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  1. #1
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    Rule of thumb for O Ring Vacuum Hold Down?

    Hi. I have built a nice little twin pump vacuum plant out of bits and pieces for very cheap on account there are about 8 pumps under my bench at work. I have sat 2 double pumps on an old 40L compressor tank. And will add in some control. The pumps reach a good strong vacuum and much much more than the high vollume dust extractor type I have seen being used.

    Looking at a fixture design to machine alloy. I am about to cut a groove out of a piece of alloy for my fixture, for my o-ring to sit in. I have bought about 5 sizes of O ring material from 2.5mmm up to 4mm.

    Hey so wondering what kind of stick-out the o-ring should have? Ie 3mm o-ring material, I am thinking I will machine a groove say 3.05mm wide by, at a guess, 2mm deep to give a 1mm stick-out seal. But is that too much or not enough? I have seen a sandpaper trick so perhaps 1mm is not enough.

    It would be good to hear what others have done before I start experimenting. Thanks in advance

  2. #2
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    Re: Rule of thumb for O Ring Vacuum Hold Down?

    Hi Boydage- For static vacuum on good surfaces its typical to have 20-30% crush on the o-ring. 30% crush would be a softer ring. 20% is typical for a normal hardness ring.

    O-ring suppliers have groove specs if you ask. Ensure the groove has enough volume to accept the crush so the surfaces come together with out nipping the o-ring. Peter

    O’ring Groove Design – Sealing Australia

  3. #3
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    Re: Rule of thumb for O Ring Vacuum Hold Down?

    Quote Originally Posted by boydage View Post
    Hi. I have built a nice little twin pump vacuum plant out of bits and pieces for very cheap on account there are about 8 pumps under my bench at work. I have sat 2 double pumps on an old 40L compressor tank. And will add in some control. The pumps reach a good strong vacuum and much much more than the high vollume dust extractor type I have seen being used.

    Looking at a fixture design to machine alloy. I am about to cut a groove out of a piece of alloy for my fixture, for my o-ring to sit in. I have bought about 5 sizes of O ring material from 2.5mmm up to 4mm.

    Hey so wondering what kind of stick-out the o-ring should have? Ie 3mm o-ring material, I am thinking I will machine a groove say 3.05mm wide by, at a guess, 2mm deep to give a 1mm stick-out seal. But is that too much or not enough? I have seen a sandpaper trick so perhaps 1mm is not enough.

    It would be good to hear what others have done before I start experimenting. Thanks in advance
    It really depends on how flat the surface you are wanting to pull down with the vacuum, if it is a flat surface then you would not want more than .020" or 0.5mm and a soft rubber is best a lot use closed cell foam this works very well for large surfaces, you don't have to go by Oring spec's for how much you have out of the grove as long as the material you are using has room in the grove to be crushed into, you can even use tubing and super glue the ends together, vacuum pods in general have very little for crush, more does not mean it will have a better seal, my grinding vacuum fixtures have no seals at all just micro size holes in the vacuum surface

    Here is a good example of not using any seals the whole vacuum table is the seal by using the foam board. https://www.instructables.com/Simple-CNC-Vacuum-Table/

    The snip is showing the closed cell foam for the seal this is very flexible and seals very well
    Attached Thumbnails Attached Thumbnails Vacuum Pod Seal.jpg  
    Mactec54

  4. #4
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    Re: Rule of thumb for O Ring Vacuum Hold Down?

    Quote Originally Posted by boydage View Post
    Hi. I have built a nice little twin pump vacuum plant out of bits and pieces for very cheap on account there are about 8 pumps under my bench at work. I have sat 2 double pumps on an old 40L compressor tank. And will add in some control. The pumps reach a good strong vacuum and much much more than the high vollume dust extractor type I have seen being used.

    Looking at a fixture design to machine alloy. I am about to cut a groove out of a piece of alloy for my fixture, for my o-ring to sit in. I have bought about 5 sizes of O ring material from 2.5mmm up to 4mm.

    Hey so wondering what kind of stick-out the o-ring should have? Ie 3mm o-ring material, I am thinking I will machine a groove say 3.05mm wide by, at a guess, 2mm deep to give a 1mm stick-out seal. But is that too much or not enough? I have seen a sandpaper trick so perhaps 1mm is not enough.

    It would be good to hear what others have done before I start experimenting. Thanks in advance
    Well it depends on a variety of conditions:
    1) It verymuch depends on the surface finish you plan to hold - wood with rough grain, plained, smooth plastics etc.
    2) The O ring material.
    3) Cutting force exerted by the cutter on the material.
    4) Precision of cut.
    5) How low does your vacuum pumps go.
    6) how fast you can replenish / restore any lost negative pressure or vacuum.

    The above all contribute for the following reasons:
    1) a rough finish tends to leak more so you would need to replenish lost vacuum quickly.
    2) a hard o ring will seal less that a softer one - (depends what material type used). If it is soft you need more interfearance with the sealing surface, a shallower and wider seating grove.
    3) If using a soft seal you are likely to have lesser cut precision under movement caused by the cutting force.
    4) The lower your vacuum pumps go the better the chance of holding your work piece in position and so more precise.

    So it is very much a give and take situation. If you plan to do repetitive work you might consider either doing a nesting or even maybe pegs at the perifery of your workpiece to help assisting with the vacuum holding.
    Martin G

  5. #5
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    Re: Rule of thumb for O Ring Vacuum Hold Down?

    Thanks for the information. I knew there was a lot to it and the reason I have not built a full size vacuum table on a guess. I will experiment with a few sizes and styles and report back. Timber and timber style panels are no problem its more aluminum that I know needs to be held tight - especially when I see the times its moved out of my vice with what I thought was a good grip. I have built a great little vacuum plant just need to put it into action.

    I do plan on building some vacuum fixtures for timber. Thanks guys

  6. #6
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    Re: Rule of thumb for O Ring Vacuum Hold Down?

    Quote Originally Posted by boydage View Post
    Thanks for the information. I knew there was a lot to it and the reason I have not built a full size vacuum table on a guess. I will experiment with a few sizes and styles and report back. Timber and timber style panels are no problem its more aluminum that I know needs to be held tight - especially when I see the times its moved out of my vice with what I thought was a good grip. I have built a great little vacuum plant just need to put it into action.

    I do plan on building some vacuum fixtures for timber. Thanks guys
    Even using removeable rails or dowel pins to locate a part can help to stop the part from moving, yes you want to use Orings for metal parts
    Mactec54

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    Re: Rule of thumb for O Ring Vacuum Hold Down?

    Out of interest last night I made a small fixture with acrylic 10mm sheet laminated together with a cavity in the centre. I also put some standoff pads inside to stop it wanting to collapse. With a 3mm ball endmill I traced around the inside of the part I need to cut out with a clearance from the edge. The slot I engraved out is 3.15mm wide and 2mm deep. A length of 3mm oring material fits in the slot really nicely with a 1mm stickout. Not having any 401 loctite (darn it) what I did was cut the oring on a very gradule angle, lay that down with the other end the same overlapping.- It actually sealed which was cool. Rubber likes being squished lol

    I also bored in 3 x 10mm holes for alignment pegs.

    The part I need to cut is a small medical theatre positioning paddle which will slot onto a piece of SS rod. It needs to go into a stereliser. The original part was made from acrylic plastic and the 138deg stereliser has destroyed it into a broken windscreen type of breakdown. I am going to replace it with Teflon. The Teflon holds down very well up and down but slides if I push hard. I am cutting around the outside on a contour cut so the locating pins cant be in there during machining. I might try the sandpaper trick. Or I was wondering if a thin piece of scotchbrite might slow the slide down as long as it doenst stop the seal. Regardless it actually took a reasonable push to break the friction and move it. I will machine conservatively.

    Next step I will try some alloy sheet.

    Have also attached a pic of my little home built Vac plant. I have about 7 of these pumps under my bench here in NZ if anyone wants one. You will only need to change the piston seals to make them like new.

  8. #8
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    Re: Rule of thumb for O Ring Vacuum Hold Down?

    Hi Boydage - what vacuum pressure does the pump hold? and why build a pump when they are so cheap? Peter

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    Re: Rule of thumb for O Ring Vacuum Hold Down?

    Quote Originally Posted by peteeng View Post
    Hi Boydage - what vacuum pressure does the pump hold? and why build a pump when they are so cheap? Peter
    These pumps came out of a range of endoscopy storage cabinets. They were replaced because the cabinets are very fussy with pressure and flow requirements. Sorry, I didn't build the pumps but did install two of them onto a compressor tank so built the plant I guess is a better description. I still need to build a control box for them though.

    In good order they will pump down to approx -90kpa which is quite reasonable. The other pumps I have under my bench are not as good and need new piston seals. If I was to rebuild one I would be looking at building a whole new piston out of acetyl or similar as the original piston design is pretty poor made out of cast alloy.

  10. #10
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    Re: Rule of thumb for O Ring Vacuum Hold Down?

    Quote Originally Posted by boydage View Post
    Out of interest last night I made a small fixture with acrylic 10mm sheet laminated together with a cavity in the centre. I also put some standoff pads inside to stop it wanting to collapse. With a 3mm ball endmill I traced around the inside of the part I need to cut out with a clearance from the edge. The slot I engraved out is 3.15mm wide and 2mm deep. A length of 3mm oring material fits in the slot really nicely with a 1mm stickout. Not having any 401 loctite (darn it) what I did was cut the oring on a very gradule angle, lay that down with the other end the same overlapping.- It actually sealed which was cool. Rubber likes being squished lol

    I also bored in 3 x 10mm holes for alignment pegs.

    The part I need to cut is a small medical theatre positioning paddle which will slot onto a piece of SS rod. It needs to go into a stereliser. The original part was made from acrylic plastic and the 138deg stereliser has destroyed it into a broken windscreen type of breakdown. I am going to replace it with Teflon. The Teflon holds down very well up and down but slides if I push hard. I am cutting around the outside on a contour cut so the locating pins cant be in there during machining. I might try the sandpaper trick. Or I was wondering if a thin piece of scotchbrite might slow the slide down as long as it doenst stop the seal. Regardless it actually took a reasonable push to break the friction and move it. I will machine conservatively.

    Next step I will try some alloy sheet.

    Have also attached a pic of my little home built Vac plant. I have about 7 of these pumps under my bench here in NZ if anyone wants one. You will only need to change the piston seals to make them like new.
    You can use the pins to keep it in place, you just have them down below half the material thickness just to hold the blank, you need 2 sets of holes when you do this (1) for the blank and (1) for the finished size, so you do the first operation and then flip the part and do the second operation.

    Super Glue works to stick the Orings together if you have to, angle cuts work well too.
    Mactec54

  11. #11

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    Re: Rule of thumb for O Ring Vacuum Hold Down?

    Great information as always. I am getting ready to machine my first vacuum fixture from aluminum, so this discussion is both timely and helpful. During my research I found some of the Pierson Workholding videos relevant. Not sure if you have visited the channel - this video was particularly useful to my planning - https://www.youtube.com/watch?v=DarGtuxrbUA

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    Re: Rule of thumb for O Ring Vacuum Hold Down?

    I made the dumbest mistake last night setting my WCS on the top of the temp VAC fixture and not allowing for the fact the oring holds the workpiece up a small amount. This made the slot in the piece I cutout too thin. Eeeh - lucky I purchased enough Teflon to make 2 pcs. The other item was setting the WCS on the rear far corner of the fixture, confused Fusion into not letting me cut a chamfer which really annoyed me. That empty toolpath thing sucks. And no matter what I did I could not get the chamfer to work - it was happy to cut into one edge only. So I ended up probing the corner of the workpiece setting it to G55

    Anyways. The other item that I am not 100% about was the larger surface of one side of the workpiece appeared to pull down more than the other. That will be the mm2 compared to the different length of oring I guess. I have other stuff in my head so wont think about it anymore ha

    Anyways. It worked. I need to put a controller on my VAC plant so its not running 100% of the time.

    This little piece (out of interest) was quoted by a company to repair at $580 they then passed it to me to repair it - the teflon cost $13. Thats medical for you. The original piece out of acrylic failed because it couldn't handle the 138deg a stereliser pushes into items. So we made it out of teflon. Madness. I reckon, that us as a world society need to train hospital surgeons. Train SO MANY of them, that they have to apply competitively for jobs. That they cant ask for $400k+ per year. This will make them think before they demand overly expensive medical positioning equipment. Making the whole surgery process for people much much cheaper. Sorry. Tangent. Soapbox. And off topic.

  13. #13
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    Re: Rule of thumb for O Ring Vacuum Hold Down?

    Quote Originally Posted by boydage View Post
    I made the dumbest mistake last night setting my WCS on the top of the temp VAC fixture and not allowing for the fact the oring holds the workpiece up a small amount. This made the slot in the piece I cutout too thin. Eeeh - lucky I purchased enough Teflon to make 2 pcs. The other item was setting the WCS on the rear far corner of the fixture, confused Fusion into not letting me cut a chamfer which really annoyed me. That empty toolpath thing sucks. And no matter what I did I could not get the chamfer to work - it was happy to cut into one edge only. So I ended up probing the corner of the workpiece setting it to G55

    Anyways. The other item that I am not 100% about was the larger surface of one side of the workpiece appeared to pull down more than the other. That will be the mm2 compared to the different length of oring I guess. I have other stuff in my head so wont think about it anymore ha

    Anyways. It worked. I need to put a controller on my VAC plant so its not running 100% of the time.

    This little piece (out of interest) was quoted by a company to repair at $580 they then passed it to me to repair it - the teflon cost $13. Thats medical for you. The original piece out of acrylic failed because it couldn't handle the 138deg a stereliser pushes into items. So we made it out of teflon. Madness. I reckon, that us as a world society need to train hospital surgeons. Train SO MANY of them, that they have to apply competitively for jobs. That they cant ask for $400k+ per year. This will make them think before they demand overly expensive medical positioning equipment. Making the whole surgery process for people much much cheaper. Sorry. Tangent. Soapbox. And off topic.
    You want the work piece to pull all the way down to the Vacuum base surface, so it sounds like you have too much Oring above the Vacuum table, or not enough room for the Oring to compress into the grove, the Oring grove should have a square bottom not round and the correct Oring grooving cutter will put like a dovetail cut around the Oring pocket here is a snip of what an Oring Grooving Cutter looks like
    Attached Thumbnails Attached Thumbnails Oring Grooving Tool.jpg  
    Mactec54

  14. #14

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    Re: Rule of thumb for O Ring Vacuum Hold Down?

    I had never seen that type of end mill before, but I can see how it helps the O ring or round gasket material spread out below the surface - I imagine that helps with proper compression as well as avoiding pinching at the mating surfaces. From what I have read the geometry also helps to keep the O ring securely in place, which might be an added benefit for vacuum fixtures. I watched a few videos showing them milling their slot. Am I correct in that they require a hole to be drilled first to allow them to lower to the correct Z depth before they start milling the slot? I think I saw that in a few videos.

    Originally I was going to purchase the one from Pierson - see below - but perhaps I may give the O ring cutter a try as well. Pierson sells them for 1/16" and 1/8" gaskets, and these cut a square bottom and chamfer at the same time. I purchased extra aluminum stock to practice on both for feeds and speeds but also to experiment with the correct DOC. I found some resources for O ring groove design/sizing that are informative, but geared more for industry users and applications, but still interesting to read through.
    Attached Thumbnails Attached Thumbnails SV2-Endmill-double.jpg  

  15. #15
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    Re: Rule of thumb for O Ring Vacuum Hold Down?

    Quote Originally Posted by TMToronto View Post
    I had never seen that type of end mill before, but I can see how it helps the O ring or round gasket material spread out below the surface - I imagine that helps with proper compression as well as avoiding pinching at the mating surfaces. From what I have read the geometry also helps to keep the O ring securely in place, which might be an added benefit for vacuum fixtures. I watched a few videos showing them milling their slot. Am I correct in that they require a hole to be drilled first to allow them to lower to the correct Z depth before they start milling the slot? I think I saw that in a few videos.

    Originally I was going to purchase the one from Pierson - see below - but perhaps I may give the O ring cutter a try as well. Pierson sells them for 1/16" and 1/8" gaskets, and these cut a square bottom and chamfer at the same time. I purchased extra aluminum stock to practice on both for feeds and speeds but also to experiment with the correct DOC. I found some resources for O ring groove design/sizing that are informative, but geared more for industry users and applications, but still interesting to read through.
    There are many suppliers here are (2)

    I normally mill the slot then use the Oring Cutter to finish the pocket, AB Tools have them without having to do a hole first

    Harvey Tool
    AB Tools
    https://abtoolsinc.com/prod/shear-hogs/
    https://www.harveytool.com/?creative...Harvey%20Major
    Mactec54

  16. #16

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    Re: Rule of thumb for O Ring Vacuum Hold Down?

    I looked at the AB Tools 'without hole drop' version. If I understand it correctly, I could do what you suggest and choose a slot milling toolpath first (with a width that matches the major diameter of the O ring end mill), and then I am guessing there needs to be two more passes made at depth that will cut the angle on each side of the slot. Is this correct?

  17. #17
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    Re: Rule of thumb for O Ring Vacuum Hold Down?

    Quote Originally Posted by TMToronto View Post
    I looked at the AB Tools 'without hole drop' version. If I understand it correctly, I could do what you suggest and choose a slot milling toolpath first (with a width that matches the major diameter of the O ring end mill), and then I am guessing there needs to be two more passes made at depth that will cut the angle on each side of the slot. Is this correct?
    Yes, that is correct just make sure you move the cutter back to center before you move Z axis up, a cheaper option you can also use small dovetail cutter but won't have the radius corners, they work quite well for not so important Oring grooves.
    Attached Thumbnails Attached Thumbnails Carbide Inverted Burr.png  
    Mactec54

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    Re: Rule of thumb for O Ring Vacuum Hold Down?

    Thank you for those tips and options.

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