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CNC PCB DRILL - CONSTRUCTION

Modified: 23:20, 26 October 2013

This project has a lot to it so has been split into the following pages:

SPECIFICATION:

  • Reads .drl gerber format files
  • 0.001" resolution
  • 0.002" accuracy
  • 265mm x 280mm table
  • Manual feeds
  • LCD display
  • Serial connection.

MAIN FRAMEWORK
The main frame is entirely made from meccano and forms a gantry with 4 supports at each corner which are screwed to the base with a right angle bracket. The base is made from 15mm plywood. Most of the beams are right-angled pieces for rigidity and also give twice the area to secure to. The stamdard 4mm blots were used and tightened very securely. The rails were 2 pieces of 15mm angled steel screwed to the base, these took time to align The structure was strengthened by 2 diagonal struts at each corner. Some beams needed to be extended to span the table. Each corner was put together using a square so there was no stress when it was fixed to the base. The result is an incredibly stable structure - I mean ROCK solid!!
X POSITION MOVEMENT
The table is slightly larger than A4 size and has 2 pieces of right-angled steel that ride along 2 others attached to the base. These are well greased. The table is made from laminated MDF with a sacrificial 9mm MDF board on to. The X axis motor is mounted to the base. It has a 6mm shaft and uses a polythene pipe as a flexible coupling to a 6mm threaded bar turned at one end to 8mm. The pitch of the thread is exactly 1.5mm. The motor has a brass gearbox with a reduction of 11:1 The min slotted opto switch is mounted next to the motor. As the table moves towards it, a small piece of angled meccano attached to the underside of the table breaks the light beam The other end of the threaded drive bar is supported in a piece of shaped nylon attached to the base to prevent it thrashing about. Secured under the table is a captive nut held in a wooden housing.
Y POSITION MOVEMENT
The y axis drives a carriage holding the drill and drilling mechanism across the frame gantry. A motor from a printer is held in place onto the side of the frame with shaped strips of meccano. Meccano brackets also support the drive shaft into the gear train.

The coupling was formed by drilling a 1mm hole through the plastic toothed wheel on the motor shaft. Then another 1mm hole in the 4 drive shaft and a paper clip through both holes and bent over. I know it sounds awful but it works a treat.

The 4mm shaft connects to a series of gear wheels that form the drive train. this feeds exactly the same turns to each of the y-axis drive threads that attach to gear 1 and 5 (counting from left). I tried using toothed belts but they were terrible and just didn't have the grunt to overcome the friction of the carriage. The drive screw threads are a pair of 4mm threaded bar so it is essential they turn together to prevent the carriage jamming..

The gears are mounted on 4mm shafts with a thread cut in both ends. One end secures the axle to the frame with a double nut, and the other has a double nut and washer holding the gear in place. The grub screw holes act as great oil points.

The cariage was supported by 2 pairs of guides formed from nylon. They moved along 2 stainless steel bars taken from an A3 inkjet printer and were guides for the original print head. Nylon is an oily material which helped but lots of grease was also added.

There are captive nuts held under the guides and secured to the carriage so the carriage moves along the guide bars as the threaded drive bars turn .The nylon supports are bolted to 2 laser cut plates either side of the carriage with 2 meccano supports to keep it rigid. Attached to the supports are 2 (one cut in half) stainless steel linear bearings which guide and support the verical motion of the drill holder. These are probably the most expensive part of the whole drill - thanks Dave - and have no play in them whatsoever and provide superb friction free movement.

The stainless steel carriage supports are secured wither end with another nylon block with a hole and tapped grub hole to keep the bars in place.

One end of the drive threads has a slotted wheel attached. The pitch of a 4mm thread is 1mm which is 0.03937 which means the wheel has 39 slots, each slot 1/1000 of an inch. The error incurred by the extra 0.00037" develops an error of nearly 0.1" over the width of the table. The PIC compensates for this by adding an extra count every 105 pulses. The 10 inch table width requires 10000 pulses so that's an extra 95 pulses = 0.095"

Z POSITION MOVEMENT
The motor moved up and down on guides formed from the stainless steel bearings mentioned earlier. These are so smooth that the motor literally glides up and down. Attached to the guides is a block of nylon measuring about 75mm square by 20mm high. The motor has a friction fit into the block of nylon. The motor diameter was exactly the size of a drill bit I happened to have. Since inserting the motor, it hasn't moved since. It can be adjusted or removed with some effortif needed.

The mechanism to raise and lower the drill and nylon mount is made from more meccano. I had a piece that slid along a flat straight and could be connected to a gear. This gave the vertical movement required. A bit of modifying and I ended up with a strut either side of the motor lifting the mount squarely.

The motor used was a 12v type with a brass gearbox on the end giving an output ratio of 148:1. This moves at about 2 turns per second at full speed with a great deal of torque. This is needed to overcome the weight of the motor. There is no friction at all due to the pukka linear bearings.

The carriage undertook quite a design change to ensure that the lifting mechanism was robust and would not distort under strain. Some pieces of meccano had to be adjsuted slightly to accomodate the holes on the motor mounting plate.

The entire lifting mechanism is fixed to the 2 vertical linear bearings with just one nut and bolt either side. Since the lift effort is balanced in line with these mountings, there is no turning force applied to the fixing and it stays in place with just these 2 bolts. They are done up quite tightly however.

Mounted on the carriage guides are 2 flat strips of metal that bread the beams i the opto-switches mounted either end of the frame to let the MCU know when the limits have been reached.

Other pages:
By Phil Townshend 2012

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