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Bridgeport Mill CNC Conversion

The date is May 2016 and the decision was made to convert my manual Bridgeport knee mill to full CNC control.

It has served me well, having helped build two large scale traction engines and a multitude of other projects and production parts for my home business but I had ideas for other projects that just could not be made manually.

I did a lot of research before going in with the spanners, the workshop was too small for all but the smallest commercial CNC milling machines as it just did not have the headroom needed, plus the commercial offerings all had very small tables.

The Bridgeport had the size I wanted with the 48″ bed and it was in good mechanical condition generally.

My Bridgeport mill


I had decided to go with a full rebuild and use the best quality parts I could afford on it. The list included 750W AC servos on the X & Y axis and the knee, 500W AC servo on the Z axis, an uprated 3Hp main drive motor with 1:1 drive, new ball-screws on X & Y axis, an industrial grade CNC controller by CS-Labs and the front-end driven by Mach-3 software – Mach-3 was getting a little old in 2016 but I was familiar with it and it has a good support backing.

The Strip-down

The strip-down commenced…
Head removed
Years of dried grease in the back-gear housing
Head off, screws removed
Base fully stripped

Repainting and build-up…

Base repainted in “Bridgeport Grey”
Knee refitted
Top ram back on and knuckle joint fitted
Saddle fitted and new oil lines going in
Head rebuilt and all mechanical feed mechanisms removed then blanked-off
Table back on and ball-screws fitted

Axis Drives

The Y axis motor drive plate being fitted, pulley ratio is 2:1
Y axis drive fitted, belt is 25mm HD
X axis servo drive mount fitted, 2:1 ratio with 30mm toothed belt

Limit Switches

Limit switches were a must-have – the servo motors and drive reduction meant that there was enough power available in a bad situation to rip the ends off the table!

X axis switches being trial-fitted, one is a limit the other is a homing switch
Y axis switches, homing and limits
Knee limit and homing switches

Wiring and Testing

The wiring was lashed together on a sheet of scrap MDF. Partly for testing and partly to judge the layout for final cabinet build.

First wiring testing

The Z Axis

The Z axis is the hardest part on these conversions – the quill drive and head is not built for automation at all. Various options have been done before and I chose to drive the axis by connecting a ball-screw to the old quill-stop mounting. The motor is a 500W AC servo and drive is via 20mm toothed belt at 2:1 ratio.

The drive mount bolts on to existing points at the top and a hole drilled & tapped in the lower head reference surface.

Limit and homing switches for the Z axis

The weak point in the Z drive is the connection to the quill/spindle tube. The first attempt broke very quickly so I had to machine up a new, stronger version. This drive setup is hard on the ball-screw & nut as it places a lot of angular load on them and ball-screws do not like that.

Revised Z drive connector

The Cabinet

The control cabinet, welded up from 2mm steel sheet
Mounts for the control cabinet bolted to the main casting
Operator station fitted to an arm mounted on the rear head post
Button labels were engraved on my mini-mill

Cabinet wiring is one of my favourite parts, I have made several during my work days but this was the biggest. Servo drives are top-left, motor VFD’s are top-right, CNC controller in the middle and the PC for the display at the bottom. Behind the PC are the safety protection fuses and breakers plus wire loom connections.

Control cabinet
The cabinet, PC removed for view

Final Testing and Debugging

Testing the setup
The finished build, almost….

The High-Speed Spindle Attachment

When I made the conversion, I wanted to be able to sell my mini-mill to recover floor space. This meant I would need a high-speed spindle on the Bridgeport. I devised a mounting bracket to enable a 24,000rpm water-cooled engraving spindle to be fitted to the main Z axis of the Bridgeport.

Having a working CNC mill now made the build easy and very fast, 6061-T4 aluminium tooling plate was used as the surfaces were finished to a high standard.

Attachment bottom plate

When in use, the attachment mounts onto the quill/spindle, the small spigot is clamped in a collet to give extra stability. The main drive motor is locked-out and the drive signal is switched to the high-speed motor drive.

The attachment stored on its bracket when not in use

For engraving use, a large sub-table is mounted on the main table, it was drilled and tapped with a grid of M6 holes for clamps.

Engraving sub-table
Engraving test – 8mm letters, not bad for such a large machine

The Knee Drive

The knee drive was added a while after the main build, another 750W AC servo and 4:1 drive was used. This was programmed to take the tool length offsets so it only moved when changing tools or setting up the job.

Knee drive being fitted

Swapping the Spindle

The spindle was changed after a bit of use – the original R8 taper fit spindle was swapped for a QC30 spindle. The QC30 is compatible with modern BT30 tooling which means I can use the tool table and preset tool offsets as they are in much larger machines.

Tooling Up

Now that I had a BT30 spindle, I could use standard tooling mounts which offered repeatable position for the tools – the old R8 spindle collets were not repeatable which meant jobs had to be programmed and run one path at a time. Repeatable position tooling means I could run the job as one process and change tools where needed.

A BT30 tooling chuck
My tool rack
BT30 tool fitting mount, bolted to the end of the mill table
Tool height setting probe

The Software

I wrote completely custom user interface screens in Mach-3 – these featured only the exact functions I needed to do what was required, no bells and whistles.

Some Videos

Axis motion tests
Circular interpolation check
Making the high-speed mount plate
Making the engraving sub-table
Testing a threading cycle
My tool-height probe/set screen

Afterthoughts and Notes

The CNC controller I used has something called “Encoder homing” – this uses a combination of mechanical limit switch and electronic encoder for setting the home/zero position of each axis. Using a switch alone is not very accurate but using encoder homing allows you to return to a job at a later point while it’s still set up and continue working on it after having turned the mill off!

Converting a Bridgeport mill to CNC most certainly does not increase its resale value to the point where you can recoup the costs involved in doing so. I knew this at the start and was OK with it, it’s worth remembering though.

The parts I used are fairly expensive – the ball-screw kit came from the USA and cost nearly 3x what I paid for the Bridgeport! The CNC controller was industry standard kit but being that good made the job so much easier as it just worked without any messing about, support is also there if needed.

After conversion, the mill went on to produce work for 4 years, at which point I retired and sold-up. It made far more money than it cost to convert 😉 and the person that bought it got an absolute bargain.