CNC Build #2: DIY CNC - phase 1: planning, continued

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Blog entry by loupitou06 posted 08-20-2020 06:34 PM 605 reads 0 times favorited 0 comments Add to Favorites Watch
« Part 1: DIY CNC - phase 1 : planning Part 2 of CNC Build series no next part

Now that you have determined your needs for the CNC machine, you have more or less determined the size of your machine and choose the material for the base/frame, the linear motion and the design of your gantry

Now let’s jump on the next planning phase:

4. Motors and drivers

In order to move your spindle – you are going to need motors, and here you have again 2 main options:

1. Stepper motors
2. Servos

Steppers are by nature open-loop: you tell them to move a N steps and they will try – but if your machine is blocked, the speed is too large or the spindle forces are too strong, then they will skip steps and fail to move – but your controller will be unaware of this and will continue the Gcode (program) as if nothing happens.
On the positive side, stepper motors are very common an cheaper.

Servos on the other hand are close loop system, they have a build in encoder and know at anytime where the axle is regardless of the inputs so if a servo skips it will feed this back to the controller. Servos are also faster and quieter but unfortunately less expensive.

High end machines are very often build around servos but today the vast majority of hobbyist and pro-level router are build around stepper.

Because of their limitations, if you choose to build your CNC with steppers, you will have to dial in the motor parameters and your gantry to work at ~80% of the capacity of the motors. In other words (and I’ll cover that in more details in future posts) you will dial in the motor parameters (speed, acceleration,...) until they skip and then backup 20 to 25% to ensure your CNC will work reliably. This could be frustrating when you find at the end of your build that your design choices limit the performance.

Stepper motors come with standard size (NEMA23, 34,...) and given a size they have different torque (expressed in N/m usually). Large motors (like NEMA34) have usually larger torque rating at the expense of speed. Most DIY CNC and pro-level kits use either NEMA23 or 34.

In my example, I went with NEMA34 motors, the corresponding microstepper drives thinking I’ll have plenty of power to drive my axis and because I made the gantry so tall (and hence heavy) I’m driving my X axis (the one moving the gantry) much slower than Y and Z. I purchased my motors as a kit on Ebay with the drives and the power supplies for $470 (back in 2018)

I can’t find the original purchase link but here is something similar

One last word on motor selection/planning, there are now closed loop stepper that include a encoder and you have guess it the price is between steppers and servos.

I personally am happy with stepper motors, but you need to realize the limitations and really dial in your machine.

5. The Spindle

The business end of the build.

Once again you have options from cheap to very expensive:
1. Dremel or small DC motor
2. handheld router
3. Full size router (1 to 3HP)
4. Dedicated spindle with ER collet (0.5, 1, 1.5 to 2kW) either air cooled or water cooled
5. Dedicated spindle with BT collet and automatic tool changer

The good news is that the spindle is largely independent of the rest of your build (as long as the rest of your build is designed to handle the cutting forces of your spindle)

I would recommend picking your spindle first because it will greatly influence the requirements of your machine – you don’t need NEMA34 motors for a Dremel spindle.

The most used options are
  • routers – often a free option if you have a router laying around
  • dedicated spindle (air or water cooled)

I went with a dedicated spindle because I wanted to be able to turn the spindle on/off and vary the speed via software and that is not easy with routers. Keep in mind that a spindle will require a VFD (Variable Frequency Drive) to operate which adds to the cost.

The benefit or water cooling is that the spindle will usually last longer, run cooler and be more quiet. The down side is that you need to add a water cooler like this one to you build.

I went with an air cooled 2.2kW spindle for my build and I’m very happy with it – very powerful and reliable, I paid ~350$ for the spindle and the VFD

6. Controller and software

The controller and software work hand in hand to drive the CNC. One must choose a software then pick a controller that work with it.

We are now entering the realm of opinion/preference/religion when it comes to software and people have very strong opinions about the software. As a hobbyist your main choices are Mach 3 or 4, Acorn Centroid and LinuxCNC because the professional software/controller like Siemens or Fanuc are out of the price range.

Mach 3 is by far the most common software used and has a very large community of user – most of them willing to lend a hand and help you. This is also the software that has the most compatible controllers. It’s highly customizable but it will require you to spend time tuning it for your need.

Mach 4 is newer and not necessarily better than Mach3 but offer more functionalities.

Centroid (Acorn) is very popular with machines retrofit (say you buy an old CNC mill of lathe and the controller is not supported anymore) but here you are locked to one controller.

LinuxCNC is very popular (just look at how many commenters on YouTube recommend it for any DIY CNC build :) ). It is know for being very stable and reliable – Tormach uses it on their CNC machines behind the PathPilot interface.

I went for my build with Mach4 and the PMDX-424 board. This bundle was attractive because the price/feature was good. After about 2 years of usage I regret not using LinuxCNC and a Ethernet based controller or a plain Mach3. I paid ~$400 for the controller and a license of Mach 4

I have struggled with Mach4 and despite the help for other users and the folks at Artsoft/PMDX I still have issues now and then.

I use PathPilot on regular basis and find it much more stable. You also have access to a virtual Pathpilot controller for testing purpose

This is only a opinion and I’m sure there are plenty of happy users of Mach3/Centroid/Mach4 and LinuxCNC.

If you want to save money on your build, you can also consider using a parallel port I/O board connected directly with your computer and Mach3. Most CNC motor package provide this board and if you have an old computer with a parallel port you can download Mach3 and get going for pretty much free. Keep in mind you have then to dedicate your computer and make sure it is never interrupted !

Planning Phase: conclusion

If you have followed along so far (contrast this is wordy) you already realized that my build

Frame: $0 (scrap wood and existing outfeed table)
Linear Guides : $800 (for a 5’ x 3’ x 12” work envelope)
Motors: $500 (Nema 34, drivers, power supply)
Gantry: at least $400 (assuming you don’t have access to a CNC)
Spindle: 350$ (0$ if you plan to reuse an old router)
Controller and Software : ~400$ (could be much cheaper if you go with simple parallel port I/O board and Mach3)
Computer: $0 to $400, you don’t need a powerful computer but it should be dedicated.
Ancillary: cables, limit switches, relays,.... these add up quickly, expect to pay $200 to $300 depending on how fancy you go

All in all a good CNC router capable to run for long hours (carving steps with small end mill can take hours) will end up costing you several thousand $. That’s why you should plan carefully and see if a simpler commercial unit is a better solution for you.

In next posts – I’ll focus on my build, first the hardware section , then cabling and finally the software configuration.

-- 100 fois sur le metier remettez votre ouvrage

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