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Do you use a vacuum table or vacuum fixtures?

If yes, then I know you’ve scrapped more than one part because it popped off the table due to too much cutting force breaking the vacuum.  Wouldn’t it be great to have a quick and easy way to stop that from happening?

I get all my best ideas for new product features from hearing customers describe their problems.  In this case I was chatting with a gentleman who was afraid he wouldn’t be able to use the full feeds and speeds our G-Wizard Calculator was suggesting because it would pop the parts of the vacuum table on his CNC Router.  This is not the first time I’d heard someone complain about the problem.  In fact, my own brother runs a Multicam CNC Router and complains about it all the time.  But the human mind being what it is, this was the first time I heard it as expressed as a G-Wizard Calculator Problem and so it was the first time I tried to think of it in terms of a G-Wizard Calculator Solution.  It turns out the problem is tailor-made for an elegant and easy solution–why not have G-Wizard simply avoid generating feeds and speeds that will result in cutting forces that are too strong for the vacuum’s holding forces?

BINGO!  The light bulb started burning very brightly above my head, and here’s what I came up with.

After you download and install version 3.32 of G-Wizard Calculator, it comes up and you’ll see there’s a new Mini-Calc button labeled “Vacuum”.  I’ve put a little red rectangle over it in this screen shot to make it easy to pick out:

Press the Vacuum Mini-Calc button to get the new calculator for vacuum fixtures…

G-Wizard incorporates a whole array of unique special-purpose Mini-Calculators to help you figure out such problems.  The others include:

•  Surface Finish:  Helps calculate stepover to achieve ballnose scallops according to your surface finish requirements.
• Interpolate:  Figure feeds and speeds to accurately interpolate a hole, or maybe just keep your tool life happy on tough materials.
• Ramp:  Same as Interpolate but intended for ramping entries to the feature.
• Plunge:  Calculate feeds and speeds for plunge roughing toolpaths

The new Vacuum Mini-Calc is made to help with the part pop-off problem and here is what comes up when you press that button:

The Vacuum Fixture Mini-Calc…

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.  The little part we used for this demo was 4 square inches, so it was 2 inches by 2 inches, let’s say.  What if we just tried to cut it out at full speed with a 1/2″ endmill?

Well, here’s an idea of what could happen:

With no torque limit, our example shows 164 oz-in of torque–that’s too much!

With no torque limit, our example shows 164 oz-in of torque–that’s too much!  You’re probably going to have trouble with those parts unless everything is working just right.  But with the torque limit turned on, G-Wizard scales back to 227 IPM on the feedrate and the cut can happen within the limit, which is within the safety margin.  All should be well!

Accounting for variability?  Inevitably, there are some variables involved that will be different from one job to the next.  For example, in every use of a vacuum fixture, there will be some leakage of the vacuum.  You may or may not be able to account for that.  If the leakage is extreme, it may exceed the ability of your vacuum pump to keep up or overcome it.  Your part may be warped and may not get a good seal everywhere.  How do we account for these kinds of variances in the calculator?

Here are some thoughts.  First, these variables are one reason why it is important to use a conservative safety factor.  If you are consistently still getting pop off, up the safety factor.  Second, you can directly change the torque limit to fine tune a particular job until parts quit popping off. Lastly, if it’s really important to a big job, you could directly measure the pop off force.  I would connect a line to the part on the vacuum fixture, run it over a pulley and add weight to a bucket at the other end until the part pops off.  Now go weigh the bucket with the weight and you know exactly what it takes in terms of force to make the part pop off.  Given that information, tweak the safety factor on the vacuum calculator until you get to that hold down force.  Add some more safety factor on top of that and you’ve now calibrated the system for your exact job.

This new vacuum fixture capability is just one of the many features designed to help G-Wizard Calculator save you time and frustration so you can be a better CNC’er.  Why not check out our free 30-day trial and see what it can do for you?

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Quit Scrapping Parts Because They Pop Off Your Vacuum Table
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