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rmfaires 11-14-2012 08:49 AM

How important is a vacuum system
I am looking to buy a new cobra, and to keep cost down I may delay the vac system until later. I have had cnc's before, but always have had a vac system. I cut mostly plywood and mdf for cabinets. How difficult is it to do without a vac system and use clamps and tabs?

rcrawford 11-14-2012 09:44 AM

I don't have a vac system, and I've cut a few sets of kitchen boxes. Its not fun. I have the Cobra 408, so the cutting area is just slightly larger than a sheet of plywood (not enough room around the edges to clamp). So I have to hold down using bolts through the plywood into my t-bar. This means I need 3/8" holes that line up with my t-bar in the table, and miss all the parts I have nested in the plywood. Takes a lot more planning. Then I use tabs so the parts don't shift when I cut them out. I have a 36" edge sander, so its not too bad sanding the tabs off, but it takes time.

If I had a good vacuum hold down system I could cut a sheet of nested parts in less than 10 minutes. Now it takes me well over 1/2 hour, plus the extra designing time to add the hold down holes and the tabs. If I were cutting lots of sheet goods, I'd definitely invest in a vacuum.

That being said, you can cut a vacuum plenum into your Cobra anytime, and simply add a vacuum system. Not that difficult. I cut my own plenum because I wanted it between my T-bar (so I could use T-bar clamps or vacuum), but I only have the Fein shop vacs right now. These work okay for holding down a whole sheet, but don't have the power to hold down pieces when they are cut out of the sheet. (they get to about 4"hg tops, whereas you need at least 15"hg and 400cfm for smaller parts).

james mcgrew 11-14-2012 10:48 AM

while waiting on my new machine I have been cutting with my digital with no vac,, I cut large parts sides and bottoms by cutting all first toolpath to .70 deep once this has cut all parts I run another path at .055- .058 this removes the remaining skin with little or no effort,, Kieth godwin shoots a small 18 ga nylon pin in his to hold parts,, me I put a hand on the part as it is releasing

works well

Denny J 01-09-2013 08:59 PM

I have been struggling with cutting parts on my cnc even with the vacuum that I have. I did just order the Raptor nylon brad gun today and that will be here in a few days to help with small part management but I am certainly going to upgrade my vacuum setup. I just located a pair of pumps that will increase my vacuum capability and will use them and sell my 7.5 HP centrifugal blower.

If you are serious about nesting and productivity my experience so far has been that you cannot have enough vacuum. Now that I found out where to get pumps at prices that don't break the bank I am going to upgrade as big as I can.

ktringle 01-09-2013 10:23 PM

Denny, What pumps have you found? I'm interested in using vacuum hold down but the pumps I've found are either 3 phase or too expensive to be practical.

rcrawford 01-09-2013 10:39 PM

I'm interested in your vacuum source as well!

I found some 12hp Becker rotary vane pumps for just under $8k, but I'd need a phase converter. Those will go up to 28"hg, and I think it was around 175cfm at 15"hg. I understand you need about 15"hg and 300-400cfm for good hold down for nested work. With those Becker pumps I'd have to be careful to cover the unused areas of spoilboard and plan my cuts right. Its not worth it for me to spend that much because I don't cut full sheets very often.

Denny J 01-09-2013 11:26 PM

OK. I have spent a ton of time looking into this stuff and I think that I found the way to go. Someone correct me if I am wrong but between the 3 types that are used-
Regenerative Blowers
Rotary Vane
Rotary lobe.

It seems to me that the Rotary lobe design is the best design from a performance and also a durability standpoint. I dug around and found out you can buy a rotary lobe pump for in the neighborhood of 2300 dollars new that is designed for the dairy business. They are nearly bulletproof and a friend who' family is in the dairy business tells me some pumps out there have in the neighborhood of 200,000 hours on them. The ones I am looking at are made by Boumatic,DeLaval and Sutorbilt. They are also belt driven which means you can add the motor of your choice and can start small and put a bigger motor on it if you need more power without having to change out the pump. Obviously there is a range that you need to stay in but you have flexibility in that respect.

The other thing that is taking over the dairy industry that I find very interesting is the same VFD's that we are using on our spindles, they are using on their vacuum pumps so you can turn down the power when you are cutting bigger part and save electricity and then crank them up when you need more vacuum for smaller parts which essentially means you can get a 15 HP motor and pump for instance and run it at say 7.5 for most of the time but then have the reserve when you need it.

So anyway, look for a Dairy supply store in your area and see if this is the right way to go for you.

This is a link to the pumps i am talking about.

Charlie_L 01-09-2013 11:36 PM

Having grown up milking cows, I would give it a try. I would expect them to work well. Some of the large operations around here operate 24 hours a day.

rcrawford 01-10-2013 12:09 AM

You can get VFDs that convert single phase to three phase in almost any horsepower, but you must remember that the rated hp is actually decreased by about 1/3 when you do this. So your 15hp pump will actually only be giving 10hp, which will affect pump performance. So if you want 15hp, you need a 22hp motor.

Denny J 01-10-2013 09:24 AM


Originally Posted by rcrawford (Post 22229)
You can get VFDs that convert single phase to three phase in almost any horsepower, but you must remember that the rated hp is actually decreased by about 1/3 when you do this. So your 15hp pump will actually only be giving 10hp, which will affect pump performance. So if you want 15hp, you need a 22hp motor.

Interesting. So is it the same way with spindles in that a VFD on a 5HP columbo spindle would only put out like 3 hp?

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