My CNC mill

MaxNC Mill with my controller and the old motor mount.

The mill was eating spindle belts faster than MaxNC could deliver ! I found the problem and this image describes it. The cheap motor mount supplied by factory is not sturdy enough to hold belt tension, and can make the belt twist and destroy itself. The people at MaxNC were too busy to bother with the problem and informed me I was "the only customer that has had this problem." ! OK that may be the case, but the original motor mount is crappy to say the least, so I fixed it myself.

Update: throughout the years I have had many emails form MAXNC users who were also told "they were the only customer that has had this motor mount problem"  ! Draw your own conclusions ! 

The fact is the basic MAXNC slides etc are reasonable value for money. I still have the original crappy stepper motors and driver electronics..... I keep it just to remind me how bad it really was.!

Anyway through the years I have spent money on this mill and recently I found a company that is producing some very nice upgrade kits look for the good news below.

New motor mount. Made without a mill, all holes and slots were cut using my scroll saw. This new four  point mount makes the belt run smooth and true. In fact it has never run so quietly and smoothly.

Update Dec. 2007: Since I constructed this new motor mount (before year 200) I have never changed the belt.

The Good news:  

If you have the same motor mount problems you can now buy a really nice motor mount ready made from this company Link: http://www.dearmondtool.com/maxaccess.htm while you are there have a look at the "CLASS" ER- 16 Spindle System Upgrade"  

I just received the ER-16 spindle upgrade and motor mount today and can assure you it is a very nice product. Well made and well packaged.

NOTE! I have NO connection with dearmondtool.com I am just a very satisfied customer.  

As soon as I get time I'll post some photographs of the new ER-16 spindle upgrade and the motor mount that comes with it. 

RPM Meter 

The Original RPM Meter

I needed a simple RPM meter for this mill, and used a PIC16C73 for the job. I wanted a reasonable update rate and "close enough" accuracy ! and a resolution of  ±15 RPM. Accuracy is determined by the 4 MHz  crystal and timer setting in the PIC chip. In this design the update rate and measuring gate are set at 0.999 second.

This design has been licensed to Camtronics Inc. It is available as a kit or ready made.

Several different options have been added, including input for a second sensor. Resolution has been improved.

Link to Camtronics Inc.  

You can buy (no longer available, but the site is still worth a visit) this design as a Kit or ready made from  http://www.seanet.com/~dmauch/ Look for it on the Products page, look or search for Optical Tachometer

Sensor

LED and photo transistor used to detect four black lines on the spindle pulley.

RPM Meter Schematic

The HEX code file is available on the Download  Page.

RPM Meter Circuit Board

Simple single sided PCB, could also be wired using perf-board.

Bi-Polar Motor Driver

I needed more powerful motors and decided to build a Bi-Polar driver based on the L297 and L298 chip set.

Here is my version of this popular controller circuit diagram. The circuit is controlled through the computer printer port. I arranged the connection based on the popular shareware DanCam program.

Circuit Diagram

New Modular layout using EAGLE 

  No longer recomended. Please use the New Modular Design. 

The circuit board has drivers for two relays as well as an expansion header a fourth axis.

I included a lowpass filter in the feedback loop, but found there was no need for it in my configuration.

Low Pass Filter(R12/C3 and R13/C4)

I made an extra effort to get low inductance carbon composite resistors for the sense resistors R14 R15 as most wire wound

power resistors are wound as a coil they can introduce ringing. Each sense resistor is made up of two  1 ohm 2 W  in parallel.

Power and Stepper Motor Connections.

The power supply is from the ubiquitous PC power supply -- where else can you get a 12V, 10A + 5V DC supply for $20?

I made a two sided circuit board, and used thick traces around the L298 output driver and ground plane on both sides, I also doubled up on the power connectors.

Here is a small section of the circuit board, the blue traces show the motor high current path. The both sides of the board carry ground planes and +V lines. Notice how close the power connector is to the L298 output stage. This layout is stable and has NO noise problems, and I suspect it's another reason the lowpass filter is not needed.
Just for the record, these circuit boards are etched not milled.

I published the full artwork, follow Modular Design link at the top of this page.

Both designs use the printer port to control the Stepper driver. I wrote a small program to help in debugging any printer port issues follow this link to LPT_SPY.exe

The three L298 output driver chips are attached to a simple heatsink, the enclosure has a fan and keeps the entire system cool.

Limit Switch, Relay and other connections. 

Although the MaxNC mill does not come with limit switches, I included them in this design.

Spindle Motor Speed Control, as supplied by MaxNC, this is the original assembly !. It does not take much understanding of electronics to form an opinion about this kind of assembly.

My version of the Spindle Motor Speed Control.

I re-built the spindle motor speed control circuit. I think my insurance agent sleeps better now !

I added three Compumotor bipolar motors rated at 80 oz-in. Upon completion, test show the holding torque of the motors when driven by this circuit is 80oz-in.

Test Setup to measure Holding Torque.

This is my way of measuring holding torque.

The arm attached to the motor is made of aluminum and the plastic cup and string weigh less than 2 grams.

For weights I used US quarter coins, they have a specified weight of 5.67grams.

When the last coin is added the motor "gives up" and the arms falls rapidly downwards.

My three motors tested as follows:-

Manufacturers Holding Torque specification for this motor is 80oz-in for 0.47 amp drive current.

.Specification for A57-83

If anyone has a simpler way of testing holding torque, please email a description.

email

My complete and ready to use out of the box MaxNC-10 mill has gone through several changes, some it could be argued are because I wanted something better than that which was supplied.  However there were a few things that simply could not be ignored or be called change for the sake of change. For example..........

The spindle motor speed control was a bad assembly and had to be rebuilt ( My opinion based on 40 years experience in the electronics industry.)

The Spindle motor was shedding and destroying belts so fast, there was no option but to rebuild the motor mount.

Would I buy this product again ? NO !

Can I recommend it ?  NO !

I'll live with this mistake and use it until it becomes a PITA, then it gets trashed and I'll look for another mill. With this hard-earned knowledge gleaned from this, my first foray into the world of CNC mills, I should be able to sort the weeds from the wheat.

Living with this mill has been about as nice as a belly full of pin worms ! (a quote I borrowed from a Texan )

Have a good day...!

PS PITA stands for Pain In The Ass, however if I were a member of a Royal family, it would be PITRA.

Copyright© 2007 Hans Wedemeyer, Houston, Texas, U.S.A. All Rights Reserved.