I built a permanently attached mill carraige very similar to the one
you are contemplating. The carraige is used to move and level my
Bridgeport-type milling maching. The frame is made from 1/4" angle
iron and plate. The front casters swivel and the rear are fixed to
make it easier to steer. The jacks are fabricated from 1" threaded
rod and are threaded into coupling nuts welded to the frame. The
jacks have swivel pads on the bottom and a hex head on the top. The
front of the frame has a clevis connection to attach an approximately
4' T handle that I use to pull the carraige. This is much easier than
pulling on the machine itself. I used bondo between the frame and the
mill to prevent metal chips from collecting in the crack.

The carraige moves easily and the jacks level the mill with very
little effort. The carraige is very stable both on the casters and on
the jacks.

I have posted the following pictures of the carraige in the
metalworking.com dropbox:

mill carraige 1a.jpg
mill carraige 2a.jpg
mill carraige.txt

This is the first time I have posted any pictures in the dropbox so
please ignore the other mill carraige files. They are just larger
versions of the same photographs. The following are the links to the
dropbox photographs:

http://metalworking.com/DropBox/mill...raige%201a.JPG
http://metalworking.com/DropBox/mill...raige%202a.JPG

Again this is the first time I have tried to insert a link so if it
doesn't work you can just paste the address into your browser.

I hope my experienc provides some ideas for your carraige.

Ron Leap