9A Thread dial stopw when half nuts engaged

[animal12]

like the title say's I was trying some threading practice today when I noticed that my thread dial stop's turnin when I engage the 1/2 nuts . is it supposed to stop , or do I have a adjustment off ? it starts spinnin again when I open the nuts
tks
animal

 

[SLK001]

It stops when half nuts are engaged (and it is meshed).

 

[animal12]

thank you sir

 

[johnoder]

When engaged the whole carriage assy including that dial move along the screw as if that assy was a big nut

No relative motion in that mode

No relative motion = no rotation