Question
I am currently comparing several quotes on CNC routers. What is the least amount of vacuum that will hold the parts? I am trying to not have to upgrade my building's electrical. I have enough amps to run the quoted machines; just want to verify what the salesmen are telling me. I will mainly be cutting melamine PC core or MDF.
Forum Responses
(CNC Forum)
From contributor K:
Do not let any salesperson talk horsepower when it comes to vacuum. Flow is what matters. There is no magic to vacuum; it is only atmospheric pressure that holds your parts down. The larger the work piece, the larger the area that the atmosphere has to apply its force to. If the total forces for machining applied to the work piece overcome the atmospheric pressure acting on a given area, then parts can move. The vacuum pump only removes pressure from under that part. Remember, perfect vacuum is not possible unless your shop is in space. If the pump cannot move enough volume to compensate for leakage created by router path or open edges, then the pressure differential is smaller and the forces required to move the part decrease.
You require more vacuum volume if cutting MDF, as vacuum will leak right through the part if it is not sealed. Melamine requires less as the vacuum will not leak through the part. Also material that is warped or bowed applies other forces that can contribute to parts moving.
I would suggest a min 300 cubic meters per hour for a 4x8 table and 500 cubic meter per hours for a 5x12 table.
This is a letter from Bill Glenn of Camaster CNC that was sent to me a couple of years ago on vacuum. I have shown it to many and it seems to be a good laymen's explanation.
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 country-boy 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. Most people including myself 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. I read a book of theirs several years ago that explains very well how it works - it was 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 are 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 what's there (15 lbs. max). If you have a 10" square you are trying to hold down, the maximum you can hold down 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 people's 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. Here's how he does it.
Our low-cost vac tables are MDF 1 1/2" thick with a grid system 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 adheres 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 unnecessarily deep into the sacrifice board. When the sacrifice board gets uneven and is affecting the vacuum, you can 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.
