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Old 09-14-2014, 12:07 PM
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Gary Campbell Gary Campbell is offline
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Default Some Facts About Vacuum

Some Facts About Vacuum


What is Vacuum?
Vacuum, by definition is extremely simple; it is the removal of air, usually in a chamber. Since that air has weight and volume there is a pressure, or negative pressure, developed. Vacuum hold down on CNC machines seems to be one of the hardest items for many Users to understand. There are volumes written about it all over the web, most of it by persons that may not understand the physics of the process completely. By definition vacuum is extremely simple, it is negative air pressure. As it applies to parts holding on a CNC table, the goal is to remove the air from beneath the part and allow the weight of the air in the atmosphere to press down on the part to hold it in place for cutting.

That weight, or negative pressure is quantified in numerous ways. Inches of Mercury (inHg) and inches of water (inH20) are the most common in the US. Negative PSI (-pounds per square inch) is also used, but not as commonly. The metric system has their own and all are easily converted to another by using online converters. The maximum vacuum pressure we can expect to generate is 29.92 inHg at sea level. That number will be lesser at higher altitude, simply because there is less atmospheric pressure due to less air "stacked" above the ground.

The conversions on maximum vacuum are as follows;
InHg (29.92) to InH2O (406.61) is: inHg X 13.59 = inH2O
InHg (29,92) to psi (-14.69) is: inHg X -0.491 = psi


This means that if you could develop 29 inHg on a vacuum table you would effectively have 14.24 pounds of pressure on each square inch of your part holding it down. That is over 65,617 pounds on a 4 by 8 sheet. Very few if any sheet size systems could contain vacuum that high, nor could we afford the pumps that could generate that pressure at the volume it would require.

Components of Vacuum Systems Used for Hold Down on CNC Machines

The Pump:
A vacuum pump is an air pump, just like an air compressor. In an air system the system of piping is connected to the outflow (pressure) side of the pump, and in a vacuum system the piping is connected to the inflow (negative pressure) side. Most vac pumps and compressors are designed and built to provide one much better than the other. Numerous types are used, impeller, diaphragm, rotary vane, rotary screw. The most common that we will use are impeller and rotary vane due to the pressures and volumes that they deliver.

Vacuum Pumps are usually rated at max pressure (inHg) and max cfm (cubic feet per minute) or volume. Both are important to what we need them for, but you must realize that these pumps will only develop max pressure at zero flow and max flow at zero pressure. Where most think that these specs are an "AND", in reality they are an "OR". You cannot have both at once.

Virtually all pump mfgr's publish a flow chart for each model they sell. Knowing what vacuum can be maintained at a given flow is how one can determine how well or if that pump will work in a CNC hold down system.
There are three levels of pumps that are most commonly used by CNC operators. These levels are determined by the intended use which then determines the specs or level of vacuum and flow required.

High Vacuum, Low flow: This type pump can be rotary or diaphragm and will usually produce vacuum between 20 and 29inHg. Similar to AC evacuation pumps. They are numerous, small and cheap. They work well with completely sealed small puck systems and vacuum bagging operations. They usually have a cycle switch to only run the pump when needed and may or may not have a reservoir tank. They typically have 5cfm or under and must operate within a sealed environment. Some porous woods cannot be held with these systems unless sealed.

High Flow, Medium Vacuum: This type pump is usually an impeller or rotary vane pump. Maximum vacuum will be 10-15inHg and max flow will be 100 to 400 cfm. Rotary vane pumps are usually 3 phase 10-20 hp units that are used in production operations where 3 phase power is available. These rotary vane pumps will have the higher vacuum, but often lesser flow numbers.
The impeller versions are high performance steam extractor motors installed in parallel multiple configurations to achieve high CFM numbers. Seldom developing more than 10 inHg, impeller pumps often will flow over 100 cfm per motor, allowing 400+ cfm flow systems.

The High Flow, Medium vacuums are the least cost, most often used systems in nested base sheet sized hold down systems in cabinet, woodworking and sign shops.

High Flow, High Vacuum: These systems are similar the rotary vane medium vacuum systems above, except that the pumps will be higher horsepower (20++), often have double rotors, and are used for nested based, high production systems.

Note: Dust collection pumps are impeller pumps that produce very high flow (400 to in the thousands of cfm) and very low (10 to 20+/- inH2O) static pressure. Most will only produce less than 1 inHg of vacuum and will not work with either a spoilboard or puck system.

Distribution System: Some system of piping to distribute the vacuum pressure to the various extents of the table is required. Often as simple as home improvement store PVC schedule 40 piping, valves and fittings, CNC controlled air or electric operated valves, regulators and diverters are sometimes added. Whatever is used, it must be able to withstand the vacuum pressure that the system will use.

Plenum Vacuum Grid: A plenum is defined as a chamber for air flow. In our use we combine a sheet of some material, a grid like network of channels and holes that allow connection to the distribution system into what is commonly called "the Plenum". Technically, only the network of channels would be the plenum. To ensure maximum efficiency, every surface of this plenum should be sealed.

The Spoilboard: Commonly a "spoilboard" is any material used in a CNC operation that is placed below the material being cut to avoid damage to the table itself. By its nature, an inexpensive discardable product like MDF is the most common. There is much confusion around the term "Spoilboard" as it commonly applied to 3 distinctly different applications. Technically a spoilboard is just that, and is described above.

A Vacuum Bleeder Board, or "Bleeder" is also a spoilboard, but must be able to "bleed" vacuum. In order to bleed vacuum the material in a bleeder board must be porous and allow the flow of air thru it. Some grades and brands of MDF are porous enough to work as a bleeder, many LDF (light density fiberboard) and most ULDF (ultra light) will also work well.

The third version of a spoilboard is a "Waste board". It is typically a thin (1/4" +/-) sheet of porous MDF placed over a bleeder board to both protect the bleeder and allow removal of the cut parts and dust on the table to an awaiting processing table in one quick operation after cutting to reduce cleanup and loading time. Each sheet is loaded on the table along with a waste board, and usually 2 or 3 waste boards are cycled in and out with the sheets and then reused once parts are removed.

All spoilboards, bleeder boards and waste boards need to be surfaced, some on both sides prior to use and again when bit tracks reduce the vacuum system efficiency. Most bleeder board will perform better if the "skin" is removed prior to use. This removes resin that accumulates on the surface during manufacturing process, opens up the interior pores and allows better air flow.

Pucks and Fixtures: Pucks are dedicated vacuum fixtures that are some usually generic shape, usually rectangular or round. They have a small vacuum distribution grid built into them, a gasket or gaskets to control leakage and a port to connect a vacuum source (usually smaller tubing) to them. Fixtures are usually very similar in design to pucks, except that in most cases they have been designed to hold specific parts for specific repetitive cutting operations. Pucks can be purchased at woodworkers supply houses or made in house. Fixtures are almost always made in house on the machine for which they are intended.
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Old 09-14-2014, 02:21 PM
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Default Info On Table Boards and Spoilboards

Table boards are usually the first sheet that is bolted to the CNC frame. It should consist of a stable material that has been sealed on all exposed surfaces. I like to bolt on a table board, then bolt or glue another layer for a plenum board if I am building a vacuum table. A single piece can be used if the material is 1" or greater in thickness, but do not cut a plenum into a single 3/4" board and try to use it without additional support.

Here are some of the most used materials for these purposes:

Phenolic: Usually considered the best material as it is the least susceptible to movement from moisture and heat and is virtually impervious to any chemicals. It is however, expensive and hard to machine. To do a good job, it will take at least a 4hp spindle to surface and/or cut a plenum grid. It is also difficult to adhere subsequent layers to, drill and tap is preferred.

Aluminum: For all practical purposes, as stable as phenolic, but may move more with heat. Easier to machine than phenolic and most used by those that cut aluminum as it is easier on bits. Most aluminum cutters use a single surfaced sheet of aluminum as a table and "spoil" board and replace as needed. Hold down is provided by drilled and tapped holes at regular intervals.

MDO: The best of the wood based sheetgood products, MDO, or medium density overlay, is an exterior grade fir plywood sanded smooth with a phenolic paper layer on one or two sides. MDO has been used as papering surface on billboard signs in days gone past. Extremely stable, not very expensive and lasts indefinitely if edges are sealed well. Accepts glued on plenum or spoilboard layers well with common woodworkers glues. May be difficult to find in some areas.

Industrial partical board: Not as stable as MDO for table board use, but it is heavier and will be better than most plywoods or MDF if sealed well. Works OK for plenum use, but likes to chip out if vac grid is cut aggressively.

MDF: (Medium Density Fiberboard) Probably the most used as a spoilboard, due to low cost and the probability of damage from wildly plunging bits! The least "stable" of the lot, but performs well as a plenum if sealed. Performs well as a spoilbnoard if surfaced when closer tolerance is needed.

Spoilboard Materials:
Usually from the MDF type of product line due to low price and by definition we are going to destroy and replace them. Aluminum works best for aluminum cutting, phenolic is not recommended as it will destroy your bits. There are numerous grades and types of MDF. All MDF is NOT created equal! Weights and resin content varies greatly and along with it its ability to hold screws, if that is your hold down method, or pass vacuum if that is your intended purpose. If you plan on using a sheet of any grade MDF for a bleeder, make sure that you surface both sides and verify that it actually allows air flow prior to gluing it down.

Here are some and what they can be used for.
"Regular" MDF: Consumer and commercial grades are both available. Most consumer grades from big box stores do not cut well, don't hold screws well, and seldom allow vacuum flow.... but there may be some exceptions. Commercial grades cut well, and SOME brands will flow vacuum. Some of the better cutting grades are touted as HDF, or high density fiberboard. It is common for those with high vac medium and high flow systems to use MDF and HDF as their vac systems are so strong that they can pull air thru them. Not so much with lower spec'd systems.

LDF and ULDF: Low Density and Ultra Low density fiberboard are what work best as bleeder boards for low and medium powered systems. Trupan is a brand that has proven to be one of the best. Others made from Radiata Pine are also very good. They do not cut well for parts as they are soft and stringy. They also may compress under high vac pressure and cause depth inconsistencies when used with high vac systems.

Extira: Is an exterior grade HDF that has a high resin content. It is resistant to moisture movement, but not impervious. It will work for a spoilboard, even for aluminum cutting with cutting fluids. It will NOT work as a bleeder. I should also perform well as a table board, but it doesn't take to glues as well as MDO does.

In any case each and every one of the above should be true and straight, bolted and/or glued as well as possible. All Should be surfaced before use, with the exception of a table board when other layers are applied, and then when required due to bit tracks or moisture movement, which can be a couple times a day.
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Last edited by Gary Campbell; 09-15-2014 at 10:07 AM.
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Old 09-14-2014, 06:52 PM
de5 de5 is offline
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Thanks for posting this information Gary. You are truly an asset to CAMaster, and I'm sure I speak for everyone here when I say thank you for taking the time to be involved in the forum to share your expertise.
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Old 09-14-2014, 09:08 PM
sheustess sheustess is offline
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Great info Gary. GO BLACK BOX!!!!
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Old 09-15-2014, 09:00 AM
Signpronfl Signpronfl is offline
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Does anyone have some info on the Black box vac systems as far as real world usage? I have not seen much statical info....specs etc. For those who have it does it work well? I am looking to hold sheet material while I 3D carve it and then cut it out. most of my pieces are large 10+sqft. How does it work with small pieces being cut out? Thanks for the info.
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Old 09-15-2014, 09:52 AM
de5 de5 is offline
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Here's a good thread on another forum about the black box vac:
http://www.talkshopbot.com/forum/sho...ighlight=black

The guy is real happy with it.
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Old 09-15-2014, 10:46 AM
rcrawford rcrawford is offline
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Quote:
Originally Posted by Signpronfl View Post
Does anyone have some info on the Black box vac systems as far as real world usage? I have not seen much statical info....specs etc. For those who have it does it work well? I am looking to hold sheet material while I 3D carve it and then cut it out. most of my pieces are large 10+sqft. How does it work with small pieces being cut out? Thanks for the info.
I'd be hesitant to use the vacuum while doing larger 3D carvings, simply because of the time involved. Rather than running a vacuum for 5 hours straight, it might be more practical to use composite nails (Raptor nails and an Omer gun).

Of course, you might be doing shorter 3D runs where a vacuum would be very practical!!

As far as hold down, 10 sf is 1440 square inches, so even if you get 8"hg, that would be around 4psi, or 5760 lbs of force on the 10sf piece. Plenty of hold down, unless your sheet goods are warped. If your pieces are usually the same size, you can route a shallow groove in the spoilboad just inside each piece, and place a foam seal that sits slightly proud (but will compress flush with the spoilboard). This will effectively double the hold down force of the vacuum through the spoilboard (I didn't believe it until I tried it).
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Old 09-15-2014, 02:30 PM
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Gary Campbell Gary Campbell is offline
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Like Russell I would not look forward to extended run times with the added noise of a vacuum unit running. The light cutting forces exerted by most 3D files don't require as robust solution for hold down.

I do have some thoughts on the gasketing adding to vacuum on a spoilboard. First off they are NEVER 100% efficient. Second, if you can add a gasket to a spoilboard setup and double the hold down, then I propose that something was wrong with the initial setup that cut it in half. There are a good number of factors in play, most of them will reduce hold down if attention is not paid. With that said, vacuum has a max (times the efficiency) you cannot do anything to get more than the max.
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Old 09-15-2014, 03:20 PM
Art Mann Art Mann is offline
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Gary,

I am not just kidding or using a cliche when I say you ought to write a book. There just is no single place to go and find this kind of information.

Art
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Old 09-15-2014, 04:10 PM
rcrawford rcrawford is offline
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Quote:
Originally Posted by Gary Campbell View Post
Like Russell I would not look forward to extended run times with the added noise of a vacuum unit running. The light cutting forces exerted by most 3D files don't require as robust solution for hold down.

I do have some thoughts on the gasketing adding to vacuum on a spoilboard. First off they are NEVER 100% efficient. Second, if you can add a gasket to a spoilboard setup and double the hold down, then I propose that something was wrong with the initial setup that cut it in half. There are a good number of factors in play, most of them will reduce hold down if attention is not paid. With that said, vacuum has a max (times the efficiency) you cannot do anything to get more than the max.
The gasketing was more for slightly warped plywood. I used it when making panels for raised panel doors, as my glue-ups weren't perfectly flat and they were too wide for my planer. Gives a seal around the piece so the air doesn't leak in as fast. I agree, if you have a flat substrate on the spoilboard, the gasket wouldn't do much.
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