3 weeks by cncdivi

Computing Feeds and Speeds accurately under all circumstances involves a significant amount of intricate math to yield satisfactory results. There’s far more depth and nuance to it than the rudimentary equations most learn, which aim to associate surface speed, chip load, RPMs, feedrates and the like.

These equations may work fine for manual machining, although superior outputs can be achieved through appropriate calculations. However, they often prove entirely inaccurate for standard CNC operations.

When I first ventured into this field, I utilized an Excel spreadsheet to keep track of everything. I would incorporate each new piece of knowledge I acquired, perhaps from an article by a tool manufacturer, into the spreadsheet.

Eventually though, this rapidly turned into a convoluted web, and I still hadn’t reached a conclusion. I eventually started to encounter problems where it was not possible to pinpoint a definitive formula to provide an answer.

These types of issues demanded the use of iterative solutions. That entails plugging in a few estimations, observing the outcome, and readjusting until the suitable variables yield the desired result. This process, known as Goal Seeking in spreadsheet terminology, can be performed to some extent within spreadsheets, but not entirely.

Hence, I created the G-Wizard Calculator, which is capable of extensive iterative solving and complex computations, processing and considering over 50 different variables to generate a result.

As a user, it’s great to have G-Wizard’s machinery working out all those 50 variables on your behalf–at least that way you don’t have to.  It’s elegant and powerful.  But, anything that involves 50 variables and non-linear iterative problem solving is going to occasionally present you with an answer that’s hard to understand.  That’s what I want to talk about today.

There are cases where GW Calculator will tell you it needs your spindle to run more slowly than it can or when it needs feedrates that are faster than your machine will deliver.  It signals these problems by turning various parameters orange (to signal you to stop and figure out why they’re orange if you don’t already know) and through various messages.

It’s important that you don’t just ignore the orange and the messages–they’re there for a reason.  For example, if you allow the chip load to fall too far below the recommended chip load for the cut, perhaps by running too low a feedrate, G-Wizard warns that your cutter will start to rub.  That’s a condition that will dull a cutter pretty quickly.

What I want to write about here is the strategies you can use to work around these roadblocks.  BTW, a lot of these problems happen because DIY CNC machines often use the parts that are available without checking too carefully whether their operating ranges match up well with the projects their owners want to tackle.

What to do if your spindle won’t run slowly enough

This is a problem I hear frequently from the CNC Router crowd.  They’re often running very high speed spindles and those spindles often can’t be slowed down to ideal cutting speeds once we move away from wood and try to cut various harder materials.  There are two good strategies you can employ to get the recommended rpms to go back up into the operating range of your spindle:

1.  Use a cutter that can take more rpms.  If you’re not already, switch from HSS to Carbide.  If you’re using Carbide, get cutters with better coatings that can run faster.  You can run TiAlN and similar modern coatings ridiculously fast.

2.  Use a smaller diameter cutter.  The smaller the cutter, the faster it must be spun for a given surface speed.  That’s why manufacturers specify a surface speed instead of an rpm in many cases–because it applies to all diameters.  So, if you can’t spin slow enough for conditions, try using a smaller diameter cutter.  In the end you can do most anything you could with a larger diameter, it just may take a little longer.

Those are the two best strategies for not being able to slow down your rpms enough.

What to do if your feedrates aren’t fast enough

In some ways, this problem of not being able to feed fast enough is related to having a spindle that won’t run slowly enough.  It’s made worse when cutting soft materials that allow large chip loads.  Therefore, this is also a problem most often seen by the CNC Router crowd.  Here is what you can do:

1.  Slow down the spindle rpm.  As mentioned, there will be limits to this, but in general, if you have maxed out the feedrate, slowing down spindle rpm can reduce the required feedrate to avoid rubbing.

2.  Reduce the number of flutes on the cutter.  Fewer flutes means you can move through the material as fast because an individual flute has nearly the same chip load regardless of the overall number of flutes.  There are exceptions in sticky materials that may benefit from fewer flutes, but ignore that for the time being.  Use a single flute cutter if you have to, but you can get the feedrates way down.  Going from a 4 flute to a single flute means 1/4 the feedrate, in fact.

3.  Try a smaller cutter.  Smaller cutters have lower chip loads and will also slow the feedrate way down.

G-Wizard will try to do all this for you

G-Wizard Calculator will try to find the best mix of parameters within the limits you set, but it can’t change those limits.  It won’t for example, suddenly change your 4 flute selection to a single flute.  But, now you know how to get around the limitations of machine, tooling, and materials using these strategies.

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