Centec 2B fast traverse ideas

[Graham Horne 2Participant @grahamhorne2]

I’m rebuilding a Centec 2B mill, and I’ve got other threads where I’m sharing photos of the progress for anyone interested. This post is more about how the half nut works on the Centec 2 mills and whether the fast traverse captain wheel on the 2B is actually useful or not. Just a heads-up, I’m not an experienced machine rebuilder, so I’m open to any advice!

The Centec 2B has a captain wheel that drives a gear running against a rack under the table. When the half nut is disengaged, you can make quick longitudinal movements using the captain wheel. I can see how this would have been great in an industrial production setting, but to me, it seems like a compromise when it comes to the half nut engaging properly with the lead screw. I’ve seen upgraded half nut designs that make it more robust and give it a more positive engagement.

My mill—despite being a later model—has what seems like a pretty weak friction lever to hold the half nut against the lead screw. Of course, you have to disengage the half nut to use the captain wheel for fast traverse.

I see two main issues with this:

I’d prefer to use a motorized longitudinal feed for both slow cutting and fast traverse, so the whole engage/disengage process for fast traverse doesn’t seem necessary. Am I overthinking this? Is the fast traverse actually really useful, and I just haven’t discovered its benefits yet?

The half nut mechanism on my machine doesn’t feel robust or provide a solid engagement.
I’m thinking about replacing it with a fixed bronze nut built into the longitudinal slide—more like the cross slide nut—so there’s no engage/disengage mechanism at all. I haven’t checked how feasible that is yet, but that’s the direction I’m leaning.

Would love to hear thoughts from others. Is the half nut mechanism actually decent, and I’m just overthinking it? Or should I just focus on making a stronger, more positive engagement half nut and keep the fast traverse?

[John HaineParticipant @johnhaine32865]

I think a half-nut mechanism is rare on mills though common on lathes.  I could imagine in a production workshop where you might have multiple jigs set up on the table it could be useful  If you’re planning on fitting a power feed anyway I wouldn’t bother with it.

[Keith Rogers 2Participant @keithrogers2]

When I had my 2B I found didn’t use the fast feed handle so I tightened the nut and locked the half nut in permanent engagement. Easy enough to just wind the handle or use power feed. Somebody once told me that it was often used for slotting screw heads and similar operations.

Keith.

[John Haine]

When I thin

On John Haine Said:

I think a half-nut mechanism is rare on mills though common on lathes.  I could imagine in a production workshop where you might have multiple jigs set up on the table it could be useful  If you’re planning on fitting a power feed anyway I wouldn’t bother with it.

I was also considering making a proper bronze nut attached to the cross slide, with a pin from the nut going through into a hole in the cross slide. That way, it would be securely attached but still able to swivel slightly. The lead screw would run through the nut, and I could possibly add some backlash adjustment to fine-tune it.

I know the lead screw threads are square, which might make internal threading more difficult. Not sure if this is the best approach, but it seems like it could improve engagement and make everything a bit more solid. Has anyone tried something similar, or is there a better way to go about it?

[SillyOldDufferModerator @sillyoldduffer]

Graham’s diagnosis and proposal sound right to me.   Seems his Centec was configured for repetitive production work, where much the same operation was repeated endlessly and as fast as possible by an unskilled or semi-skilled operator.

As general purpose milling rarely calls for fast repeat, I see no reason why Graham shouldn’t remove it.

Repetition work tends to concentrate wear and tear on short parts of the ways and lead-screws so check for it over the full range of travel.   Depends on how hard the machine was pushed, over what distances, and for how long.   Worst case, so much metal has been lost over a short distance that the mill’s accuracy goes to pot in that zone, best case, mild wear spread evenly with no ill effect.

Replace badly worn lead-screws and/or nuts.  Worn ways can be ground flat, at least a few times, and there are potions that can replace missing metal.  No experience of them, but reports seem positive.

Dave

[Graham Horne 2]

On Graham Horne 2 Said:

I know the lead screw threads are square, which might make internal threading more difficult. Not sure if this is the best approach, but it seems like it could improve engagement and make everything a bit more solid. Has anyone tried something similar, or is there a better way to go about it?

This seems an ideal opportunity to ditch the original, and install a BallScrew

MichaelG.

[Gary WoodingParticipant @garywooding25363]

I’m with the other responders here. The capstan got in the way and I never used it. I removed the capstan and immobalised the half nut. After adding a VFD to the traversing motor I’ve got the best of both worlds.

[duncan webster 1Participant @duncanwebster1]

Dumped mine and fitted a stepper motor drive.

[Author]

Posts