I just (finally) was able to find some decent dual piston wobble pumps -
this is the kind I like for wood turning - they are now on my web page
www.wbnoble.com, click on the vacuum pumps & chucks link, they are at
the top of the page. There is also a short, and a very long article
that I wrote a while back on the theory and practice of vacuum chucks
that may be helpful to you.