Loose Fixtures and How to Repair

January 25, 2017 Blog

Parts can become loose or break on any kind of machine. On CNC machines, specifically, there are many parts that can loosen and go unnoticed and break off which can cause more damage. One CNC operator had the vacuum fixture shown in the article below break off. To find out how he remedied the situation read the full story and make note so if it happens to you you know what to do.

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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Source: http://blog.cnccookbook.com/2017/01/17/scary-vacuum-fixture-mishap-possible-cures/