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Vacuum Pump

Emiliano Achaval said:
View attachment 21103 found this pump in Craig's list. Is it the right type for a vacuum chuck? Thank you in advance. Aloha.
Click to expand...

All vacuum systems will leak some. The pump must remove more air than the system leaks.
In the 13.4 AAW journal, Dave Lancaster reccomends a pump that evacuates at least 3 cfm
In the 27.3 AAW journal John Hiem has an article on leakage comtrol.

The system you show removes 1.8 cfm - migh work.
If your system leaks 2 cfm it won't.

Al
 
Emiliano Achaval said:
So, unless I do some testing with a pump I wont know how much my Stubby 750 leaks... Hmmmm...
Thanks for the reply!
Click to expand...
@Emiliano Achaval there air leakage in many places.

The coupling to the latte, the vacuum Chuck threads, the vacuum Chuck seal with the work, and the pourosity of the wood.

Usually the two biggies are the porosity of the wood and the seal with the workpiece.
I don't think the stubby will leak.
 
Emiliano Achaval said:
So, unless I do some testing with a pump I wont know how much my Stubby 750 leaks... Hmmmm...
Thanks for the reply!
Click to expand...

There's probably zero leakage in your lathe spindle, but:
  • the rotary coupler leaks around the seal and through the bearings
  • the connection between the vacuum chuck and the spindle leaks
  • the seal on the lip of the vacuum chuck leaks where it contacts the wood
  • the system uses a bleed valve to regulate vacuum being applied and this can be quite significant if the piece is thin and you don't want to apply full vacuum
  • air leaks through the wood ... an amazing amount of air. And, if it happens to be ring porous wood like ash, it's almost like trying to pull a vacuum on a sieve.
Under the best of circumstances with a pump such as a rotary vane type with a high flow, the attainable vacuum vs. system leakage will be a linear function like that shown below.

vacuumvsleakage.jpg
It's pretty clear that if the system has much leakage then the maximum achievable vacuum is significantly impacted. This means that in the real world for small thin light turnings where the vacuum might be as low as 10 in-Hg the system could only tolerate a leakage of 1.2 cubic feet per minute (please excuse the obvious typo in the chart that says cubic inches per minute, my brain was on autopilot). Likewise, if the system really needed to pull a strong vacuum of 25 in-Hg for a large heavy piece then the amount of leakage that the system could tolerate and still be able to provide that vacuum would be only 0.3 cubic feet per minute ... and that really isn't very much. I would say that would be where this pump couldn't do the job. Anyway this illustrates why vacuum pumps need a high flow rate. When the specs say that the pump has a maximum flow rate of 1.8 CFM, that's NOT while pulling full vacuum ... it's with zero vacuum being pulled.

The vacuum pump that you show is the kind designed for air conditioning work where it needs to pull a hard vacuum, but flow rate is not a priority. This almost certainly means that it is a small piston type pump and the chart that I showed above is probably too optimistic for that pump. If it's an off brand pump then there probably isn't much, if any, published performance data available, but you might consider the chart below as a worst case scenario.

vacuumvsleakagenonlinear.jpg
While this second chart might be a bit pessimistic, It's probably not too bad an idea to use it as a guide for maximum achievable vacuum as a function of leakage. And once again, please excuse the typo for the units on the X axis. It should be cubic feet per minute.
 
I think that what Al and I are saying is to get a vacuum pump that has plenty of flow. That is more important than the value of the peak vacuum. While a wooden bowl might hold water, it won't hold air at all. You know the famous soda straw model used to teach beginners about cutting so that the grain is supported ... well that soda straw model is pretty good for describing what air thinks about bare wood.
 
When I bought my Stubby many years ago, I did part of the plumbing for the vacuum, now after reading you guys help. I'm reminded as to why I didn't get it done 10 or more years ago, lol. Nobody has one here in Maui, if a club member had any experience would be easier to get some help...
 
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