Threads not deep enough

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rronald

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I've been cutting some threads in the last week or so.....mainly just to get in practice to do them correctly.

Had a real usage today....making another stud for a QCTP. The thread is a 9/16-18. I'll talk about this one, but the problem seems to occur on any thread that I single point cut.

The fundamental problem- when I get to where I think the thread should be done, the thread is still too big. I always need to cut substantially deeper to make it mate with a nut.

For the 9/16-18 thread I started with a diameter of .557". Deliberately a bit under(-.005) the published major diameter of .562 so that the OD would not create any fitment problems. I ran the carbide thread cutter in on the diameter till it scratched the surface and entered the micrometer measured diameter into the DRO as a negative number (-.557"). Started cutting the thread.

The minor diameter is supposed to be .4958". I started trying to screw on the nut about .5". Then cut a bit more. And more. And more. No success at all until .490". And I was cutting about .001" at this point, so I'm pretty sure cutting forces were not pushing the part and the cutter apart. Even have tried doing some spring passes.

I'm sure that I did not have any backlash when I set the initial diameter and when I measure the cuts.

I was cutting at the 90 degree angle using the cross-slide (instead of doing the 29.5 degree cut with the compound.

The thread looks great. And it's a very good fit to the nut....tighter than using the same nut on the original AXA toolpost. If anything, the one I cut is more precise/tighter fit than the purchased stud.

I realize I need to be able to measure the minor diameter to do this right. At least that way I can see where my cuts are removing less than expected. So I guess I'll pick up some thread wires. But I'm wondering why measuring the initial diameter and then moving in the correct amount always seems to cut less than the numbers indicate that it should.

Thread.jpg
 
If you cut that long thread, you do need to support the part using the tail stock. If you don't, you will need to cut deeper and the nut will have more play towards the chuck.

The cutting depth depends on the tool. There is a lot of difference if you have a sharp ground HSS tool, a ground HSS tool with a tool nose radius, a half profile insert or a full profile insert.
If the half profile insert has a minimum pitch that is larger than the required pitch, you also need to cut deeper. If the half profile insert has a maximum pitch that is smaller than your pitch, you have to cut a lot deeper.
If the full profile pitch is less than you pitch you have to cut a lot deeper.
If you use a ground tool without a tool nose radius, you have to cut deeper but that is something you can calculate.
If you use a ground tool with a tool nose radius, you can only guess how deep to cut.

Check the thread depth using the 3 wire method. Any 3 wires will do if they have the same diameter and aren't to thin or to thick. The shaft of a drill will do well. Glue them on the thread using some thick grease and measure the diameter using a micrometer. There are web sites where you can find the requested diameter like this one. More info about 3 wire thread measuring.

If you can read the thread charts, know the tool nose radius of the threading tool and are comfortable using CAD, you can draw the thread and the tool in CAD to find out how deep to cut the thread.

Last year I start skipping the cleanup (spring) passes that you really need to do and cut the threads 0.02 mm deeper (adjusted the tool length). Now the threads fit without cleanup (spring) passes. So I do not have to worry about work hardening any more.
 
Have you accounted for radius vs diameter on your infeed? Thread depth is a radius number, but if your DRO or dials are calibrated to "direct reading" (diameter reduction) you'd only be going half as deep.

GsT
 
If your major diameter is correct , it's pretty simple to know when you're very close to your pitch diameter . It just comes with experience . Thread wires are $10 a set from PeeDee or anyone else . Thread mics are expensive . Easy way would be to get close and run a good die down the threads
 
GeneT45 said:
Have you accounted for radius vs diameter on your infeed? Thread depth is a radius number, but if your DRO or dials are calibrated to "direct reading" (diameter reduction) you'd only be going half as deep.

GsT
Click to expand...
I was shooting to hit the minor diameter, so the DRO should properly read that location.
 
rronald said:
I was shooting to hit the minor diameter, so the DRO should properly read that location.
Click to expand...
Minor diameter is not important on a OD thread . Major and pitch diameter is what you need correct . If the tool is a sharp V or has a radius would make a heck of a lot of difference on the PD if you go your route . :grin: Minor diameter is clearance .
 
mmcmdl said:
If your major diameter is correct , it's pretty simple to know when you're very close to your pitch diameter . It just comes with experience . Thread wires are $10 a set from PeeDee or anyone else . Thread mics are expensive . Easy way would be to get close and run a good die down the threads
Click to expand...
Yeah...that is a plan I've followed several times.....except that I'm single-pointing the thread because I don't have a die of that size.

Plus it seems like cheating.

I have noticed that if my blue sharpy coloring is still visible on the crests, there's probably more cutting to do.
 
mmcmdl said:
Minor diameter is not important on a OD thread . Major and pitch diameter is what you need correct . If the tool is a sharp V or has a radius would make a heck of a lot of difference on the PD if you go your route . :grin: Minor diameter is clearance .
Click to expand...
So...do thread wires and thread micrometers measure pitch diameter?
 
rronald said:
Plus it seems like cheating.

I have noticed that if my blue sharpy coloring is still visible on the crests, there's probably more cutting to do.
Click to expand...
No such thing as cheating if making a good part . And yes .005 on the crests would get you very close depending what you cut the major diameter to .
 
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