This post is intended to help those who are looking into converting their Bridgeport or Bridgeport clone manual mill to CNC. This topic has been discussed many many times in the past but I'd like to list my observations after completing my 2 axis conversion. First off, I bought a Taiwan-made BP clone, in mint condition, from a local community College. I recently decided to add 2 axis (X-Y) CNC controls to it so I could make more complicated parts at my home shop. Now, I work at a machine shop where I have access to both a Haas VF3 and VF4, both with 4 axis rotaries, but I like to come home and be able to work on my own projects. The technical specs of my conversion are as follows:
-Hiwin ground ballscrews
-Nema 34 1128 oz-in hybrid steppers
-KL-8020 hybrid stepper motor drivers
-Inductive proximity sensors for homing
-Custom made yoke and motor brackets
-Custom made belt and motor guards
-Centroid Acorn controller
-Centroid MPG pendant
-Fusion 360 (free hobby use)

First, I'd like to say that I'm very pleased with the performance and accuracy of the Hiwin ballscrews. They are preloaded so they have only a few tenths of backlash and they are silky smooth.

Second, I initially thought I would still be able to use my mill as a manual machine after the conversion. I really didn't think this through because the only way I know that one could achieve this correctly is by removing the belt between the leadscrew and stepper motor. With the guards/shields in place, this is very difficult unless you have a tensioner that you can slack the belt with.

Third, if you are thinking of converting a BP clone, you may have to make a custom yoke to mount your ballscrew nuts to. Some Taiwan mills use the same size yoke so you may get lucky and could use the old Elrod yokes now sold by Servo Products. I started out by making a bullet aluminum yoke for my machine. Once installed, I quickly realized that the force exerted by the y axis ballscrew can flex the yoke and cause significant lost motion....about 0.002" on my machine. Don't make the mistake I did, use steel if you have to machine your own. Speaking of making your own yoke, these are a precision component. The perpendicularity of the ballnut mounting surfaces is critical!

Fourth, the cast aluminum bearing retainer mounts on the ends of the table and knee are not rigid enough for cnc use. The y axis retainer on the knee is probably okay but the x axis retainer will flex and cause loss of motion.....I've measured up to 0.0015" on my machine. This can be remedied by making the retainers out of steel and use thicker proportions where you are able.

Fifth, be aware of the limitations of a Bridgeport type mill when deciding on a cnc conversion. These knee mills, especially those with R8 spindles, cannot be pushed hard or you run the risk of knocking the head out of tram. I've been using a 6 flute, insert, 2" diameter shell mill for facing for quite some time and I can easily push the cutter hard enough to overcome the tram setting. These mills are capable due to their versatility but only with a loss of rigidity. You can't "feel" how the machine is cutting with CNC control so sometimes it is difficult to know just how hard you are pushing the machine.

Last but not least, as others have pointed out, some milling operations are just faster on a manual machine. The MPG pendant makes a world of difference when setting the work coordinates and jogging the machine around but at the end of the day, basic operations such as bringing a part to length on the side of a vice can, IMHO, be performed quicker manually.

Hopes this helps anyone out there considering a cnc conversion on a BP or BP clone. It can be a fun project but it is a labor of love. You can achieve great results and have a fairly accurate machine after a conversion but you could also save yourself ALOT of work if you can find an older oem cnc that needs a control retrofit.