G9729 Half Nuts

[LX Kid]

I have the Grizzly G9729 17x31 and would like to try my hand at making my own half nut. At $75 a pop I need a spare or two. I've already replaced it once and they wear fast even when frequent cleaning and lubing. Question "is" does anyone know what the Acme type/TPI/pitch is for the lead screw. A tap is around $100 and want to order the correct tap. American Standard/ Metric etc.

I called Grizzly Tech Support and they didn't know. The guy said he would try and find out but was not too optimistic. Thanks

 

[LX Kid]

The lead screw is a left handed thread and because the lathe cuts towards the chuck there is more pressure against the half nut. If the G9729 had a reversing lead screw the half nut would not wear so fast because it would with the thread instead of against it. From the pic above you can see the wear on the right side of the half nut "fighting" against the lead screw.

 

[Bob Korves]

I think half nuts are more usually single pointed than tapped. You might also look into material choices. The photo looks a lot like brass, and I think some bronze alloys might serve better. Also, does the carriage move freely with the handwheel? If not, that could be a source of the wear.

 

[LX Kid]

I think half nuts are more usually single pointed than tapped. You might also look into material choices. The photo looks a lot like brass, and I think some bronze alloys might serve better. Also, does the carriage move freely with the handwheel? If not, that could be a source of the wear.

Well yes I've had a lot of problem with the binding of the rack and gear box pinion gear. I had to add about .008" shims to the gear box to make it so it wasn't binding. Moves "much" more smoothly now just since last week. Lathe has been sitting for past six months un-used. First half nut failure was because I had locked the saddle and engaged the lead screw! (Dumber than dirt!) I replaced the steel lead screw pin with a brass one now hoping this will save my half nut next time. When I take the lead screw out of gear, and engage half nut, turning the hand crank runs smooth. When I hand crank back towards the chuck it is a lot harder to crank. Wondering if I already have wear on the half nut for that direction?

 

[LX Kid]

I'm also wondering if it would help to file each sharp corner of the lead screw at the swarf opening?

 

[Bob Korves]

I was speaking about moving the carriage with no feeds or half nuts engaged, carriage lock loose. Does the carriage run freely then?

 

[LX Kid]

I was speaking about moving the carriage with no feeds or half nuts engaged, carriage lock loose. Does the carriage run freely then?

As I explained above it was binding until I added shims to the gear box. The pinion gear was too tight against the rack.

 

[Bob Korves]

OK, it sounded like you were also traversing it with the half nuts engaged but the gear box disconnected. I have never tried that, could be hard on the half nuts. If it was tight traversing the carriage earlier, that may have added some wear to the half nuts as the carriage was being powered.

 

[LX Kid]

I just ordered another half nut, $75, so I can have a spare. As you can see, from my first half nut, the wear on right sides of the acme thread. Gotta be something that I can check, adjust better, different lead screw lube etc. I believe the wear is caused by the lead screw, as described above, from the pinion and rack having too much pressure against it when engaged. I haven't removed the one I'm running on now but can see with a flashlight that it is wearing and will be another early failure. I also found the belt idler bearing starting to make noise so I ordered another, $3, from Grizzly. If I don't get this particular bug worked out this lathe may soon be on Craigslist!

 

[mmcmdl]

Amco bronze or possibly oillight may be a good replacement material . Acme taps are available but chasing the thread is a better option . They sell inserts if you have a holder . TIP..... thread that bad boy with a 60 degree tool then dig the rest out with the Acme insert . Make 2 or 3 extras also !