View Full Version : Knee mill z-axis question.

05-08-2006, 01:25 AM
I have a used Rockwell milling machine that is similar to a Bridgeport in knee design for z-axis. My question is regarding the torque necessary to operate a "normal" knee mill vertical table axis. Mine is really hard to move up and down. I have taken apart the assembly and checked for burrs on the gib and ways. I have cleaned and lubed everything and made sure it was reassembled properly. The problem is, I've never had the opportunity to operate a knee mill and I don't know what a "normal" amount of torque is for this function. Any of you guys or gals have any common trouble shooting ideas for me? Any help at all would be much appreciated. :) Best regards, Eric.

05-08-2006, 02:39 AM
My first thought was to check the knee locks. But since you've been through it I guess that isn't it.

Let us know what you find out. I hope it isn't that metal on metal kind of grind.

On mine there is some effort required but not more than you would expect for that weight. Check your lube system. Mills are oily and dirty when everything is oosing properly. They just didn't bother to design a way to handle the drippings.

05-08-2006, 02:55 AM
On my Bridgeport, I normally do about ten revs of the handle (= 1") in five seconds. I can do that using the web of my hand to push down on the handle and my index finger to pull it up on the other half of the stroke. As the previous poster said, make sure the lead screw is properly oiled, i.e. wet.

And check the gib adjustment. The knee should be noticeably loose on the column with the lock released, and dead solid with the lock engaged. That's the way my BP is set up, and I expect it would be true of any similar design. That's one reason you don't feed the knee during a cut. Set it for the proper initial height, lock it down, and feed the quill.

05-08-2006, 06:22 PM
"That's one reason you don't feed the knee during a cut. Set it for the proper initial height, lock it down, and feed the quill."

I know everyone has their reasons...but I had always been taught the opposite. You never want to feed the quill during the cut if you can help it for several reasons. With a spiral cutter as it is not as rigid as the knee and something can grab and pull itself in.

More importantly, because of the rigidity, the knee ends up being superior to the quill in that the moment arm for the cutter gets longer and longer as the quill comes down, making for a bigger lever and less overall rigidity with the quill extended.

Thirdly, if the head is even slightly out of tram, the error will be multiplied as the quill is extended (the angle gets longer and you get further and further from concentric). I read several postings here or somewhere about frustrations with co-ax type indicators because (due to their length) there is a substantial difference in quill extension from setup position to cutting position....thus any slight angular error in head tram becomes really obvious when your center changes as you move the quill back down for the cut from the position it had when you were indicating center.


05-08-2006, 06:37 PM
Dweller, what model#.I have a wells Index 747 with the Power Z axis,there is a lot of weight there a bit more than a Bridgy as It is a beefier machine all the way around,But someone will argue that.

Anyway, I use a synthetic grease on the screw,and of course the one shot luber for the vertical ways.

05-09-2006, 07:52 AM
I don't know the model number. It's a Rockwell vertical mill and is smaller than a Bridgeport. After reading these responses, I know that something is wrong with the Z gib/way. I'm going to do some more investigating when I can muster up some time. Thanks for your help guys, Eric.