Official Luthiers Forum!

When the resevoir fails to work (doesn't create an instant seal), you now have gallons of air volume for your little pump to pull into a vacuum rather than the tiny amount of volume in the hose and fixture channels. It works exactly in reverse of positive airflow: if you have a bigger 'tank', it takes longer to fill and longer to reach a given pressure. In the case of vacuum, you might need to reach a specific 'negative pressure' before the part will stick on its own.

Just as an addendum to what others have said: I've never used nor required any more vacuum flow than what I can pull through a 1/4-3/8" ID hose and one port in any fixture.. I don't believe it's of significant benefit to have any more than that unless you're fixturing parts at least larger than, say, a guitar top (and as thin). To get a good grab, you need to have your part covering and compressing your gasket before you turn on the juice rather than relying on airflow to try and make up for a bad initial seal.

_________________
Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Nelson would one of the vacuum pumps like I use for bagging work for your fixture? It doesn't have the high flow like the shop vac, but it does suck! That's kinda like that "gas" statement you made !

Bob, purely from the curious.....could you do a little pic from Rhino to show an end view of what a plate, gasket, and grooves should look like? I have never used vacuum for flat work and it sounds like it might be the answer to my flat work holding problems.

Mike

Google 'vacuum chuck'. Most people just use a plate with a grid of slots in it, deep enough that the gasket won't 'pinch' over the top but not too much deeper.

Most of my vacuum fixtures are purpose-built, and for 3D parts. I didn't bother making a drawing of the grid plate, I used a macro in the G-code.

_________________
Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Trev--Thanks for the tip on the rubber mallet. That worked wonders. Guess it just needed a little coaxing.
Here's a shot of one of the parts after rough contouring the outside. And thanks to everyone else for the excellent suggestions!
Nelson

Looks great Nelson! If I see it correctly, looks like a lovely back about 2/3s roughed...is it a single, deep pass you're taking? (If so, climb cut of only a few thou each pass?) Is the lexan a sort of dust collector?

_________________
Dave
Milton, ON

Dave--I sometimes take it one pass but usually break it up into two rough passes.
The stepover is .125" for each cut and running about 160IPM at 24KRPM.
Yeah, the lexan is a shroud for the dust collector with the outlet back behind the bridge.
It's adjustable for height with a couple thumbscrews on the back side for tightening against the 1/2" vertical rods.
The door is hinged on the left to allow opening for tool changing access.
The tape is a temporary "lock"---need to fix the mechanical door lock.
There's quite a volume of chips coming off that maple when roughing with the 3/4" ballnose. I need to replace the skirt of neoprene strips with a bristle setup to better contain the wild shavings.
I'm leaving the flange an 1/8" over thickness and then rough out the inside the plate before coming back to bring the flange and outside down to near thickness.
The maple warps during roughing so taking some off the inside tends to equalize the stresses prior to finishing the outside.
Here's a little "action shot".
Nelson

Really? Probably apples to oranges, but I was trying to relate it to my duplicarver, using a 1/2" cove bit in an old B&D router. Takes me 7 or 8 passes or more to get it close. Takes forever.
The dust shroud is cool! (Know what you mean about all the chips....garbage bags full!) In my case though, by hand, I have to be able to see the bit.

_________________
Dave
Milton, ON