The construction of a gantry CNC router is shown with a full description of the resulting machine including the pro's and cons of the particular design - FabRap

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The construction of a gantry CNC router is shown with a full description of the resulting machine including the pro's and cons of the particular design

Dabula Kahle

Having made the decision to proceed with the project of designing and building a CNC router it was off to explore what scrap material I could find.  Luckily I had scrounged a fair amount of scrap Aluminium bar.  The dimensions varied considerably over the length of each bar, but I could work to an acceptable scale.  This was a lucky break as, in the final analysis, 75 per cent of the material used came from this scrap.  A major cost saving.


All axis rails would be made of 20 mm steel bar with Phosphor Bronze bearings.  I found suitable shells at a reasonable cost.


The first parts made were the Z axis carrier and the X axis carrier.  I visited the local importers of ball screws and found that I was being quoted exorbitant prices.  This was far beyond my modest budget. I then researched the process of turning the lead srew with an Acme thread.  I even went to the stage of maching some sample threads. However the process of machining Acme Threads into the lead screw nuts proved to be a major challenge.  I had decided, as a matterof expedience, to use rolled 14 mm thread for the lead screws.  This was a compromise but had the advantage of low cost.  

Once both carriers had been completed, they were bolted together and the carries shafts and lead screws were tested.  The initial lead screw nuts were machined from Nylon as in the picture.  This did not prove to be satisfactory so new lead screw nuts were machined from Phosphor Bronze.  This proved to be far better.  The shaft bearings were force fitted into the carriers and they had to be reamed to a good fit.  

Backlash was minimised by using 2 lead screw nuts on each carrier and adjusting them to reduce play.  The overall movement of the carriers was satisfactory and showed no appreciable lateral movements.

The next task was to manufacture the Y Axis carrier.  This also used the phosphor bronze bearings.  The uprights are made from a deep channel with the sides of the channel providing strength against any bending.  This part of a gantry type CNC router has to be exceedingly strong as the lateral and twisting forces can be strong.  

No additional support structure was planned for between the uprights.  The two 20 mm axis rails were deemed to be strong enough.  In practice I have not found any problems caused by this construction.  


The lower section of the carriage has also proved to be strong enough to withstand any flexing due to the forces when machining.

The next task was to make the bed of the router.  This would also provide the support for the Y axis carriage.  The end plates are made from 100 mm bar and this is not strong enough to stop any bending.  The top of the bed will be completed with a chipboard covering of 25 mm.  On top of the chipboard, I will be making T slots by adding MDF strips and capping these with Aluminium.


The idea of the T slots is convenient but for most operations it is wise to use some sacrificial material between the work  piece and the router bed.  

Once the basic parts were completed, the router was assembled and the witing started.  The black wires are the motor feeds while the grey are for the limit switches and the E Stop.    

The motors are NEMA 23 57BYGH115-003B steppers from Wantai Motor.  The motors have a holding torque of 30Kg per cm and a detent torque of 0.89 Kg per cm.  The motors are all 4 wire 3 Ampere windings with a maximum Voltage of 6.3 Volt.

The limit Switches and the emergency stop button are wired as normally closed contacts.  This ensures the circuit is fail safe as any lack of continuity will show up as a fault condition.

The motors are connected using 9 pin Molex connectors.  The wiring will allow for both 8 wire and 4 wire motors.  All signal wires are terminated on a terminal block mounted on the rear of the router.   

The motors have the shaft extended on both sides of the motor.  The external shaft has a hand wheel fitted for easy manual operation.  Of course the motors need to be disabled to allow for manual operation.  A calibration wheel is also fitted between the hand wheel and the motor.  Calibration engraving on this wheel will only be completed when I have built a rotary axis.

The spindle is a modified old Bosch hand router. The handles used for manual operation have been cut off and the on off switch has been removed.  The standard collet was for a 6.35 mm ( 1/4 Inch ) cutting tool.  I have made collets for both 3 and 6 mm to allow for a wider selection of cutting bits.

To complete the machine, I purchased an old workbench from a friend who was forced to close his workshop.  The workbench was modified to allow for a computer with screen to be added.  The control circuits are also housed in this unit.  The workbench is equipped with casters so that the machine can be easily moved to an operational area or storage area.

In Conclusion, I have a CNC router that is accurate and stable.  The backlash causes the machine to slow down at each change of direction as compensation takes place.  For an amateur this is not a major problem as we are not as time conscious as indutry applications are.  Once backlash compensation has been adjusted the machine will remain within one hundredth of a millimeter.  This is adequate for the type of project I am likely to undertake.

Problems and inadequacies during the building of the router were as follows:

1. The lead screws were not attached to the frames properly.  I had to retrofit thrust bearing to each axis to overcome the problem.
2. Some cost saving decisions have compromised the design.  It would have been better to build a workbench from the start.  The idea of having a table top within the case is not the most practical method.  It can be awkward to input manual instruction and with the keyboard being below the router, the worksurface is not in sight during the operation.  An inconvenient process but one I can live with.
3.  Access to the electronics is not easy.  As a result fault finding can be difficult.
4.  I should have used Stainless steel for the axis shafts.  Steel corrodes and the axis have to be lubricated each time to stop the corrosion.  next design will used linear bearing or similar carriers.

On the positive side the advantages are:

1.  Having the unit fully self contained, it is easy to move into position to work and then to pack away once the work is completed.  This is essential for an over committed work space.
2.  The router is adequate for the tasks I have in mind.  

 
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