Half Nuts?

[great white]

I just started using the power carriage feed on my old "new to me" atlas TH42.

It seems when I engage the half nuts, there's a fairly long delay and then the carriage starts to move.

Once moving, there's no stopping it without disengaging the feed lever.

Is it normal to have a large amount of "backlash" is the carriage feed or does this indicate worn half nuts in need of replacement?

I'd really rather not have to take the apron apart to only find they're fine "as is"....

 

[RJSakowski]

I would expect half nut wear is the issue. All the lash in the gear train and the lead screw should have been taken up before you engage the half nuts and they are the only thing left. When you say "the carriage starts to move" do you mean actual movement of the carriage or of the hand crank beginning to rotate? If the latter, there is typically a fair amount of lash in that gear system on an older lathe. You can assess this by looking at the amount of rotation necessary before carriage movement begins when you reverse direction.
My old Craftsman/Atlas lathe had white metal half nuts and I had to replace them years ago.

 

[great white]

kinda both the wheel and the carriage.

I'll pay more attention next time I use it.

I'm suspecting there's a couple zamak gears inside the apron that have decent amount of wear on them.

It works the way it is, I'm just a proactive maintenance kind of of guy.

Among other things, I build engines and things within service limits can stay, outside has to be replaced. Not having those specs for something that is supposed to be a precision instrument like a lathe kinda drives me nuts.....

 

[RJSakowski]

kinda both the wheel and the carriage.

I'll pay more attention next time I use it.

I'm suspecting there's a couple zamak gears inside the apron that have decent amount of wear on them.

It works the way it is, I'm just a proactive maintenance kind of of guy.

Among other things, I build engines and things within service limits can stay, outside has to be replaced. Not having those specs for something that is supposed to be a precision instrument like a lathe kinda drives me nuts.....

I just checked my Atlas and it has a collar which controls axial play in the lead screw. It is held in place by a single set screw. If that collar slips, when you put a load on the lead screw by engaging the half nuts, it will tend to pull the lead screw out rather than move the carriage in (assuming power feed toward the headstock). A severe load like running into a stop under power feed could have moved the collar. Another possible is build-up of crud in the half nuts. Grease combined with machining swarf or other dusty or dirt can build up, preventing the half nuts from fully closing. Since the Acme threads have a slight taper, this would manifest itself as additional lash.
I have noticeable wear in my half nuts but not a significant amount. I have .015" play in my Atlas; my year old Grizzly G0602 has .003". In normal use, you engage the half nuts outside the work piece so the lash is removed.

 

[great white]

I'll have a look at those items.

I don't know if it's ever been crashed, but I don't recall the lead screw moving in and out on engagement. Worth a check though...

 

[Bill C.]

With the power off engage the half-nuts and measure how much play there is while turning the feed handle. If the lead screw moves back and forth as soon as the nuts are engaged then check the left end of the lead screw. See if there is adjustment or if you need to make a spacer to take up the gap to remove the play.

Good luck

 

[Wreck™Wreck]

I just started using the power carriage feed on my old "new to me" atlas TH42.

It seems when I engage the half nuts, there's a fairly long delay and then the carriage starts to move.

Once moving, there's no stopping it without disengaging the feed lever.

Is it normal to have a large amount of "backlash" is the carriage feed or does this indicate worn half nuts in need of replacement?

I'd really rather not have to take the apron apart to only find they're fine "as is"....

Are you using the half nuts for power feeding or just for threading? Backlash in half nuts while threading is unimportant because you can't cut the same thread in both directions. Lash in a power feed becomes a problem when it is required to move accurately in both directions when interpolating a radius for example.

 

[great white]

Are you using the half nuts for power feeding or just for threading? Backlash in half nuts while threading is unimportant because you can't cut the same thread in both directions. Lash in a power feed becomes a problem when it is required to move accurately in both directions when interpolating a radius for example.

Feeding.

Haven't attempted screw cutting yet.

 

[randyc]

I'll just toss this out there for whatever ... I have read of people refurbishing half nuts. This is what was described:

• Disassemble and clean the half nuts THOROUGHLY so that all traces of lubricant are removed (solvent + submerging in boiling water)
• Grease an area of the lead screw about halfway down the ways
  
• Mix some "J.B. Weld" and apply a generous coating to the threads of the half nut.
• Reassemble, move the carriage to the point where the half nut is over the greased lead screw area
• Engage the half nut (note the lathe is NOT turned on)
• Wait 24 hours then disengage the half nut
• Disassemble again if necessary and trim away the excess cured J. B. Weld (may not be required)

Personal observations:

• Never tried this myself
  
• Probably can't hurt anything unless parts are inadvertently epoxied together
• Epoxy won't survive a crash
• Epoxy may not survive heavy feeds (I'm presuming that the lead screw is the feed mechanism)
• But as Shakespeare said "Nothing ventured, nothing gained"

 

[great white]

Interesting thoughts, bit I'd rather just buy new half nuts if that is the problem.

The leadscrew doesn't move when the half nuts are engaged. I also watched more closely when engaging the half nits to feed. It doesn't seem to be as slow to move as I though. I believe its just the slow speed at which it was moving.