at least six iterations of my neck/fretboard fixture and probably an equal number for my bridge fixture.

Vacuum is good vertically, but it doesn't take shear well. If you're running conservative feeds and speeds then you can get by without using pins, but if you're running 'proper' feed rates then even a 1/8" cutter can knock a fretboard out on the short side. That's with a full 27" vac on a smooth surface. Small routers have very low feed rates compared to their spindle speeds, but I wouldn't risk it unless you're certain that shear isn't going to be a problem.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Thanks Jim. I'll contact them on Tuesday.

What kind of plastic Bob?

I've been using King "Star Board" material for vacuum jigs and it's very nice. It cuts easily and smoothly with regular wood bits (it's made as a substitute for wood in the boat building industry). Holds a vacuum very well. You can buy scraps of it in good jig sizes off of Ebay at a reasonable price too. On the downside, it tends to be warped when you get it (it comes in plywood sized sheets, so I guess this isn't a surprise), but a quick pass with a table flattening bit or a jointer, and it's good to go. Stays flat once you surface it. The same material is also sold as "Sea Board."

Dave

Whatever I've got lying around

I use aluminum for anything I'll reuse, but I've used chunks of acrylic, ABS, PVC, and Delrin before for temporary fixtures.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

I've used urethane tooling board from General Plastics before but not the 50 lb. stuff. I got a sample of the 50lb stuff the other day and I have to say it looks terrific for tooling plates and vacuum fixtures. It machines like cold butter and is quite stable and rigid.

Truth be told...I use aluminum for just about everything but only because I have a LOT of it on hand. My company used to process between 1500 - 2000 lbs. a month of 6061 T-6 so I had about 4000 lbs of it leftover when I sold the business...in all kinds of sizes. That's going to last me a while making the odd tool but I'd switch over to this urethane tooling board in a heartbeat if I had to.

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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

Is that the FR-4500?

This is excellent info. I think I'm going to try and buy as turn key a package as possible from this outfit: http://www.veneersupplies.com/categorie ... ess__Kits/

I'll probably have to call him up to get what I need without a bunch of extra stuff.

Another question about the seals - in the above links, it looks like he uses some sort of tape to the HDPE so under vacuum, the part will probably deform unlike the fixtures discussed above. So, when cutting the grove for the sealing strip, do you cut a wider but shallower grove to give the sealing strip somewhere to go? How high above the fixture does the sealing strip protrude?

I've never actually seen a vacuum fixture up close, just far away shots from things like the taylor videos etc. so if someone's willing to post a pic or two, specifically with some close ups on the seal, I'd be much obliged!

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

Andy, Here are three pictures of one of my jigs--it's the one I use to hold the backs of my banjo rims. It's made out of King Star Board, and I'm using the 1/4" McMaster foam cord as a gasket. The groove is slightly over .125" deep, and the cord just presses in place. The center is hollowed out ever so slightly just to give some breathing room for the vacuum. The first picture shows the vacuum hole in the jig, and you can see the hose coming out of the side of the jig. Hope this helps!

Dave

I think so...yeah. That number sounds too familiar to not be it.

Whatever the exact number it's the 50 lb. foam in that product line.

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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

I just got off the phone with them. The FR-7100 can be machined with hand tools and the offal is more like dust. The FR-4500 machines more like metal. They both come in 50# densities. I've got some samples on the way.

I'm using 1" and 1.25" Paperstone, which is similar to Richlite. It's a "green" countertop material. Scraps can be had for a very reasonable price if you have a fabricator near you. The stuff is fantastic.

Don

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"I want to know what kind of pickups Vince Gill uses in his Tele, because if I had those, as good of a player as I am, I'm sure I could make it sound like that.
Only badly."

OK, so i'm almost there but the one final thing I can't figure out is whether I need a continuous run system or and Auto Cycling system.

If you look at this chart: http://www.joewoodworker.com/docs/vacuumpresschart.pdf

I'm looking at the EVS + 3CFM pump for $298 or the Excel 1 continuous 1.1CFM pump for about the same. The 1.1 system works out a little cheaper as the clamping kit has less parts.

The "man" part of me says to go with the 3CFM system but it's much louder and needs a lot more assembly, some stuff needs to be purchased at the hardware store plus, it's not clear but I might have to add a continuos run mode for vacuum clamping.

The 1.1 cfm version is nice because it's much quieter - I could see myself using this for bracing and maybe bridges as well.

I'm looking for the kit or complete setup because I'm tired of building machines already - I want to build instruments already!!!

Any advice?

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

Andy,

If you have a decent compressor then using a venturi is perfectly adequate. I have a $500 reed pump that I never use anymore because the venturi does the job with no moving parts. The venturi doesn't use much cfm.

_________________
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

I have and use the cycling kit that Joe Woodworker sells. It works very well, and I use it primarily for vacuum bagging. I like the cycling arrangement because once the vacuum is pulled, it pretty much holds with very little cycle-on time, and even with the muffler, the venturi is louder than I like to hear. My compressor is way louder than I like to hear and my shop is too small to really isolate it. For my vacuum clamps, though, I use a continuous running Gast pump. The reasons I use the pump instead of the venturi are multifold: First, the Gast pump is far quieter than the venturi, especially if the compressor kicks in; Second, it's more portable--I can carry and use it in places that I don't have enough air hose to reach (I use vacuum clamps on non-CNC projects too, and that sometimes takes me out of the shop itself); and third, I only have one 220V circuit in my shop. If I'm running the CNC, I already have my spindle and a dust collector running off of that circuit. If my compressor kicked in at the same time, it would push my circuit to the limit and could trip the breaker. Not worth taking the chance.

But bottom line is that the Joe Woodworker venturi pump works great. Either the cycling or non-cycling model would work, but I'd recommend the cycling unit since it will greatly reduce the time your compressor spends running.

Dave

Oh yeah--I got the Gast pump as new old stock off of Ebay for less than I paid for the parts for the Venturi machine.

I use Venturis on my short-run work, and have a few for different purposes, but if I had a small shop then I wouldn't want to hear my compressor run any more than it needed to. I have one of the quiet little electric pumps that I use on anything I'm going to leave under vac for a long time, and it's quite adequate for non-leaky work in the CNC.

If/when I get rid of my turbine spindles, I'll probably get a bigger electric vac pump to eliminate that last bit of compressor usage besides my ATC, because silence is still awesome even when you're in a big shop with a relatively quiet compressor

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Doing some price comparisons.

Paper based Phenolic (Garolite XX) is about $800 + freight for 4' x 8' x 3/4" (McMaster-Carr)
Urethane tooling board is about $1500 +freight for a 4' x 8' x (3/4" or 1" I can't remember) (General Plastics)
Cast aluminum is about $2300 + freight for 4' x 8' x 3/4" (McMaster-Carr)
Paperstone is about $4200 for a 5' x 12' x 1-1/4" (local supplier)

Definitely not as cheap as paper based phenolic, but aluminum plate is much cheaper if it's not the cast plate and flat bar is very much cheaper than normal plate.

A 4'x8'x3/4" sheet worth of 6" flat bar is around $1100 from OnlineMetals (8x6"x8')

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

King Starboard on Ebay can be had for $77 for a 3/4 x 54 x 24 inch piece (plus shipping). Other sizes available too, but even in 4x8 pieces, it's way cheaper than any of these and works great for fixtures.

Dave

What is Starboard? It looks like a polyethylene.

You are correct. It is HDPE.

So, do any of Y'all use double sided fixtures? i.e. ones that clamp themselves to the table and the workpiece simultaneously?

I have a t-slot table on my machine right now but am considering replacing it with phenolic plywood or something like that so that I can stick the fixtures right down to the table with the part. Any luck with that?

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