Vacuum clamping very small parts

[cubicdissection]

I need to set my shopbot up to machine very small parts from solid hardwoods (I'm a puzzlemaker). I've been thinking about various implementations including pneumatic clamping and toggle clamps, but if I can get vacuum clamping to work it will vastly increase productivity because there will be less time fussing around between runs. However, I'm having trouble quantifying whether or not it will actually work, and I'd like to know it will before I drop a ton of money on the project.

For instance, cutting notches in burr puzzles. Because some material in the middle is usually removed from the pieces, and they are machined on two of four faces, my total area for vaccuum is two square inches. Say I mill out slots for the pieces to fit in to 1/4 inch depth, and use 1/8" sunken gasket to achieve excellent fit. Absolute vac is something like 14 psi, so by the time I use up the gasket area and leave a .05 shoulder for support, my total area to hold the piece down is really .845 square inches. Assuming less than perfect vaccuum of 8 psi, lets call it 6.75 lbs of presure holding the piece down.

Is that enough to hold down a 1"x3" stick of hardwood (reinforced on the sides by the recessed slot) while it has grooves cut in it? Is my guesstimate of 8psi achieveable with say a fein?

To get an idea of the sort of pieces I need to clamp, here's a picture:
http://www.toysfromtimespast.com/toys/318-2.jpg

[angus_hines]

No expert here but maybe a vacum jig that has all the different shapes you use would work.

And I still haven't figured out that puzzle you left...LOL

[cubicdissection]

Well, I never said it was an *easy* puzzle ;)

How's your bot running? Made anything yet?

Man, here I am getting OT on my own T.

[ljdm]

Doubt you will get 8" vac from the fein, best I get with 2 feins is less than 7". I use double-sided tape for small hardwood parts, seems to work pretty good.

[angus_hines]

Eric I've made a couple of small 3-D things and chopped up some ply wood. But that about it right now.

Will be back at it when we get back home from FL.

[cubicdissection]

Thanks Lou, that's the kind of information I need. Would I be able to get more vac with a dedicated pump, or venturi valve setup? Assuming higher vac, will the force I get combined with lateral reinforcement on the bottom via the cutouts get me enough holddown to mill these pieces?

DS tape would not work; this will be a production environment and I'll be making lots of these hopefully.

[Gary Campbell]

Eric...
Check out Brady's column. http://www.shopbottools.com/BradysTricks.htm#BradyVac%20II

He has some very good ideas there for small parts. Also check the archives as there are many special purpose fixtures that others have made to accomplish similar projects to yours.
Gary

[knight_toolworks]

myself I would make a jig with pockets that the pieces set down into. this way the vacuum only needs to hold them down. You will want to use downcut bits too for the cleanest and most accurate cuts.
with the pockets these would be easy to clamp down in rows too. using a high vac pump just on the ends may work.

[cubicdissection]

Hi steve,

that is exactly what I was envisioning and trying to explain...guess I didn't do a great job


Even if I can get 5lbs of vacuum, which doesnt seem like much, when you extrapolate that to such a small piece which has lateral support via the pockets, it seems like it might be ebough.

Gary, thanks for the link. I've read through Brady's stuff in the course of my research and none of it specifically addresses the approach of pockets and small parts. I was hoping someone on the forum has done that sort of implementation and could offer some guidance.

[Gary Campbell]

Eric..
Brady has posted a good number of examples of gasketed special purpose hi vac fixtures. Some from Trex type decking and a good number of other examples that I am sure will get you working. I wish I could remember the post headings... but cant. If you take the time to find them, it will be worth it.

You are right... with parts that small, 5 inHg probably wont make it.
Gary

[fmihm]

Eric --

Try a Google Custom Search of the Forum (NOT the Forum's Search Board) -- it's at:
http://www.google.com/cse?sa=Search&cx=007058511302222020632:fqbvgi8jgoa &cof=FORID:0
for: gasket watson

It gives you 17 hits. Some of these appear to be really worthwhile postings about gasketed pockets.

Hope this helps.

[Brady Watson]

Eric,
There are limitations to what vacuum can realisitically do for holding down small parts. Before I forget, here's the link to the Veranda Vac Jig (http://www.talkshopbot.com/forum/messages/312/26527.html?1199656102)

I have held down thinner parts down with the Fein with only .65" surface area, but the material was very thin. Once you start getting into thicker materials, you really need to reduce your stepdown and feedrates to reduce chipload and ultimately cutting force on the part. In many cases, especially in wood based materials, a single bridge/tab at the start point greatly aids in the hold-down effort. I'm afraid that you are going to have to do some experimentation in order to get the right hold down setup for your specific application. My shop is riddled with customized vacuum jigs and clamp fixtures for various parts for different customers.

My recommendation for the type of parts that you show in your 1st post is to use Destaco-type push-pull clamps mounted on a 2X8 piece of lumber that is screwed down you your table. The lumber is then flattened and dadoed to accept your parts. I did a run of similarly shaped parts for Disney not too long ago. I found it was too small for vacuum and I needed to get crafty in order to properly hold down the parts. The solution was to use a push-pull in the Y direction (NOT down from the top/Z) with a deep dado that acted as a locator and fence to resist the clamps. Worked perfectly on 600 parts.

You may find some more insight into mathematically understanding what is going on with vacuum & how it relates to hold down in this thread. (http://www.talkshopbot.com/forum/messages/312/18079.html?1185224641)

...but for now, you may find this equation helpful:

Your vacuum's max pressure in Hg" / perfect vacuum @ sea level (29.92 Hg") X standard air pressure (14.7 psi) = How many pounds of downforce will be holding down your part

(E.G.) 7.3"Hg / 29.92"Hg X 14.7 psi = 3.6 psi

So to pare it all down...you need to ask yourself if you dropped a 3.6 pound weight on top of your part to be held down, would it move? What if I cut less aggressively? What geometry bit should I use? (Is my upcut spiral actually hindering my hold-down efforts???!!)

-B