Phil Coleman's Web SiteConverting a Bridgeport to CNC on a Budget
A cheap add-on for 3D CNC
I became fascinated with CNC having finished a clock and deciding that firstly crossing out was not very fulfilling and secondly that converting the mill to CNC would be a good project to keep the mind occupied with technological developments and improve my understanding of this subject.
I have a beautiful Bridgeport “J” head machine, old but little used and therefore in great condition and it seemed logical to use that as a basis for a CNC conversion.
The basic requirements for converting a standard milling machine to CNC are:
Fit motors to provide horizontal movement to the table (“X” & “Y”) and to the spindle to provide up and down movement (“Z”). I was not able to convert my Bridgeport directly to ball screws because the cost was too great. There are kits available for this machine and if you can afford them that would be the way to go but they were beyond what I could justify. So I went for a compromise and bought a moderately priced X-Y compound table from a local supplier and converted it to ball screw operation.
It is highly desirable in any CNC system that there be no backlash in the drives to the table and spindle. For this reason it is practically mandatory to fit ball screws to all the drives. This is to prevent dead spots in the work that would show up wherever there is a change in direction in the cutting process such as in curves. Fortunately ball screws are readily available at reasonable prices and can be retro-fitted to machines with not much complication. I bought mine on eBay and they have proved satisfactory. The ball screws are fitted with opposed angular contact ball races and have shims to give preload so that there is no backlash in them. The ball nuts are fixed to the tables with robust brackets. The ball nuts don’t have any facility for taking up backlash and fortunately I haven’t found that necessary; they seem to be quite well made. They have a lead of 5mm.
I used a 6:1 toothed belt drive reduction on all the drives on my machine in order to reduce the likelihood of stalling the motor on heavy cuts and to give finer resolution. Having used the machine now for some time I think that this was overkill and that 2:1 or 3:1 would have been OK.
The “Z” drive uses the tapped hole in the quill for an extension bracket that supports the ball nut for the ball screw. This is shown in the photos. It works well and also has a 6:1 reduction.
All things considered I think the hardware is the simplest part of the enterprise.
Electronics: the electronic side of things for me presented the most difficulty and I was fortunate in having a friend who gave me invaluable assistance in setting this up. I would have been in dire straits without his help. In the end it all seemed reasonably logical and to my great relief, worked perfectly in the end.
The Bridgeport with the removable X-Y table. The X-Y table is easily removed so that the mill can be returned to normal operation when needed.
This is the X-Y table with its ball screws fitted:
This is the "raw" X-Y table before any modifications:
Another shot of the X-Y table:
How the ball screws are fitted to the X-Y table:
Here is how the electronics are set up:
This is how the computer is set up. I use "Mach3" software for the control of the machine. I use "Sheetcam" for generating the Gcode from DXF files.
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