I tried another blank with a radius cut in the circumference
as per Dave G's suggestion.
I attempted to make this radius the thread root depth,
again as Dave suggested.
A few pics of this week's progess.....
Pic 1 shows the index/faceplate I use to bolt blanks to
so that machining an outer diameter and inner bore are
perfectly concentric.
Pic 2 is the blank ready to start, along with radius cut.
Pic 3 is the first teeth beginning to cut and 'self index'
Note the first points of contact are both outer shoulders
of the radius.....this shows that my radius cut was too small.
There is no sign of any point contact on the centreline yet.
(this pic was dark because the flash didnt go off...so I histo
stretched it ...hence the grainy b/w
Pic 4. close up of the self index error.....obviously the diameter
was wrong...note the 2nd smaller set of cuts.
Pic 5. It was obvious that the self index hadn't worked out.
You can see the radius has now got teeth cut almost through
all the way so that the first points of contact on the shoulders
almost meet up.
I decided to stop here and not go any deeper.
One thing learned from this try was that the hobbing was smoother
because it was trapped between the shoulders.
I will now skim this all off and have another try to get the root thread
radius more accurate.
Steve