Our team doesn’t have a CNC mill. I have been proposing to get one for many years but our mechanical mentor who prefers to mill manually told me that CNC doesn’t buy you much. He said, you still have to watch the job so you can’t go away and let it run. He also said you have to get into CNC programming which is quite time consuming so unless you are milling several copies of the same thing, the overhead of doing CNC milling is far greater than the benefits if you are making one of a kind part.
I admit I am a software/electrical mentor who doesn’t know much about CNC mills but I imagine if you are going to CAD the robot anyway, isn’t it as simple as telling Inventor to generate a CNC routing file that will control the CNC mill similar to 3D printing? I can’t imagine why we need to get into the CNC code. Regarding the argument about having to watch the job’s progress, it may be true if a cheaper mill doesn’t have an automatic coolant recycle system so you need to keep checking the temperature and may have to manual squirt cutting fluid at the bit. So the question is: am I understanding this correctly or is the mechanical mentor correct on his arguments?
Your colleague is actually correct for this specific application. Many FRC robots rarely ever need complex parts that require the intricate toolpaths and controls that only a CNC mill can accomplish. Staying simple with your design to a scope that requires you to only use your manual machines and likely an in-house or out of house waterjet/router will take you very far. There are a handful of teams that I can point out that do just this who have played on Einstein and in championship division finals in the past few years.
Then what’s the difference between CNC mill and CNC router aside from the fact that they are different sizes. There is another thread that said everyone should have a CNC router. You also seem to say a waterjet/router is very useful. How does a waterjet compare to a CNC mill?
Reality lays somewhere in the middle.
The short answer is that CNC machining does have an overhead and it absolutely can be worth it.
The somewhat longer answer(s)
isn’t it as simple as telling Inventor to generate a CNC routing file that will control the CNC mill similar to 3D printing?
It isn’t. While modern Computer Aided Machining software is really good and can do a lot of work for you, it is still encumbant upon the CAM user to make a lot of decisions regarding what tools to use, what the stock looks like, how the work will be held, what order to do operations in, what operations to use, the cutting parameters and so on. CNC programming is an entire career and it requires a solid understanding of both programming and machining.
That said, tools like HSMworks and MasterCAM make programming 3 and 4 axis work fairly easy. It’s certainly nothing a competent manual machinist can’t work out with a few weeks of effort. Once you’ve worked out the mechanics of the programming and the specifics of the machine you’re programming the time to generate usable code can be quite short; perhaps an half an hour for a modestly complex part. A lot of older machinists think of CNC programming as typing out each command line by line, and that is a time consuming process, but that’s hardly ever done anymore.
Regarding the argument about having to watch the job’s progress, it may be true if a cheaper mill doesn’t have an automatic coolant recycle system so you need to keep checking the temperature and may have to manual squirt cutting fluid at the bit
This is another case of the truth being in the middle. The concern for me when running new code isn’t necessarily coolant recycling, but that the machine might crash. That is to say that the machine can be programmed in such a way that stuff can break, and these machines are powerful enough to really break stuff. There are also just often optimizations I realize I can make as the program runs the first time.
After a success run or two it’s not unreasonable to start a program and let it run. Whether I let a tool run ‘dark’ is dependent on a lot of factors, but at the very least I can usually walk away from the machine for a bit to do something else.
So the question is: am I understanding this correctly or is the mechanical mentor correct on his arguments?
To come back to this having some context; he’s not wrong. CNC machining requires a knowledge base and tooling beyond the mill. It’s not like a 3d printer that you just unbox and go to town with (and I’d contend very few 3d printers are like that either). If you don’t have someone already present who can tackle all that the learning curve can be steep and you won’t be able to take advantage of the capabilities that your new mill offers.
To put it in perspective, going in with only a casual understanding of CNC stuff, it took me about a year of regular use with a CNC mill and CAM before I felt like I wasn’t constantly screwing up and scrapping parts and breaking tools. Years later and I only just now feel like I’m a genuinely competent programmer and operator.
But those aforementioned capabilities are real, and they extend beyond just making the same part twice or making a part without an operator present. There are things that you can do on a CNC mill that simply aren’t reasonable on a manual mill, and can be done with speeds that just can’t be achieved cranking dials.
If you wanted to explore CNC machining, I’d look at a CNC router before a mill. Because the parts you’ll produce on one are generally flat, most of the programming will be 2.5d and relatively simple to produce and understand. The work holding is simpler, the consequences of a crash are generally milder and its a somewhat more FRC applicable tool (though CNC mills are totally useful in FRC).
From my teams experience, learning how to operate a CNC mill has been instrumental to our success. Learning how to generate G-code to run on a CNC mill is a very useful skill to develop and use in the future. Some of our graduates have used this knowledge in their College classes/clubs developing a passion for machining.
I will also argue that a CNC mill can make many things that a manual mill could never make in a reasonable amount of time. Things such as accurate hole patterns, consistent press fits, and accurate center-center distances for chain/belts.
With practice, generating these sequences can be substantially faster than manual machining for intricate geometry.
There are many more talented and experienced machinists on this forum than I, but I’ve had some experience working with both and have appreciation for the strengths of each.
The manual mill, particularly with digital readouts, is great and can produce just about every part you’ll NEED for FRC. For one-offs, you can go straight from a pencil sketch to a part. You can quickly whip up a prototype or modify a slot to make it slightly wider or longer. It is particularly useful for teaching students about speeds and feeds and what a machine should sound like, feel like, and look like when it is running properly. In the hands of a skilled operator it can produce amazing results.
The CNC mill is also great and can produce just about every part you’ll WANT for FRC. It has the advantage of being able to cut custom radius curves very easily, both in 2D and 3D surfacing operations. The foundations for CNC machining are soundly based in manual machining… you have to know about the different tools that you have available and how they behave when cutting “up” vs cutting “down”. And that whole “speeds and feeds” thing is kind of critical too. Not to mention jigs and fixtures for holding your workpiece, and how to set up your workpiece in the mill. It’s not nearly as simple as 3D printing as the interaction between the tool and workpiece is much more complex than a simple extrusion, and much greater forces are involved. The computer can generate gcode based on your drawing, but you have to understand the process in order to tell it how to generate that code. Maybe a good analogy would be autorouting software for circuit boards? Sure the computer can autoroute, but if you don’t know how to do it manually, you don’t know how to do it automatically, either.
And while I have been known to walk away from a CNC machine while it is cutting, that really isn’t recommended practice… particularly if you haven’t proven the jigs and gcode work properly or if you need to change tools manually. It can get expensive quickly when things go sideways and there is no one there to push the pause button.
So neither of you are wrong. Everything the mechanical mentor told you is correct. Once you’ve got the workflow worked out, however, a CNC mill does allow students to learn new skills and produce parts that would be very difficult to create on a manual mill. Most of the time you won’t really NEED to make those parts, but you might want to. More importantly, I’d suggest is that adding CNC capability to your team’s inventory will give students the chance to learn about it and experience both the strengths of each type of machine.
Perhaps you might want to consider a CNC router as an entry point to CNC machining. A good CNC router can cut aluminum (slowly) and can make amazing parts from plastics and wood. It uses the same concepts as a CNC mill, but costs considerably less for a machine of the same size.
Most importantly, it will hopefully not offend the machining mentor, as it sounds like he knows his stuff and has some strong opinions on the topic.
Jason
*every part you’ll “want” and “need” that is capable of being produced on a mill, that is. 
**when I started typing this, there were no replies… I think Andy and I must have had some long distance mind link as we were typing our responses… he beat me to posting, though!
I would defiantly agree with your mechanical mentor on this one. While there is some expanded capability to a CNC, it really isn’t much. I was lucky enough to come from a team that did have access to a CNC mill. We were able to use it to do some nice tasks such as machining triangular pockets and machining our own bearing blocks, but there were other ways of designing the parts in order to get the same effects (ie, just fork over $20 for a Vex Bearing Block).
Also, Having spend some time working on the connection between Inventor and the CNC machine, I can assure you it is not as easy as a 3D printer. You have to remember that with a 3D printer, you are just melting some plastic and making it into a shape. With a CNC mill you are cutting metal. You have to worry a lot about speeds and feeds of your machine, and one error can cause damage to the machine or injury. The best way to look out for errors and to generate the code effectively is to get very familiar with milling, which is best done on a manual mill. In addition, this means that their is a lot more time generating code for the part to ensure that everything will work fine (it not as easy as just hitting print). While you may never look at the code if you have a good understanding of the software, you are still spending a considerable amount of time generating the code. If the part is extremely complex, or you need to make a lot of them, writing the code could be a better idea, but it would probably have just been easier to design a simpler part. In short I would venture to say that although we used it, it was certainly not worth the cost.
On the flip side, I will say there can be a lot of advantages to some other types CNC equipment. While I was a student, I learned a lot about manufacturing through using CNC equipment. I would highly recommend a CNC router, as making tool-paths for large flat sheets is a lot simpler that would you would do on a mill. There can be a lot of nice things designed on one, from artwork for the team to some basic aluminum parts. In addition, a small CNC engraver/milling machine could also be beneficial in giving the students some basic CNC knowledge with out spending the huge dollars of a full size CNC mill. I have used some small CNC engravers in the past, which have been very fun, especially on the decorative side. The team that I now mentor will be getting something similar, but from what I read has a bit more “milling potential” without breaking the bank. http://carbide3d.com/nomad/. We are excited to see what we can do with it, but I imagine most of the parts we will still use our manual mill for.
The difference between a CNC router and a CNC mill is not only the size but also the capability of what the machine can do. While both are essentially the same from a operational standpoint, the CNC router is usually limited to 2D parts.(usually .25 inch thick or less) The power associated with the spindle and drive motors is unable to handle the speeds and feeds required for preforming large tool passes which will expedite the completion of your 3D part. While it is still feasible for you to machine 3D parts on a router, the runtime will increase tremendously.
As for a waterjet, you completely remove the option to cut any complex 3D features that are not thru the material because of how a waterjet operates. It is a high pressure stream of water, much like a power washer, that cuts through all of the material in the path provided.
Simple answer: YES.
FWIW, I downloaded and spent about 30 minutes with HSMworks this morning (free for students) and ran a part on 299’s new CNC router this evening despite almost never running a CNC or using a CAM program before. That being said, I am a fairly experienced manual machinist who has a lot of interest in CNC machines, so take that with a grain of salt. Your manual machinist mentor could definitely get the hang of Solidworks or Inventor integrated CAM plugins very quickly- I was astounded at how easy it was to use.
HSMworks setup went like this:
- Make the part in CAD
- Make a new “job” that holds all the machining operations
2a. Define the tool you’ll be using, and setup feeds and speeds (Ex: 40 IPM feed, 20 IPM plunge) - Select the “boring” operation and select all the holes and set your helix rate (how fast it goes down)
3a. Use a drill operation or a smaller endmill if needed to get small holes. I want to get 4mm endmills in the future to
cover most holes.
3b. Add a finishing pass if needed (I left 0.007" for finishing) - Select the “2D contour” operation and select the outside, set your cut depth per pass (0.06" per pass in my case)
4a. Use 2D contour to make any pockets as well, using finishing passes - Run a “stock simulation” which basically shows the tool cutting the part and make sure nothing bad happens
- Screw a plate down to the bed of the CNC router
- Zero the axes and hit “go”!
We already figured out how to zero the axes and stuff in the day previous, but it was no big deal.
CNC machining is an extremely useful tool. It lets you take a large plate and turn it into a robot’s worth of parts in just a few hours. I manually machined 2.5 robots for 115 during my high school years and I can tell you that CNC machining would have saved a LOT of time. Especially when it comes to parts like gussets or large plates that require repetitive drilling on many parts and large setups, the setup and execution for CNC machining is way faster. Just make a tubing/vise jig like what 1678 has and use the rest of the bed for holding down plates.
Some stuff is simply impossible or incredibly time consuming on a manual mill. My cycloidal VP stage, CGX-108, took four days of machining on a manual mill but could have been done in one using a CAM program and CNC. It also required me to do rotary table setups and spend extra money buying pins and making setups. CNCs let you bypass using a rotary table or a boring head, which is where a lot of time goes depending on your style of manual machining.
All that being said, manual machines can really shine in certain situations. When you need to do on-the-fly changes to a part, square up tubing, or make a single simple part, manual machining is far faster. However, if you change your style of building robots to accommodate it, CNC machining can decrease the manufacturing time and prototyping time of robots by dozens of hours. 1072 might be getting a PCNC 1100 soon, so then I’ll have a bit more to say about what’s realistic with those.
EDIT: as far as router vs. mill goes, I would say routers are easier to setup and faster to CAM for but less forgiving. It’s more important to use the right bits and have the right feeds and speeds compared to a mill. On a mill, you can take faster, heavier cuts, at the cost of not having as much work area for things like long tubes or bellypans and needing to actually have defined setups and zero positions. If I had to choose a single machine, it would be a CNC router, then a manual mill, then a CNC mill, but just having a CNC mill would work too if you set it up like a CNC router (one giant plate to machine).
What router did y’all pick up?
A 4’ x 4’ Gerber router (Can’t remember the P/N off the top of my head but it’s at least 20 years old). We still have a couple issues with backlash and poor quality endmills, but I’m going to use some of my personal stash to figure out the backlash issue soon (Sunday maybe?). I took a crack at it the other day and thought I had it, but we’re still not getting the quality I want.
If we had to, we could do bellypans and gussets easily. Anything with small holes is fine, but bearing holes are just tantalizingly out of reach at the moment.
We have a spare Gecko G540 controller btw. 
We have a manual mill and we keep a squirt bottle of oil by it. Yes, you can get circulating cutting fluid for any mill, but it completely changes your workspace because you have to catch it.
Of course the obvious (but so-far unstated?) difference is that on a router the motor and tool move while on a mill the part moves. The result of this is that a router is usually less powerful and can remove material as a slower rate. Some discussions have said the routers can’t work so well on hard materials. Also the router can’t be expected to be as accurate, but I’m sure there are people that will defend router’s accuracy.
The main reason I think you’d like to have a CNC (and I don’t have one) is because many robot parts are duplicated left-right; You may have four swerve modules; You may have six chain tensioners; many teams build a practice bot. When you go into serial production, there aren’t very many people/students that can build the same part repeatedly on a manual mill well and even those that can will find it tedious and ‘boring’ (pun intended).
Our team has had access to a CNC mill since 2015. It is a Bridgeport style mill that still runs on floppy disks. For a number of reason we do always have a student who is trained watching the mill as it runs, but I would argue for a robotics team a CNC machine should never be left alone, the risk of the machine damaging its self is too high.
What you would mostly want a CNC mill for are parts that have gear, belts, and/or bearings. Gears and belts need precision spacing and bearings need precision holes. The precision holes on a manual mill are possible but they require a different reemer for every size bearing. And the precision hole placement becomes a very daunting task consider that any variation can cause expressive wear and tear on systems. For these reasons we view CNC milling as precision tool not a hands off tool.
Besides a router, a good option could be retrofitting your manual mill with a ProtoTrak, depending on what mill you have. We have three ProtoTrak Bridgeports at work and I think they would be useful to most FRC teams. I could have a student milling pockets and drilling hole patterns in less than an hour.
I’d say that going from nothing, a CNC knee/bed mill would be a good choice if one can be found for a good deal, but already owning an manual mill, a router seems more valuable for the same price, if not less, than a retrofit kit.
Hmmm…
How much and who’s paying for stepper motors? 
If you have a few people who can master CAM, it seems to be absolutely worth it. Our robots are consistently underweight simply due to the pocketing that’s made simple with a CNC mill. As a programmer, I can still see the difference that getting a HAAS has had since we can get a practice bot much earlier in build season.
That being said, make sure you actually know how to use it efficiently. It is easy to waste countless hours and dollars when you have to run a part multiple times because you messed something up or broke a tool.
I agree. I would rather have a router and a manual mill than just a ProtoTrak equipped mill but one of OP’s fellow mentor’s qualms was the difficulty of programming.
I may be reading this differently than it was meant to be read, but a CNC router can do the same 3D operations that a mill can, and can handle similar size parts… in fact the router will often handle much larger parts.
The primary difference is that the router is optimized for wood (and plastics), while the mill is optimized for metals. The router head turns at a higher RPM than the mill head, and (as mentioned elsewhere) the steppers may not be as powerful and the frame not as stiff. This also means that you’ll have reduced precision in a router… routers are normally good down to one or two thou, while mills will get down to a fraction of a thou.
You can definitely cut aluminum on a CNC router, but you do have to watch your speeds and feeds carefully and take smaller cuts than you would on a mill. I would not recommend cutting steel on a CNC router.
The other difference is that a CNC router will be considerably less expensive than a CNC mill of similar size, making it a good training platform for CNC machining operations. I have had students build nice CNC Routers for less than $600. Check out some of the kits and plans… a CNC router kit might make a really interesting off-season project!
Jason
As many have stated, the CNC mill is not necessary.
Just for a quick note of my skills: I have done manual machining for 8 years, CNC machining for 4 years, and worked in a maker space that had 3D printers, a router, wire bender, and much more, for 3 years.
Depending on how many manual mills you have (hopefully with Digital Read Out’s (DRO’s)), I would instead of purchasing a CNC Mill, buy more manual mills so that you can have more students working on mills at once increasing your part production. But this would be conditional on team size, a number of students actually machining on a given day, space allowed, your teams specific needs, etc.
I have been a mentor for a team the past 4 years of a team that had access to a full machining lab at a university. We constantly were using our manual machines, and only used the CNC machine for parts that needed high tolerances and at a quantity. for example, we used a CNC machine for making the housings of our swerve drive that we used this past season. the year before that we used the CNC machine for making our stepper blocks for our drive train because we needed a large enough quantity of them that it was faster than machining that quantity. These are the two main instances we used the CNC mill.
Our students were able to produce everything else on the manual machines. (we also had access to engine lathes and CNC lathes. but only used the engine lathes)
As for the differences between CNC milling and 3D printing, I think it has been summed up fairly well. 3D printing is simple due to it being an additive manufacturing process. I have worked with FDM and SLA printers, and the splicing software does the majority of the work for you, with very little skill needed. On the other hand with CAM software you need to know the speeds, feeds, tooling to use, making sure your coordinate plane is corrupt, etc.
Depending on your CAD/CAM software, you can set up templates for your processes to make programming your tool path for the CNC mill faster. For instance, Where I was they have Catia V5. I had a template for drilling, pocketing, circular milling, facing, and a few more. It decreased my tool path programming time, but I still had to know what needed to be changed based on the material, tool, or some other factor.
In summary, he is correct in his arguments, but there are some who prefer to use one over the other. (note: I have used a CNC mill as a manual once or twice out of necessity of it being the only open machine)