First vacuum fixture.....

[Max Girouard]

I'm working on my first vacuum fixture. I've got this prototype made up in MDF. The first one I made we tested and it seemed to hold very well. I still have to drill the holes in this to attach the vacuum on this one, so first question is: Can you tap a hole in MDF? I'm sure it won't be very strong so I'll probably tap it and epoxy the fitting. Next, should I seal this with shellac. I've read posts about people saying that MDF is very porus, but on the first test fixture we made, we were unable to pull the work piece off once vacuum was applied. Once I get everything worked out, I'll end up making this out of something a little more stable and resistant to humidity changes. Any suggestions on what material I should use? Last, I have been running the zero in a hole routine, and it works really well. I tested it a few times and I'm getting the jig zeroed to within 0.001 every time. The only problem is when the routine is done running, it closes out of SB3. How do I get it to stop doing that. Also, thanks to all in my previous post that recommended the copper cap idea in the first place!

[hh_woodworking]

Yes you can tap mdf I have done several times before, but they do strip out after a while of use.

[donek]

I would coat the mdf with epoxy or urethane. You are likely to get many many uses from that tool. If you want to thread it, I would drill to your tap drill size, heat the hole and saturate with epoxy. The heat will thin the epoxy and the MDF will absorb it. With multiple coats, you will have a plasticized material that will hold threads quite well.

It's likely easier to simply use a pressure fit hole or to silicone in a plastic fitting with your vac or threaded connection.

[Brady Watson]

You don't need to go nuts tapping a hole. Just drill it with the proper drill size for NPT, and slather some kind of sealant like PVC cement, RTV, epoxy etc on the threads. Run it up and you're done. You can do this with the Bot if you go slowly with let's say an 1/8" bit with an inside profile tp with spiral plunge.

I would use a 90 on the bottom of the fixture, coincident with the extrusion channels, deep enough so it lets the panel stay flat & then run the tubing along the channel out to your vac source. Your vac source should be a high suction type (one that does 20+ Hg" or more), and err on the side of larger tubing than smaller if you have the choice. Small tubing doesn't make full use of the pump's airflow and this can make it difficult to create the initial seal. Sealing off the MDF will be important, since those smaller pumps do not generate a lot of CFM.

What are you using for a vacuum source?

-B

[Max Girouard]

I've got a Gast 1.1 CFM pump that I use for my vacuum clamping bag, and have used it for clamping work to my benchtop, so I'm hoping it will be strong enough for this application, if not I'll have to get a larger pump, but tests on the prototype I made before this one looked promising. I threw a board on, and within a second the board grabbed and I could not pull it free.