Help please? Threading on a 618

[machinistnoob]

There are two screws behind the half nut lever. These mount the half nut cam plate. I noticed that with the half nuts engaged and moving the carriage with the hand crank, I could move those screws from side to side slightly.

Normally, this would be taken up with lash but it is possible if the screws are sort of tight and the carriage moved to the left against the engaged half nuts, you could move the half nuts slightly to the right and they might stay in that position while making a threading pass. One the following pass, if the carriage was moved to the right against the engaged half nuts, the half nuts would move slightly to the left and a threading pass wouldn't track with the previous pass. Ironically, either loose or tight screws wouldn't see this issue. If the screws were loose enough to allow free movement,the movement would be accounted for when taking up the backlash before starting the thread

Tightening the screws eliminated the movement on my 6x18.

Thank you. I will look into this.

 

[machinistnoob]

Update. I cut 2 different thread pitch. A 32 tpi, and a 20 tpi. No issue at all... worked as expected. The only difference was the change gears. I didn't think the gears could be the issue, but it appears that might be the case. I will try different combinations. The gear train for the original, 16tpi, has gears in all positions. The 32 and 20 tpi only has one, along with different lead screw gear.

 

[machinistnoob]

2nd update: I successfully, set up and tried two more tpi's. Then set up for a 16 tpi which started all this trouble. To my surprise, it went perfectly...
It appears the problem was me the whole time. Must have been something in the gear setup. (?)
Lesson learned.

Thank you all for help. This is a fine community, indeed.

M. Noob