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2 Tools for Optimizing CNC Milling Cut Depth and Cut Width/Stepover

Nov 8, 2016   //   by Bob Warfield   //   Beginner, Blog, Cool, FeedsSpeeds, Manufacturing, Products, Software, Techniques  //  3 Comments

higher-mathematicsThe depth of cut and width of cut (also called stepover) are crucial variables when milling pockets, profiling, facing, and any other machining operation where you’ll be cutting with the flutes on the side of an endmill.  Aside from the machine’s capabilities, the material, and a description of the tool to be used, they’re the most important variables for determining feeds and speeds.

In conjunction with your feeds and speeds they can determine your Material Removal Rate, which in turn will determine how long it will take to complete the operation. Given a material removal rate and the machinability of the material, we know the spindle power required for the operation.  Turning that back around we can compute the potential for deflection of the tool.  Too much deflection ruins tool life, makes for inaccurate cuts, and is also bad for surface finish.  Balancing the cutting forces to avoid excess deflection is often best done by optimizing Cut Depth and Cut Width combinations, rather than by simply reducing the feedrate or rpms used on the cut.

Given all that, one might say that your Cut Depth and Cut Width are going to determine just about everything of interest in terms of what you want to get out of the cut–how long it will take, how it will impact tool life, and the surface finish.  Cut Depth and Cut Width are very important variables indeed!

Yet, most machinists do not have analytical tools that help them to decide on the best Cut Depth and Cut Width.  Instead, they rely on guesswork, rules of thumb, and what has worked reasonably well in the past.  Rules of thumb can be useful memory aids when they’re used to refer to simple phenomena that are easily explained.  But, with Cut Depths and Cut Widths, there are so many complex phenomena that interact with each other that rules of thumb are useful.  Worse, they can conflict.

rules of thumb

Are your rules of thumb at odds with your productivity?

Here are just a few of the factors that Cut Depth and Cut Width can impact:

  • Material Removal Rates.  Of course, all other things being equal, we may be able to remove the most material by burying the cutter at maximum depth and width.  Of course, all things are never equal and this strategy usually doesn’t work well at all.
  • The ability of the cutter to clear chips.  Exposing more of the cutter through shallower cut widths makes chip clearing easy.  Forcing the cutter into narrow slows with wide cut widths and making the slots very deep relative to cutter diameter makes it hard to clear chips and so you’re probably recutting chips and seriously reducing tool life.  In the worst case, the cutter can break very quickly.
  • Heat.  With proper feeds and speeds, most of the heat goes out in the chip.   But even so, heat can build up on the cutter and if left unchecked, it will kill that cutter quickly.  The time a flute spends in the cut is time when heat is building.  The time out of the cut is time it is exposed to air and coolant–in other words, it is cooling time.  Trade offs of MRR’s versus cooling “duty cycles” are a big part of calculating the best HSM toolpath feeds and speeds.  Maximizing that is one of the reasons HSM toolpaths are so productive.
  • Power Requirements.  The higher the Material Removal Rates, the more spindle power will be required.
  • Tool Deflection.  The more power we pump into the cut via more spindle power, the higher the cutting forces, and the greater the tendency of the tool to deflection.
  • Wear:  If you can spread the wear along more of the flute length for a given amount of material removed, your tool will obviously last a lot longer.  But, this is traded off against the greater likelihood of Tool Deflection, which can also be bad for wear.

There are actually many more such factors, and they’re all inter-related.  Choosing the best Cut Width and Cut Depth are a function of optimizing a lot of trade offs!

One can do systematic experiments given time, but there are so many variables involved that any combination of machine, material, and tool requires a completely new set of experiments to be done.  A big matrix of Cut Depths and Cut Widths must be assembled and Feeds and Speeds together with MRR’s computed.  Very few have time to do all that, so they fall back on what’s worked in the past, which may not be even close to optimal in the present.

That is, unless you have our G-Wizard Calculator Software.  GW Calculator has not one but two tools for optimizing Cut Depth and Cut Width.  In fact, since it can calculate Scallop Heights for 3D profiling with ballnosed endmills, it actually has three tools, but we want to focus on the two.

Defining Cut Width and Cut Depth

Let’s start by defining the terms “Cut Depth” and “Cut Width” (also called “Stepover”).  Since a picture is worth a thousand words:

Cut Width Depth Stepover

Cut Depth is the length of the side of the flutes that are actually cutting.  So, if you’re doing a pocket in several steps or layers, it is the thickness of the current layer you’re cutting and not the total depth of the pocket.

Cut Width, also called Stepover, is the total thickness of the cut when the endmill is viewed from directly above.  It can be no more than 100% of the cutter’s diameter.

Optimizing Cut Depth and Cut Width Using G-Wizard’s Cut Optimizer

We introduced the Cut Optimizer some time ago and it has been an extremely popular part of G-Wizard. To this day, only G-Wizard has a Cut Optimizer.

The Cut Optimizer is easy to use, and it lets you optimize one of the two variables (Cut Depth or Cut Width) while you hold the other one constant.   For example, suppose you need to machine a slot that is 1″ deep with a cutter that is the diameter of the slot.  You will be holding Cut Width constant–it’s the diameter of the cutter.  But Cut Depth is the variable to be optimized.  What the Cut Optimizer does is to calculate the deepest cut that can be taken without creating excessive tool deflection.

Or take an opposite example.  You’re profiling a wall and you’d like to do it in a single pass instead of having to step down for multiple passes.  You want to do this because the wall finish will look prettier without having the steps visible and because it spreads the wear over a longer flute length, which means your tool will last longer.  In that case, Cut Depth is the constant and the Cut Width is what you’d like to optimize.

Cut Optimizer is good with both kinds of problem and we show you exactly how it works in our latest G-Wizard University Video:

Cut Optimizer in action…

G-Wizard University provides short videos on specific topics that make it easier to learn our G-Wizard Software.

Optimizing the Depth or Width to keep Tool Deflection under control is super easy.  Just click the title of the one you want G-Wizard’s Cut Optimizer to optimize and it’s done:

Just click the label of the variable to optimize (Depth or Width) and it’s done!

How Much Deflection Should You Allow When Optimizing Cut Depth and Width?

MillDeflect

How much tool deflection should we allow?

This is a complex topic, and I’ll refer you to a dedicated article for more information.  Suffice it to say we allow more for roughing than finishing because too much deflection is bad when you want a good surface finish or need to hold tight tolerances.  Therefore, finish passes can tolerate less.

For roughing, deflection is roughly equivalent to runout as far as tool life is concerned.  Even a little (as a % of max chipload), can really shorten tool life.  Thinking of your endmill as the paperclip that got too many times, and with deflection it is getting bent at whatever your spindle rpm is as times per minute.  Worse, deflection excites chatter like crazy.

G-Wizard’s default deflection limits are pretty conservative, and are aimed at minimizing chatter.  But, you can set them to whatever you’d like.

Optimizing Cut Depth and Cut Width Using CADCAM Wizards

cadcamwiz

The CADCAM Wizard for finding optimum parameters for pocketing…

Cut Optimizer was a great advancement–finally a tool that made determining the best Cut Depth or Cut Width a science.  But the CADCAM Wizards are even more powerful and strangely enough, simpler to use.  CADCAM Wizards are a feature included with every copy of G-Wizard Calculator.  They’re accessed by clicking the CADCAM tab.  Imagine having an assistant who was really good at figuring out “Cut Recipes.”  A Cut Recipe is everything you need to know in order to tell the CAM software how to generate a toolpath to cut a particular feature:

–  Feeds and Speeds–naturally!

–  Which tool to use

–  Cut Depth and Cut Width

Moreover, you want the recipe to address both roughing and finishing without you having to tell your assistant anything additional.  Lastly, you want this assistant to ask you as few questions as possible.  After all, your time is valuable.  Let the assistant worry about the details.

That’s exactly what CADCAM Wizards do.  It’s easier to watch them in action than to try to explain further, so check out another new G-Wizard University video for the details:

CADCAM Wizards:  Let your smart assistant figure it out…

CADCAM Wizards really put some amazing power at your fingertips.  Like I said in the video, when was the last time you got a chance to run 1100 different scenarios to find the best combination of parameters for a job?  I will never have enough time to do that even once.  Yet as a customer said to me, G-Wizard makes it possible to optimize every cut.  It even makes it easy to do so.

What About Stepovers for 3D Toolpaths?

The astute reader will see that a lot of what’s been said above is great for 2 1/2D machine, but may not apply to 3D Toolpaths.  It’s true, and that’s a whole new area.  We’ve got a great article on selecting the best stepovers for 3D profiling that talks through the theory.  But check it out:

G-Wizard Calculator has a CADCAM Wizard that can optimize stepover for 3D profiling toolpaths using all those same principles.

Conclusion

Both of these tools are unique to G-Wizard, so if you’re looking to make Cut Depth and Cut Width a little more scientific (and you know you really ought to), you’ve found the right place.  Put them to work for you, they’re quick and easy, and before you know it you’ll find your cycle times are shorter and your tools last longer.  It’s almost as good as sliced bread.  Well, not quite, but close.  If you’ve never tried G-Wizard, be sure to check out our free 30-day trial.

 

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2 Tools for Optimizing CNC Milling Cut Depth and Cut Width/Stepover
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