Size & Source for Vacuum Pump

[johnm]

Folks -

I am looking for what would be a suitable vac pump for a 4x8 table on a PRTalpha... There are hundreds of pumps that come up in a search on e*ay, from the ones that fit over your dongle (I assume tho' these might be fun, they wouldn't really work on a 'Bot) to the larger units that look like they'd make your ears pop at 50 paces. I know that it should be able to pull "X" inches of mercury, but they don't always list a spec. Would an amp or HP rating provide an equivalent? Also, what type of pump - I've seen piston and rotary vane. I am hoping I wouldn't need a "supercharger" style - I don't think I have the power available to drive such a beast.

Any tips, tricks or input would be welcome.

Thanks!

John

[bleeth]

John: You have just entered the domain of an area that is as deep as "What kind of cnc should I buy".
The first step is to search all that has been previously written on the forum about this. There has been a lot.
To hold an entire 4x8 sheet down the answer starts coming in the field of regenerative blowers. Remember that all vacuums lose strength once your board is cut through and therefore, for example, if you are going to cut a large piece into many small ones it takes a lot more to keep holding it and therefore a lot of people will leave a thin skin and cut the parts out afterwards with a razor knife. For surface cutting smaller parts a lot of guys use vacuum pumps like the vak-matic. As long as you are not planning to make use of every inch of the board, just as many people screw and/or clamp it down to the spoilboard and leave tabs to hold the parts in place. Many botters manage with a stronger shop-vac, or for a slightly larger investment but better quality buy a Fein Vacuum which draws it's vacuum from an outside source and so the motor stays cooler.
After you have researched the site, talked to the guys in Durham, and figured out the type of cutting you plan on doing while the piece is vacuumed down, you will be a lot closer to knowing a little more about what questions to ask for what specific uses.
For me, I have gotten by without much vacuum at all for two years now and havn't found anything I can't do without alternate methods.

Dave

[johnm]

Dave -

I am thinking I would have the parts either tabbed, or skinned so that the vacuum wouldn't be broken. This raises another issue - is it more work to do hand work on parts with tabs/skins than is saved by the speed of vacuum clamping?

The smallest component I think I'd be cutting would be about 4" square, most would be closer to a square foot, or larger.

John

[billp]

John.
"Pods and pucks" would be VERY useful for this purpose. If you get a chance go to my web page ( www.baycraftdesigns.com (http://www.baycraftdesigns.com) )and look under the "Shopbot"page where I have a few examples of how they can be used, and how to make them. For the size items you are speaking about a small, rotary vane pump pulling over 20" of mg would be plenty (depending on your material of course...).
Remember you can't use "tabs" with vacuum unless your gasket material is INSIDE the tabs, otherwise the tabbing process itself will break your vacuum seal. Full length "skins"(.02" thick or thereabouts) will still give you sufficient vacuum, and be easy enough to cut out when your piece is finished.

[bleeth]

John, Bill's advice is right on assuming your are cutting these a few at a time. If you were cutting a load of parts out of one sheet the situation may be different. I recall you saying something about gingerbread parts. Some of these can be quite intricate. I ran a shop that made many of these for Victorian Screen doors and we used a system similar to the "Pods" and left a skin. Remember that no matter what there will be clean-up. A very well known botter in New Jersey calls himself (only half jokingly) "The King of Sanding". I think that is a title shared by most of us.

Dave

[gene]

has anyone used or is using the 7.5 hp vac that shopbot sells ? How well does it work ? the unit i am speaking of is the single phase .
thanks for your advise Gene