09-17-2008, 06:00 PM
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| | | Join Date: Oct 2007 Location: usa
Posts: 2,299
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here is a repost of bill glens letter on vacuum | |
bill glenn wrote this for some of us who were learning vacuum.
There is much confusion about what kind of vacuum pumps are needed for different applications regarding hold-down on CNC Routers. Many of us think that more horsepower automatically results in better hold-down and opt for more HP and of course more electric bills. I will try to give an unbiased view on how it works and what works best from a (kuntryboys) way of saying it. I have used everything from reverse blowers from Grainger that will work on 220 single phase to 25 HP regenerative blowers to Positive displacement pumps such as Becker, Busch, or others. I have built CNC routers for 13 years and just lately feel like I understand it enough to really help someone else understand it. For most people including myself, we may think that a pump holds down parts because it "sucks the part down with air". When we understand what really holds the part it will help us understand more about holding down parts. Piab is a company that makes small vacuum cups, small pumps, etc. Several years ago I read a book they had and it explains very well how it works and it was very interesting. Basically without getting too deep into science this is it:
On what is called a standard day which is 59 degrees F at sea level air weighs about 15 lbs per square inch. Think of it as a column of air 1 inch square that goes all the way toward outer space until there is no air molecules. If you live in the mountains say at 2000 ft in elevation then your column of air is going to weigh less because there is not as much of it to weigh. If any of you are airplane pilots then you already know that the hotter the temperature the farther apart the air molecules are and the longer it takes to get off the ground and the slower you will fly. The same is true with vacuum. It will not hold down with as much force the hotter it is and the higher in elevation you are. But remember this column of air is fluid so it will try to replace any vacuumed areas. THIS IS VERY IMPORTANT. Air only weighs 15 lbs per square inch maximum. It cannot be made to weigh more with any size vacuum pump because the most you can vacuum out of anything is whats there. (15 LBS. MAX) if you have a 10" square you are trying to hold down the maximum you can hold down with is (10" x 10"=100 square inches) = 150 lbs of force. This is only with a pump that will pull 28-29 inches of vacuum. This can be accomplished with a positive displacement pump such as Busch, Becker, or others that can pull this kind of vacuum. Regenerative blowers on the other hand cannot pull this kind of vacuum but are more on the order of 10-12 inches which would make its hold down force per square inch be more around 6 or 7 lbs per inch. If you were holding the same 10 inch sq piece of material it would be held down with more like 60 or 70 lbs.
Positive displacement pumps would probably be the ultimate but we have to remember that they do not put out the same volume as a regen blower. They put out much less per given HP because they are doing more work so you must find a balance. They also require more HP to get the job done. So if you use residential electricity you may find that you want to go with a 5-7.5 HP positive displacement pump such as Busch, Becker, etc and make sure you don't have leakages etc. I will mention some tips there later. If these pumps are too high new there are several companies that sell rebuilt ones with a guarantee. If you have plenty of electricity you may want a high volume regen blower such as Fuji, FPZ, etc of 20 or 25HP and when you have that kind of volume you don't have to be as bothered by leakage. On a 4 x 8 table I feel that 10 HP regen and below is a little bit marginal. Of course there are huge vacuums that will pull huge volume and high vacuum but most of us are trying to do CNC on a budget and could live off some peoples electric bills. Positive displacement pumps generally cost nearly 2-3 times the price of a regen blower.
I have a customer who has 2 of our machines. I was surprised to learn that he is running both tables with 7 1/2 HP rebuilt Busch pump that pulls 28-29 inches and has decent volume. He paid about 3300.00 for the pump. He is careful with his leakage. Heres how he does it:
Our low-cost vac tables are MDF 1 1/2" thick with a grid sytem cut into the table with 4 zones on a 4 x 8. Each zone is 24" x 49" with a 2" PVC pipe running to each zone that has a PVC cut off/on for each zone. He mills a sheet of 3/4 LDF or MDF on one side. He then adhere this to his grid table being careful to only put the wood glue on the grid itself so as to not seal up the bottom of the sacrifice board. This keeps the sacrifice board from becoming sealed. Then he mills the top of the sacrifice board. He also then seals up the edge of the sacrifice board to not let air escape through the edges. When you cut your parts be careful to set your z so that it doesn't go unnecisarily deep into the sacrifice board. When the sacrifice board gets uneven and is affecting the vacuum you can then remill the board just enough to clean it up. When you get down to the grid again simply do the same with another board and start over. This works very well. (Thanks to Greg Westberry for the enlightenment on how he does his vacuum system and some help on the above info as well)
I have also found that if you have limited electricity available such as only 220 single phase and don't want a phase converted setup that a Dayton 10 HP blower working in reverse works fairly well because it has such a huge volume but uses a lot of electricity to start and you have to build a different vacuum box to handle the volume.
Hope this helps a little in making your decisions. I will try later to give some tips on how to cut parts with low volume or no vacuum at all.
Bill Glenn
Camaster CNC  |
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