Lead screw nomenclature

[IQChallenged]

I'm doing two things at once: gearing down my router CNC and converting my knee mill to CNC. Before placing my order for lead screws for the mill I need to confirm the distinction between free-end and fixed end. Can someone tell me which is which? My mill will ultimately have a stepper on one end and a bearing support on the other end. Neither of them will actually be "free". They will both be fixed. I realize it's just semantics but I need to know the language so I get the ends machined properly.

[ger21]

Free end is usually supported by a single bearing which doesn't restrict axial movement. Fixed end is usually in a block with double angular contact bearings, which prevents axial movement of the screw. You might want to have whoever your ordering from clarify their terminology for you, to make sure I'm right.

[IQChallenged]

I just now think I got it. Please tell me if I'm wrong. One "angular" bearing keeps the leadscrew from moving toward the operator. When I say "moving" I'm talking about unwanted movement which is only one part of backlash. But the shaft would still be able to move away from the operator (depending on which way the machine is traveling as it cuts a workpiece). So an opposite hand angular bearing stops it from doing that. And since these two bearings (theoretically) eliminate the movement of the screw toward and away from the operator (forget axial, linear, radial talk), the other block can simply support the screw with low-end tech and low-end cost. Am I right or wrong?

[Al_The_Man]

Usually a free end screw is used where the total length does not warrant a double end support, an example is in the Lathe X axis, where the total stroke is often only 12" - 18"
In this case there is usually a thrust bearing at the motor end and the nut is attached to the cross slide saddle and there is no outer end bearing support.
With longer lengths, if there is no support bearing on the other end, whip will ensue due to droop when running at high speeds.
Al.

[IQChallenged]

Originally Posted by Al_The_Man Usually a free end screw is used where the total length does not warrant a double end support, an example is in the Lathe X axis, where the total stroke is often only 12" - 18" In this case there is usually a thrust bearing at the motor end and the nut is attached to the cross slide saddle and there is no outer end bearing support. With longer lengths, if there is no support bearing on the other end, whip will ensue due to droop when running at high speeds. Al.

I get your point. And I notice that my mill has no support on the short screw. But-- isn't singular versus dual support (whip versus no whip) really different from fixed versus free (axial movement restricted versus not so restricted). I know I'm splitting hairs but I just want to make sure I have a good understanding. Sometimes the devil's in the details.

[Al_The_Man]

It has always been my interpretation that fixed ends refer to a ball screw supported at both ends, this is generally done with a double opposed thrust bearing at one end to prevent any axial movement, and the opposite end bearing is a standard non thrust type, or any combination of the above as to what bearings are used.
Maybe it would be wise to check with the supplier as to their interpretation.
Or decide on what bearing method you prefer and supply them with a drawing.
Al.

[RICHARD ZASTROW]

Many of us refer to the unfixed end bearing as the "floating" end, Meaning that end can move or float axially.

Also, some refer to the end that is fixed as the "held" end.

I'm sure you have figured it out.

Dick Z