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  #11  
Old 07-16-2018, 07:36 AM
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james mcgrew james mcgrew is offline
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Using Ldf , Mdf Is an Industry standard used in tens of thousands of commercial and private shops, That said you are right you do not have to use it, As someone who cuts 40-60 sheets daily i for one would want to save power as it is a business expense, my power bill could buy a cnc every few months.

Try it the way you want, I have a thread on here some where about Buddy Easlers Vac system on his smaller machine, are there other ways yes, is there a better way for larger sheets without cutting the plenum "Maybe"

the first 4 x 8 i got in 07 had a .25" thick hardboard "pegboard" with holes. worked great for holding parts until the first one was cut thru then all came loose due to the loss of vacuum. Now if you are not cutting thru then it worked well.
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  #12  
Old 07-16-2018, 08:15 AM
rcrawford rcrawford is offline
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Vacuum hold down is one of the least understood mechanics in woodworking. Common sense would tell us that the vacuum 'sucks' the workpiece to the table, therefore the more open the table, the stronger the vacuum force is under the workpiece, sucking it down.

This is completely wrong. The hold down force actually comes from atmospheric pressure pushing the workpiece down onto the table, NOT the vacuum 'sucking' the workpiece down. Without vacuum, that atmospheric pressure pushes equally on all 6 sides of your workpiece and there is no hold down pressure. The purpose of the vacuum is to create an area of lower pressure under the workpiece, so the sum force of the atmospheric pressure is now pushing down on the workpiece.

This can be achieved with any method, as already mentioned by previous posters. The beauty of the spoil board method is that it is much easier to produce a large area of low pressure under the whole spoil board, because the MDF offers just enough resistance to airflow so that you do not need a seal around your workpiece.

Hold down pressure from the atmosphere is directly proportional to the surface area that the atmospheric pressure is pushing down on. The spoil board method creates a low pressure under the whole surface of your workpiece. In contrast, the 'pegboard' method only creates a low pressure area directly under each 'hole' in the pegboard, so you have a greatly reduced surface area that the atmospheric pressure can push down on.

If it were possible to create a perfect vacuum under your workpiece, then you would have approximately 15psi of force holding your workpiece to the table. With a well sealed jig and a really strong vacuum source, you might get close to this. But if you cut through the workpiece, you instantly lose this seal and your workpiece goes flying off the table from the force of the tool.

With a spoil board, you won't get close to that ideal 15psi of force pushing down. A really strong regen blower might give you 8-9psi (15-18 "hg), and a large Becker type rotary vane vacuum can give you 10-12 psi. If you are holding down a 4x8 sheet, you are still getting between 36,000-55,000 lbs of pressure holding the whole sheet down. Even a little shop vacuum will give you 3psi of hold down force over the whole spoil board, or 14,000 lbs of force holding a full sheet down.
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  #13  
Old 07-16-2018, 09:23 AM
Jim_in_PA Jim_in_PA is offline
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Options, the small pump(s) you have are fine for fixtures and you don't generally pull through MDF for that kind of application. In fact, I don't believe what you have or what I have (small Gast pump) could actually pull through MDF. Where you use the MDF spoilboard with vacuum is with the larger, vac table setups to do things like full sheets, etc., and yes, those big pumps draw some power. So it's important to differentiate things here. That doesn't mean a fixture can't or shouldn't have an MDF surface because you want that when you need to do through cuts unless the fixture is tailored and gasketed to allow you enough clearance to do a cut through without cutting into the fixture. There are SO many ways to work with vacuum...
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  #14  
Old 07-16-2018, 01:19 PM
Options Options is offline
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Quote:
Originally Posted by james mcgrew View Post
Using Ldf , Mdf Is an Industry standard used in tens of thousands of commercial and private shops, That said you are right you do not have to use it, As someone who cuts 40-60 sheets daily i for one would want to save power as it is a business expense, my power bill could buy a cnc every few months.
I know that feeling and have owned an air duct cleaning business since 2001. At 65 though I am getting very tired of dealing with the weather, rising costs and doing free estimates that consume most of my free time. The prospect of working in a climate controlled building a few feet from my back door has become very appealing and once I learn how to make something other than sawdust I have 17 years worth of satisfied customers to market to. Until then though and Uncle Sam starts helping to pay the bills I need to keep operating costs down.
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  #15  
Old 07-16-2018, 02:16 PM
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james mcgrew james mcgrew is offline
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I get that, Just one thing I also have to avoid being pennywise and dollar foolish,

I hope this works out for you

A vacuum is not for everyone nor every application
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