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    Default Learning Vacuum Workholding

    I recently purchased a vacuum plate system and spent a lot of time on this forum and the manufacturers website learning about how to best use it. I learned a lot but there were still many questions I had to answer myself. This post will hopefully contribute a few more pieces of information that someone else learning vacuum workholding might benefit from.

    Many other sources already detail how vacuum workholding actually works so I'll just state the basics here. The holding power of a vacuum system is directly related to surface area of vacuum on your part. The relationship is surface area multiplied by atmospheric pressure. If you were holding a 10" square part then you'd have 100in^2 of surface area under vacuum. Multiply this by atmospheric pressure, approx 14psi, and you'd have 1400 pounds of down-force holding your part. There will always be some inefficiency from the sealing gasket, and quality of the part surface against the gasket. Correct me if I'm wrong, it doesn't matter what vacuum pump/generator you use so long as it's flow rate exceeds the leakage rate in your system.

    I purchased a Pierson Workholding system.
    SmartVac II Starter Kit SmartVac II Starter Package - 9.5" x 14" Base
    It included the 9.5" x 14" base plate and the Venturi vacuum generator.

    I chose this model because it was narrow enough to allow holding it in standard 6" vises with the jaws reversed.

    Attachment 243536

    As a job shop I like to keep my two vises on the mill as much as possible. The 9.5" x 14" size is large enough to accommodate most jobs we'd run on this machine. And if we need a larger plate in the future then we'll be more comfortable spending the extra money now having used a vacuum system.

    The system itself is pretty well defined on the manufacturers website using photos, videos, and the provided installation documentation. The only tools I really needed were some scissors and a T-Nut.

    After gripping the base in my vises I quickly verified that it didn't bow very much from the vise pressure, the plate itself is Blanchard ground and has peripheral slots that allow for bolting down to a T-slot table, this should keep the plate flatter than holding it in the vise but I was still able to keep it within 0.002" flatness over the whole plate.

    The base plate has a grid pattern of gasket grooves, I've ran a couple parts directly on this grid, but everything else I have done has been using their sacrificial top plates.

    Additional to the starter kit I also purchased a matching Top Plate and their Gasket Slotting End Mill.

    SmartVac II Top Plate - 9.5" x 14"
    SmartVac II "Gasket Slot" Endmill

    The top plates look to be aluminum tooling plate with four accurate holes in the corners. There are four shoulder screws provided in the starter kit which locate the top plate onto the base plate using those screws.

    The top plates allow you to customize your gasket groove pattern as well as serve as sacrificial components allowing you to drill through or add locating features. After bolting on a top plate I noticed no significant deviation from the base plates flatness. There is a video on Pierson's website showing their preferred method of machining your top plate. They show adding gasket groove all around the outer groove in the base plate then bolting on the top plate. They advise you to machine your gasket groove and any blind features with the vacuum generator running, then to release the vacuum before doing any through features such as the drilled hole(s) for porting the vacuum up to your gasketed area. Pierson also recommends drilling through using a standard tap drill size such that you can plug the hole later if you wanted to. I believe they recommended the tap drill for a #10 screw, but I chose instead to use M3. I am not sure but I do not think this would significantly affect air flow rates.

    Pierson sell a custom 0.118" end mill for cutting gasket grooves (for their 1/8" gasket cord). I used this end mill at 6000 RPM, 18 IPM, with a 2 degree ramp entry, and a depth of 0.095". This cut the slots great and left the designed chamfer around the top edges. Pierson do also mention the potential benefits of cutting shallower "distribution" channels to help air flow from the extremities of your gasketed area to the vacuum port hole. I don't know if these channels have much effect on vacuum performance but I added them anyway since I can't think of a detrimental effect. For these distribution channels I used the same end mill and just slotted 0.040" deep then used a 1/8" chamfer tool to break the edges, having any burrs under your part wouldn't help your sealing efficiency.

    Note that the vacuum gasket cord has a round cross section. However once you load the plate and turn on the vacuum this 0.125" cord gets squished down into the 0.095" slot and forms a square cross section. This drastically increases the sealing surface area of the gasket. I therefore think the depth of slotting for the gasket groove is fairly important. By following Pierson's directions for inserting the cord into the groove I had no issues with sealing, cut the ends square and push into the groove to create compression.

    Attachment 243535

    I have only used 1/8" gasket though I have also purchased an assortment of other sizes from Pierson. The 1/16" gasket might be worth experimenting with if you have lots of drilled holes through your part, as the 1/8" gasket begins to take away a lot of your vacuum surface area if there are many through holes to seal. Also gasket has a minimum bend radius, therefore limiting how precisely you can seal your part with through features, see the photo below where the gasket has to snake around features where it can't bend tight enough to enter. I was able to seal a small through hole with an inner radius of 0.1", i.e. the ID of the gasket groove was 0.200" and the OD was 0.436". This seemed really sketchy to me but it worked through half a dozen parts, each with 7hrs of cycle time. So I don't know if you could get away with a tighter bend radius, but I would recommend stopping at R0.125" while using the 1/8" gasket.

    Attachment 243533

    In actual use of the vacuum system I found the Venturi to be reliable and tolerant of coolant as advertised. Many years ago we experimented with a vacuum pump and found out that you had to be cautious of getting coolant into the pump. The Pierson Venturi unit just gets clamped into a T-slot and you run push-connect air tubing between the vacuum base, the Venturi, a pressure regulator, and a air supply. On our Haas machine there are spare air ports on the side of the machine. All hardware was included in the starter kit, I cut the tubing to the lengths required for the travels of my machine and installed everything. The supplied pressure regulator comes with a magnet glued to it for mounting on sheet metal. I stuck this right to my machine cabinet and didn't have any issues with vibration. I did not have to drill any holes or do any machine modifications to install this system.

    Attachment 243534

    When I ran my first part it was a 3/8" thick Delrin strip, about as long as my 14" plate and a few inches wide. I had difficulty holding this part without it shifting. I realized I needed more surface area for the vacuum. I was able to modify my top plate pattern to get a few more square inches of area as well as extending the gasket out to every corner of the part. I believe the end mill created enough lifting force in the corners of the part to break the vacuum and shift the part. So getting the gasket as close to the edge while still leaving some buffer distance is critical. I also noticed that cutting on the shorter 3" sides was much more likely to shift the part than when cutting on the long sides. I mitigated this by a combination of methods. I switched to a smaller diameter tool and isolated my cuts to single edges rather than cutting around corners, I had the most trouble with lift when the end mill rounded an external corner. I don't know for sure if this next decision made a difference or not, but after a couple trial and errors I switched my coolant off, I just had it on by default and didn't need it for this part anyway. I suspected that the face milled bottom side of this Delrin strip may have been sliding due to the coolant too.

    The next part I cut was a 1" thick 10" square slab of aluminum tooling plate. This part would end up as a "W" shaped part. To hold this on my smaller plate I actually rotated it 45 degrees, such that the section that now overhung my plate was all getting milled away and all the usable vacuum area was contained within my 9.5" vacuum plate. I didn't want to throw this part if it lifted cutting that overhung area so I did something different. There were a couple through holes that happened to be nominal inch sizes like 1/4" and 3/8". I added a couple interpolated holes to my Top Plate. Then while running the part I initially drilled those holes and dropped dowel pins through the part and into the Top Plate (sealed with gasket and also not drilled entirely through the top plate). These dowel pins allowed me to go much more aggressively after this part and mill it in a reasonable time as compared to how we would have usually done it on a sub-plate with toe clamps. If I had a spare piece of material I would have bumped up my feed rates and widths of cut, but I easily was able to use a 1/2" mill at full depth profiling the part at 120 IPM and 0.050" WOC. The two dowel pins seemed to do the trick.

    With the dowel pin trick I now felt much more comfortable taking on some work specifically for the vacuum plate, the kind of work which would have been tricky and extremely tedious to do with clamps or tape. These parts were aluminum plates that each were a few inches wide by 12" long, 1/4" starting thickness with nearly the whole parts pocketed out to a floor and wall thickness of 0.040" and completely surrounded by rings of drilled and tapped holes. Each part required about 3.5hrs of pocketing and o-ring groove slotting, with a +/-0.002" tolerance on may pocket sizes and locations. These parts could not be allowed to shift at all, and luckily they had a couple dowel pin holes in them. I initially faced the parts on both sides using the vacuum plate. Then the first features I added were these dowel holes. I had those dowel locations reamed in the Top Plate, allowing me to drop in a couple tightly fitting pins to keep the part from shifting. This worked excellent and I had no issues with movement. But one plate style had through holes while the other had the associated tapped holes. The plate with through holes therefore had significantly less surface area. Then I learned another trick, nesting parts. I was able to fit one of each part on the same top plate. This let me buy fewer, larger sheets of material. And run both parts simultaneously as a set. And aid the part with less surface area by having it on the same setup as the part with more surface area, this lessened the chance of the far corners lifting and combined with the dowel pins produced a very secure setup.


    So hopefully the information above helps someone else get into vacuum workholding, feel free to reach out with questions as I'm sure I'm forgetting some things I learned.

    In summary...


    The Venturi "SmartVac II" system from Pierson works great.

    I have only used 1/8" gasket with their top plates and their custom end mill, but I will probably try the 1/16" gasket at some point and probably use the same ration of depth and width as for the 1/8" stuff.

    Maximize all usable surface area to get the most holding force and prevent lifting when cutting around corners.

    When milling external corners...either don't...or at least reduce feed rates.

    Coolant may have an effect on holding ability.

    Vacuum gasket has a minimum bend radius, particularly when sealing drilled holes, right angles are doable for other areas but they do cause "bunching" of the gasket at that angle's corner.

    Unknown whether distribution channels have an effect on performance

    Only use the largest tool diameters as needed to keep lifting forces as low as possible.

    Dowel pins or other "locational" references dramatically increase rigidity, they do not prevent lift but do eliminate shifting. Maybe adding a bolt partway through a cycle would be the best method since it'd eliminate lift and shift. But distant areas could still lift.

    Nest parts to increase holding power, leave thin webs between parts to keep them connected as a whole sheet. Design webs to be easily removed manually after machining.

  2. #2
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    This may be food for thought for anyone who reads this post. Getting the most out of your vacuum fixture requires you to maximize vacuum surface area as you stated. The problem with using grooved channels is that when your part comes in contact, you only have true vacuum pressure in THOSE areas. That means in your first example, you don't have 100^in2 anymore, you only have the area of the channels and a little more that bleeds off under the rest of the fixture. Take a look at this video and it may help explain the difference a bit better.

    Metal Cutting with VACU-GRIP™ Vacuum Work-holding - YouTube

    Quote Originally Posted by 23rd Century CNC View Post
    I recently purchased a vacuum plate system and spent a lot of time on this forum and the manufacturers website learning about how to best use it. I learned a lot but there were still many questions I had to answer myself. This post will hopefully contribute a few more pieces of information that someone else learning vacuum workholding might benefit from.

    Many other sources already detail how vacuum workholding actually works so I'll just state the basics here. The holding power of a vacuum system is directly related to surface area of vacuum on your part. The relationship is surface area multiplied by atmospheric pressure. If you were holding a 10" square part then you'd have 100in^2 of surface area under vacuum. Multiply this by atmospheric pressure, approx 14psi, and you'd have 1400 pounds of down-force holding your part. There will always be some inefficiency from the sealing gasket, and quality of the part surface against the gasket. Correct me if I'm wrong, it doesn't matter what vacuum pump/generator you use so long as it's flow rate exceeds the leakage rate in your system.

    I purchased a Pierson Workholding system.
    SmartVac II Starter Kit SmartVac II Starter Package - 9.5" x 14" Base
    It included the 9.5" x 14" base plate and the Venturi vacuum generator.

    I chose this model because it was narrow enough to allow holding it in standard 6" vises with the jaws reversed.

    Attachment 243536

    As a job shop I like to keep my two vises on the mill as much as possible. The 9.5" x 14" size is large enough to accommodate most jobs we'd run on this machine. And if we need a larger plate in the future then we'll be more comfortable spending the extra money now having used a vacuum system.

    The system itself is pretty well defined on the manufacturers website using photos, videos, and the provided installation documentation. The only tools I really needed were some scissors and a T-Nut.

    After gripping the base in my vises I quickly verified that it didn't bow very much from the vise pressure, the plate itself is Blanchard ground and has peripheral slots that allow for bolting down to a T-slot table, this should keep the plate flatter than holding it in the vise but I was still able to keep it within 0.002" flatness over the whole plate.

    The base plate has a grid pattern of gasket grooves, I've ran a couple parts directly on this grid, but everything else I have done has been using their sacrificial top plates.

    Additional to the starter kit I also purchased a matching Top Plate and their Gasket Slotting End Mill.

    SmartVac II Top Plate - 9.5" x 14"
    SmartVac II "Gasket Slot" Endmill

    The top plates look to be aluminum tooling plate with four accurate holes in the corners. There are four shoulder screws provided in the starter kit which locate the top plate onto the base plate using those screws.

    The top plates allow you to customize your gasket groove pattern as well as serve as sacrificial components allowing you to drill through or add locating features. After bolting on a top plate I noticed no significant deviation from the base plates flatness. There is a video on Pierson's website showing their preferred method of machining your top plate. They show adding gasket groove all around the outer groove in the base plate then bolting on the top plate. They advise you to machine your gasket groove and any blind features with the vacuum generator running, then to release the vacuum before doing any through features such as the drilled hole(s) for porting the vacuum up to your gasketed area. Pierson also recommends drilling through using a standard tap drill size such that you can plug the hole later if you wanted to. I believe they recommended the tap drill for a #10 screw, but I chose instead to use M3. I am not sure but I do not think this would significantly affect air flow rates.

    Pierson sell a custom 0.118" end mill for cutting gasket grooves (for their 1/8" gasket cord). I used this end mill at 6000 RPM, 18 IPM, with a 2 degree ramp entry, and a depth of 0.095". This cut the slots great and left the designed chamfer around the top edges. Pierson do also mention the potential benefits of cutting shallower "distribution" channels to help air flow from the extremities of your gasketed area to the vacuum port hole. I don't know if these channels have much effect on vacuum performance but I added them anyway since I can't think of a detrimental effect. For these distribution channels I used the same end mill and just slotted 0.040" deep then used a 1/8" chamfer tool to break the edges, having any burrs under your part wouldn't help your sealing efficiency.

    Note that the vacuum gasket cord has a round cross section. However once you load the plate and turn on the vacuum this 0.125" cord gets squished down into the 0.095" slot and forms a square cross section. This drastically increases the sealing surface area of the gasket. I therefore think the depth of slotting for the gasket groove is fairly important. By following Pierson's directions for inserting the cord into the groove I had no issues with sealing, cut the ends square and push into the groove to create compression.

    Attachment 243535

    I have only used 1/8" gasket though I have also purchased an assortment of other sizes from Pierson. The 1/16" gasket might be worth experimenting with if you have lots of drilled holes through your part, as the 1/8" gasket begins to take away a lot of your vacuum surface area if there are many through holes to seal. Also gasket has a minimum bend radius, therefore limiting how precisely you can seal your part with through features, see the photo below where the gasket has to snake around features where it can't bend tight enough to enter. I was able to seal a small through hole with an inner radius of 0.1", i.e. the ID of the gasket groove was 0.200" and the OD was 0.436". This seemed really sketchy to me but it worked through half a dozen parts, each with 7hrs of cycle time. So I don't know if you could get away with a tighter bend radius, but I would recommend stopping at R0.125" while using the 1/8" gasket.

    Attachment 243533

    In actual use of the vacuum system I found the Venturi to be reliable and tolerant of coolant as advertised. Many years ago we experimented with a vacuum pump and found out that you had to be cautious of getting coolant into the pump. The Pierson Venturi unit just gets clamped into a T-slot and you run push-connect air tubing between the vacuum base, the Venturi, a pressure regulator, and a air supply. On our Haas machine there are spare air ports on the side of the machine. All hardware was included in the starter kit, I cut the tubing to the lengths required for the travels of my machine and installed everything. The supplied pressure regulator comes with a magnet glued to it for mounting on sheet metal. I stuck this right to my machine cabinet and didn't have any issues with vibration. I did not have to drill any holes or do any machine modifications to install this system.

    Attachment 243534

    When I ran my first part it was a 3/8" thick Delrin strip, about as long as my 14" plate and a few inches wide. I had difficulty holding this part without it shifting. I realized I needed more surface area for the vacuum. I was able to modify my top plate pattern to get a few more square inches of area as well as extending the gasket out to every corner of the part. I believe the end mill created enough lifting force in the corners of the part to break the vacuum and shift the part. So getting the gasket as close to the edge while still leaving some buffer distance is critical. I also noticed that cutting on the shorter 3" sides was much more likely to shift the part than when cutting on the long sides. I mitigated this by a combination of methods. I switched to a smaller diameter tool and isolated my cuts to single edges rather than cutting around corners, I had the most trouble with lift when the end mill rounded an external corner. I don't know for sure if this next decision made a difference or not, but after a couple trial and errors I switched my coolant off, I just had it on by default and didn't need it for this part anyway. I suspected that the face milled bottom side of this Delrin strip may have been sliding due to the coolant too.

    The next part I cut was a 1" thick 10" square slab of aluminum tooling plate. This part would end up as a "W" shaped part. To hold this on my smaller plate I actually rotated it 45 degrees, such that the section that now overhung my plate was all getting milled away and all the usable vacuum area was contained within my 9.5" vacuum plate. I didn't want to throw this part if it lifted cutting that overhung area so I did something different. There were a couple through holes that happened to be nominal inch sizes like 1/4" and 3/8". I added a couple interpolated holes to my Top Plate. Then while running the part I initially drilled those holes and dropped dowel pins through the part and into the Top Plate (sealed with gasket and also not drilled entirely through the top plate). These dowel pins allowed me to go much more aggressively after this part and mill it in a reasonable time as compared to how we would have usually done it on a sub-plate with toe clamps. If I had a spare piece of material I would have bumped up my feed rates and widths of cut, but I easily was able to use a 1/2" mill at full depth profiling the part at 120 IPM and 0.050" WOC. The two dowel pins seemed to do the trick.

    With the dowel pin trick I now felt much more comfortable taking on some work specifically for the vacuum plate, the kind of work which would have been tricky and extremely tedious to do with clamps or tape. These parts were aluminum plates that each were a few inches wide by 12" long, 1/4" starting thickness with nearly the whole parts pocketed out to a floor and wall thickness of 0.040" and completely surrounded by rings of drilled and tapped holes. Each part required about 3.5hrs of pocketing and o-ring groove slotting, with a +/-0.002" tolerance on may pocket sizes and locations. These parts could not be allowed to shift at all, and luckily they had a couple dowel pin holes in them. I initially faced the parts on both sides using the vacuum plate. Then the first features I added were these dowel holes. I had those dowel locations reamed in the Top Plate, allowing me to drop in a couple tightly fitting pins to keep the part from shifting. This worked excellent and I had no issues with movement. But one plate style had through holes while the other had the associated tapped holes. The plate with through holes therefore had significantly less surface area. Then I learned another trick, nesting parts. I was able to fit one of each part on the same top plate. This let me buy fewer, larger sheets of material. And run both parts simultaneously as a set. And aid the part with less surface area by having it on the same setup as the part with more surface area, this lessened the chance of the far corners lifting and combined with the dowel pins produced a very secure setup.


    So hopefully the information above helps someone else get into vacuum workholding, feel free to reach out with questions as I'm sure I'm forgetting some things I learned.

    In summary...


    The Venturi "SmartVac II" system from Pierson works great.

    I have only used 1/8" gasket with their top plates and their custom end mill, but I will probably try the 1/16" gasket at some point and probably use the same ration of depth and width as for the 1/8" stuff.

    Maximize all usable surface area to get the most holding force and prevent lifting when cutting around corners.

    When milling external corners...either don't...or at least reduce feed rates.

    Coolant may have an effect on holding ability.

    Vacuum gasket has a minimum bend radius, particularly when sealing drilled holes, right angles are doable for other areas but they do cause "bunching" of the gasket at that angle's corner.

    Unknown whether distribution channels have an effect on performance

    Only use the largest tool diameters as needed to keep lifting forces as low as possible.

    Dowel pins or other "locational" references dramatically increase rigidity, they do not prevent lift but do eliminate shifting. Maybe adding a bolt partway through a cycle would be the best method since it'd eliminate lift and shift. But distant areas could still lift.

    Nest parts to increase holding power, leave thin webs between parts to keep them connected as a whole sheet. Design webs to be easily removed manually after machining.

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    Tag team spamming...or spammer #2 is eating off the dead carcass of spammer #1
    Herbert R. Tarlec Jr. & Sr. Excellent !

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    ZShorn has no clue what he is talking about. Seriously, related to the guy with horns who stormed on Jan 6? Organic prison food?
    A vacuum clamp DOES NOT PULL DOWN ANYTHING. The vacuum does not need to "get to it" from below. It is atmospheric pressure from ABOVE THE PART! Atmospheric pressure pushes it down. A vacuum clamp will hold much tighter at the lowest point of Death Valley than it will at the top of Denver. The open grid just makes it faster to evacuate the air from below, and place your gasket in a shape that fits your part.

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    Quote Originally Posted by Scruffy887 View Post
    ZShorn has no clue what he is talking about. Seriously, related to the guy with horns who stormed on Jan 6? Organic prison food?
    A vacuum clamp DOES NOT PULL DOWN ANYTHING. The vacuum does not need to "get to it" from below. It is atmospheric pressure from ABOVE THE PART! Atmospheric pressure pushes it down. A vacuum clamp will hold much tighter at the lowest point of Death Valley than it will at the top of Denver. The open grid just makes it faster to evacuate the air from below, and place your gasket in a shape that fits your part.
    I tried a test on one of my vac plates where I didn't pocket for a gap under the part and found it didn't hold the part worth a shit. When I cut the .005" pocket to create space under the part it then did in fact hold the part. Yes Virginia, you do need some space under the part for the vacuum/atmospheric pressure to hold the part tight to the plate.

    Also, as much as I love venturi pumps they DO NOT like ingesting coolant. A little coolant will drop mine from 26psi to 15 or less . They too need a coolant trap.

    Here is my HarryHomeShop version. Once you install a gage you will never use a vac system without it. It is handy as hell to see exactly how much vacuum you have before pushing the green button, and reassuring to watch while machining.

    vac-pump.jpg


    Whoaa, $1300 for a little vac plate kit!!! That's nuts!!! After using vacuum workholding for over 20 years the only thing I think they got right is using foamed cord stock, or at least it looks like it. Also, you REALLY want to dovetail your O-ring channels so they stay put.

    Damn OP, I'm with Digger on this one.

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    Quote Originally Posted by Scruffy887 View Post
    ZShorn has no clue what he is talking about. Seriously, related to the guy with horns who stormed on Jan 6? Organic prison food?
    A vacuum clamp DOES NOT PULL DOWN ANYTHING. The vacuum does not need to "get to it" from below. It is atmospheric pressure from ABOVE THE PART! Atmospheric pressure pushes it down. A vacuum clamp will hold much tighter at the lowest point of Death Valley than it will at the top of Denver. The open grid just makes it faster to evacuate the air from below, and place your gasket in a shape that fits your part.
    Well, yes and no. If your seal is perfect, then I agree, eventually you'll get a nice vacuum everywhere under the part and it doesn't matter. But if you're cutting a little close to your gasket, or a little hard, or have any small leaks, especially with flexible materials like delrin, being able to evacuate quickly can make the difference between a scrapped part and not even noticing. For example, if you're cutting 1/8" delrin sheet close to the gasket, you can get enough lift to momentarily break the seal.

    Also, if your material is soft enough to seal to the fixture under atmospheric pressure around the vacuum channel, you can get a trapped pocket of air under the rest, and that really cuts down your effective clamping area.

    One of those "in theory, there's no difference between theory and practice" sort of things.

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    All good info but WTF is a vacuum? Correct answer is nothing. In the absence of air there is nothing. Nada, Zilch, nothing. There are other factors that determine if a given fixture will work well. Leakage is one big factor. Flexible workpiece? Yup, lift a corner and all is lost. I use a shitload of .250 EPDM sponge cord stock in a 5 mm groove, about 5 mm deep. Yes, that cord stock also creates side to side friction. So does peel and stick sandpaper, and also .125 grooves with .139 O ring cord stock. The atmospheric pressure pushes down and the coefficient of friction determines the rest. Any pocket may also increase that friction.
    I cut some other parts from .375 canvas phenolic, CNC router with LWMDF spoil board. Part is about 5" long, maybe 4 sq inches total. I cut hundreds from a big sheet nested and all at once. Sheet is 36.75 x 48". I use an Onsrud down spiral and onion skin the first pass, and I climb cut. Second pass is a bevel bit to ease one edge. Last pass sets them free. They stay put most all of the time. That machine has a 25 hp RV pump. Throws off a ton of heat.

    Theory and reality? Vacuum gauge 2 sim fixtures, but one has pockets, other does not. If they both read the same vacuum level then it will take the same pressure to pull them straight up from the fixture. It is the friction designed into the fixture that keeps the part located side to side. Down spirals can be your friend here.

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    A perfectly flat piece of aluminum material on top of a perfectly flat aluminum vacuum plate only has pressure over the vacuum hole(s). Since production material and production vacuum fixtures aren't perfectly flat you normally get at least "some" vacuum pressure all over if you let the pump evacuate for long enough. Any areas where the part and the fixture actually touch has no vacuum.

    When the material/vacuum plate IS flat enough that only a couple thousandths gap remains in large areas causes problems. The pump can't evacuate fast enough across a large surface area for atmospheric to hold everything together. That's were the shallow channels help the most giving the air a place to go quickly. Just scratches or a rough surface can accomplish the same thing.

    MDF works since it's porous and doesn't need channels for evacuation but takes a big pump to keep up if you leave large areas uncovered. Also, MDF sucks in shop dust in the uncovered areas eventually making those areas "solid" and unable to draw a vacuum.
    Last edited by UncleFrank; 02-28-2021 at 03:51 AM. Reason: addition

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    No such thing as vacuum pressure. A vacuum is the absence of pressure. Two perfectly flat surfaces in contact with each other have a vacuum between them, gasket or not. Because they are perfectly flat there is no room for air between them. You will not be able to separate them until air gets between them.
    I need to be very careful when picking up 4x10 foot sheets of 16 Gauge 304 with my vacuum lifter. It will pick up 2 sheets at once quite often. I have even picked up a sheet from flat, tilted it, placed it on an A frame cart. It was harder to tilt for some reason. That reason was 2 sheets stuck together by vacuum, and not vacuum from the lifter. The lifter frame has 8 pods to keep the sheet flat and not peel away. I have had the same thing happen when picking up a sheet from the A frame cart. 2 sheets lifted and the second sheet slid away and chopped an extension cord. Could have been my foot. Knowing this I always lift about an inch and check for a second sheet.

    A vacuum DOES NOT pull anything down. It is the atmospheric pressure that PUSHES it down.

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    Scruffy, what you have said about not needing any space between the part and fixture did make me think. When making a fixture to hold a 10"x14"x3/4" polyethylene plate I did everything but the .005" relief under the part, installed the o-ring seal, set the part in place, and pulled 26"of vacuum. Then I pushed on the part and it moved quite easily. Next I cut the .005" of relief, set the part on, pulled the vacuum and it would not budge. Dropped the vacuum to 15" and still it would not budge no matter how hard I pushed on it. Pulled the part off and there were no marks on it so the extra hold was not from sharp edges around the relief cuts. You DO need some space between the plate and part for the vacuum to work. My venturi pump only uses .5cfm @ 60 psi so it is pretty intolerant of leaks, so that was not my problem when I didn't have any space between the part and plate. I understand what you are saying, but it don't work dude.

    The same applies if your seals leak and coolant floods the space between your part and vac plate. Where the coolant fills the gap you will not have any holding pressure.

    The polyethylene plate was as extruded so it wasn't perfectly flat. I do not use channels in any of my vac fixtures, just .005" of relief no matter how big they are.

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    Quote Originally Posted by Scruffy887 View Post
    Two perfectly flat surfaces in contact with each other have a vacuum between them, gasket or not.
    Nope. Two perfectly flat surfaces in contact have no "between". The upper surface is supported by the lower surface in perfect balance to the down-forces on the part. If the part begins to lift, then there would be vacuum, but it has to lift a bit first.

    As you've observed with your sheet: Take two perfectly flat plates, and stack one on top of the other. Give it a few seconds for the air to get out. Pick up the upper plate quickly, and the lower one will follow for a while before falling. But slide the top one sideways and it'll move just fine, unless adhered by an oil film or magnetism.


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