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Scary Vacuum Fixture Mishap and Some Possible Cures

Jan 17, 2017   //   by Bob Warfield   //   Blog, Cool, FeedsSpeeds, Products, Software, Techniques  //  13 Comments

I jumped about two feet when this plate broke loose from the vacuum fixture while the pocket floor was being machined:

He had actually dialed things back quite a bit to try to keep it happy, and I would have thought the majority of the worst cutting forces were done, but the part still broke loose.  Nobody harmed, fortunately, but the part was scrapped.

Here’s a still frame of the plate in mid-air:

vacuum fixture cnc crash

That cutter grabbed the part, lifted it off the table, and had it spinning fast not long after.  In this case, the part was light enough and the cutter large enough diameter, it all held together long enough to hit the E-Stop.

A heavier part might have broken the cutter right away and gone sailing off into the enclosure.  A super heavy part might’ve sat there without spinning at all and just shifted a bit.

It’s always scary when a part gets loose from the workholding (especially big parts on CNC Lathes–oy vey!).  Enclosures and definitely eye protection are important.

So what’s the cure?

Your holding force is proportional to the amount of vacuum times the surface area it can act on.  Stronger vacuum helps, but you can only get strong to a point.  A perfect vacuum gives you 100% of your atmospheric pressure pushing down on every square inch of the part that has vacuum.  Getting a stronger vacuum is a function of having a strong enough pump and making sure leaks are minimized.

More surface area for the vacuum is ideal and can often increase holding force more than increasing the vacuum.  If the gaskets crush so the workpiece sits directly on the vacuum fixture, there’s not much surface area for vacuum to work with–just the area of the grooves.  Ideally, you want the entire under surface area of the part to have vacuum so you generate a lot more holding force.

Another answer is to limit cutting forces so they don’t exceed the amount of downforce the vacuum fixture can create.  Our G-Wizard software has a built-in mini-calculator that is aimed at doing just that.

Here’s what the Vacuum Mini-Calc looks like:

vacuum table force calculator

G-Wizard Table Vacuum Cutting Force Calculator…

The calculator is pretty simple.  First thing it wants to know is your pump type:

  • Positive Displacement
  • Regenerative Blower
  • Venturi driven by compressed air

These are the common vacuum pump types nad each one produces a little different amount of vacuum.  If you have a vac gage you can even enter the exact amount into the vacuum field.

Next, you want to enter your part’s surface area.  Easy for square parts.  For irregular parts, your CAD package can almost always calculate the area.  Note that if your vacuum fixture is aluminum or something non-permeable, you actually want the surface area of the vacuum chambers under the part.  It’s okay to estimate this information without getting too crazy, as the very next thing is a safety factor.  By default, we use a 2x safety factor on the calculator.  So whatever we calculate the hold-down force to be, we will halve that (or divide by whatever factor you give) just to give some margin for error and things like vacuum leaks.

Note the “Max Part Height” info.  This is valuable because tall parts with not very big bases are tough on vacuum fixtures.  When you machine the part that’s way up high, you gain leverage against the hold down.  Such parts are not the best idea for a vacuum fixture, and if you exceed the height, I would hump up the safety factor to be sure things stay put.

The last thing is G-Wizard Calculates the maximum spindle torque.  Here again, we make a fairly conservative assumption in these calculations.  We are protecting against ALL of that force being translated into an upward force that pulls the part off the table.

Okay, now here’s the nifty part.  Click the “Limit Spindle Power” button and you come back to G-Wizard:


Note the new torque limit, marked in red…

Note the new torque limit, marked in red.  You won’t see that red in the product, but you will see the torque limit of 112 oz-in.  Now any feed and speed you calculate will be automatically adjusted to stay within that limit so the part won’t pop off the vacuum table.

Having the ability to limit cutting forces with a safety margin can really help with vacuum fixturing!

If you’ve never tried G-Wizard, check out our free 30-day trial–it’s chock full of useful utilities for CNC’ers.  And the best part is even if you don’t purchase, you get to keep a lot of the calculators fully operational after the trial ends for free.



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Scary Vacuum Fixture Mishap and Some Possible Cures
4 (80%) 1 vote


  • That is a great G-wizard feature and a good example of the limitations of vacuum tables and appropriate tool choice. In this case however, he was done with the clearing. It appeared that he was starting a boring operation and then feed stopped and then the crash. I’m guessing he had a program error since the spindle didn’t stop but without a user comment, who knows. When something doesn’t feel right, it probably isn’t. He wasn’t pocketing though.

  • “If the gaskets crush so the workpiece sits directly on the vacuum fixture, there’s not much surface area for vacuum to work with–just the area of the grooves”

    Great article and cool new features, but I’m not sure that this statement is right. The part clamping is not achieved by vacuum acting on the bottom of the part, but as you said it is atmospheric pressure acting on the top that creates the clamp. When the work bottoms on the fixture the down clamping force doesn’t change at all, and in fact you should now get a better hold because you have contact between the work and the fixture to provide additional friction for resisting side forces. Before bottoming you are relying 100% on friction from the gasket to prevent side to side movement, this seems like it would be a far inferior situation, not to mention the height of the work will vary slightly with fluctuations in vacuum pressure.

    For what it’s worth this is just a quick comment based on intuition, I don’t even have a vacuum chuck to test it on. Do you agree or did I miss something?

  • Pretty sure the vacuum force is holding the part – there is already no atmosphere under a flat part sitting on a flat surface, but it will move quite easily!
    Likewise, vacuum can hold a plate up(working against atmospheric pressure/gravity) as well as down , which further debunks your intuition.
    So yes, you missed the fact that the vacuum is in fact applying a “force” to the part, not simply removing atmosphere from the back side. In this example, the force of the cutting tool simply exceeded the vacuum and gravity forces holding the piece down(quite scary, just like a part pulled from a vise).

    • “Likewise, vacuum can hold a plate up(working against atmospheric pressure/gravity) as well as down , which further debunks your intuition.”

      Mike, atmospheric pressure is not like gravity with a working direction – it works in all directions. The force from vacuum comes from air molecules physically hitting into the plate on the side that still has air on it, while those molecules are largely removed on the side with vacuum.

  • Alex, you can test your intuition about whether it is the surface area exposed to air or the surface area exposed to vacuum very easily. Make a vacuum fixture that consists of a flat plate with nothing but a 1/8″ hole in the center. Make sure it’s flat enough we can’t cheat!

    Now, drop a part 1″ in diameter onto that plate and measure the pull-off force. Compare that to a part 10″ in diameter, but weighing exactly the same as the smaller part.

    You’re going to find out pretty quickly you need a surface area with vacuum to generate the clamping force. Adding more air surface area that has no vacuum underneath won’t help you.

    Now, as to what really happened to this particular part–could be cutting forces exceeded the vacuum hold down, or it could be the end mill pierced the pocket floor and let air into the vacuum area. Hard to say for sure.

    Regardless, if you’re going to use vacuum fixtures, you want to maximize the vacuum area, minimize the leaks, and make sure cutting forces don’t exceed the available holding power.

    • Bob – Ok maybe I will do a test to demonstrate my point. I think it won’t matter at all how big the vacuum hole or vacuum channels are, holding force will increase linearly with surface area inside the gasket.

  • Looks like he cut through the bottom of the part, so his hold down force went to 0.

    I’m pretty sure Alex is correct. It’s the atmospheric pressure holding the part down, not the vacuum. I think the issue with too much surface of perfectly flat fixture under the part is that tiny pockets of air can get caught that will still push up on your part, but I think in reality this is not a major concern most of the time.

    In the example of a 1″ diameter part vs a 10″ diameter part, you need to define where the seal is. If your seal is at the periphery rather than at the the hole, then the 10″ part is going to have 100x the holding force. If it’s at the port, then the holding force will be the same. In the case where the seal is at the port, there is atmospheric pressure pushing up on the bottom of the part everywhere except within the seal.

    • Jonathan, no, I think you mean to say the force is NOT the same at all if the seal is at the port versus the periphery of the part. That’s exactly my point.

      If you doubt it, why then bother to ever create a vacuum fixture with more than 1 port, sealed right at the port, with the port located under an area of the part that is guaranteed not to leak? Making the area where you pump the vacuum smaller greatly reduces the chance of leaks, is much cheaper to make, and would be superior in every way.

      Except it doesn’t work that way. The holding force is proportional to the surface area under vacuum times the difference between atmospheric pressure and the vacuum pressure.

  • Looks like a programming error to me. His tool toward the end made an agressive cut through the part and broke the vacuum. Even looked to me like the end mill dug into the vacuum fixture.

  • Looked like a real A.. PUCKER moment.
    Much like the drill press moment:
    Wish I would have clamped that SOB down.

    • Yup, drill presses can get nasty in a hurry if you don’t clamp the parts down. Especially with sheet metal–spinning blades of doom!

  • P.S. Glad nobody was hurt!
    Looks like maybe the ways cover didn’t do as well.

  • Wow, I just bookmarked this as “Scariest CNC Moment”. I do think that the vacuum was lost, quite quickly to. Whether the tool broke through the bottom, or the the thinness of the floor allowed a vibration to set in and allow air in under the part, I don’t know, but I can’t see the cutting being done be enough to “pull” the part off the table.

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