521,872 active members*
2,215 visitors online*
Register for free
Login
Page 1 of 3 123
Results 1 to 25 of 75
  1. #1
    Registered
    Join Date
    Dec 2015
    Posts
    137

    Another Vacuum Table Question

    Hey guys,

    Been a while since I've posted. Last year I built a 4x2 CNCRouterParts machine. I cut a lot of guitar parts and a lot of plywood on it and it does a great job. I am starting to do even more plywood work and I'm looking into vacuum holding and I have no idea what I'm even looking at. I've read as many threads as I can find about vacuum holding and various table designs and while it seems easy enough to do, I have no idea where to begin as far as the actual vacuum goes.

    I have seen a few videos of simple shop vac holding tables and I know it isn't good for the vacuum, but I think this might be an easy way to get started. I have a shop vac that never gets used and what I'm really wondering is how much work holding can I expect from a shop vac?

    Assume I build a basic table with a pegboard top for the airflow. Let's say 24x48 inches, so when I put my cut down sheet of plywood on there, it complete covers the whole vacuum table. I have been cutting parts at 250ipm at 0.3" DOC in 1/2" prefinished baltic birch. The parts aren't small, but they aren't huge either. Some are roughly 12x12, while others are maybe 3x12. I am trying to eliminate the use of tabs and screws for work holding so an onion skin doesn't really interest me. I'd love to be able to just pull parts off the machine when the cutting is done.

    Is a shop vac sufficient for this kind of cutting or will I just ruin plywood? I don't have the exact specs of the shop vac, but I want to say it's in the 4-5hp range.

    Thanks for any insights. I've seen the shopbot links to those $130 vacuums, so I might be interested in that eventually but for now I want to know if I can make a shop vac work and upgrade later.

  2. #2
    Registered
    Join Date
    Dec 2003
    Posts
    588

    Re: Another Vacuum Table Question

    You need to do more homework before cutting anything or building anything and its a good sign that you are asking the questions here.Have you seen the MDF zoned vacuum tables that are offered as kits which allow you to build your own small vacuum table?They use ball valves and sit on a flat bench top-to which the vacuum holds them.A shop Vac is sufficient for a hand router and may be enough for a small CNC if the parts cover a large enough area.The area is important because the more area subjected to a vacuum-the greater the clamping force and the chance that the piece will stay put.The ball valves are an important part of the concept because unless they are pushed open there isn't an air passage and the vacuum beneath the job is more complete.Your idea of using pegboard is almost certainty doomed by the amount of leakage it will allow around the piece you are trying to machine.You also need a sufficiently rigid surface to keep the Z axis where it believes the job to be located and pegboard will very likely allow the job to move below where it is expected to be.I am trying to remember who makes the kits for the small tables I have seen.

  3. #3
    Registered
    Join Date
    Dec 2015
    Posts
    137

    Re: Another Vacuum Table Question

    Right, I should be more clear on the design I have in mind. From the videos I've seen, it looks like I can build an MDF box that is glued together to prevent as much leakage as possible. Painted as well to lock in as much air as possible. I want to build it with tabs sticking out on the bottom that can be anchored into the spoilboard (mainly so I can remove it when necessary). Obviously there is a hole in the side with a vacuum port that the vacuum connects to. The top of the box is MDF that is also glued on, but with a grid of 1/4" holes spaced to 1". This gets surfaced flat, and everything gets some paint or poly to seal it. Essentially this is the spoilboard and gets replaced every so often. Any area of the grid that is not covered would be covered by a piece of plastic to seal up the table.

    Basically, this design: https://www.youtube.com/watch?v=B4HDGnJVe_o

    It appears to work in the video, I guess my question is how to calculate if a shop vac will provide enough holding power for the cuts I'm making.

    Now, I haven't considered ball valves before and I'll be doing some research on those for sure. There are a lot of things with a vacuum table that I am not familiar with and wanted to ask here first. The youtube videos for shop vac tables look like they work...I just don't know how hard you can push it on those tables before things move and you ruin material.

  4. #4
    Registered
    Join Date
    Dec 2003
    Posts
    588

    Re: Another Vacuum Table Question

    I just watched a bit of that video and it looks like a lot of work for a so so outcome.Take a look at one of these https://www.sitebox.ltd.uk/trend-min...-oTRE_MINIMACH .It might not be exactly what you had in mind,but the principle is one you can adapt-a single piece of MDF with holes for the ball valves and some lengths of rubber gasket.As for holding power-bigger area is better and this applies with any vacuum system.If a part might be likely to move you can reduce the cut depth to make it less likely or you can reduce the feed rate a bit.It depends on the amount of material being removed and some trial and error may be encountered.

  5. #5
    Registered
    Join Date
    Feb 2012
    Posts
    74

    Re: Another Vacuum Table Question

    I've been using a similar setup as that vacuum table for a few years and it works but has some downsides. I started with a single shopvac and it did a fair job as long as parts were big but when I started doing some displays that contained a lot of small parts it wouldn't cut it. I built a box that holds 4 of the motors that you've probably seen on the shopbot site. Huge increase in performance and I've been able to hold parts as small as a little over 1 square inch (18 per display and I've made around a 1000 displays) with an onion skin with the initial cuts using a 1/4" down spiral at 250 ipm and 3/8" doc. I cut through the onion skin so the parts do just pull off the table. Probably lost 20 of the small pieces in that time with a couple where the plywood separated and half went up the dust collector and half was still on the table. Large parts I cut with no onion skin. The way I was able to make this work was to make a cover sheet where I omitted any holes where the bit would cut for any production type jobs like the displays. For one off stuff I would cover the rest of the table with boards/tape/whatever like Frank did for his. Again.... it works but can be a pain and if you cut through very many holes you lose vacuum fast. For some jobs I would need to stop and tape over cuts so it would be able to finish without losing vacuum. I also used a huge hepa filter before the vacuum motors because with the big holes you suck a lot of dust into the system. It had to be cleaned out a lot and was a mess that I hated dealing with.

    Having grown tired of having to hassle with the vacuum loss when cutting and exposing the holes along with having to clean the filter a lot I decided to build a new base and vacuum table recently to replace my initial build and so far it has had mixed results. I started with trying to just pull vacuum through mdf with no filter and wasn't happy with the results. Not sure if the particular sheet of mdf isn't working right but I get very little vacuum loss with the whole thing uncovered and I took off a lot from both sides to remove the "sealed" part of the mdf. It did hold large pieces well. I'm going to go with some light weight mdf as soon as I get a chance to see if there is an improvement over the mdf I used. I'm also pulling less vacuum because I'm at nearly 7k feet in altitude. So far I've tried to drill 3/8" pocket holes on a 1" grid and leave about .1" so that there is no through hole. That was a great boost to the performance but still wouldn't come close to holding the small parts and if the plywood was warped at all it wouldn't hold. Tried drilling a very small hole where the small parts were located and so far the results have been a whole lot better.

    My initial tests were using a piece of plywood that was a little over 12"x12" and after drilling the pocket holes I was able to make some test cuts where it didn't move and that was with the rest of the table open (3'x4
    total area). Then I tried another piece that was the same size and after I cut a dado in the middle of it then I lost all suction to it because it warped just enough that it wouldn't hold. If I would press it down so that it would catch vacuum it would hold but as soon as I let it warp up it was as if there was no vacuum and the warp was so small you could barely see the gap. I then put it over the place where I had drilled the small through holes the piece sucked down to the table and it took a lot of force to move it.... like put all my weight behind it and push kinda force. Even with just 2 of the motors running it had a lot of holding power and that was with the entire rest of the table open. I'm going to add a small filter back in and then drill a lot more of the small holes and play with that. I'm hoping with the small holes I will have the same holding power I had before without the loss of vacuum when the holes get uncovered.

  6. #6
    Registered
    Join Date
    Dec 2015
    Posts
    137

    Re: Another Vacuum Table Question

    Thanks for the input guys, very good information here. If I'm understanding the ball valves correctly, they are tied to a specific area of the vacuum table, so if you don't need vacuum there you turn off the valve and I imagine you get even better vacuum holding power in the rest of the zones. I like that concept (if I'm correct in understanding it).

    Lambodesigns--I appreciate your detailed feeds/speeds. Very good baseline for comparing what I'm doing. Now if I'm understanding you correctly, you are saying a small part is roughly 1 square inch?? That is tiny!! I'm trying to think of my smallest part, and I'd have to say my smallest is between 5-10 square inches. If I need to add tabs for some of the small parts I'm OK with that. It would be really cool to have some kind of vacuum holding for at least the bigger parts.

    At what point, with the shop vac, did you find parts starting to move on you?

  7. #7
    Registered
    Join Date
    Feb 2012
    Posts
    74

    Re: Another Vacuum Table Question

    I didn't really do much experimentation once I got the order for the displays and I hadn't used the vacuum table much before that. I had to make it work and the single shopvac didn't allow me to cut them out so I had to upgrade and do what it took to make it work.

    Yes, the parts were just little wedges for the display to hold it together so they are really small as they are roughly a little over 2" long and a 1/2" wide (just checked in fusion and they are 1.81 square inches in size to be exact). My table is on 1" grids so they just barely cover 3 holes. Without an onion skin there is no way they would possibly stay put but with only cutting .010 to 0.15 inches of material they tend to stay in place. It is also important to cut all the small parts first. If you have a large piece of plywood held down and you cut out a small part it will stay in place especially with an onion skin that is really thin. It has been a few years since I set this up but I also remember that it was important as to what direction you were cutting so that the onion skin pass would press against the outside and not against the small part due to bit/router deflection from the cutting forces. It only takes an air gap to make all the difference. I'm pretty sure it was a climb cut to make that happen. Cutting with a 1/4" bit at those speeds and doc does put a lot of force on things.

    How well a part will hold with through holes really depends on how much vacuum you are losing during cutting. If you have a small part and don't lose any vacuum it will stay put... start losing vacuum and things go sideways fast. That is where using the pull through mdf method really differs. With pulling through the mdf you can have a huge area open and still have vacuum on the part. With through holes you lose a small fraction of the vacuum and it is game over.

    One other interesting note on using the pull through. I had a piece of really thin cheap plywood from a drawer bottom that was probably 1.5 square feet. It wouldn't stay put at all. Turns out it was sucking through it. I put a piece of plastic over it and it sucked down to the table. I put it over the area that had the few small holes and it sucked down hard to the table even without the plastic on top. That's where the cfm vs vacuum pressure comes into play. Bottom line... there is no simple answer with vacuum hold downs and what will work for a given piece but if you have something that works for what you need it will make your life easier.

  8. #8
    Member
    Join Date
    Apr 2007
    Posts
    164

    Re: Another Vacuum Table Question

    one other thing that might be useful is your vacuum cleaners are rated with inches of water lift on a sealed vacuum. when you get into a situation where the material you are holding down is non porous and have a high vacuum you get into inches of mercury for you vacuum ratings which you also see the cfm rating for the volume of vacuum. in metal work holding your well over 25 Hg and every thing is sealed up with gaskets so the cfm requirements are not that demanding. with wood since it is porous i think the cfm is pretty important. one of the benifits with a zoned vacuum table is only using the section you need to cut down on the high cfm requirements. high volume vacuum sources are pretty expensive to say the least.

    but to keep this short when you do the conversions between shop vac ratings and a commercial solution you see the short comings of using a shop vac.i would say if your lucky it might have half the vacuum you find on a commercial wood working system and nowhere near the cfm needed for a big vacuum table.

  9. #9
    Registered
    Join Date
    Dec 2003
    Posts
    588

    Re: Another Vacuum Table Question

    Quote Originally Posted by canman77 View Post
    Thanks for the input guys, very good information here. If I'm understanding the ball valves correctly, they are tied to a specific area of the vacuum table, so if you don't need vacuum there you turn off the valve and I imagine you get even better vacuum holding power in the rest of the zones. I like that concept (if I'm correct in understanding it).
    If you followed the link I gave to the Trend table you will have seen that a rubber seal divides the top surface of the table into zones.The ball valve in each zone is opened by the workpiece being pressed down and allowing the air in the zone beneath the workpiece and within the boundaries of the rubber to be extracted.Thus there is no manual opening of anything.The really neat feature is that the same seal is used on the underside to hold the table in place while also acting as a manifold for the various zones.

    If you were mainly repeating the same part it wouldn't be hard to create a custom version with the seals a short distance from the edge of the cut in order to achieve maximum grip.

  10. #10
    Registered
    Join Date
    Dec 2015
    Posts
    137

    Re: Another Vacuum Table Question

    Quote Originally Posted by routalot View Post
    If you followed the link I gave to the Trend table you will have seen that a rubber seal divides the top surface of the table into zones.The ball valve in each zone is opened by the workpiece being pressed down and allowing the air in the zone beneath the workpiece and within the boundaries of the rubber to be extracted.Thus there is no manual opening of anything.The really neat feature is that the same seal is used on the underside to hold the table in place while also acting as a manifold for the various zones.

    If you were mainly repeating the same part it wouldn't be hard to create a custom version with the seals a short distance from the edge of the cut in order to achieve maximum grip.
    Oh wow, yes that's a rad feature! I like that a lot. I'll have to do some more research on how to implement that into my design. I do a lot of one off work so cutting tabs off for most jobs is fine, but I do have a few production-style jobs that would be nice to use vacuum holding with, so creating the custom version you are talking about is probably the way to go.

    machinedude -- I don't remember if I mentioned, the baltic birch is the prefisnished two sided stuff so I'm hoping it is at least a little less porous than other wood products that I cut. The CFM requirements are where I'm getting stuck...perhaps you guys can point me to some websites where I can get adequate research done. What I've found so far is only confusing me more! As far as the high CFM vacuums go, I definitely have seen how expensive they are. Certainly not interested in that for sure. But the little shopbot vacuums seem like a good deal, especially if I'd only need one for a 2x4 table size.

  11. #11
    Gold Member
    Join Date
    May 2005
    Posts
    3919

    Re: Another Vacuum Table Question

    You have gotten some good comments here but I have a few to add.

    1. No matter what it says on the vacuum cleaner you can’t get 5HP out of a 15 amp 120 VAC outlet.
    2. Leakage and slippage doom many vacuum systems.
    2A. One way to combaT slippage is to use fences on your vacuum table.
    2B. Leakage can be combatted in a number of ways but a big one is to think zones!
    3. The max clamping pressure you can get at sea level is about 14 PSI. That is it and generally it will be less than that by a significant amount. There is no magic to address this, rather it is physics.
    4. Since you will likely never get 14 PSI differential pressure on your work piece clamping forces are minimal for small objects thus the technique is that f little use if your goal is to cut out small parts.

  12. #12
    Registered
    Join Date
    Aug 2005
    Posts
    82

    Re: Another Vacuum Table Question

    Sorry folks but home shop vacuum systems generally won't begin to approach 14 PSI (roughly 28" HG at sea level) vacuum pressure. Shop vacs standard measure is inches of Water Gage, a far cry inches HG. Furthermore, most vacuum hold down systems (this includes the big industrial CNC's) rarely measure vac pressure at or near the table. A simple test of holding power is to mount a vacuum gage (HG) onto a gasketed phenolic panel (perhaps 6" x 6") and place that where you would be placing your work piece with vacuum on. Divide the value you get on the gage by 2 and THAT is what you're getting in the way of PSI at your work piece. That will quickly point out many flaws in a vac hold down system.

    Another thing to consider (wizard mentions this) is side slippage. Cutting forces are generally parallel to the XY plane. If you are cutting melamine, chances are greater that it will slip as opposed to a veneered piece of MDF. I've used peel and stick abrasive paper on my work piece fixturing to combat this. Obviously, the abrasive wouldn't work well on a conventional spoil board.

  13. #13
    Registered
    Join Date
    Dec 2015
    Posts
    137

    Re: Another Vacuum Table Question

    Wizard and MARV: Thanks for the comments. Very informative. It is sounding more and more like a futile effort to do any kind of vacuum work holding for the cuts I am making...if I were simply engraving parts, it might be different. But I am in a semi-production environment and the goal is to hog parts out as quickly as possibly. If that means tabs, so be it. I suppose my goal now is to optimize my tab efforts. Nothing worse than the tabs breaking during cuts...or having to break off a million tabs after the cut..

    Thanks again for all of the information. This is giving me a great starting point to do a lot of research and see what is actually feasible for what I want.

  14. #14
    Registered
    Join Date
    Aug 2005
    Posts
    82

    Re: Another Vacuum Table Question

    canman7, if you are trimming tabs by hand afterward, you may want to try to onion skin the whole part during your roughing operation and take a final pass (perhaps at a slower pace) and clean the onion skin out at the CNC. Keeps things a bit simpler when programming and you won't be exerting as much lateral cutting force on the work piece. And you may not have to do any hand work afterward. Just a thought.

  15. #15
    Gold Member
    Join Date
    May 2005
    Posts
    3919
    Quote Originally Posted by canman77 View Post
    Wizard and MARV: Thanks for the comments. Very informative. It is sounding more and more like a futile effort to do any kind of vacuum work holding for the cuts I am making...if I were simply engraving parts, it might be different. But I am in a semi-production environment and the goal is to hog parts out as quickly as possibly. If that means tabs, so be it. I suppose my goal now is to optimize my tab efforts. Nothing worse than the tabs breaking during cuts...or having to break off a million tabs after the cut..

    Thanks again for all of the information. This is giving me a great starting point to do a lot of research and see what is actually feasible for what I want.
    Gee I hope I didn’t turn you off to vacuum fixturing completely because it can have a use in a production environment. The problem is it has to be carefully considered. Sometimes it might even pay to have a custom vacuum box for a specific part run. This is no different that setups done on production mills, fixtures are designed for the part to be made.

    Other things to consider are hard hold downs in areas that are known to be waste. You can run a couple of screws into threaded holes to supplement the clamping for rough milling and rely upon vacuum for final cut through. You can even use the vacuum to hold the part in place to “drill” the holes for those clamping screws.

    In other words vacuum tables can be part of the solution or the whole solution. You don’t want to think in terms of all or nothing in production. Rather think about what is cost effective and minimizes difficulty on the floor.

  16. #16
    Gold Member
    Join Date
    May 2005
    Posts
    3919
    Quote Originally Posted by MARV View Post
    Sorry folks but home shop vacuum systems generally won't begin to approach 14 PSI (roughly 28" HG at sea level) vacuum pressure.
    Yeah I probably wasn’t clear on that, simple vacuum systems will not pull down that far. Even though I said substantially less I’m pretty sure people don’t realize how much less it can be.

    I work on industrial vacuum processes so regularly work on systems pumping down into the tens of millitorr range, this isn’t going to happen with a vacuum cleaner. The pumps themselves at work cost more than most of the routers here.

    The other thing that probably needs to be addressed better is leakage, this can be a huge problem. Leakage greatly reduces holding power which I think people understand but it doesn’t do so evenly across the vacuum fixture. This especially if there are obstructions to air flow in place. It is counter intuitive but vacuum systems require good air flow within the fixture itself to avoid large pressure gradients.

    This is one reason to like zones in your vacuum fixture. If the parts cut result in lots of leakage it can be a good idea to shut off vacuum in one area before moving onto the next.
    Shop vacs standard measure is inches of Water Gage, a far cry inches HG. Furthermore, most vacuum hold down systems (this includes the big industrial CNC's) rarely measure vac pressure at or near the table. A simple test of holding power is to mount a vacuum gage (HG) onto a gasketed phenolic panel (perhaps 6" x 6") and place that where you would be placing your work piece with vacuum on. Divide the value you get on the gage by 2 and THAT is what you're getting in the way of PSI at your work piece. That will quickly point out many flaws in a vac hold down system.

    Another thing to consider (wizard mentions this) is side slippage. Cutting forces are generally parallel to the XY plane. If you are cutting melamine, chances are greater that it will slip as opposed to a veneered piece of MDF. I've used peel and stick abrasive paper on my work piece fixturing to combat this. Obviously, the abrasive wouldn't work well on a conventional spoil board.
    Another thing to watch out for is the original poster had a picture of a table with an array of holes drilled in it. If these are the only way to draw a vacuum against a part it can result in issue if the part seals off that hole a little too good. If that happens then your effective area is the size of a drilled hole. Thankfully most wood doesn’t seal off that well but finished products maybe an issue.


    As for slippage nothing will ruin you day more than to have this happen during machining. It doesn’t matter if it is a mill, lathe collet or a router table. Generally it leads to starting over.

  17. #17
    Community Moderator ger21's Avatar
    Join Date
    Mar 2003
    Posts
    34946

    Re: Another Vacuum Table Question

    What you want for guitar parts are dedicated, gasketed fixtures. You can definitely achieve 14" Hg, IF the material is not porous. If it's porous, vacuum will leak through.

    I have a friend that makes necks, bridges, and fretboards, and uses vacuum fixtures to hold everything. If possible, using pins in hidden areas can greatly improve functionality, as they'll keep parts from sliding, which can be an issue with vacuum.

    For small, and/or repetitive parts

    IMO, any system using pegboard or similar (with holes) is a waste of time. All the holes do is leak.

    Pulling through an MDF spoilboard is the best method for sheets, but requires BIG pumps, or at least a LOT of CFM to overcome leakage.

    Individual pods may be an option for bigger parts.

    If possible, I'd always prefer to onion skin rather than use tabs. If cutting forces are low, you can sometimes cut to within a few thousands, and just clean up what's left with a sanding block
    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  18. #18
    Member
    Join Date
    Dec 2019
    Posts
    11
    Very interesting discussion and a lot of good information

  19. #19
    Registered
    Join Date
    Feb 2012
    Posts
    74

    Re: Another Vacuum Table Question

    Got in another order for some displays and thought I would give an update since I had a few ups and downs getting them to run properly with the new vacuum table.

    I started off tying to use mdf as the spoil board and wasn't having the best results with living at 7k feet and cutting some very small parts. To get the smallest parts to hold I ended up having to drill some small holes to increase the holding power. Having drilled a hole in the spoil board I decided to add back in a small filter to protect the motors. Life is good and I start cutting... first 4 runs are perfect and I'm on a roll. Then I lose a shelf on the last cut. Chalked it up to having a piece of sawdust under the plywood or something so I load the next one and it cuts fine but I see the last shelf is starting to slide at the end. Made sure the table was extra clean and started the next one.... didn't get 2 minutes into the run and the whole piece starts moving around.

    From there I was back in testing mode and it was as if I had lost nearly all of the vacuum so I tried everything from resealing the edges, resurface the table, and even going back to more of the pegboard style with small holes drilled on a pattern. No gain. That's when I decided to go ahead and install the ldf for the spoil board.... wouldn't have been a big deal but it was at the bottom of my order of 30 sheets of plywood so I had to restack all the plywood to get to it Once I had shaved off the old mdf I found the real problem. My mistake was I had installed the 3" pvc into the vacuum table using a press fit (it was really tight so I thought it was good) and when I put the filter in I had slid the pipe up in vacuum table. When I started cutting it was still letting air through but as it ran it ended up sucking the pipe against the spoilboard so I lost nearly all suction. At least at that point I found the problem so I went ahead with the ldf installation and made sure the pipe isn't going anywhere.

    I was expecting a little gain with the ldf but it was a huge gain. With the mdf if the plywood warped at all it would lose suction. Now it sucks it down like it used to do with my old pegboard style table but I don't have to worry about cutting through any holes. The only real negative is the small parts don't hold as well as they did with the pegboard so while they would cut fine as the dust collector would move over the top of them they would get sucked up. I added two very small tabs that I can almost cut with a finger nail and they are staying put. By adding the tabs the top of the table stays flat and I don't have to worry about covering any holes.

    For me this has been a great upgrade over the pegboard. For each run of 30 displays I would have to clean the filter and I also had to use templates that would only have holes under the parts and cover the ones that I would cut through. Each new project would require a new template. The few tabs I have to cut are well worth the extra time saved and now I can cut one off parts without worrying about losing vacuum. That said... the pegboard did work to cut well over 500 sheets of plywood and make a healthy profit on them.

  20. #20
    Registered
    Join Date
    Dec 2015
    Posts
    137

    Re: Another Vacuum Table Question

    lambodesigns, thanks for posting this. I've been doing a bit of research and haven't gotten around to posting here yet. I am planning on building a little vacuum table this weekend as proof of concept. I have plenty of MDF and plywood lying around, and sealing the box won't be difficult. I have a shop vac that I plan on using, which likely only pulls maybe 1-2" HG? If so, I'm looking at 1 psi max. Which is fine for proof of concept, because I am working on little closet job where the smallest panels are 6x12". So at 1psi, a 6x12 inch panel has 72lbs of force pulling down on it, if I understand my research correctly. 72 pounds doesn't sound like much but I've lifted 70 pound dumbbells and they're freakin heavy so I think it's at least worth a test. The majority of the panels are 12x12 or 12x40, which is 144lbs and 480lbs respectively. That seems like enough hold down for to at least run some tests.

    If the tests are marginally successful, I think it's worth investing in one of the shopbot vacuum motors for $130. I think they pull around 10" HG, so right around 5psi, which would equate to 360 pounds of hold down force on the smallest closet panels. I live at sea level as well so I don't have to worry about drop in psi due to elevation.

    Anyways, I think it's worth at least trying. As long as I'm understanding these basic calculations correctly. There's always a good chance I've interpreted something completely wrong!

  21. #21
    Registered
    Join Date
    Aug 2005
    Posts
    82

    Re: Another Vacuum Table Question

    I think you guys are getting close, but don't discount the notion of lateral slip. After all, a good deal of the cutting forces are in the XY plane. Anything you can do to minimize slip is to your advantage. Sometimes a reference edge on the work piece doesn't need to be cut, so a temp fence will work (think dbl stick tape). Other times sticky back abrasive is the answer. Here is a place that I used to buy from that may offer some interesting solutions (no affiliation);

    All Star Adhesives - gasketing products for CNC routers and v-groove tape for v-grooving and coving applications

    As a side note: my vac hold down system is powered by an eBay sourced, lubricated rotary vane pump made to pull down to 29"+, run all day long, was rebuilt/warranted and cost me $400 ($2400 list I think). They are out there. Its intended to service my 3' x 4' CNC gasketed and sealed MDF top as well as a vacuum press (not together). No spoil board. The key to making this system work is few leaks and some thrown together fixturing..

  22. #22
    Registered
    Join Date
    Feb 2012
    Posts
    74

    Re: Another Vacuum Table Question

    Some more lessons learned from a few hours of cutting this morning.....

    My tabs are really a combined onion skin and tab of .02" which worked great.... most of the time. The plywood is 3/4" prefinished maple and not what I would call stellar quality so some it is warped "a bit". If the area where I'm cutting the small parts is warped at all and not absolutely flat to the table it ends up trashing the parts because the dust brush pushes them into the bit. I'm going to up the thickness and see how the rest of the day goes but out of around 10 runs I've lost 2 sets of the parts which is more than double what I lost in over a 1000 runs before Trying to look on the bright side with the tabs.... it is overall quicker because I just lift the whole piece off the table and set it aside instead of picking each one out. Then after I get the next run going I pop them out while the cnc is running so overall at the end of the day it is faster with the tabs.

    Canman77.... I got started on this because a friend gave me a small vacuum pump. It was one that required a lot of air to be moving to cool it down so a good seal meant it was overheating in no time. Without a good seal it didn't generate enough force to hold much. Worked fine but only for a minute or two at a time. Found out my shopvac worked the best of anything I had and used it for a little while. Really is amazing how much force they generate. Then when I got the large order in it justified the upgrade to 4 of the good ones and a dedicated box for them. Good luck on your testing and I think you will be happy with the results. If the mdf and a shopvac works for your test then ldf and a shopbot vacuum motor should be the ticket.

  23. #23
    Registered
    Join Date
    Aug 2005
    Posts
    82

    Re: Another Vacuum Table Question

    I should mention that when I first started out years ago, someone handed me a cast off refrigeration pump from a milk house operation. Two stage piston pump with separate motor drive. It didn't owe me anything, so I swapped output to intake, ran it all day at 24"HG and it never quit. Sometime, you make do with what you've got.

  24. #24
    Community Moderator ger21's Avatar
    Join Date
    Mar 2003
    Posts
    34946

    Re: Another Vacuum Table Question

    I think you guys are getting close, but don't discount the notion of lateral slip.
    Yep. Parts will slide with far less force than it takes to lift them off the table.
    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  25. #25
    Registered
    Join Date
    Feb 2012
    Posts
    74

    Re: Another Vacuum Table Question

    I use that same trick when I mount ice tires on my go karts. We cut the sidewalls off of old tires and then run screws through them. To mount the "skins" we use the intake from an air compressor to suck the tires down so that we can slip the skin over them. Funny to see a round tire get sucked into a square shape

    I also have a regular vacuum pump that will pull a ton a vacuum as long as it is perfectly sealed that I picked up for $20. Uber quiet and will run forever. Once you lose any seal and it doesn't have enough cfm so vacuum goes to zero in a heartbeat. Someday I'm going to add a vacuum bag to my shop for gluing stuff up but I've just never found the justification for one because they aren't cheap.....and I am

    Ding... fries are done... time to go load more plywood

Page 1 of 3 123

Similar Threads

  1. Vacuum table - power question
    By Dadnatron in forum General Off Topic Discussions
    Replies: 1
    Last Post: 01-28-2016, 08:58 PM
  2. Vacuum Table Question/Help
    By andy_con in forum General MetalWork Discussion
    Replies: 1
    Last Post: 10-09-2014, 01:37 PM
  3. Vacuum Table Question
    By Myshadowboxes in forum DIY CNC Router Table Machines
    Replies: 13
    Last Post: 04-08-2011, 09:23 PM
  4. vacuum table question
    By ironman014 in forum DIY CNC Router Table Machines
    Replies: 3
    Last Post: 12-29-2009, 08:23 AM
  5. yet another vacuum table question....
    By boomerize in forum DIY CNC Router Table Machines
    Replies: 9
    Last Post: 08-18-2009, 06:16 PM

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •