Help with single point threading from one of y'all machining gurus!

[nced]

In the past I've made custom springer pistons and even extended piston latch rods for TX200s, however I've always used a good adjustable die to cut the 10x1mm threads on the end of the o1 tool steel rods I use. After threading one end of the latch rods and machining the "catch" on the other end I harden and temper the "catch end". This has worked well for me however actually cutting a 10x1mm thread with a die takes careful heavy torque to turn the die.

Well.....I had the notion to practice cutting some threads on the end of a 10mm o1 tool steel rod so I viewed a bunch of YouTube "how to" vids, ground my lathe bit with the recommended 60 degree point and relief grinds, added the recommended change gears for a 1mm pitch, set the compound to 29 degrees, made sure the tool was on center of the 10mm stock that was mounted in my 4 jaw chuck and centered within .001 tir, slowed the rpm of my LatheMaster bench top lathe to min, touched the stock with the tool bit and set the cross slide scale to "0", advanced the compound .005, engaged the half nut when the "thread dial" hit "1" on the dial, then proceeded to cut a thread, the first pass was just fine.

After the first pass I disengaged the half nut, retracted the cross slide, moved the cutter past the right end of the stock, advanced the cross slide to "0", advanced the compound another .005, when the "thread dial" again rotated to "1" on the dial I engaged the half nut and took a deeper pass over the previous cut. Well....how about that.....the second pass actually followed the first pass cutting the thread deeper. This worked fine for several passes following the same procedure till (for some reason) the next pass didn't follow the previous cuts very closely messing up the thread!!! 

OK, I cut the messed up thread from my work piece, advanced the work and re-centered in my 4 jaw chuck and started all over again. This time I ended up with the full thread I was looking foe but I noticed that the thread was pretty ragged even though the last few passes were only .001 deep. 

I'm wondering a couple things.........
1. How did the first threading attempt have a mis-aligned pass when I engaged the half nut at what I thought was "thread dial at 1"? I did notice that when the thread dial was at "1" the half nut didn't engage till the thread dial was a bit past "1". Is this normal? 

2. Is the rough thread finish of the successful thread a product if the material (o1 tool steel) or poor grinding geometry (I thought I followed the YouTube instructions)?

[JHB51]

Ok it has been 45 years since trade school and cutting threads on a lathe. If I remember correctly the half nut has to engage exactly on the numbered line. As far as the roughness of the cut try some of that heavy cutting oil used for black iron pipe. Good luck

[michaelthomas]

Ed......the only thing I have run into that may have caused your first issue is the stock slipping in the jaws or possibly getting bumped between passes.

For problem #2......I would guess that maybe your sfm is on the slow side.  Hard to say.  It's normal to want to turn the spindle slow when threading due to the "rapid" disengage that is often necessary, but I get much better finishes running as fast as I can and being very quick on the disengage......lol.

If you are threading into a shoulder, you can cut a relief at the end of the thread to give you a bit of leeway on the disengage.

Mike

[eeler1]

Use the die to clean up the threads, after you get them close.

[sixshootertexan]

I don't know about your lathe but some lathes you can not disengage the lead screw while cutting metric threads. What RPM selections do you have on your lathe?

[michaelthomas]

Ricky made an excellent point. 

If that's the case on yours......you're going to have to stop the spindle at the end of the thread instead of disengaging the half nut.

Mike

[Bob H.]

Ricky made a very good point, if your lathe was not designed as a "metric" lathe, engaging and disengaging the half nut is a game if chance. I have used small engine lathes most of my life, and on small critical parts, I have turned the spindle my hand. RPM can and will eat your lunch, if you can't control the spindle.

Threading makes a lot of tool pressure, especially on a small lathe.  Typically, my last 2 or 3 passes are made without advancing the compound rest, this relieves the tool pressure and improves the thread finish.  BTW, there is no substitute for a good cutting fluid that keeps a buildup from forming on the cutting tool. Keep the tool overhang to a minimum and avoid allowing the work to flex.

BobH.

[nced]

Use the die to clean up the threads, after you get them close.

That is a possibility since it would remove a lot of the pressure on the die thread cutters.

[nced]

Ok it has been 45 years since trade school and cutting threads on a lathe. If I remember correctly the half nut has to engage exactly on the numbered line. As far as the roughness of the cut try some of that heavy cutting oil used for black iron pipe. Good luck

Heavy cutting oil? I'm currently using Rigid thread cutting oil and I'll look for some "heavy stuff". I do have some chainsaw bar oil that's pretty thick stuff.

[nced]

Ed......the only thing I have run into that may have caused your first issue is the stock slipping in the jaws or possibly getting bumped between passes.

For problem #2......I would guess that maybe your sfm is on the slow side.  Hard to say.  It's normal to want to turn the spindle slow when threading due to the "rapid" disengage that is often necessary, but I get much better finishes running as fast as I can and being very quick on the disengage......lol.

If you are threading into a shoulder, you can cut a relief at the end of the thread to give you a bit of leeway on the disengage.

Mike

LOL......I do use an undercut and I was even considering the purchase of a reducer to cut my spindle speed in half! LOL

[nced]

I don't know about your lathe but some lathes you can not disengage the lead screw while cutting metric threads. What RPM selections do you have on your lathe?

Well....my lathe is a LatheMaster bench top lathe that I bought years ago and it's larger than the normal minilathes weighing in at 200 pounds......still real light as far as lathes go. Here is a pic of the LatheMaster behind the Harbor Freight minilathe that I used in the beginning.........

Anywhoo......the LatheMaster has an "inch thread" lead screw and the stepped pulleys give 125,210,450,620,1000,2000 rpm.

I think I'll turn the threaded end  my 10mm stock to 3/8" and change the gears for 24 threads per inch (3/8"-24 is real close to 10x1mm) and give it a try again. Perhaps this will work better with the LatheMaster inch lead screw for using the "thread dial". I tried cutting the metric thread by turning off the lathe without disengaging the half nut then reversing the motor.......problem was judging the amount of "over run" as the chuck was coasting to a stop so I attempted using the "thread dial".

[nced]

Ricky made a very good point, if your lathe was not designed as a "metric" lathe, engaging and disengaging the half nut is a game if chance. I have used small engine lathes most of my life, and on small critical parts, I have turned the spindle my hand. RPM can and will eat your lunch, if you can't control the spindle.

Threading makes a lot of tool pressure, especially on a small lathe.  Typically, my last 2 or 3 passes are made without advancing the compound rest, this relieves the tool pressure and improves the thread finish.  BTW, there is no substitute for a good cutting fluid that keeps a buildup from forming on the cutting tool. Keep the tool overhang to a minimum and avoid allowing the work to flex.

BobH.

Would there be any benefit to adding some top rake to the bit? I do know that when the compound is set at 29.5 degrees only the sharp "leading edge" is doing the cutting when advancing the compound for increasing the thread depth but I'm wondering?

As far as turning the spindle by hand......I've considered fabricating a hand crank that can be inserted in the spindle bore (only 3/4" for the Lathe master") and is of a "split collet design" with a draw bar that would get tightened to the MT3 part of the spindle. That way I could have a REAL slow cutting speed and a lot of torque from a long crank.

[JHB51]

Try a little relief on the top of the tool bit the back right corner should be the low spot. The cutting oil you are using should be good for the job may be add some grease to thicken it up a bit if it runs off the part un less you are using a flood type application.

[sixshootertexan]

You said you have a thread relief. If you have a bar for threading ID's use that. Thread from the back side starting in the relief and thread coming out in reverse. This will allow you to run the RPM you need and be able to stop the chuck to manually run the chuck forward to get the tool back to the thread relief.

[nced]

You said you have a thread relief. If you have a bar for threading ID's use that. Thread from the back side starting in the relief and thread coming out in reverse. This will allow you to run the RPM you need and be able to stop the chuck to manually run the chuck forward to get the tool back to the thread relief.

Humm....I use a quick change tool post so perhaps I could put the bit in the holder upside down, adjust the bit height, and run the lathe in reverse to accomplish the same thing.

[sixshootertexan]

[/quote]
Humm....I use a quick change tool post so perhaps I could put the bit in the holder upside down, adjust the bit height, and run the lathe in reverse to accomplish the same thing.
[/quote]

I've done that before. Especially running brass, it throws the chips down instead of up and all over you.

[nced]

Ricky made a very good point, if your lathe was not designed as a "metric" lathe, engaging and disengaging the half nut is a game if chance. I have used small engine lathes most of my life, and on small critical parts, I have turned the spindle my hand. RPM can and will eat your lunch, if you can't control the spindle.

Threading makes a lot of tool pressure, especially on a small lathe.  Typically, my last 2 or 3 passes are made without advancing the compound rest, this relieves the tool pressure and improves the thread finish.  BTW, there is no substitute for a good cutting fluid that keeps a buildup from forming on the cutting tool. Keep the tool overhang to a minimum and avoid allowing the work to flex.

BobH.

Finally some single point threading success!!!

I changed the "change gears" for 24 threads per inch, took a piece of 10mm O1 tool steel, mounted and centered it in my 4 jaw chuck, then successfully cut a 3/8"-24 fine thread on one end. I don't know if it helped but prior to cutting the thread I annealed the rod by heating to "blue temp" and let it cool slowly. Using the 3/8"-24 thread will work out great for some custom R9 pistons I have in mind which will feature a screw in latch rod similar to a TX200 latch rod. I think that this success has given me the confidence needed to attempt  a second feature of the custom R9 piston..........threaded in piston end plug. To do so I'll need to do an internal and a mating external thread so this old 67 year old dog is still learnin' some new tricks.

The cut threads aren't as smooth as I would like however they are uniform and the 3/8"-24 nut screws onto the thread without wobble!  Definitely a closer fit than a normal nut to bolt fit! The reason I've been using O1 tool steel is the fact that I can easily harden the "latch end" with a propane torch and oil for quenching, however it's not the easiest allow to machine!

Anywhoo.....thanks for the help y'all!
When cutting in Imperial thread with my lathe having an inch lead screw I had no problem cutting the thread by disengaging the half nut after a pass, moving the carriage past the end of the work piece, then engaging the half nuts after returning the cross slide to "0" and advancing the compound a couple thou. I also took the advice to make a couple final passes without and extra tool bit advancing. As mentioned, the thread is a BIT rougher than I would like, however it will be a good base for the LocTite to bond when the latch rod is installed in the piston!

[sixshootertexan]

That looks good. You have about as good of a finish as you're going to get on tool steel.

[SeanMP]

I cut a lot of shop made taps from O1 and your main issue is is speed. For 3/8 O1 you should be up around 900-1000 rpm.

Give it a try on a piece of scrap. The difference in finish will be day and night. What's happening right now is the material is tearing off. Have a look at it under a strong magnifying glass and you'll see the tears.

I know the thread chase is whipping past like a Ferrari but one of the other posters gave you a good tip. Flip the tool upside down and run in reverse. Then your not threading to a stop ....which can be real intimidating at first.

[nced]

I cut a lot of shop made taps from O1 and your main issue is is speed. For 3/8 O1 you should be up around 900-1000 rpm.

Give it a try on a piece of scrap. The difference in finish will be day and night. What's happening right now is the material is tearing off. Have a look at it under a strong magnifying glass and you'll see the tears.

I know the thread chase is whipping past like a Ferrari but one of the other posters gave you a good tip. Flip the tool upside down and run in reverse. Then your not threading to a stop ....which can be real intimidating at first.

OK....I'll give it a try. LOL.....at least I won't have to wait as long for the "number lineup" if I miss the "number lineup" on the threading dial the first time around!