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...
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.
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 start?
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 pretty!
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 change gears?
Michael Koblic, 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?
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 got.
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 train.
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 lathe.
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 hardened steel).
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 successful.
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 crank) 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 workpiece axis). 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 over.
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?