Whether you're thinking about converting your existing manual mini mill to a CNC Mill or planning on buying a new mini-mill to convert to a CNC one, there are some things you need to know.
Converting a mini-mill is obviously not an easy plug-and-play solution with a kit.
It involves planning, skill, and the right components for a successful transformation.
Through this guide, I intend to give a general overview of this process for beginners.
I also recommend the best CNC mill retrofit kit for several models of mills.
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Should you convert your Mini Mill to a CNC one?
Before you embark on this process, I'd like to say that this process would take at least a week of effort and probably a month if you can work only on weekends.
You'd also need another person to help you, especially due to the hoisting of heavy parts that's required.
Also, you need a minimum amount of knowledge about the parts of a mini mill. It also helps a lot if you have built or operated at least a small CNC mill before.
However, with the right mini mill CNC kit and a proper plan this conversion can be achieved.
You can also keep the total cost of this, including the mini mill to be well under the price of a brand new entry-level Tormach CNC benchtop mill.
If you didn't know, the Tormach is probably the least expensive entry-level CNC benchtop Mill that can cut steel. I provide a general overview of prices of CNC machines in various categories here- how much do CNC machines cost?
With a GRBL based controller and an inexpensive harbor freight X2 mini mill, you could build a CNC mini mill in under $2,000.
This certainly won't be anywhere near enough for any kind of commercial operation, but I'd say it's an excellent choice for a hobbyist looking to play with a CNC mill in his garage.
However, if you find the conversion intimidating you can opt for a ready-to-use CNC benchtop mill. I talk in detail about all the options here- Best Benchtop CNC Mills in 2022 for Machining Metal.
Which is the Best Mini Mill for converting to a CNC Mill?
If you're going to buy a new manual benchtop mill for converting into a CNC Mill and you're wondering which one to get I suggest the Precision Matthews PM-25 MV which costs ~$2,000.
I recommend this primarily because it comes with a belt drive system as opposed to a gear drive system found in most mini-mills.
If you're ok with spending a bit more money for a lot more power, I also recommend the Precision Matthews PM-833TV benchtop mill.
For the benchtop mill CNC retrofit kits that I recommend, I've listed the compatible CNC mills as a table in the next section.
The Mini Mill to CNC Conversion Kit
Converting the mini mill or benchtop mill consists of fixing a new set of mechanical components and electronic components into your mini mill.
Mechanical Components (kit)
The mechanical part consists of replacing the lead screws in the mini-mill with ball screws and double nuts.
The reason why you need a new mechanical system is that the lead screws in benchtop mills have backlash and are not really built for CNC operation.
Ball screws have very little backlash and are an excellent choice for the drive of a CNC.
There are a few companies that sell the conversion kit with all the mechanical components, but I strongly recommend that you buy it from Arizona CNC (CNC conversion kits).
They are the market leader and the most reliable company for CNC conversion kits.
Their conversion kit is customized for specific models of tabletop/mini-mills.
They offer their kit for the most popular mini-mills of companies such as SIEG, Precision Matthews, Grizzly, Harbor Freight, and Little Machine Shop.
The price of these conversion kits ranges from $700-$1200 depending on the quality of the ball screws and the motor shaft size you choose.
A key takeaway is that almost all benchtop CNCs have retrofit kits available for purchase.
For Grizzly G0704, PM-25MV, PM728-VT, and PM-30MV, you can get the readymade converted mill from Procut CNC.
This way, you don't have to risk the conversion going wrong.
Electronic Components (kit)
The second part of the conversion is the electronics to control the CNC.
You would need stepper drivers, a controller, limit switches (3), mechanical test probes for controlling the X, Y and Z movements.
I recommend some viable choices for electronics in the latter part of this guide.
For controlling the spindle you can buy a VFD if you want to control the RPM manually using buttons.
If you looking to choose a spindle for your CNC, then check out this guide- Best CNC Spindles in 2022 for a Smooth Cutting Experience
Of course, you need a computer at the other end of the controller to operate your CNC software and give the mill instructions.
Unlike the mechanical kit, there are multiple routes to take here depending on your preferred software (Mach3, GRBL, or LinuxCNC are some examples).
You might be interested in this-Best CNC Mills -Desktop, Benchtop, and Industrial.
How to Convert a Mini-mill to CNC
The exact steps on how to go about this would depend on the model of your mini mill and your choice of electronics. I only intend to give a general overview of how to go about this.
The entire mill needs to be disassembled for doing the conversion. It's not as simple as replacing the handles with stepper motors.
For starters, each axis of the mill is to be taken apart and ball screws are to be fixed instead of lead screws in each axis.
The threaded shaft in a ball screw is shaped to be helical raceways for the ball bearings inside the special nut.
The weight on the Z-axis of a typical benchtop mill can run into a thousand pounds, and so you will most probably need an engine hoist to pick up and move the axes.
Your mill may be screwed to the floor using nuts and bolts and you will need to take it out.
With each axis taken out, you'll need to add the components for CNC to them individually.
Take out the lead screw in each axis and replace it with the ball screws.
Note that when installing the ball screws, you need to be careful not to snag any oil lines.
Next, the lead screw nut block needs to be replaced with the ball screw nut block.
If the mounting holes in the nut block do not match, you'll need an adapter to fix the ball screw nut block to the axis.
Also, you might need to do some manual grinding to make the whole thing fit correctly. This is to be expected as this is no plug-and-play solution.
The other way around this would be to fabricate some of the parts yourself.
The oil lines have to be put back on after you place the ball screws inside the axis.
The next step is to attach the stepper motors to the axis. Make sure that the stepper motors can handle the heavy Z-axis weight of the mill.
I talk a bit about stepper motors for CNC applications here- Stepper Motors for every type of CNC.
Repeat the same steps for the other two axes as well.
Put the X and Y axis back on the base first.
Next, you need to place the Z-axis or the column block. Before you can operate the mill you need to make sure that the Z-axis is perfectly perpendicular to the table by properly squaring it. This is called tramming the mini mill.
This is done using a tramming indicator which is available for buying. Note that there are detailed youtube videos on how to do this.
If you check with the indicator and find that the mill is not trammed, then you need to use thin sheets of metal under the base to adjust the leaning. This is called shimming the column of the mill.
Whichever way the column is leaning to, you should put the metal sheet on that particular side of the base to push the column to the other side and compensate for the lean.
you'll need to do a trial and error with sheets of different thicknesses and test the lean each time with an indicator to see if the tramming is complete.
Once you're done with tramming the column, you need to tram the spindle to ensure there is no tilt to it. The spindle needs to be square to the bench or base on which the mill is placed.
Similar to the column, you need to use the tramming indicator to check the tilt on the spindle. The exact way to do this is shown in several youtube videos. Once you find the tilt, you need to tap on the headstock in the compensating direction.
Loosen the nuts and tap using a mallet for compensating for the tilt. Once again, you need to retighten the bolts and test with the indicator to see if it's corrected.
You'll need to do some trial and error here as well and make it perfectly square to the table.
Fixing the electronics for the CNC to the mini mill
Once the mechanical part is done, you need to set up the electronics for the CNC. Note that you have to install the stepper motors for each axis before this.
Now you need to fix the brain/control system for the stepper motors, which is ultimately what makes it a computer-controlled system instead of a hand-operated manual mill.
There are a lot of options for building the electronics for the CNC. I'll just explain how to make a basic system
You'll need three limit switches and two probes, and an unregulated DC power supply.
Also, you'll need three stepper motor drivers. For driving NEMA 34 stepper motors, I recommend lead shine DM860T stepper drivers (up to 7A).
Geckodrives are another reliable option (for Mach3).
If you are new to stepper drivers, I've done a detailed review of the main stepper motor drivers available on the market, over here-Best Stepper motor drivers for CNC's.
You could probably manage with NEMA 23 stepper motors for the X and Y-axis. Just verify how much torque you need and select the appropriate stepper motor.
If you like linuxCNC software, then the Mesa 7I76E is a good choice for the controller.
For the Mach3 what you need are ethernet breakout boards from warp9.
If you don't want to purchase each electronic part separately, what I recommend is the Probotix CNC conversion electronics kit. It has some heavy motors and drivers to make sure you have enough power for your benchtop mill.
It has everything you need in terms of electronics for the mill.
With a mechanical kit from heavy metal CNC and the electronics kit from probotix, you should be covered in terms of components.
Whichever way you go, make sure that you use a NEMA 34 with at least 900 oz.in of holding torque for the Z-axis.
Note that the limit switches give the mill spatial awareness and avoid it from exceeding its physical limits.
The probes are for assessing the position of the stock before beginning the milling.
Also, you certainly need a computer that connects to the controller, either using a USB/Ethernet/Parallel Port Cable. That depends on your choice of controller.
Next, you need to retrofit the spindle system. If your mini-mill spindle runs on a gear-driven spindle motor system, then you need to convert it to a belt-driven system.
This needs a conversion kit which can be found here-Belt Drive Conversion Kit for Mini Mill
For controlling the spindle, you'll need a VFD motor controller. You need to get the VFD controller that's appropriate for your spindle motor.
The VFD will usually have manual buttons to decrease or increase the spindle RPM. You can also reverse the direction of rotation of the spindle as well.
You can also connect the VFD to the controller if you want to control the spindle speed using your computer instead of the manual buttons.
The software that you use to control the whole thing from your computer would depend on which controller you got. It could be either Mach3 or GRBL or LinuxCNC.
It also could be something else. Really there are a ton of choices here.
Mini Mill CNC Conversion Plans
If you want a well-documented plan for converting your mini mill, Little Machine Shop offers one- Mini Mill CNC conversion plan
This plan is a bit pricey but can make things easy for you.
The compatibility table lists the Mini mills for which the conversion plan can be used.
These include brands like Sieg, Grizzly, Bolton and Clarke.
In conclusion, whether you're retrofitting your existing mini mill or buying a new mini mill for conversion to CNC, make sure you know what you need and how to go about it.
Done correctly, it can give you a functioning home CNC Mill at a total cost that's much lower than a new entry-level CNC mill from brands like Tormach or HaaS.