[jime]

I've got some small steel parts to machine and my knee mill is tied up digitizing and running other stuff. I thought about putting together a benchtop cnc to handle some of the smaller parts. I see many on this forum using the box types in the z axis. Most of those seem a little low on HP and light weight for a machine cutting only steel. I was thinking something like this one I saw on EBAY:

http://cgi.ebay.com/ws/eBayISAPI.dll...6924&rd=1&rd=1

Anyone want to offer any free advise?

[Runner4404spd]

i would use a square column mill at a minimum. they aren't that expensive. you can get an industrial hobbies one for about $1800 or a wholesale tool one for around 1400. both of those will offer significant advantages over the round column mill.

[wizard]

Just to reinforce what Runner... said, Absolutely DO NOT get a round column mill for CNC. No way no how. These are not the platforms to go out and buy new for CNC. The primary problem being keeping the vertical axis in registration.

If you have one laying around that is another story. You would still settle for lesser performance and registration issues but may be able to work around those issues with smaller parts. In the end it may make sense to take a loss and sell anything like that on hand.

Anyways back to the issue of square column mills. These vary widely in quality. Rong Fu seems to set the standard here. Many of the lesser machines though will need work to perform as well as you knee mill. I would go into this project expecting more work on the conversion than at first seems obvious.

Also due to the time and money invested in a conversion, you might be better off just converting another knee mill if you have the room. The Iron isn't that expensive relative to the time and materials for the conversion. A knee mill will be more flexible. The only issues being room in your plant and the size and volume of your smaller parts.

Dave

[jime]

Wizard, I have a beautiful old bridgeport that I have considered converting. Unforunatly, whenever I look at the cost of the ball screws and the Z axis conversion, I'm right back to a benchtop. Hardware to convert the quill is about $1500, ball screws for x/y another $1400. That doesn't include adaptation to mount the motors. Shouldn't I be able to have a benchtop with ballscrews and ready for motors for less than 2K (including the mill)? Am I looking in the wrong place for upgrading my bridgeport? Thanks! Jim

[Runner4404spd]

To do a high quality conversion will cost money no matter what. yes the bridgeport parts will probably be more expensive, since it is bigger. also neither one of us is knocking the use of a bench top machine, just yur choice of bench top machine. especially for milling steel, you want rigidity, your bridgeport will have that, the round column mill won't have nearly as much. that being said, you can get a tormach mill for about $7000, ready to go, or you can get an industrial hobbies mill for a little more. both offer significant advantages over the round column mill.

[phantomcow2]

Don't get that for CNC. Like others said, round column does not make for a suitable CNC. The X3 might be a little underpowered for you, though it may be worth checking out. It's a popular mill now that Grizzly has it in stock. I've done steel with excellent results, my CNC is almost done.

[jime]

Okay, you guys have convinced me. Looks like I'll save my $$ and go with fitting up the Bridgeport. Thanks! Jim

[phantomcow2]

You've made the best decision. It won't be cheap, but buying a large enough benchtop and converting it would not be either. THis way you've at least got the mill cost covered. I believe Hiwin makes ballscrew for Bridgeports, so maybe you can just install those with no modification needed. If not, give Roton a call for some decent rolled ballscrew. If you can get access to a lathe, you can turn the ends yourself after you've annealed it.

[jime]

Thanks for all the info. Based on what I see, I'll transfer this up to the Bridgeport forum.