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CNC Machining Of Plastic Sheet

This article is about a very compact CNC machine, called a GearBest. This machine comes as a kit with all parts needed to create a working CNC except a build instruction. The frame is composed of just a couple of parts, so that the product photos on the Gearbest pages were sufficient to put it all together.

The photos I took during the assembly are now part of my build instruction that is available on my pages in order to make things easy for you. When using the product photos as template, the CNC table of the assembled frame isn’t really underneath the milling motor… …but only a few screws have to be loosened to get a better result. All three axes are driven by bipolar stepper motors through spindles.

Gearbest tabletop CNC milling machine

Great Methods To Machine Plastics

To adjust the guiding rods in parallel to the axis, loosen the screws on one end… …and drive the carriage to that end by manually turning the spindle. Now, tighten the screws again. Repeat that procedure at the other end of the X axis as well as on the Y and Z axis so that the carriages move as smoothly as possible along the guiding rods. A soldering iron as well as heat shrink tubing or insulating tape are needed for the wiring. Here I am soldering the cables of a stepper motor. There is a plug on the other end of the cable that has to be connected to the CNC shield on top of the microcontroller. Besides the cables on the steppers, the terminals on the spindle motor as well as the female connectors of the power supplies have to be soldered.

cnc table top plastic milling machine

You can find more information about the wire connections on my pages. The machine needs two power supplies, one with 12 and one with 24V output. The adapters of the mains cables are a weak point of the kit. The plug of the 24V power supply can easily be replaced by a normal cable. I cut the cable of the 12V power supply to connect a new plug. Use a switchable multi plug to have an emergency stop function for your machine.

Only a few hours passed by for the assembly of the machine. The dimensions of the machine fit into a cube of 27cm edge length, thus it is easy to carry with you. The X axis can move for approximately 12cm,… …the Y axis can travel for about 10cm… …and the Z axis for 4cm. The size ot the CNC table is 15 times 9cm. An Arduino Uno clone with a CNC shield on top is used to control the motors. Grbl is running on the microcontroller, thus the machine can process G code. Here I am using a terminal program to send G code instructions directly to the engraver. In doing so, you can use this CNC to learn G code from scratch. The first instruction sets the current position to 0 for all axes.

Using Millimeters To Input

The machine takes numbers as millimeters to move for the X axis… …the Y axis… …as well as the Z axis. You can also transmit a move command for all axes in a single line that will be executed simultaneously. M3 turns the milling motor on… …M5 turns that motor off. The feed rate is encoded by an F followed by the speed in millimeters per minute. Here it is set to 80mm per minute and the X axis is commanded to move for 10mm with that feedrate. Machine operation becomes more simple when using an appropriate control software. With the software I have written for this machine you can control the axes with the cursor keys. The spindle motor is turned on or off by another key press. Usually the command sequences used to machine parts are stored in a file. After selecting a g code file you can send all commands to the microcontroller by a simple key press.

Setting Up A Microcontroller

The frame of the machine isn’t very solid – you can easily deflect the V bit noticeably with your finger. That’s the reason why this machine is sold as engraving machine rather than calling it CNC router. The kit comes with three 10 degrees V bits. 3mm Dupont Delrin is cheap and easy to cut, thus it is a good material to start your experiments with this CNC. The revolution speed of the spindle motor is relatively low and can’t be set by pulse width modulation, because a relay is used to turn the motor on or off. The appropriate feedrate of a CNC router depends on many parameters such as the material to cut, the diameter and number of flutes of your milling bit, the spindle speed and so on. The V bit has only one cutting edge and the diameter of the tool varies with the depth of the cut. Only the tip of that engraving tool is meant to dive into the material, thus V bits are inappropriate cutting tools. Caused by the small diameter at the tip of the tool, the revolution speed of your spindle must be as high as possible, thus it would make sense to operate the motor with more than just 12V.

Using a small CNC milling machine for engraving

There is a lot to try out to find the best feedrate of this engraving machine. In general there is: The harder the material to process, the lower the feedrate. You can process or Dupont Delrin with the maximum speed of this engraver – which is 200mm per minute – without causing damage of the machine, however lowering that speed to get better results makes sense. A deep engraving is processed in several runs. Besides the feedrate, the depth per pass is another machine parameter to be considered. You can not only engrave the 3mm Dupont Delrin, but also cut the material with ease. Put another plate underneath the material to be cut to avoid the milling bit from diving into the aluminum table of the machine – with the high feedrate the V bit would crack quickly if that happens.

You can also process wood with this machine, but not with maximum machine speed, thus you must reduce the feedrate Here I have set this parameter to 80mm per minute. The depth per pass is set to 0.3mm. The total depth of the engraving is 1.2mm. Have a vacuum cleaner at hand to remove the dust from time to time. The power consumption of this machine is no more than 35W. The temperature of the driver chips doesn’t exceed 50 degrees. The motor of the Y axis heats up to about 47 degrees during operation. The temperature of the steppers at the X and Z axis is even lower, thus it’s within an acceptable range. The temperature of the spindle motor exceeds room temperature just barely, by what you could increase the operating voltage clearly without causing damage of the device. It took around one hour to process the G code file. The engraving is clearly visible, but caused by the fiber structure of wood, the edges are a bit rough, thus to get a perfect result you must use some sandpaper to smooth the surface. 1mm plywood can be cut with ease. Once more, the cut is done in several runs.

Using G Code – Machining Plastics

The feedrate is set to 80mm per minute and the depth per pass is 0.3mm. The templates are cut with really good precision, however the wood fibers result in rough cutting edges at the top and bottom of the plywood. Once again, sandpaper makes it look much better. Next material being processed is 2mm acrylic plastics. Plastics iare a tricky material to process with a CNC machine. If the milling bit moves to slow along the path, the plastics starts melting by what the miller can no longer cut the material.

You can get full resolution images of all examples on my project pages. You can easily carry that compact engraver around, thus it is an appropriate machine to teach Computer Numerical Control to a wide audience. Another advantage is the relative low noise emission while running. This low cost engraver is a good device to start experimenting with computer controlled machines. As a beginner you will stumble over many pitfalls on your way from a 3D CAD file to appropriate machine commands, as well as there are things that will go wrong while operating your first CNC.

With the T8 you can start experimenting without damaging expensive components whenever something goes wrong. You should rethink your business plan, if your idea is to start manufacturing in an industrial scale with a sub 200 Euros machine. This CNC is a great gift for all tinkerers and it is meant to get modified by all home constructors on this planet. I will have a look at ways to modify and so improve the design in another blog post showing what else you can do with this interesting machine.

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