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| New CNC
Router
Project
The
problems of my first CNC machine are detailed below and theirs a lot
that can be done to improve its performance without breaking the bank
and investing in linear guides, ball screws and proprietary stepper
motor drives. This project is to see if I can make a low friction
version of the existing machine using readily available parts or
components that can be made on my Chester Model B lathe/miller and
castings produced with the small furnace I made the other year.
Design
Peramiters
- Use as much of the old
machine as possible. The Bed, 18mm BDMS guides etc.
- Replace the guide bushes
with V rollers to reduce friction.
- Design a simple roller
nut to remove the friction from the leadscrews.
- Utilise
608 bearings wherever possible, available from skateboard suppliers
much cheaper than bearings from mainstream bearing suppliers.
- Keep the components
simple and small enough to machine on my Chester Model B and using
castings made in my home foundry.
- Design
the components and assembly using Blender producing a series of
tutorials based around the machines components to help others get to
grips with producing accurate 3D models and assemblies with Blender.
- Investigate other drive
electronics, possibly a bipolar chopper driver.
The problems
with my first CNC Router | |
| | My
first attempt at a CNC
router detailed below has some major
limitations in its design and performance. Notably it was built on a
shoestring budget with the cheapest possible configuration. The bed was
made from bonded aluminium box section. These were
architectural
trims acquired at cost price.
This has proved to be
sufficiently rigid
and provides good fixing via the ‘T’ slot sections.
This is
possibly the most successful part of the machine. Most of the other
parts of the structure were machined from a length of 3inch by 1 1/2
inch extruded aluminium bar, where this wasn't big enough sections were
bolted together. | |
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Aluminium
section bonded together to form the Bed | | |
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Guides
and Slide Bushes | | The
guides are
lengths of 18mm bright drawn mild steel rod with sintered bronze bushes
as the slides. Again the 18mm diameter bar has proved amply rigid
enough for this size of machine, however the friction from the bushes
causes a sufficient “slip stick” effect to be a
problem and
light oil on the guides quickly becomes contaminated with dust binding
up the slides and causing the stepper motors to lose steps. The machine
then loses position and messes up the job.The positioning of the
carriage is carried out using lead-screws made from threaded rod with
brass nuts. To minimize backlash a split nut tightened by a small grub
screw was used. | |
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Lead-screw
and Phosphor Bronze nut |
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| The threaded rod was easily accurate enough to position
the router within 0.1mm of the intended position, but backlash was a
problem and as the backlash was adjusted out friction increased again
causing the stepper motors to lose steps. The
electronics was also a problem. Not something I have a great deal
of experience in I managed to trawl the web and put together a basic
stepper drive but extremely inefficient and not capable of fast step
rates. The motors relied on dropping resistors to regulate the power
resulting in much poorer performance than could be achieved with better
electronics.
My First CNC
Router
The
machine bed was made from epoxy bonded aluminium extrusions and the
support components machined from 3 x 1 1/2 solid bars. The design of a
traveling bridge and twin X-axis drives was chosen to allow larger
pieces to be mounted on the table without requiring extra torque to
drive them. The Y-axis guides can be raised within the frame to
accommodate thicker pieces. The machine performed
well on slow feed rates but occasionally missed steps on the X-axis
when the feed rate was increased it seemed obvious larger motors were
required. After some searching I found a suitable replacement with the
same frame size but a substantial increase in torque. A slight
modification was made to the coupling to accommodate the increased
shaft diameter and the new motors fitted. The machine hasn't missed a
beat since. The step generator
circuit was built using a simple TTL logic circuit. This gave the
correct 4-phase sequence of steps for the stepper motors from the
serial step signal generated by the computer. The output from the TTL
circuit was then used to control the power transistors to the motors. | |
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