I see that all the bench grinders and buffers have their spindles
threaded in UNF - either 1/2-20 or 5/8-18. Is there a reason for this?
I am asking because I just tried to cut a 1/2-20 thread on an adaptor
I made and could not do it - most likely because of the Crappy Tire
die. I do have a good 1/2-13 SKF die. OTOH it might be God's way
telling me to finally learn how to cut the threads on a lathe.
Before I do anything so heroic I thought I would ask...
Campbell River, BC
Try the lathe, it's not that difficult. The first time I did lathe
threading was because I needed to make a temporary pull stud, something
like 3/4"-16 for a hydraulic punch set I got that had been inadvertently
packed with the metric versions of the pull studs. I just followed the
setup instructions in one of my machining books and gave it a shot. The
first pass cut a bit ragged and I determined that the profile on my
cheap brazed carbide cutter sucked. I spent a few minutes improving the
profile with a diamond hone and then tried again an got a fairly good
cut. A few passes and it was done, the stud threaded smoothly into the
hydraulic puller and the punch, and held up just fine punching 2" dia
holes in 12ga steel.
Pete's recommendation is good advice. Cutting a thread on a lathe generally
gives much better results (and greater gratification).
Eventually one will need a thread which will require using a lathe, so it's
better to get some experience before that need arises.
My recommendation would be to avoid carbide cutting tools. Using HSS will
increase your chances of success with better results and eliminate much of
the frustration of using carbide on smaller benchtop machines.
Grinding HSS isn't extremely difficult, and some of the best advice you can
find has been written by Harold V. Some of that advice can be found in RCM
archives, but his concise articles can be found in the Chaski forums.
Harold has included details regarding proper grinding wheel selection and
use of a dressing stick.
Sets of pre-ground HSS cutting tools can be found at Grizzly, LMS and some
eBag sellers, which can be very helpful for getting started. These sets will
show angles of rake, relief and chip breaker examples while eliminating the
need to try to duplicate cutting tools from HSS tool blanks and drawings.
You can cut most of the thread depth on the lathe and finish with a die,
which will cut more easily if it doesn't have to remove much metal. The die
will track better and the threading bit doesn't have to be ground as
precisely to form and finish. You also get the correct root shape without
having to regrind the bit for each pitch. That matters if the thread will be
highly stressed. Usually I grind the tip correct for about 32 TPI and leave
it that way, and finish with a die if the thread will be in tension.
The die will start straight if you screw a short piece of threaded rod into
the back of it and hold the rod in the tailstock chuck. Pressure from the
face of the chuck may be enough to start it straight.
Ah, that is the sort of pragmatic approach that I like!
I have die holders for the lathe. I tried them and could not even get
the thread started. I tried pushing with the tailstock. I tried brute
force by hand. Not a thing.
As I see another one of these in the near future followed closely by a
need to cut 3/4-16 I shall have a go. I suspect the whole thing will
be done manually as I have no confidence of my ability to stop the
lathe at a speed of 150 rpm just at the right moment.
BTW I have developed a great liking for carbide tools but I see that
for this job HSS will be better.
Campbell River, BC
Mine turns at about 50 RPM in back gear and still demands close attention
and quick reactions to stop at the end of the thread. I shut off the motor
when it's about one thread away and then lift the belt tension lever to stop
the spindle quickly.
The spindle is fairly easy to turn by hand if the bit is sharp and the chip
1-3 thousandths thick. It's certainly less effort than cutting the full
depth thread all at once with a die. Could you make up a safe balanced hand
crank for the spindle and use the power only to run the bit back to the
You can always run the spindle backwards and flip the cutter upside
Then you don't need to worry about stopping at the right time.
Milling attachments showed me that neither of my lathes behaves well if an
upward cutting force lifts the saddle off the ways. The hold-down clamps and
their machined guides aren't precise enough to take out all the play and
still move freely.
OK, I had a go on a piece of scrap today. Everything went fine until I
cut the first pass at 40 tpi. Re-reading the chart I corrected the
gears and tried cutting ostensibly 20 tpi. I did, BTW what you
suggested and run the lathe under power both ways just finishing the
last three threads manually.
The big disappointment was that the thread ended up 21 tpi. Once I was
able to get the die on it I was able to correct that with a resulting
pattern of periodic double cuts and a variable major diameter (reminds
me of beat frequency oscillators). I can get the nut on but it is not
The other thing that I did not expect was that starting with a rod of
0.493" the final major diameter (before the die application) was
0.507". Even after filing it was still 0.502".
So the big question: Is there a reason for the 21 tpi *other* than
Campbell River, BC
BTW what's with the deafening silence on the question as to why these
threads are UNF rather than UNC? Could it be that it does not matter?
You want me to guess this from the other side of the continent?
If the bit didn't cut freely it just might have pushed metal forward,
increasing the diameter and shortening the rod. Did you sharpen the bit edge
to at least the full depth of the thread?
And backing the cutting tool out before running in reverse?
There is usually some backlash in the leadscrew/half-nuts interface, so
it will follow a slightly different track in reverse, and will tend to
dull the cutter and do weird things to the thread.
Strange. Is this with a quick-change gearbox, or a set of loose
gears which you have to change on the far side of the headstock? If the
latter, I would suggest that you go through the gears and count the
teeth on each, and compare them to what the manual claims they should
be. This is *every* gear on the way from the spindle to the leadscrew,
not just the ones you expect to change.
Another possibility is that instead of using the half-nuts to
engage the drive, (if your lathe is so equipped) you have the feed
clutch engaged instead. This does not make the carriage move a precise
distance per spindle rotation -- and might have been about the ratio you
would get between your intended thread with the first setup and what you
The tool is not cutting -- it is mashing metal out of the
groove, causing it it build up on the crest of the threads. This may be
happening in the reverse feed if you did not back the tool out of the
groove before reversing.
I forget what model lathe you have, so I don't know what
features to expect. Perhaps it is made for metric, and you can't get a
precise 20 TPI without playing games with conversion gears -- or it may
be set up for inch threads, but there are conversion gears in the gear
If you want to be sure what you are getting, set up for 20 TPI,
and set up a dial indicator on the bed to measure the travel of the
carriage. Engage the half nuts, zero the indicator, and rotate the
chuck precisely one full turn. This should move 1/20 of an inch, or
0.050". If it doesn't, check the gears -- all of them. Count the
teeth. If it is a lathe which has not been used for cutting threads
before, it may have come with a gear with the wrong number of teeth, and
the number stamped on the gear may be what you really wanted, not what
you got. :-)
Mark a tooth with layout die before counting so you don't count
the same tooth twice.
I don't remember even seeing the question. :-)
It does not matter what you call it when cutting it on the
It only matters when you are trying to *buy* the tap and/or die.
National Fine or National Coarse are just names for what family the
threads are in. What matters are the diameter and the TPI (or in metric
threads, the pitch (mm per turn). In your case, 1/2-20, which is NF
(The 'U' in front comes from "Unified", when thread standards between
the US and the UK were somewhat merged.
I dunno the reason for the dies not starting, but there are some really poor
quality die sets on the market and for China/India standards, the products
just looky-like dies but are barely sitable as thread chasers for cleaning
up a slightly damaged good thread.
Almost always, the side of the die that's marked with the size is the
starting side and the exit side doesn't have the widened opening for
starting onto the workpiece.
Also, the end of the workpiece should have a fairly uniform chamfer which
aids in starting the cutting action.
If no cutting action can be felt in the first 1/4 to 1/2 turn, it's likely
that something is wrong.. either the quality of the die is very bad/damaged,
or the workpiece is excessively oversize (assuming that the workpiece isn't
As DoN has mentioned, using a DI and turning the spindle with the carriage
feed engaged will show if there are errors in the selected gear set.
I generally just rotate the chuck the number of turns equal to the TPI to
make sure the travel is 1.00".
Before I assembled a variable speed drive, I was using a hand crank fitted
into the back/left end of the spindle for short threaded sections such as
lens adapters which may only use 3-4 turns of thread.
When cutting up to a shoulder, the variable speed drive is a huge benefit.
For a fine thread, using a hand crank is fairly easy work, but there are
several other required additional steps which make cutting the thread
This is a basic outline:
Unplug the power cord for hand crank method
Setup the compound at about 29 to 29.5 degrees
Cutting lubricant applied to work area
Setting the cross feed dial to zero for the scratch cut
Set the compound dial to zero for the scratch cut
Make the first pass scratch cut
Backing out the cross feed enough to clear the workpiece
Reverse feed to locate the carriage for a second pass (by hand if using a
Set the cross feed dial back to zero for the second pass
Advance the compound for a reasonable cut (depending upon hand or power cut)
Make the second pass with cutting lube
Repeat from Backing out the cross feed
It helps to have a (quality) nut of the desired thread nearby to use as a
gage.. otherwise, there are cutting depth specs on on threading gages for
setting up the cutting tool prior to threading (perpendicular to the
A quality nut would be one that doesn't rattle when run onto a quality tap..
which will insure that the resulting thread will fit any other standard nut
of that size/pitch.
Adhering to the thread specs will generally prevent cutting too deep,
although getting familiar with a machine generally includes seeing how well
it responds to operator input, so when nearing the final recommended depth
of the cut, trying the nut can prevent undercutting and the need to start
The edges of the thread crests can be sharp and possibly a little rough..
removing the hand crank and running the spindle at a reasonable speed will
allow the operator to wire brush the freshly-cut thread (carbon steel brush
bristles for a steel workpiece) and/or run some emery cloth over the area
(apply a protective cover/shop rag over the ways to keep any shedded
abrasive off them).
A 3-sided file can also be used to very lightly chase along the thread to
break the sharp edges of the thread.
I had to generate a spreadsheet threading gearbox chart since the original
was missing. I added the compound infeed at 29 degrees, assuming the proper
bit tip width, and the indicator line-matching rules. As I mentioned before
I usually leave the bit more pointed and the calculated infeed cuts a
somewhat shallow thread, so I can cut in to the number quickly before
measuring with three wires.
The bit will cut the proper thread form if I then disconnect the tumbler and
move the spindle a tooth backwards, to cut the thread wider but not deeper.
Which brings up the question: Why did they create both families? Why
are there two different pitches for each standard diameter of thread?
Does coarse grip better/stronger than fine due to its smaller core
diameter? I don't recall reading about that in tech school.
If you're trying to take a roomful of people by
surprise, it's a lot easier to hit your targets
if you don't yell going through the door.
-- Lois McMaster Bujold
The coarse thread standards were developed in the mid 1800s and are well
suited to steel bolts in cast iron (or aluminum) tapped holes. The fine
series is a better strength match with steel female threads and the larger
root diameter makes the bolt stronger. Originally it was called SAE after
the Society of Automotive Engineers.
group introduced the fine thread standard in 1906 after the coarse
series proved inadequate for the developing automotive industry..
Properly it should be BSPP and BSPT as British Standard Pipe can be
parallel or tapered, the parallel thread largely being used for
mechanical fastening as for instance bulkhead fittings, well not BSPP in
the US I expect. See
Thanks. Pretty much all that I did except for the indicator thing.
I could only find a scanty reference to the depth of each pass - they
recommended 0.003" on the compound. That makes it about 0.0025" depth
of cut per pass. For the 1/2"-20 the depth of thread is about 0.060"
so that is about 25 passes. Soudns about right? Does this vary with
how beefy one's lathe is?
Campbell River, BC