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Homercide
03-11-2015, 11:21 AM
Hello, I have an Atlas QC-54 10f lathe and the half-nut engages the right way about half of the time. You have to fiddle with the apron wheel or power feed to find the sweet spot to get the lever to go all the way down and engage. Is this normal? It can happen anywhere along the lead screw. The screw seems fine and I took it all apart and cleaned everything very well. (that may be my problem). Is there an adjustment procedure that I should do to get this corrected? I tried tightening and loosening the two nuts holding the lead screw in the bearing. I also tried moving the lead screw bearing around within the limitations of the two screws to see if that made a difference. Shouldn't the lever engage every time it is moved downward?

I went in again and cleaned everything and lubed it again last night with no change.

Thanks

RichR
03-11-2015, 11:50 AM
Hi Homercide
Welcome to the forum. A half-nut will only engage when its threads coincide with the threads of the lead screw. I apply gentle pressure to the lever then
as the feed screw comes into alignment with the half-nut I can feel the lever start to move and finally engage completely.

Homercide
03-11-2015, 12:00 PM
So what I am experiencing is normal?

flylo
03-11-2015, 12:04 PM
Probably as you have to"feel" the sweet spot.

Doozer
03-11-2015, 12:35 PM
The half nut threads are probably as sharp as brush bristles.

-Doozer

TGTool
03-11-2015, 01:40 PM
The half nut threads are probably as sharp as brush bristles.

-Doozer

Hmm. That could well be, but it shouldn't exhibit as the symptoms he's experiencing. In theory, if the nut thread lands are very thin it should provide a broader timing window for engagement.

Hopefuldave
03-11-2015, 07:51 PM
So what I am experiencing is normal?

Hi Homer,

Yep, leadscrews are square threads or Acme (flat-topped and flat-bottomed), if the peaks and troughs aren't lined up they'll ride on each other while either the leadscrew turns or the carriage (and so half-nuts) move and they align.

The usual procedure is to engage drive to the leadscrew and apply gentle pressure, just like Rich has described - if you're threading, it's normal to watch the thread dial coming round and apply pressure as one of the numbers is approaching the pointer, and use the same number (I'm simplifying!) for each pass.
If you're cutting Metric on an Imperial lathe, or vice-versa (or DP, Module, B.A. threads) it's normal to have to leave the half-nuts engaged, stop the lathe at the end of the threaded section and reverse back (after withdrawing the tool!) to beyond the beginning again, as the thread dial won't help...

If your lathe doesn't have a separate feed shaft and uses the leadscrew for power feed it's not important to pick a number and stick to it, but the feeling it in is exactly the same procedure, even with a keyed leadscrew driving a worm and wheel for power feeds the gears will need time to come around and mesh.

vpt
03-11-2015, 08:08 PM
Mine does sort of the same thing (qc48 10f).

If I put slight pressure on the half nut lever I can feel the half nuts ride across the top of the leadscrew threads until they line up and the half nuts fall into place (engage the lead screw). However sometimes mine will "half engage" where the carriage would start to move but the half nuts aren't fully engaged. I will back them off and then try again. Like mentioned it is a 'feel thing' that you will get used to as you use the lathe. I know my half nuts are worn some and that may be part of the problem. I am going to keep using them though till they wear completely out.

Homercide
03-11-2015, 11:55 PM
Thank you all for your input, I appreciate your help.