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  1. #41
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    Default Re: Another Vacuum Table Question

    Plywood that has a slight warp will actually hold down better with the convex side up, as it pulls the middle down and keeps the edges tight. With the convex side up, you have almost zero chance of holding it.

    Gerry

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    Default Re: Another Vacuum Table Question

    Lambo - Its all about the smiles Peter



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    Default Re: Another Vacuum Table Question

    Rather than start a new thread, I figured I'd update this one.

    I finally managed to build a vacuum box, only to test the hold down power a shop vac would have. I glued a bunch of MDF together to make a box and hooked up the vacuum before I put the top on the box...I put the top on, turned on the vacuum, and I could not move the top to save my life. I was really impressed. Didn't take much effort to seal any seams, and it's all MDF so I know there is leakage all over the place. Instilled enough confidence to glue the top on and install it on the bed of our machine. I cut a random shape about 100sq in at 150ipm, 0.3 depth of cut, in 1/2" baltic birch. First two passes, climb cut with 0.02" of radial and axial stock to leave. Final pass, conventional, full depth and cleaning up the climb cut. No movement.

    Basically, this test was for proof of concept and I am beyond thrilled with the outcome. Realistically, I could build a vastly superior box right away and use the shop vac, but I don't want to burn it up.

    So, I'm seriously looking at these lighthouse motors now. I read somewhere that the shopvac likely is only pulling 2-3inHg, while these dedicated motors pull 10. I notice on the website that it says to drill a 1/4" (or 3/8" depending on motor model) for filtered air cooling. I assume this hole goes in the plumbing somewhere, is the location critical? Does it need to be close to the vacuum source? And do you just cover the hole with some filter material?

    Second question, what size plumbing is required? I believe earlier in this thread someone mentioned that airflow is not the critical parameter, so the plumbing holes do not need to be huge. Would 1/2" be sufficient?

    Lastly, I am looking at three models: https://www.centralvacuummotor.com/shopbot.htm

    The first two are labeled 116765-13 but show a difference in price of $10. I can't see what the difference is between those two, so where is the price difference coming from? The third model I am looking at is further down the page, the 116765-OD model. It's a little more expensive but pulls more vacuum. I have a dedicated 120v 20 amp circuit at my disposal for the motor, so I'm thinking I might as well go for the beefier model.

    What do you guys think?



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    Default Re: Another Vacuum Table Question

    Good to hear your test went well. I used 2" plumbing for my first vacuum table because of the shopvac and I ran the intake on the side so that was all the room I had. When I rebuilt the CNC base I went with 3" pipe to the main 2'x4' area and then used 2" for the back 1'x4' zone mainly because a 2" ball valve was the biggest one I could find local and didn't want to wait to order one. No science to back it up but you don't want to use 1/2". Most of the ones I've seen use 3" and it is proven to work well.

    I didn't drill a hole but based on the airflow out of the bottom of the box I have enough leakage to keep the motors alive. They do put out a lot of heat but they have for a lot of years, sometimes running 12 hours a day, so I'm not too worried about it. My old setup definitely had enough leakage to make up for a lack of a hole and the new one seems to be about the same. I generally don't cover the entire area and as soon as you start cutting through material the airflow goes up so it is only completely blocked for a short time.... thus the need for bigger plumbing than 1/2".

    Go for the beefier model.... and get 2 : ) I have no regrets getting 4 of the 220v models but I also have altitude to compensate for.... so at lower levels 2 probably works as well as my setup. Again.... no science but just gut feel on that one and having a good vacuum hold down makes using the CNC so much easier.



  5. #45
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    Default Re: Another Vacuum Table Question

    The second one has Long Life brushes. The numbers are different. LH6765 vs LLK6765
    LL = long life. All the models below clarify this.

    These motors create vacuum through CFM, so you need bigger diameter plumbing to move the air.

    Gerry

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  6. #46
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    Default Re: Another Vacuum Table Question

    Something I've found useful over the years is to make a test panel 6 x 6 x 3/4 out of MDF. Seal the edges but not the top surface and paste a vacuum gage in the top side. Gasket the bottom side. Place this on your spoil board anywhere you expect to be placing a workpiece and turn on your vacuum. If the gage reads in HG, halve the number and multiply by approximately 35 (square inches). That is the SUM TOTAL holding power in pounds available to secure your workpiece. I don't give a hoot what the gage at your vacuum pump is reading, or what your specs say you should be getting, you want to know what your getting at the spoil board.

    Now, place a piece of plastic laminate or melamine board or anything impervious on the spoil board next to the gage panel and watch the value on the gage go up! Hell, Suran wrap will work just as well. What your witnessing is the biggest variable in a vacuum hold down systems; leakage. Beat that (and have a good vacuum pump) and you've got it made.

    One last note; I believe the vacuum motors you are looking at are replacement dust collection/vacuum cleaner motors? The 2 stage models separate the motor cooling air from the dust laden air. You can plug those guys at their dust collection end and still run them all day long without burning them up. Worth the extra expense.



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    Default Re: Another Vacuum Table Question

    Marv, you hit on exactly why I'm looking at these motors again. They won't burn up...I don't really want to keep replacing shop vacs. Not to mention, the improved performance. I'll also pick up a vacuum gauge and measure what I'm getting at the spoilboard. I'm interested to know exactly what the shop vac gets me right now. What gauge would you recommend?

    Gerry, thanks for pointing out the model numbers...can't believe I missed that. Clears up the pricing difference, especially where it notes down below the LongLife option. I was hoping to get away with a skinnier vacuum box but it sounds like that won't be an option since I need the CFM. No problem. I guess since the diagram shows 1.89" for the diameter of the intake, it would be unwise to go any smaller. In order to keep the table as thin as possible without compromising CFM I will plan on 2" plumbing.

    Lambo, thanks for sharing your plumbing experiences. Since I'm not exactly setting up my table to be 100% vacuum all the time (rather, it will be a removable vacuum table) I will need to keep it somewhat thin. Hence the 2" mentioned above. I hope this will still work well. I only plan on really cutting sheet goods on the vacuum box so I don't need an insane amount of Z clearance. I have 8" to work with, and the box right now takes about 4". I will probably use 1/2" MDF for the top and bottom. And my table is also only 2x4 so I am hoping one vacuum motor will be sufficient?



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    Default Re: Another Vacuum Table Question

    It is all about what you use the CNC for on a regular basis and how small your parts will be. I cut a lot of very small parts so I would like to have more than the 4 motors I use, but that isn't going to happen, because I'm at the limit for power. I only have 2 switches for the 4 motors so I can only gauge the difference between 2 or 4. 2 would work fine most of the time but for small parts it takes the other 2 to hold things down and you can really tell the difference when those 2 are turned on.

    I had the removable table and in the last few years it never came off of the CNC so when I went with the base rebuild I made it a part of the base and used t-tracks on the side to put a piece of plywood over the vacuum table for hold downs for times when the vacuum table just won't work...... like for the Sam Maloof style barstools and chairs I'm building at the moment. Parts like that just don't have enough surface area and I'm having to use a 1/2" bit which generates way too much cutting force. I also built in a front vise so I can clamp a piece of wood in it and then attach plywood to it for a spoil board to screw things down to. Using the vise slot as a reference you can put jigs back in the same place for things you use a lot.

    One other thing that worked well on my removable table was I extended the top over the side by an inch or so which allows you to clamp things on the sides if the vacuum won't be enough to hold the part.



  9. #49
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    Default Re: Another Vacuum Table Question

    Hi - Why not buy a proper vacuum pump? Peter



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    Default Re: Another Vacuum Table Question

    Quote Originally Posted by peteeng View Post
    Hi - Why not buy a proper vacuum pump? Peter
    Hi Peter, cost is largely the issue. I have a few hundred I can allocate to vacuum holding right now, not a few thousand. And for the cuts I am making, a few hundred will do the trick.

    Quote Originally Posted by lambodesigns View Post
    It is all about what you use the CNC for on a regular basis and how small your parts will be. I cut a lot of very small parts so I would like to have more than the 4 motors I use, but that isn't going to happen, because I'm at the limit for power. I only have 2 switches for the 4 motors so I can only gauge the difference between 2 or 4. 2 would work fine most of the time but for small parts it takes the other 2 to hold things down and you can really tell the difference when those 2 are turned on.

    I had the removable table and in the last few years it never came off of the CNC so when I went with the base rebuild I made it a part of the base and used t-tracks on the side to put a piece of plywood over the vacuum table for hold downs for times when the vacuum table just won't work...... like for the Sam Maloof style barstools and chairs I'm building at the moment. Parts like that just don't have enough surface area and I'm having to use a 1/2" bit which generates way too much cutting force. I also built in a front vise so I can clamp a piece of wood in it and then attach plywood to it for a spoil board to screw things down to. Using the vise slot as a reference you can put jigs back in the same place for things you use a lot.

    One other thing that worked well on my removable table was I extended the top over the side by an inch or so which allows you to clamp things on the sides if the vacuum won't be enough to hold the part.
    Right now I am working on kid furniture parts. I am beyond the prototyping phase but not even close to full production mode. I suppose I'm in the middle where tabs are becoming highly annoying, but a full on 20HP vacuum system is overkill. Much of my CNC work right now is furniture parts, albeit not full time. Enough to warrant a vacuum system, I'd say. I was looking over my files/models today, comparing the surface area of the components. About half of the components are in the 40-50sq.in range, while the other components hit well above 200. Since I have the small parts and the bigger parts nested together, my strategy will be the same for both...cut everything out with an onion skin, and then go back over and remove the 0.01-0.02" skin on a final pass. I'm hoping this will prevent the small parts from moving since the force exerted to remove the skin will be pretty small.



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    Default Re: Another Vacuum Table Question

    It doesn't take much of a tab to hold the part in place when you are just cutting the onion skin away. A couple of tabs the thickness of your onion skin will work magic and will easily break by hand once the part is cut. I've started using them a lot more because they don't really add any time to cutting something out and I don't have to worry about things moving and getting ruined.



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    Default Re: Another Vacuum Table Question

    Quote Originally Posted by lambodesigns View Post
    It doesn't take much of a tab to hold the part in place when you are just cutting the onion skin away. A couple of tabs the thickness of your onion skin will work magic and will easily break by hand once the part is cut. I've started using them a lot more because they don't really add any time to cutting something out and I don't have to worry about things moving and getting ruined.
    That's a great point. I'm sure you've mentioned it earlier in the thread too, but thanks for the suggestion. How thick are you leaving your skin? I experimented with 0.02" yesterday and it seemed to work well enough...for all I know 0.02" is considered a thick skin.



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    Default Re: Another Vacuum Table Question

    Something I've found useful over the years is to make a test panel 6 x 6 x 3/4 out of MDF. Seal the edges but not the top surface and paste a vacuum gage in the top side. Gasket the bottom side. Place this on your spoil board anywhere you expect to be placing a workpiece and turn on your vacuum. If the gage reads in HG, halve the number and multiply by approximately 35 (square inches). That is the SUM TOTAL holding power in pounds available to secure your workpiece.
    Be aware that this is the force holding your workpiece to the table. The force required to slide the part on the table may be considerably less. With our two 10HP pumps, when we lose parts, it's because they slide on the table. They never left off the table.

    Gerry

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    JointCAM - CNC Dovetails & Box Joints
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    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


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    Default Re: Another Vacuum Table Question

    Hi - But you can buy vac pumps for $200 used for air conditioning systems,. That's what I use. They pull 99% (that's 10 tonne pr sqm) vacuum and will run continuously? Peter



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    Default Re: Another Vacuum Table Question

    But,
    1) They don't have the CFM required to pull through an MDF spoilboard, where you lose a LOT through leakage.
    2) They usually spray large amounts of oil into the room in their exhaust.

    This is the ideal pump for a 4x8 table.
    https://www.cncpd.com/product/becker...mp-vtlf-2-250/

    Gerry

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


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    Default Re: Another Vacuum Table Question

    Hi Gerry - I don't pull through the spoilboard, I make a pocket in some mdf that matches the shape of the part so it could be a small part or a big part. Pulling vac thru a board is fine for production of big parts at a commercial level. But if the parts are varied and small volume there's cheaper ways to do it via pulling air through a spoilboard. On the point of oil in air, put the pump outside, put a filter over the outlet or buy a pump with an oil re circulation feature or buy a dry pump. Commercial solutions are about volume and convenience, low volume/hobby should be about seals with really good vac especially if parts are small. Peter



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    Default Re: Another Vacuum Table Question

    Gerry and Peter both make goods points, but illustrate what hobbyists have to deal with; compromise for the sake of saving money. The key is knowing where to compromise and still assemble an effective system. I ran 2 of those Beckers on the first CNC I bought for commercial use. When it came time to rebuild, just the replacement vanes on one was $1,000. Now, one of those Beckers would have paid for my for my entire home CNC system. And what I currently have in the way of a pump generates more absolute vacuum then what the Beckers did.

    Although it can be a bit disappointing for hobbyists, they have the luxury of experimenting to find what works. You just have to be willing to try different things and not be too quick to commit. And for those that are resourceful, try a used air compressor piston pump or an old style milk house refrigeration pump. My first pump cost me nothing, was easily converted to suck air and would pump down to 26" HG all day long. It was a 2 stage twin cylinder powered with a 1 1/2 HP motor and didn't lack for CFM. And it was still working when I eventually gave it away.



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    Default Re: Another Vacuum Table Question

    That's exactly what it is Marv. Cost vs. compromise. To me (a small business with minor production), saving thousands by using a $150 pump and not getting the best suction makes more sense than spending potentially thousands to get 26" HG. But as things progress and production becomes a major operation, 26" HG is worth the extra cost.

    The other aspect is as a hobbyist/small business guy, I only know enough about vacuum pumps and HG to get me going on something that will at least work. I don't even know where to begin looking for a used air compressor piston pump or old style milk house refrigeration pump. I did some googling and ended up with a bunch of pumps that didn't seem like they were the right kind of pump. So knowing exactly what pump you are looking for is also important, and I haven't spent any time around these kinds of pumps to even know what I should be looking for.

    So basically, the Lighthouse motors simplify life for guys like me and make compromise easier.

    I plan on doing a few more tests today on the shop vac table, just to see what the limitations are with the onion skin/very shallow final pass technique. I figure if I can successfully machine my smallest parts with the shop vac table, a lighthouse motor will hold even better.



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    Default Re: Another Vacuum Table Question

    If you make the same part over and over, you might be better off with a dedicated vacuum fixture, and a small high vacuum pump.

    I have a small Gast pump that pulls 26"Hg at 3CFM that I plan on using on my new machine with a pod system.
    I paid $90 for it.

    Gerry

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  20. #60
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    Default Re: Another Vacuum Table Question

    I do make the same small part over and over, but it is nested with other parts that are larger. The small part is a little cupholder that attaches to a little desk, so I make one or two every time I cut out the pieces for the desk. Unfortunately, not worth making a dedicated vacuum fixture.

    My tests were mostly successful today. I found that I need to put an entire sheet of plywood on the vacuum surface. Smaller pieces of wood with plastic wrap covering all the other vacuum surfaces did not work, the wood moved too much. But my smallest part is 16sq.in and using the onion skin technique worked perfectly. I will probably add two or three tabs just to be safe, but it was successful enough to build a better box and buy a vacuum motor.

    The question now is regarding the OD motors...the website indicates that this is an "overdrive conical fan" which I know nothing about. Will this mean I need to add anything crazy to the system, or can I plumb it as one would expect? Building a housing for the motor is not an issue, I just don't know what the OD means other than what the website says. The specs look better for the 120v OD motor for sure.



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