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GCode is Complicated
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Set Up G-Wizard for Small CNC Machines

Jul 29, 2016   //   by Bob Warfield   //   Beginner, Blog, CNC Router, DIY CNC, FeedsSpeeds, Products, Software, Techniques  //  8 Comments
Shapeoko

Shapeoko

I get asked all the time how to set up G-Wizard Feeds and Speeds Calculator for small machines that aren’t very rigid.  Folks know intuitively (or after finding out the hard way) that these machines often can’t handle feeds and speeds that are as aggressive as other heavier machines.

G-Wizard has a well-designed set of features that are specifically designed to cope with the limitations of small or otherwise not very rigid CNC machines.  The thing to keep in mind is that when a cutter is spinning at the correct speeds and feeds, but before that cutter gets into the material it will cut, it doesn’t know the difference between whether it’s being spun by a fancy industrial VMC or a lightweight DIY CNC router.

What then is the difference and how should we account for it?

The differences come into play when the cutter starts cutting, and they fall into three categories:

  1.  The spindle and toolholders on the lightweight machine may not be as precisely made as on the industrial VMC.  The chief measure of this is called runout, and it is basically a measure of how far from a true circle the cutter is spinning.  Whenever the axis of the cutter isn’t exactly on the axis of the spindle, you get runout.  Excessive runout is very hard on tool life.
  2. The power available in the lightweight machine will almost certainly be a lot less than the power available to the industrial CNC.  If the requirements of the cut exceed the power available, the spindle will bog down and the cutter will likely be broken.
  3. The frame of the lightweight machine is nowhere near as rigid as the big machine, and so it deflects as cutting forces are transferred into the machine frame.  This deflection or bending can cause the tool to be forced to cut too deeply, to chatter, or it may simply cause the cut to be inaccurate and of poor finish.

There are likely other differences too, such as the availability of flood coolant or how rigid the workholding is, but let’s stick to our big 3 as they matter the most to tuning Feeds and Speeds to fit your machine’s capabilities.

Compensating for Runout

In case #1, spindle and toolholder runout, we pretty much have to trust that it isn’t too bad–well under a thousandth of an inch, for example.  You can measure the runout pretty easily, if you’re curious.  If you find you are continuously breaking cutters on after the other, particularly if they are small, it is worth looking into runout.

Runout vs Tool Life

Runout versus Tool Life

Ideally, the measured runout should be less than 20-30% of the chipload for the cutter you’re using.  For example, a 1/2″ carbide endmill has a chipload allowance of 0.0143″ by default in G-Wizard.  You can read that in the “Mfg” row.  So, max runout for that cutter is something like 0.0029″.  If, however, we look at a 1/8″ cutter, the chipload becomes 0.0115″ and max runout will be 0.002″.  The smaller the cutter, the more sensitive it will be to runout.

Unless you’re just curious or have reason to believe you have a runout problem, we trust that all is well there.  I remember one afternoon breaking 5 small end mills in quick succession.  I knew I had a runout problem after the third one broken.  Numbers 4 and 5 involved swapping different ER collet chucks to see if the new one I started with was bad.  After the 5th one broke, I dug out my Dial Test Indicator to figure out what was really going on.  It turned out all the Collect Chucks were fine, but I had one brand new ER32 collet that had about three thousandths (0.003″) of runout.  That was the culprit!

Toolholders and collets are often the source of runout problems.  You can also measure the runout of your spindle taper with a DTI to check the spindle for runout.  Most inexpensive CNC Routers have a fixed collet chuck that can’t be changed.  In their case, measure the inside taper the collet rides against, and try switching collets to see if runout problems go away.  If they don’t, you have a spindle problem which is going to be beyond the skill of most beginners to fix.

Making Sure Enough Spindle Power is Available

G-Wizard automatically limits Speeds and Feeds to the spindle power limits that are set on the machine profile. However, most spindles will not make the same power at all rpms.  They will have a power curve.

SpindleVFD

Because of that, G-Wizard has the ability to vary the power limit based on rpms. That last link tells how to set the feature up, and it is very helpful.

In some cases, you may still encounter spindle stalls.  This is likely because your motor doesn’t put out quite the power that’s advertised.  Perhaps its rating is optimistic or the belt is slipping a bit.  The best thing to do is record the calculated power required for the stalled cut, and back G-Wizard’s power curve down below that number at the indicated rpms.  The ratings are usually pretty close, and you’ll quickly cover any inaccuracies in this way.

Compensating for Machine Rigidity

Industrial VMC’s weigh thousands of pounds yet the actual size parts they can machine may not be a lot larger than a mid-sized desktop CNC Router.  Clearly those thousands of pounds do something for the VMC that the little machine isn’t getting, and that something is machine rigidity.

Spindle Power (HP in Imperial and KW in metric) is a measure of how much power the machine has available to try to deflect the machine’s frame.  It doesn’t just go to deflection, it also goes to cutting, but let’s take the worst case here.  In the industrial world, the relationship between machine weight and spindle power is pretty predictable.  Here it is visually:

Machines fall within a pretty narrow zone…

G-Wizard uses this relationship, together with your machine’s work envelope (volume of its travels in X, Y, and Z) and weight to find an “ideal spindle power” that is right for the estimated rigidity of your machine.  If that adjusted spindle power is less than your spindle’s rated power, then it derates to the adjusted power.

Many hobby CNC machines actually have spindles that are monstrously powerful for their rigidity.  This happens because things like Trim Routers are relatively cheap, fairly powerful, and easily adapted for use on a small CNC.  If you use too much spindle power and it overcomes your machine’s rigidity, you’ll have problems.  That’s where G-Wizard can help by limiting the power.

We have a video that describes how to go about using the features to limit power by machine rigidity.  My recommendation is to take that video and use it to create two machine profiles.  One will be spindle power limited based on these rigidity calculations.  One will simply use your spindle’s rated power.  Reality for your machine lies somewhere between the two, but probably closer to the rigidity limited rating.

Start at the conservative end and experiment with going more to the aggressive end.  The way to do this is to start from the calculated spindle power limit with weight adjustment and increase that by 10% or so for each test you perform.  Eventually, you’ll notice an indication that you’ve hit a real limit in the machine’s capabilities.  You’ll see one or more of these symptoms:

  • Sudden reduction in accuracy of parts.  If the machine’s frame is bending too much, it’s hard for it to be accurate.
  • Lots more chatter on multiple projects.  Chatter is something we all run into sooner or later.  But if it becomes extremely common all of a sudden, too much flexing in your machine’s frame may be at fault.
  • Sudden loss of tool life.  Flex in the machine frame behaves almost exactly like runout, which we’ve mentioned is very bad for tool life.
  • Sudden loss of surface finish.  Overly wavy walls on pockets and deteriorating surface finish are also good symptoms of too much machine flex.

If you encounter any of those, drop the power limit back a notch and call it done.  You will likely discover the power limit is quite a bit less than your rated spindle power if you machine is particularly lightweight.  There’s no harm in that.  As long as you can hit the right rpm and feedrate, and you don’t use so much power that things are flexing all over the place, a small machine can cut almost anything.  You just have to be more patient.

If your symptoms from flex aren’t too bad, you could also consider keeping a couple of machine profiles around with different power limits.  The higher limits would be for more aggressive roughing, while the lower limits would be for finishing and precision work. That’s also a good strategy.

The Tortoise Hare Slider is for Temporary Fine Tuning

A lot of folks like to use G-Wizard’s Tortoise Hare slider to fine tune for their machines.  That’s fine, but the techniques described above are a better more scientific way to go about it.  The Tortoise-Hare slider is more intended to help adjust for the balance between Finish and Roughing and to facilitate adjustments to individual special case jobs that are being difficult.  What’s described above is a calibration to make your machine better at all the jobs you tackle.

Conclusion

With the right Feeds and Speeds and not too much spindle power, small CNC machines are capable of amazing results!

I want to close with some words of encouragement for small machine owners.  Like I said, you can make almost anything with these machines if you’re patient.  I have seen many fabulous projects done on them in some of the most challenging materials such as titanium.  I’ve talked with numerous owners of small machines like Sherline, Taig, Shapeoko, and Carbide3D Nomad.  Using the techniques I’ve described, they’ve found their machines become well behaved and predictable.

Are you working with a lightweight CNC machine?  Tell us your experiences in the comments below.

 

 

 

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8 Comments

  • please inform on the difference
    between
    router and milling machine

    I am confused.

    keep up .

    Michel Kun

    • Michel, practically speaking there is no difference between a CNC Router and a Milling Machine as far as G-Wizard is concerned. You can tell it in the Machine Profile which kind of machine you have, but it behaves identically with either.

      Best,

      BW

    • Michel-

      CNC Routers *are* milling machines, but the term tends to be used to describe a specific type of machine, which is more accurately called a “gantry mill.”

      Easiest way to think of the difference is that a CNC router has the cutting head attached to what looks like a gantry crane and moves it around over a stationary workpiece. Your more standard milling machines will have a stationary cutting head (or almost stationary- most will move in the Z axis, but not X or Y) with a table underneath which moves the workpiece around under the cutting tool.

      • ***********************************
        I have been triing to evaluate the
        steel cutting capability of
        small cnc milling machine
        using 4mm diam carbide end mill.

        NEEDED informations that could help me

  • This article is timely for me as I’m finally “build complete” and starting to look at fine tuning performance. My machine is medium weight at #1100 but suspect my 2.7 HP motor is slightly oversized as g-wizard gives higher speeds and feeds for curve compensation than using weight adjust. I’ll be using your suggestion on “what to look for” and running a series of test cuts to flush out my real top limit. Thanks for the article!
    Cheers

    • Rob, thanks for posting. I enjoyed visiting your site to see your router build. After watching the first cut video, I was pleased to see that you’d already addressed my first two reactions which were to protect the ballscrews from chips and add a misting system, LOL.

      That machine has a lot of potential. At some point you’ll want to experiment with the much higher performance you can get from servos. I will also note that with it running nicely, it’s the perfect machine to make a CNC conversion kit for your RF-45 mill. I enjoyed the heck out of an RF-45 I ran for many years. Perfect for steel and other hard material work that the router’s high speed spindle wouldn’t like.

      • Bob, I have had some Clearpath servos on my wish list for some time but want to get some performance data first as I may find that my steppers are sufficient wrt feed limits (I hope not! – need some more justification to buy those servos).
        For performance testing my goal is not only to find out the limits for my machine but also to create a blog post with video stepping through an approach which others can duplicate (with g-wizard having a big part of the process). It would be great if you could give some feedback on the below. So to determine how my spindle HP matches up with my machine rigidity I was considering the following test. I’m going to use a 3/8” TiAIN 2 flute end mill on aluminum, full slotting, Min RPM set to 18000 (spindle full torque limit), and then the only variable will be depth of cut as g-wizard maintains spindle speed and feed at 18000 and 155-140 through this test. I’ll start at 1/32” which results in 0.3 hp and increase depth by 1/32” each test run until I reach 0.281” which will reach spindle max 2.7 HP (1.4 HP is my adjustment for weight). Do you think this is the best way to perform this test? Is slotting the correct feature to use? This is only one test…. Would you suggest other operations?
        Thanks, Rob

        • Rob, I sent you a detailed email with a G-Wizard screen shot and idea for testing. Keep us posted on how it goes. I may also post those suggestions on the blog if there is any interest in an article.

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