CNC Machinery

First Cuts with my new PM-728V-T CNC Milling Machine

I bought a PM-728V-T Milling Machine from Precision Matthews and converted it to CNC using their Ultra Precision CNC Ballscrew kit. I'm using Closed Loop stepper motors and controllers from StepperOnline and an NVCM6V2.1 6KHz 125KHz Motion Controller purchased from Amazon to control it using Mach3.

pm728-vt conversion milling machine

Here is a closer photo of the control box

pm728-vt conversion

Here is a list of the items needed to build this CNC milling machine setup.

Description (with Link) Cost
24V 2.2A Power Supply (Controller, Fan) $15
48V 12.5A Power Supply (X,Y,A Axis) $80
60V 6.6A Power Supply (Z Axis) $40
80mm 24V Fan $11
80mm Fan Filter $6
Assortment of Glands (Cable Ingres/Egress) $16
Circuit Breaker 250VAC 20A $13
Enclosure (NEMA 24) $84
Extension Cord (for power input) $33
NVCM USB CNC Controller $95
Screws, #6-19 x 1/2" $8
Solid State Relay (for future use / coolant / fan) $12
Terminal Strips $13
Thermometer / Hygrometer $14
Toggle Switch (125AC 20A) $6
X, Y Coupler (10mm to 10mm) $13
X, Y, A Stepper Kits (Qty 3) $345
Z Axis Coupler  (14mm to 10mm) $13
Z Stepper Kit (NEMA 34) $195
PM-728VT Mill w/stand, ballscrew kit, clamping kit, and delivery $5600
Total $6604


Here is a table of speeds and feed for a Sieg X2 Mini-Mill (Harbor Freight, Grizzly, etc.). These are for the most common bit sizes that I use.

Depth of cut must be less than 50% of the bit diameter. This is adjusted according to the rigidity and power of your machine. For example, with my machine, I can do about 0.055" depth of cut with a 0.250, 3 flute Endmill into aluminum. My machine is bolted to a 1" steel plate and the column has been filled with melted lead to reduce vibrations.

Anti-Backlash / Anti-wobble Mod for Solidoodle 4

In my quest for the perfect 3D printer settings, I decided to try the lift option which lifts the extruder from the surface during non-extruding moves. I found that my stock Solidoodle 4 had significant backlash in the z-axis. I tried to print out an anti-backlash nut/spring assembly from Thingiverse but it was made for previous version and my Solidoodle 4 didn't have a long enough z-axis threaded rod to keep the nut on it when in the highest position.

After thinking about this for a while, I considered just printing a solid piece that holds a nut but thought that it needed some give to keep it from wearing the rod or becoming sloppy over time. I settled on using a rubber O-ring below the nut to provide springiness. It uses a 3/8" ID O-ring, 5/16-16 Stainless Steel Nut, and 4-40x3/8 screws w/nuts to hold it to the machine. There are only two screws in the unit since the nut is snug on the O-ring, the mount only has to keep the nut from turning. Here are the components and assembly

Here is a video of the part in operation:

The files are available here as SLDPRT and STL files.

CNC Vacuum Power Control and Z-axis Auto-zero

I finally got around to adding Z-axis auto-zeroing to my Mini Mill. With my C10 Parallel Port Breakout board, it was pretty simple. Just connect a ground wire to the machine from the ground of the board/power supply and connect the probe wire to the Pin 12 (or any other) input on the breakout board. Make sure the jumper it set to pull-up on the inputs and you're done.

The code to make this work is described in a previous article on this page.

Also, the small vacuum is now controlled by Mach3. It turns on and off with the spindle control which isn't implemented on my system yet since the Mini-Mill can't be turned on/off with power directly.

This is also a pretty simple mod. I used a Solid-State Relay (10A with 3-30V control input) and mounted it inside of a power box with two duplex receptacles. The control is connected to the ground and output pin 14 on my breakout board and Mach3 is set to control this pin. I need to add a charge-pump into this since the pin is high when the PC boots and turns on the vacuum until Mach3 is loaded. This isn't that big of a deal if you are only controlling a vacuum but if you use this for a spindle, you should use a charge-pump for safety.

Here is what the inside of my Mini-Mill control box and the breakout board looks like now

3D Printed Magnetic Microscope Camera Mount for CNC Zeroing

It has always been a pain to use the mechanical or laser edge finder in my CNC mill. Although I am usually pretty close, it takes time and requires me to get up close to the work to look at it from different angles. I have always liked the Microscope Camera systems on professional CNC machines.

I recently purchased a Supereyes B008 for electronics inspection and have been happy with the performance so I bought the less expensive Supereyes B005 for about $28 on Amazon to use in my CNC setup. I know that Mach3 has a camera interface and the B005 shows up as a standard camera in Windows so I thought that it should work. I had some concerns about making the mount adjustable in both X & Y directions so that the unit can be aligned so height above the work piece won't have significant effect.

I opted to use strong 1/2" x 1/4" Neodymium magnets from Amazon to mount the unit. The top section has two on the bottom and the bottom part has two on the top so that the top pulls toward the mill head (left) and the bottom part pulls up to keep the unit securely attached to the head. The magnets are epoxied into the top and bottom recesses.

1/4-20x3/4" stainless steel set screws are used to hold the Microscope Camera into the holder and to provide adjustment in the X direction. Adjustment in the Y direction is accomplished by loosening the 1/4-20 screws that hold the top to the bottom and tighening when aligned properly. I've got bolts installed right now but can't tighten them too much but will get some screws and washers on this soon.

This is the assembled unit

Here on some photos of it attached to my machine

Here is a screenshot of SuperEyes software viewing a nickel

I've got less than $40 in this project and can't wait to get it tested on my machine. Here is some video of it in operation on my machine

Here is the crosshair overlay I made for the SuperEyes Software

The files for the top are available here as SLDPRT and STL files.

The files for the bottom are available here as SLDPRT and STL files.

Custom Vacuum Nozzles for My Loc-Line® Setup

The nozzles that came with the Loc-Line® kit are meant for liquid and aren't shaped well for use as a vacuum so I decided to make my own with my Solidoodle.

The walls on these are thin enough to be somewhat flexible but are suprisingly strong. It took a few tries to get the fit correct to mate up to the Loc-Line®

These mate to 3/4" Loc-Line®. The photo in the previous article shows them on the machine.

The files are available here as SLDPRT and STL files.

SECOND TRY: Adding a Dual Loc-Line®; Vacuum to a CNC Mini-Mill

I overestimated the flexibility of the Loc-Line®. With the connections pointing down there is no way to make the bend that I need to get the nozzle close to the bit,

This will bolt onto the front of the Mini-Mill using two existing holes that used to hold on the shield. Here are some photos of the Vacuum Manifold being 3D Printed

Here is the setup installed on my CNC Mini-Mill

The files are available here as SLDPRT and STL files.

Adding a Dual Loc-Line®; Vacuum to a CNC Mini-Mill

I've been machine more things on my CNC Mini-Mill and am getting tired of sweeping up or holding a Shop-Vac®; hose up to the machine

My latest CNC project

So I found that Loc-Line®; is available in 3/4" diameters which is about right for two of them to be connected to a small Shop-Vac®;. They will need to be rigidly mounted to the head of machine to maintain the same position relative to the cutter and also split the Shop-Vac®; hose to two 3/4" NPT connections. There is no adapter that fits the requirements so I decided to make one from ABS on my Solidoodle 3D Printer.

I designed the manifold using Solidworks and the files are available here as SLDPRT and STL files. It looks like this

This will bolt onto the front of the Mini-Mill using an existing hole and will be tapped with a 3/4" NPT tap to accept the Loc-line®;. The other side is sized at 1.3" to connect to the Shop-Vac®; hose with the end removed. Here are some photos of the Vacuum Manifold being 3D Printed and the finished product

The effects of missed steps on a CNC router

When a CNC router using stepper motor "misses steps" the physical reference to zero on the workpiece moves. Here are some examples.

Normal Circle Circle Missed Steps

These 2" circles are cut .1" deep by cutting .005" at a time (the bit travels around 20 times to cut this) The left one has square sides as you would expect. The right one shows that the CNC router is missing steps -- note the slope on the sides caused by the machine "floating" over a little bit on each pass.

On a real object this can manifest itself by giving really poor results. Here is a chair back that was done for my neighbor to replace a broken one. Note that the rungs "X" and "Y" are supposed to be the same width.

Chair back missed steps

Problem and Fix for the Univelop / TB6560 stepper motor driver missing steps

Photo of Univelop TB6560


I purchased the bipolar stepper drivers for my new CNC machine from because I liked that they have a modular design (each axis with its separate driver board). I got the electronics put together on the workbench and connected to motors and everything seemed to be fine but after I got the machine together, I noticed that it was missing steps in every axis and this was reproducible with exactly the same error and not speed dependent. After messing with nearly every setting on Mach3 and the computer parallel port, I suspected the driver board. I probed the step and direction pins on the TB6560 with my oscilloscope and found the following:

Univelop Step and Direction Pulses

The top trace is the step pulse and the bottom is the direction. Notice that the direction pulse is changing AFTER the step pulse changes. This causes the motor to step in the WRONG DIRECTION every time there is a direction change. This adds up quickly! I looked at the input to the board and saw the following:

Correct Step/Direction Timing

Notice that on the input to the driver board, the step pulse is after the direction pulse by 16uS which is what should be happening. I probed the output of the P521 optocoupler and found the following:

Univelop optocoupler rise time issue

The top trace is the output (collector) of the P521 optocoupler and the bottom trace is the input (LED). Note the VERY significant rise time. I found that the collector is being pulled high with a 100kohm (SMT0805 / 104) resistor which is WAY too big for this application. The parallel port breakout has a 330 ohm (SMT0805 /331) resistor on its output that is in series with a 100 ohm (SMT0805 / 101) resistor which gives 430 ohms total. With a 5V source and nominal 1.15V across the LED, we have about 9 milliamps which is a bit low for this part (datasheet recommends 16 to 25mA) but probably ok.

The real issue is that the step and direction lines go through two different types of optocouplers. The step pulse goes through a high-speed 6N137 (10Mbps rated) optocoupler and the direction pulse goes through a standard type P521 optopcoupler which is 200KHz in the best case (10V supply / high current). The direction signal is delayed by the rise/fall time of the slow part (5 to 25 microseconds) and the step signal is delayed by the rise/fall time of the fast part (20 to 100 nanoseconds).


I replaced the 100k ohm (SMT0805 / 104) resistor(labelled R18) with a 220 ohm (SMT0805 / 221) resistor, shorted across the 100 ohm (SMT0805 / 101) resistor (labelled R6 ), style="color: rgb(255, 255, 255);">and changed Mach3 to use 10uS pulses (the motor tuning says 1-5 but you can enter 10 and it will work). The board now provides the direction change to the TB6560 IC about 5 to 10 uS before the step pulse which isn't optimal (should be the 16+uS output from the parallel port) but functions correctly without missing steps.


The board design seems ok except for this issue but the vendor/design takes quite a while to respond (I emailed him about this issue but haven't received a response yet). If you aren't comfortable soldering SMT components then this board isn't for you since in its off-the-shelf state, it WILL MISS STEPS.

How to set up a touch plate for auto-zero of the Z-axis in Mach3

The idea is to use the cutter itself and a soft conductive plate as a switch that triggers the probe input on Mach3. The touch plate is connected to a parallel port input pin that is pulled up to +5V with a 10k resistor and the ground is connected to an alligator clip. The ground is clipped onto the cutter bit and the touch plate is held on the work surface. Clicking the Auto Tool Zero button runs the Visual Basic code that lowers the cutter until it gets a signal from the probe input, sets the Z-axis readout to the thickness of the touch plate (.060" in my case) then raises the cutter to 1 inch above the surface.

For the touch plate, I used .060" thick PCB material. This is inexpensive and can be gotten from many sources. I recommend looking on ebay and searching for copper clad board.

Here is a demonstration of the setup in action.

First we set up the input pin for the probe. We need to have it configured to the proper pin with an active low input since it is normally pulled up to 5VDC.

Then we need to add the Visual Basic code to the button. We go to the Operator menu then click on Edit Button Script. The configurable buttons will now be flashing and we click on the Auto Tool Zero button. This will bring up an editor that may have a Not Implemented message in it. Copy the code below into this editor replacing any existing code. You will want to modify the .060 value to match the thickness of your touch plate. Save the file (same name) and exit the editor then restart Mach3. You should now be able to press the Auto Tool Zero button and have the Z-axis lower until the touch plate connection is made. I suggest that you try touching the alligator clip and plate together the first few times with the Z-axis nohwere near the table to make sure that this is working.

VB Code for Auto Tool Zero Button

		Message( "Auto Zeroing..." ) 
		If IsSuchSignal (22) Then
		code "G31 Z-3 F20"
		While IsMoving()
		Call SetDRO( 2, .060 )
		code "G1 Z1"
		End If

"Moving Knot" Cable / Pulley System to Reduce/Eliminate Racking on a Moving Gantry CNC Router

After designing and building my latest CNC Router (23" x 36"), I found that I was still seeing a lot of racking of the gantry during cutting that shows up as chatter when the router is far from the middle of the table. I was able to measure about .050" of racking movement by pushing on one side of the gantry. After a lot of research and not wanting to add a second leadscrew (dual x-axis setup), I found a discussion of the "moving knot" on a CNCzone forum. This design has been used for 100's of years on drawers and drafting tables to eliminate the racking action in their mechanisms.

The system works by having two cables that each prevent the gantry from rotating. By having these two forces oppose each other, rotation of the gantry (the racking movement) is prevented. Here is a diagram of what is going on:

Moving Knot Diagram

The installation requires 2 lengths of 3/16" cable, 4 cable crimp sleeves, 4 pulleys, 2 turnbuckes, 4 eye bolts, 2 1/4-20 "t" nuts, 2 1/4-20 fender washers, 2 1/4-20x2" cap screws, and some loctite.

Moving Knot Parts

The original forum discusses using sliding door wheels from the hardware store but these are made from plastic and most don't have ball bearings. I was concerned that these may break or wear out quickly so I found some

The pulleys are mounted on the 1/4-20 cap screw with a fender washer between the pulley and the wood (reduces stress on the plywood by distributing the lateral force) then screwed into the 1/4-20 "t" nut mounted in the gantry bottom. The pulleys are mounted vertically so that the cables can't contact each other where they cross in the center.

Pulleys Mounted to Gantry

The eye bolts are mounted to the machine to be lined up with the pulleys and are adjusted with the turnbuckles. If your gantry is narrower than mine, you may want to use the eye bolt as a pulley and put the turnbuckle below to adjust the tension so that the gantry pulley doesn't hit it. The cables are terminated on the eye bolts by swaging them with the crimp sleeves. If you don't want to spend the money on a swaging tool, then you can crimp these using a hammer -- just tap them together then rotate 90 degrees and do it again. Don't overdo this or it won't be as strong.

Cable swage

After the turnbuckes are mounted, snug them up by hand (not too tight) and do some 90 degree corner test cuts and accurately measure them. Alternately adjust the turnbuckles and perform test cuts until the machine is cutting perfectly square. When you are done, the cables should be taught but not crazy tight. I get about 1/2" of deflection over a 2 foot span using moderate finger pressure.

After installation, I tried to push on the gantry and was able to measure no deflection on the opposite side. The gantry is now "Rock Solid"!

Here is a video of the system in action:

Spindle Dust Collector that Really Works

Vacuum Pickup Vacuum Pickup Side View

I have tried several dust collection methods over the past few years including everything from a vacuum hose wire-tied to the router to several different kinds of brush/vacuum systems. The problems with many of the existing systems are that you can't get the vacuum close enough to the bit at high enough velocity to pick up the dust and still leave room to be able to change the bit. I have come up with a design that uses strong neodymium magnets to attach the lower dust brush. This allows quick and easy removal of the brush assembly while allowing the vacuum to be directed where it is needed. The grey strips on the right photo are pieces of weather stripping to keep the unit from sliding and reduce vacuum leakage. The brush is installed (hot glued) into a 1/8" slot cut into the wood ring. The magnets are glued into 1/8" deep holes and are flush with the wood.

Router Bottom Dust Brush Dust Brush Top

Notice that the fan output of this router (Porter Cable 890 Series) is redirected away from the vacuum while still leaving access to the spindle lock to change bits.

Vacuum Pickup Top

The design is pretty straightforward. The magnets and magnet washers were purchased from Rockler (Woodcraft will also have these). The brush was purchased on ebay -- just search on "dustbrush router"

Dust Brush Magnet Washer Magnet

Here is a video of the dust collector in action: