Engaging half nut

Thanks for your answers.

yes, I was talking for normal feeding, not for threadening. Sorry about my mistake not saying at first. Ok so, when you are regular feeding, you simply try until is engaged right? I try 2 or 3 times until engages, but I don´t know if there is a better way without using the thread indicator.

But if the best way is using the threading dial, As some of you said, I´ll practise with it. I wasn´t using it because I think I heard that it weard down
 
Alberto-sp said:
Thanks for your answers.

yes, I was talking for normal feeding, not for threadening. Sorry about my mistake not saying at first. Ok so, when you are regular feeding, you simply try until is engaged right? I try 2 or 3 times until engages, but I don´t know if there is a better way without using the thread indicator.

But if the best way is using the threading dial, As some of you said, I´ll practise with it. I wasn´t using it because I think I heard that it weard down
Click to expand...
Use the thread dial to help you know where to engage the half nut. It will engage either exactly on the line, or a little before, or a little after. If you need or want to have the dial exactly line up, you can install a shim or two in between the thread dial and the apron. I did this on my mini-lathe to make the half nut engagement on the line. Shim will probably be around 0.5mm or so. Vary the shim thickness until the thread dial lines up and you are able to engage the half nut without friction. I would use the 1 on the thread dial for the alignment procedure.

If you practice engagement, even when you are feeding, it will help you develop the muscle memory needed for threading. Then threading will become a lot easier.

You will wear the half nut if you try to forcibly engage it when it does not want to engage. (Many many times.) It is best to time the engagement, by using the threading dial. Start out with the lowest RPM and learn to engage cleanly and consistently. Then you can increase the RPM and practice some more. Eventually you will be able to do it even at higher speeds without thinking about it. It is just a practiced skill.
 
Most lathes will also have an adjustment for the half nuts to seat so the engagement clicks in with reasonable force - not too loose of too tight. Dave
 
Improper alignment of the lead screw w/regard to the half nuts can cause difficulties in engaging the half nuts. To check for that, engage/disengage the half nuts with the motor turned off. If the lead screw moves up & down or back & forth (or a combination of those) while engaging and disengaging, you have an alignment issue that needs to be addressed.

In the usual procedure for alignining your LS and half nuts, the carriage is moved all the way to the right, then the screws holding the pillow block are loosened. Engage the half nuts, then without disengaging them tighten the screws back down. You may find it useful to do the same thing with the left-hand pillow block; but on a mini lathe you will have to remove the speed control box to gain access to the screws. You also may want to remove the gear attached to the lead screw so the adjustment range isn't limited by it; or loosen the banjo to free up the gear.

There is yet another adjustment that can help w/regard to LS misalignment. There are two screws that attach the apron to the saddle, located on the top front left and right sides of the saddle. They are recessed in slotted holes. To perform this procedure, again run the carriage all the way to the right. Engage the half nuts, then loosen/re-tighten those two screws. This will only work for misalignment that produces a back & forth motion of the lead screw, since that's the only direction that can be accommodated by the screw slots.

BTW the bottom of the screw heads on my lathe were rounded so they always sought the center of the slot. This caused me all kinds of grief when attempting the alignment, until I replaced them with better screws.

If these don't eliminate or greatly reduce the misalignment you may have to shim the rack. Also check for debris in the half nut mechanism, proper adjustment of its dovetail, etc.

On my minilathe, which has two half nuts (some models only have one) there also is an adjiustment which sets the gap between the half nut threads and the lead screw threads. Due to the fact that single-nut versions can be easily converted to the dual-nut version, it's likely they have the same adjustment. It's possible that a very small gap might also cause difficulties when attempting to engage the half nuts.

You don't want the gap to be too small because that causes increased friction, nor too little because the half nuts could slip loose. While the latter wouldn't be that big a deal while power feeding, it would be a disaster if you're cutting threads.
 
FWIW, I regularly engage the half nuts at turning speeds on my current machine which doesn’t even have a thread dial. I use to do the same on my previous machine which did have a thread dial that I ignored. IMHO, the thread dial is only useful when you’re cutting threads at low rpm. At turning speeds, it's spinning too fast to be useful anyway. The trick at turning speeds is to close the half nuts firmly by feel rather than quickly slamming them in like you can do at low speed threading rpm. With a little practice, you'll get the feel for it and it's not going to prematurely wear out your lead screw if you keep it lubricated.
 
Is the half nut the only way you can feed the tool? The half nut is for threading, usually there is a feed lever independent of the half nut to engage the feed for normal cutting.
 
T Bredehoft said:
Is the half nut the only way you can feed the tool? The half nut is for threading, usually there is a feed lever independent of the half nut to engage the feed for normal cutting.
Click to expand...
Depends on the lathe. Usually, the less expensive models have the leadscrew doing double duty for threading and feeding. My Atlas 10F is like that.
 
My Boxford lathe has a reliable thread dial indicator but I never use it. In fact if you are cutting a metric thread on an imperial lathe or vice versa you cannot use the dial indicator, and it is best not to disengage the half nuts at all. After installing a variable speed DC motor in my lathe I find it quite simple to cut threads without disengaging. At the end of one pass you withdraw the tool and wind the tool back to the start using the motor at high speed in reverse.

There will always be some backlash between the half-nuts and the leadscrew. When making the next pass cutting the thread you have to make sure that the carriage has taken up the backlash, otherwise the tool will not engage with the groove of the thread you are cutting. Simply wind the tool a few cm past the beginning of the thread while running in reverse, so that as the motor starts it takes up the slack before hitting the thread. During this stage I put my hand on the big carriage wheel and apply a bit of friction to make sure it takes up the slack.

While actually cutting the thread I start slow, speed through the middle and slow right down at the end so that I can stop precisely where I want it. Cool!

My chuck is screwed on but I do not have any problem with it unscrewing with this technique because it is not under load when using reverse.

I think you already have a lathe with a variable speed motor, but for those who do not, you might find my YouTube videos useful. I just posted them on another thread, but thought you might also find it useful.

Evan’s You Tube Channel which includes stuff about steam engines. See the Playlist for engineers lathe.
https://www.youtube.com/evan-e-cent

Engineers Playlist

#35 Installing a DC motor

#37
How speed controllers work

If you need an online computer program to work out geat trains for cutting any thread, see
 
T Bredehoft said:
Is the half nut the only way you can feed the tool? The half nut is for threading, usually there is a feed lever independent of the half nut to engage the feed for normal cutting.
Click to expand...
"Power feed" on mini lathes is just a combination of change gears that produces the finest-possible thread. That's 256 TPI for an imperial lead screw and the maximum gear ratio available in the standard change gear set ( 20 80 20 80 for A...D respectively). Due to physical limitations it's difficult to get anything higher.

Some folks have implemented an ultra-fine power feed by attaching a gearmotor to the lead screw.
 
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