Today’s Mini, Desktop, and DIY CNC Machines are capable of some truly amazing work…
Are you struggling to get the best performance from your mini CNC machine?
Maybe you built a DIY CNC Router Kit like a Shapeoko (CNCCookbook has one!) or Inventables X-Carve. Perhaps you’ve converted a mini milling machine like one of the Siegs or a Grizzly mill.
I have good news for you–you can get good results from any of these desktop cnc machines in terms of clean cuts and decent surface finish. It’s just a matter of being patient and knowing what techniques to use. This article will help you out with having the right techniques for your mini CNC machine.
Let’s get into it!
What’s Different About Mini CNC Machines? (Hint: Mostly Feeds and Speeds)
Most people think their mini cnc machine will only be able to machine the softest materials like wood and plastics. Here’s your first big surprise:
You can do metals like aluminum or even scary tough materials like titanium and stainless steel on a desktop CNC machine with one condition:
Machining difficult materials like Titanium and Stainless Steel is a scary business if you own a smaller CNC machine. Even doing aluminum versus plastics and woods can be daunting. Our article on machining aluminum for CNC Routers is one of the most popular we’ve ever done, for that reason. But here is the surprising thing:
Provided your machine will achieve the correct feeds and speeds, you can successfully machine most any material you want.
I don’t say that lightly, and I will hasten to add that it isn’t just about feeds and speeds and that getting to the correct feeds and speeds may not be enough. I should also add that successfully machining the material does not mean you’ll achieve the accuracy or surface finish of a high end Vertical Machining Center. But, you can machine the material and you can get surprisingly good results.
The answer is that despite the fact you have a small machine rather than a full VMC, you still get to use some of the same tooling as that VMC. You can pop a premium endmill into your machine and cut with it. That’s not the whole battle, but it is hugely helpful. The tool has the hard job–it’s stuck between your machine and that nasty tough material. But, with proper feeds and speeds, the tool is already feeling more up to the task. You’ve eliminated a big part of what’s different about your small machine and that VMC. Keep minimizing those differences from the cutter’s perspective, and you’ll be doing well in no time.
I don’t say that lightly, and I will hasten to add that it isn’t just about feeds and speeds and that getting to the correct feeds and speeds may not be enough. I should also add that successfully machining the material does not mean you’ll achieve the accuracy or surface finish of a high end Vertical Machining Center (VMC). But, you can machine the material and you can get surprisingly good results.
Nearly all of this perspective goes under the rubric of Feeds and Speeds. In other words, how fast are you spinning the cutter (or workpiece for a min cnc lathe) and how fast are you trying to push it through the material you’re cutting.
Given that you’re very likely a beginner with one of these machines rather than a pro who has been using industrial CNC machines for years, it’s important for you to get help with your feeds and speeds.
I don’t mean the kind of help that comes from searching in Google or asking in forums. You’re not set up to tell good advice from bad advice. Heck, I’m an expert and I can’t really tell good from bad on the forums either. On the Internet, nobody knows that you’re a dog:
Instead, you need to get yourself a good feeds and speeds calculator. And you want one that’s specially designed to deal with the needs of your mini cnc machine, like our own G-Wizard Feeds and Speeds Calculator. What’s even better about G-WIzard is that it is not only a great all-around calculator, but it also has special features that cater to the needs of cnc router users. That’s pretty unusual for a feeds and speeds calculator.
Starts With Your Mini CNC Machine’s Raw Limits
The first thing you need to do is tell your Feeds and Speeds Calculator the raw limits of your machine. In G-Wizard, you use this simple setup screen:
Simple setup needs to know the 4 major limits of your mini cnc…
The Simple Setup needs to know the 4 major limits of your mini cnc:
- Maximum Spindle RPM
- Minimum Spindle RPM
- Spindle Power
- Maximum Feedrate
All of that should be easy to obtain from the website or owner’s manual for your machine. Once you have it set up, G-Wizard will respect those limits and confine itself to giving you Feeds and Speeds that don’t exceed the limits.
Work Within the Limits
You probably noticed above when I gave the disclaimer, “Provided your machine will achieve the correct feeds and speeds.”
What that means is the following:
- You can spin your spindle slowly enough so that the correct rpm from a feeds and speeds standpoint is more than your spindle’s minimum rpm. Many cnc routers have a fairly high minimum spindle rpm. You have to slow down the rpms more and more as you choose harder and harder materials to cut. In some cases, your spindle won’t go slowly enough to cut a particularly hard material. There are ways to work around this that I’ll mention below, but sometimes the difference is just too large and you have to give up machining that material.
- You can feed fast enough to prevent the cutter from rubbing. If the cutter rubs, it can radically reduce your tool life.
That’s it. You can’t really get in trouble because your spindle won’t spin fast enough.
I mentioned there are ways to workaround these limits.
G-WIzard has a “Cheat Sheet” button. If you click it, a list of “cheats” will pop open to help you figure out how to achieve various things. It looks like this:
G-Wizard’s “Cheat Sheet”…
Experienced CNC’ers will know a lot of this, but the Cheat Sheet is very helpful to hobbyists and beginners. Note the section at the bottom that’s all about “I Want Faster RPMs (Hit lower limit).”
G-Wizard will show red on the numbers when you hit a limit, such as the lower rpm limit. To speed up the rpms on the cut, you can use the Cheat Sheet’s recommendations. For example, try a smaller diameter cutter. Note that G-Wizard also trys to give you a little perspective on “Why”. On hitting the lower rpm limit, it says, “Careful: Too many RPMs will burn the tool!” Trust me, that’s good advice. You don’t want to force a cut to go at more than the recommended RPM’s unless you know exactly what you’re doing.
Take Light Cuts But Not Too Light
All things in moderation. Our first tendency with a small machine is to take lighter cuts. I frequently hear beginners cry that they can’t believe their machine will survive the Feeds and Speeds G-Wizard recommends! The truth is, G-Wizard is fairly conservative, but you may wish to dial it back even more. The key is to figure out how to dial it back correctly. If you simply take too light a cut, you will encourage rubbing. Rubbing can also happen if your feedrate is too slow in relation to your spindle rpms.
When the tool rubs, it’s because the depth of cut is too shallow relative to the sharpness of the tool’s edge, so it plows along on top leaving a nasty surface finish, rubbing more than cutting, getting way too hot, and generally making its life much worse than a normal cut would’ve been.
If the cut isn’t deep enough relative to the cutter’s edge, you get rubbing and the tool burns up quickly…
G-Wizard’s answer is a thing we call the “Tortoise-Hare Slider:”
The Tortoise-Hare Slider…
“Full Hare” is what the tooling makers would recommend for roughing. It’s more aggressive than settings to the left, but it isn’t really hare-raising (sorry!) levels of aggression.
For now, recognize that even on a small mill, full hare may be fine. But if you want to back off, either because you’re seeing problems or because you want to start out easy, just move the slider towards the Tortoise. It will do the appropriate things to improve tool life, improve surface finish, and avoid rubbing. A special note–“Full Tortoise” is what G-Wizard determines is the slowest you should go without inducing rubbing. It will result in the finest surface finish without rubbing, and is quite handy to have.
You never have to run more aggressively than you’re comfortable with, and going full Tortoise can really calm down most cuts to more comfortable levels.
Minimize Tool Deflection
Tool deflection is just plain bad!
Tool Deflection is kind of a silent killer because most beginners don’t know about it and most professionals ignore it until it’s too late.
Tools can deflect, and when they do it’s a bad thing. We’ve often used the “bent endmill” as a logo device for this phenomenon. Tool deflection is just plain bad for a number of reasons, including:
- It is additive to chipload and runout and can break a tool in a hurry. See below for more on that.
- Ever bent the paper clip too many times until it broke? Tool deflection does the same to an endmill. Once per rpm it deflects in a particular direction. That’s a lot of bending and it will shorten the life of the tool.
- Deflection is like tapping the side of the endmill with a little hammer once a rotation. Imagine the vibration that causes. If it happens to resonate with the rest of what’s going on, you get what’s called “Chatter.” Chatter ruins surface finish and destroys tool life. If the tool doesn’t deflect, there can be no chatter.
- Deflection means inaccuracy. An endmill can’t cut accurately if it is deflected away from where the g-code expects it to be.
- Surprising side effects. I saw a case one time where deflection allowed a tiny wall to form when making a pocket that prevented coolant from getting into the cut. The cutter broke not long after.
Here’s a graph that shows the effect of runout on tool life:
Now here’s the crazy scary thing–deflection behaves identically. If your tool deflects to 50% of your allowable chipload, the tool life will only be 60% of what it should be. It takes a surprisingly small amount of deflection to get there!
OK, we know deflection is bad and why, but what can we do about it?
First thing is to be able to tell when it is a risk. Our G-Wizard Calculator will keep track of it for you and light up when it is too much:
Too much deflection shows up in Red in G-Wizard…
Too much deflection shows up in Red in G-Wizard. Whenever you see numbers in Red in G-WIzard, you need to deal with it.
There are a lot of possible solutions, but in general, you can either change the tool setup or the cutting conditions. Changing the tool setup might mean using a larger diameter tool or reducing tool stickout. Changing the cutting conditions would mean reducing Cut Width or Cut Depth so there is not enough force to create excessive deflection. G-Wizard has excellent tools to help you find the right Cut Depth and Cut Width that removes material well while minimizing tool deflection. Once you’ve used them, you’ll wonder how you ever did without.
This stuff is particularly important with smaller machines because you’ll be using smaller cutters that are more prone to deflection plus the machines are more prone to chatter. Chatter is vibration that often sounds like screeching. It’s a destructive resonant vibration that really shortens your tool life in a hurry. Because small machines are less rigid, they vibrate more easily.
For much more information on Deflection, try this video. Also, the Cheat Sheet will give you deflection fighting tips too!
Account for the Lower Rigidity of a Mini CNC Machine By Derating Your Power Curve
A really light machine has a lot of flex in its structure. Machine deflection isn’t as bad as tool deflection, but it’s not good. The thing is, deflection happens because we are applying enough force that it overcomes whatever rigidity the machine’s structure has available–so it moves. Everything moves to a degree when we apply force. Stick a Dial Test Indicator on a VMC spindle and chances are with a little leverage you can apply enough force to create a measurable deflection. It’s just much harder to deflect the VMC than a small machine.
What that means is that if we reduce the force, we can reduce the deflection. How much do we have to reduce it? It turns out we can approximate that by looking at the relationship of work envelope (roughly, the size of the machine’s travels) versus its weight. It’s an approximation, but it is a pretty good one. Check out this data:
The relationship between horsepower and weight holds up pretty well: about 600-1000lbs per HP on average…
I was pretty surprised at how well the numbers trend. There are some outliers–the knee mill and an older Cincinnati, for example, are way up there at over 1000 lbs per HP. I conclude that 600lbs per horsepower is the lower end of “professional grade” rigidity, 1000lbs per horsepower is the upper end for well-made beefy machines, and 800lbs is perhaps a happy medium.
I had plotted a horizontal mill in the data out of curiousity, but it was a major outlier.
I will speculate that the weight requirement declines at higher spindle speeds because the mass that contributes to rigidity starts to be a greater percentage of the machine’s overall mass for those machines, and hence that is perhaps a truer number. After all, the weight of the motor sitting atop the spindle isn’t contributing much to rigidity!
For pros, you might consider plotting this information when trying to choose between several different new machines to purchase for your shop. Some machines have a reputation of being better for aluminum because they’re too light weight for more demanding jobs. You should be able to see some of that from this kind of data.
You can get a sense of the performance of some very light hoobyist machines too. Consider:
– A Sieg X3 (Grizzly G0619) weighs 418lbs with a 1HP spindle = 418 lbs/HP. Not bad, but you might derate the 1HP.
– A Sieg X2 (Grizzly G8689) weighs 149lbs with 3/4 HP = 199 lbs/HP. That’s getting iffy. I would definitely derate to perhaps 3/8 or even 1/4HP if rigidity matters at all.
– A Grizzly G0720 (like the G0704 Hoss is modding?) weighs 641lbs with 2HP = 320 lbs/HP. I would again, derate this mill slightly. To get to rigidity similar to the professional machines would mean derating to 1 HP, for example.
There are even tinier mills out there. Taig says their mills weigh 65lbs. Forget horsepower, lets measure in watts for a machine like this. If 600lbs/HP is right, and 1 HP = 735W, then 1lb = 0.8W. About 52W for the Taig. Any more will start to compromise the machine’s rigidity.
I should note that after numerous conversations, the average hobby machinist doesn’t like to push their machine too hard anyway, so they may instinctively be avoiding the edge of the rigidity envelope. Still, it’s useful to know when a cut should be watched or reconsidered. It can be even more useful to derate the spindle power of a machine so that the cutting forces stay within the limits required by rigidity. This isn’t something you’d need to do all the time, but it’s very useful when you encounter rigidity issues on a light machine.
The G-Wizard Feeds and Speeds Calculator has the ability to use this kind of information to automatically derate horsepower so Feeds and Speeds fall in the “Green” region of the chart above. It’s ideal for lighter weight machines.
Here is a video that discusses both the machine rigidity adjustments and the spindle power curve adjustments in G-Wizard:
Non Feeds and Speeds Techniques for Small Machines
Everything is not about Feeds and Speeds. Sometimes other changes and techniques are necessary for success and no amount of Feeds and Speeds tweaking can fix your problems. Below are the non-Feeds and Speeds Techniques you should be familliar with.
Use Decent Cutters
Like I said above, you can use the same cutters (barring not using cutters that are too big and aggressive) in your small machine as one can use with a VMC. You should buy a decent brand of carbide endmill (we have a survey that tells what the most popular brands are). It doesn’t need the ultimate solid green unobtanium coating, but make it carbide for rigidity and toughness. A decent brand will have a little better geometry than a no-name. If you’re buying the absolute cheapest cutters around, it’s going to make your work a lot harder.
Another thing, use cutters intended for light cuts. This is not so much an issue for endmills, but when looking at indexable tooling, you want the sharpest possible inserts. In all likelihood, you may not even want indexable tooling for mini cnc mills, but it works fine on small lathes. On a light mill, forget a big (3″ is BIG for these machines) facemill with 6 inserts. Try a flycutter or a smaller facemill.
For roughing, instead of a fancy indexable endmill, try a corncob (serrated rougher). These work great on lighter machines if your machine will take one of a decent size.
Corncob roughers can be work well on small machines…
One really nice feature of these cutters is the serrations result in smaller chips, which makes it easier to keep the chips clear (see below).
Use Decent Toolholders and Keep an Eye on Runout
Cutters generally fail in one of two ways:
- If spun too fast relative to the material, they get too hot, they soften, they get dull very quickly, and they quit cutting. This causes the chipload to rocket, which leads us to:
- With too much chipload, they either can’t get rid of the chips and jam, or it just creates so much force it snaps the cutter.
I hate to see one of the poor little beasties lose its life this way. Chipload kills faster than speed, and it is so much harder to stay on top of it. Chipload is the thickness of the chips being peeled up. Accurately calculating it is hard enough, but it gets worse. You see, the actual chipload a cutter experiences is the sum of:
- Chipload due to the cut. This is the chipload we calculate in G-Wizard Calculator.
- Chipload due to runout.
- Chipload due to tool deflection.
Most machinists, especially beginners are totally focused on #1 and not even thinking about runout or deflection. That’s why you can be bogeying along, making chips like a bandit, well within recommended chiploads, and all of a sudden PING! There goes another cutter broken in half. DOH! I hate that.
Runout is somewhat relative to cutter size, if only because small cutters tolerate little chipload. See the chart above under tool deflection for more details.
For the pro crowd, a rough rule of thumb is each 0.0001″ (one ten thousandth of an inch) of runout is worth 10% less tool life. So 0.001″ of life should kill the cutter immediately, no? Well, it may very well for small cutters. I remember one day in the shop breaking 5 consecutive 1/8″ endmills due to runout. First one I dialed down the feedrate. Second one, I suspected something else. Third one, I suspected my ER collet chuck and switched. Fourth one, it broke again on a different chuck–time to put the DTI clock on it. 0.0014″ of runout–yuck! That’s an awful lot. No wonder EM’s were breaking. Turned out it was due to a brand new ER collet from a name brand. It came right out of the packaging with a runout defect.
Small machines, unfortunately, can have a fair amount of runout baked into the spindles and cheap toolholders. Take an afternoon sometime, get out your 0.0001″ DTI, get a precision pin, and start checking runout on various toolholders, collets, and so on. Clock the inside taper of your spindle to check its runout. Know what you’re dealing with. If it is too excessive, you may have to dial back chiploads to offset the problem. You may identify a few “Bad Apple” collet chucks, collets, or other toolholders you can and should toss.
Use larger diameter tooling and avoid set scew style endmill holders for anything but 1/2″ and larger diameter cutters. They have more runout.
Use Coolant to Lubricate If the Material Calls For It
Aluminum welded to cutter due to lack of lubricant!
Some materials require coolant, while some simply benefit from it. Aluminum requires coolant, at least a mist, because it has a tendency to weld itself to the cutter. Aluminum has an affinity for carbide, it seems. The coolant in this case is acting as a lubricant. Titanium and Stainless like coolant too, for different reasons. Titanium doesn’t conduct heat very well, so the heat will tend to build up. It’s hard to get it all to leave with the chips. BTW, proper feeds and speeds will maximize the likelihood heat leaves with the chips–it’s just one more reason you have to have good feeds and speeds. In the case of Stainless, good use of coolant will help reduce but not eliminate work hardening. When you cut stainless, it wants to get harder, making it even harder to cut.
So use some form of liquid coolant, even as a mist, if you’re going after tough materials with an endmill.
Aluminum is the most common thing you’ll cut that needs lubricant. CNC router users are particularly reluctant to use flood or mist coolant, but here’s a tip:
Even a spritz of WD-40 can help tremendously when cutting aluminum.
Be Paranoid About Clearing Chips!
This is something I tell people with small machines over and over again–you have to be totally paranoid about clearing chips. Let’s consider stainless steel. Each and every chip has become work hardened. Imagine taking bits of carbide and scattering them over the cut so your tool is repeatedly slamming into those little landmines. Work hardened isn’t quite that bad, but it is no picnic. Recutting chips just makes your tool’s job harder than it has to be. Whether you used an air blast, mist, or flood coolant, make sure you are successfully clearing chips as fast as they are being made. Some things to think about:
- A dribble of flood may do much more poorly than a decent air blast. If the chips sink to the bottom of a pool of flood, it takes a lot of pressure to get them back out of there.
- Aim matters a lot. Position the nozzle so it does the most good. You will quickly see it takes multiple nozzles to do the most good no matter which way you are cutting.
There’s a reason why the inside of a good VMC looks like a high pressure car wash when it’s running–they understand the importance of clearing chips!
Use Rigid Workholding
You’ve got tool deflection under control, CHECK! You’ve derated spindle power to account for machine rigidity, CHECK! There’s no more flex left to worry about, right?
The workpiece and workholding may still flex, and that’s bad. One could right a giant treatise about how to maximize workholding rigidity. Cutting features with thin walls is a special world of pain all its own where chatter is near impossible to avoid. But, there are things you can do to improve setup rigidity and they’re very important on small machines. Fundamentally, they boil down to leaving as little of the part having out in the wind unsupported as possible. This requires various measures to achieve, but here are some quick examples:
- For plate work, think about vacuum fixtures, glues, and waxes that grab as much of the bottom surface of the plate as possible.
- For vise work, figure out how to support the part properly. It may help to use blocks to support parts not being machined:
Give that tall thin plate a little extra support…
- On lathes, use a tailstock if the part sticks more than 3-4x its diameter out of the chuck.
This list could go on a long time, but you get the idea.
Use Decent CAM to Enter a Cut Properly
A lot of the worst damage to tools and tool life comes from how the cut is entered. It can be very jarring to the tool if not done properly. There are a few things to consider, but it all depends on what your CAM package can do for you. It’s important to be aware of your CAM’s capabilities and if you are evaluating CAM, be sure to check for these things.
First, when milling, entry to cut out a pocket is typically done by plunging, ramping, or helixing into the cut. The easiest thing on the cutter is helixing, followed by ramping, followed by plunging being the least desireable. It all has to do with how long the cutter making how wide a cut and how much of the wear can be spread over the maximum flute length. With plunging, only the bottom of the endmill cuts and the wear goes there all the way down the hole. With ramping, you get a full width cut for as long a distance as it takes to ramp down to depth. With a helix, you generally will open things up so the whole diameter isn’t cutting the soonest, and that’s why it is the most gentle approach. Another thing to think about is coolant access and chip clearing. Helixing, again, opens things up so you can clear chips and apply coolant that soonest.
Note that the feeds and speeds on entry are slower than the feeds and speeds once the cutter is all the way in. Your CAM software needs to allow for that. G-Wizard will help you out too. Note the little area to the right of the feeds and speeds with the dropdown:
See how the little menu gives the different entry options (Plunge, Helix, Ramp)? Just to the right is the feedrate to use for the selected entry option. Convenient, no?
One more to think about is using a twist drill to make a big enough hole to start a pocket.
Second, with work hardening materials like stainless, get down to depth as quickly as possible. A shallow ramp or helix angle will give work hardening more opportunity.
Next, make sure your CAM has different feeds and speeds for entry versus the actual cutting. Entry speeds must be slower! Plunge speeds are typically a lot slower, while Ramping and Helixing are only slightly slower. G-Wizard can figure the right Feeds and Speeds for all three, but it does no good if your CAM program won’t let you put them in. Actually, that’s not quite true, you could use G-Wizard Editor to insert them at the proper place in the gcode if needed.
There are more CAM-related entry issues to consider. If you’re going to face mill or profile an edge, you generally want to arc into the cut. Going straight in is a bad idea. Much more on this sort of thing is available in our Feeds and Speeds Course in the chapter on, “Toolpath Considerations.”
Gain Experience With Software Easier Materials
Many would like to start with aluminum or worse. It’s doable, but it’s not the best place to start. Choose something more forgiving, like wood. “Forgiving” in this case, means that the material has a larger Sweet Spot. That means a wider range of Feeds and Speeds will work. For metals, you have to be pretty dialed in. For Wood or Plastics, there is a greater margin of error.
Start with easier materials like the wood in this awesome rubber band gun project–it has a much bigger Sweet Spot than metals…
Once you’re ready to tackle something more challenging, be sure to read up on any special techniques you may need to know about. For example, aluminum is probably the most common metal you will want to try to cut. Be sure to check out our article that gives 10 tips for cutting aluminum with a CNC Router. It turns out that sometimes Feeds and Speeds are not enough to keep you out of trouble. When cutting aluminum, there are some other things you’ll need to pay attention to.
Prefer Larger Diameter Cutters
Sometimes I think small machine owners are afraid of larger cutters–they just look too scary for these little machines. It’s like they’re going to impart entirely too much violence into the workpiece. Or maybe it has to do with cost. Bigger cutters can look a lot more expensive, I suppose.
But the larger cutters have two important advantages:
- They’re much stronger
- They’re less sensitive to runout
Let’s talk about strength first. G-Wizard Calculator has a nifty Rigidity Calculator built into the Quick Reference tab. Let’s compare the strength of a 1/8″ End Mill versus a 1/4″ End Mill:
Doubling diameter resulted in 16x more strength!
Doubling the tool’s diameter results in 16x more rigidity which is a good indication it’ll be 16x harder to break. I know which one of these two I want to use for my early ham-handed experiments–I’m taking the bigger stronger tool every time.
More rigidity means less deflection, which means longer tool life. Larger diameter cutters are also less sensitive to runout which also means more tool life.
Sorry this was such a long guide, but I did promise to be comprehensive.
Your mini CNC is capable of quite a lot if you follow these techniques. Just be patient with it as it can’t work as fast as a bigger machine. That and a little lower accuracy and less nice a surface finish are really it’s only limitations.
So be patient, learn the tricks, and pretty soon you’ll be getting results you hadn’t dreamed were even possible on your small CNC. We’re here to do what we can to help. We put out the information, and we offer Free Trials on our CNC Software so you can try it for yourself and see some of the things I’ve been talking about.
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