View Full Version : Threading tool question

05-11-2009, 03:30 PM
I've cut cut threads (metric and American Standard) with my lathe on several occasions in the past using a sharp pointed 60 degree HS tool bit with lantern tool post and a tool holder with good results. Recently I bought a new AXA style QC tool post and wanted to grind a new threading tool for it. Thinking back and remembering reading about different 60 degree threads I decided to look in South Bend's How to Run a Lathe before grinding the new tool bit. There I learned that while the general form for 60 degree threads is the same, the root is different for American Standard, International and Metric and appears to vary from pitch to pitch. Now, here's the question: For general threading can the sharply pointed American Standard type form tool be used for all 3 threads mentioned above? It seems that it would work because the sharp point will leave more clearance at the thread root than either the International or Metric forms require.

Looking at my taps and dies, metric and standard, it appears that they all have sharp pointed threads. I was thinking of using a sort of compromise for my tool bit by grinding a very small radius on the tip, just breaking the sharp corner, thereby reducing the chance of damaging that sharp point, making the thread root slightly less sharp as well. Since breaks or cracks can start in sharp corner it might also be helpful in preventing fatigue cracks in stressed threads.

Bear in mind I'm not building space shuttle parts, but would like to turn out threads that fit well. Any experienced machinists here have thoughts that might be helpful regarding this?

05-11-2009, 07:51 PM
I'm not a machinist, just a HSM but I've cut a fair number of threads and march to the beat of a different drummer on threading so take this for what it is worth. I had similar concerns with grinding the tip of the threading tool for a specific tpi and then having to regrind for each different tpi.

I don't use a sharp pointed threading tool. I round the tip to match a 32 tpi machine screw, the finest thread I commonly cut. In rounding, I touch the tip to the grinder wheel so the tip has 7 degree relief. Note that a threading bit typically has 10 degree relief on each side so the tip has about 20 degree relief. If you round the tip by following this existing relief angle the tip will be very weak and will tend to break down much earlier than with 7 degree relief. The difference in the way the tool looks is small but important.

I leave the compound perpendicular to the cross slide while threading. The compound is advanced half the amount the cross slide is advance for each pass. This isn't critical so if it isn't exactly half it will still work fine. This results in the tool moving down the flank of the thread, similar to setting the compound to 30 degrees. Using this method, the thread depth is read directly from the cross slide dial, assuming you zero it to start.

Once you arrive at depth for the tpi you're cutting you can widen the base of the thread by advancing the compound. This allows one threading bit to cut any tpi less than the original tpi used when doing the original rounding. And, by using a 7 degree relief at the tip the bit will last well despite being used for many different tpi's.


05-11-2009, 08:10 PM
Good call John...I had the same experience. Just slightly rounding the nose seemed to give a "cleaner" thread in my experience,of course mostly I work with crap that probably shouldnt be threaded to begin with. I had alot of good advice from all angles,ultimately it ended up with me throwing a few pieces in the scrap bucket,but experimenting for a little while did the trick.

05-11-2009, 10:49 PM
I like to touch the tip at about 7-10 deg with a diamond hand stone.
I do put only the slightest of radii on the tip maby .005-.010 max.
Run 'er slow and wet, works for me.........

05-11-2009, 11:55 PM
Cutting the thread with a sharp V is the best way as it will allow clearance in the bottom of the V for the opposite thread to fit. It is best to use a file to take the sharp top of the V from a male thread and I use a scraper or emery cloth on a stick to make a flat on female threads.

The compound should be set from 29 to 30 deg to cut threads and any other way is iffy and may not produce a good thread. Also, using the compound at 29-30 deg to feed the cuts makes the cutter cut on the leading edge and chase on the trailing edge.

Using the crossfeed to cut the threads forces the cutter to cut on both sides and produces a poor thread with heavy load on the cutter and work.

05-12-2009, 01:08 AM
I don't normally cut threads that have to last for decades and keep people flying safely or any such, so my threads are intended to satisfy my simple requirements first and last. Threads I cut that go into equipment that may outlast me (my Harley, frinstance) get better treatment and I do clean up the cutter radius and try hard to make the best part possible.

I'd feel like crap if something I did on my bike hurt a new owner of it down the road and I've no doubt it will outlive me by a good margin.

That kind of thinking cost me my first and only job as a machinist. I refused to rebuild aircraft propeller pitch change units that had defective over-run speed controller springs in them. It was a $5.00 spring and I rejected 80% of them for being out of spec in the high-speed direction where prop blades become ballistic objects. I've actually been in an aircraft that had had a cabin insertion of a propeller blade event occur. There's not much to see except holes on both sides of the aircraft. One is much larger than the other.

05-12-2009, 05:43 AM
Yep, and sharp Vee's are stress raisers.

Regards Ian

05-12-2009, 10:03 AM
It sounds to me like my thoughts are going in the right direction. After posting, I did what smiller does, used a diamond stone to knock the sharp corner off ever so slightly. I think I'll make a comparison to a 32 TPI thread and decide whether to radius more or not. In the past I've used carld's idea of hitting the crests of the threads lightly with a file, it makes sense in any case as those sharp crests are more easily damaged than flatter ones. Ever had a whisker of steel from a thread get caught in the thread when mating two threaded parts? It binds and can ruin your day! I haven't tried making a flat on the crests of female threads but will do so from now on. Good idea!
I've studied several commercially made threads closely and noted that most have crests and roots that are slightly flattened or radiused. That may be by design or due to method of manufacture, i.e. rolling vs. cutting. In any case, there seems to be enough latitude in thread designs that these ideas will work well and be "commercially acceptable."
Thanks all who took time to provide input. That's what makes this a great site.