Threading Silver Steel rod

[Lew Hartswick]

That looks like insufficient clearance on the left side for that coarse a thread. So the
side force breaks the point. Notice the direction of the break. It's as though it as
"pushed" to the right in the pix. The 60 deg point must have clearance for the angle
of the thread. I once did a chart for that for the common threads but forget where
I did it. When I made the fixture for grinding the points on a surface grinder I picked
the worst case of the common threads.
...Lew...

 

[mu38&Bg#]

What size is that thread? Les might be on to something with the side clearance. Are you threading under power?

Greg

 

[Ken I]

Very good point - you need to clear the helix angle of the thread on the leading face.

Remember the helix is steeper deeper in (ie at the tip) so you might be cutting just fine on all but the tip - and the problems doubles from bad to worse if your tool is above centre.

Ken

 

[picclock]

Thats an interesting thought and it may be the solution. The tool is ground on a jig (many thanks to gadgetbuilder) which sets the 30 degree angle and 10 degrees side clearance. This automatically gives good front clearance, and the tooling is always set to the correct height. The Thread is M6. Most of the tip breakages seem to occur that way with the break in that direction, which means that it may be the left hand side clearance that causes the problem. Trig calculations give an angle of 9.46 degrees which is very close to the 10 degrees of clearance that the tool is ground to. So does that mean I should allow 20 degrees of relief on the left had side of the tool ? It would also explain why the tip never breaks on the first cut but only when some thread depth is present.

I completed that thread shown in the picture using the angled cross slide method and final finish with a die, and whilst its acceptable its nothing to be proud of.

Its also affected by the silver steel, as when threading normally I never need a die finish, and the thread surface looks more polished, cut rather than torn. Perhaps it is polished because the tool is rubbing due to insufficient relief.

I have used silver steel from two suppliers but I think they are both
made by Stubbs. I have not noticed any difference between the two.

I will regrind the tool with more side relief and see if things improve for the next thread. This really sounds like it could be the solution. Why didn't I think of it :'(

Many thanks for your help. :bow: (especially if that's the answer)

Best Regards

picclock

 

[mu38&Bg#]

At first glance, 10° side clearance looks like it should be enough. I'll look at it closely in the CAD tonight.

Greg

 

[Ken I]

If you want to experiment put some packing under one side of the tool to tilt it by 10° (approx.)

Ken