Threading dial question

[rake60]

An internal thread changes the game completely!

A threading bar takes away the rigidity.

No professional machinist machinst could cut an internal 1 1/2-8 thread straight in
unless they were sound asleep in a sweet dream.

I won't be turning a machine on in my hobby shop this long weekend for just that reason.
In the past 2 days at work I made 6 high pressure caps that called for a 4 pitch internal
Acme thread. The only way to get the machine to cut that thread cleanly was to open
the machine doors at that process and push a length of 2 X 4 wood against the tool holder.
57 passes. I have a bruised hand and chest from making that work out, but I also have
6 high pressure caps that fit perfectly with no chatter in the internal threads to be
delivered to the customer on Tuesday morning.

Rick

 

[chuck foster]

years ago i did some internal acme threads (not sure what they were for) but i made a wooden piece that went on the backside of the boring bar and i kept the bar from springing and chattering............if i remember right the pitch was about 1/2 thread per inch.
it was a rotten job but i got it done.............but the worst part was since i could do the job i got about 60 more of them to do, they were big brass castings.

one of the scary things i did was a huge bronze bearing, and i mean huge, it had a 6' o.d and a 4' i.d and was 7' long.
it was something for the mining industry, it had a grease groove from one end to the other and in the 7' length the grove went one turn. if you think the carrage on rick's lathe was moving fast in the video you should have seen this one go
the grove was about and inch deep and an inch wide, i had to make about a hundred passes to get it to the right depth.

any way rick............hope you heal fast and get back in the shop..............but wasn't it you that said the body is not a machine :-\

take it easy my friend and rest well cause monday will be another work day

chuck

 

[rake60]

LOL

Yes I did say a body is not a machine.

But, pride in your work sometimes intervines.
Bruised ribs will heal, but when some redneck oil field worker is standing
at a well head and that pressure cap spins on before he gets hurt....
Well it's all relevent.

Knowledge is a wonderful tool!
When you actually know what HAS to work the very first time you add a little
extra effort to it.

What we do at home is a different story.
If it doesn't work the first time we are not happy but we can fix it.

Try telling a pipeline rough neck, "Oh I'm sorry, bring that back and I'll fix if."

Drill pipe joints are 50 feet long.
Not something I'm willing to be shown where it should fit!
:big:

Rick

 

[NickG]

Thanks for all this information, but I still am not understanding the swivelling over of the compound slide! , is the tool is still at normal angle to the lathe spindle axis when you set the compound slide at 29.5 deg, i.e. you have to swing the tool post back 29.5? If so what difference does it make using that slide or the cross slide? The tool still ends up in the same position for each cut? Or do you set it all up at 0 degrees to cut the thread, then just move the compound slide over? Then you don't get the right thread form!

??? :-[ :-\

 

[Circlip]

When you have set the Top slide over to half the thread angle Nick, you set the cutting tool point at right angles to the workpiece using a thread gauge or whatever you use. Wind the tool in to touch the job and zero the top slide dial. Now, by feeding in using the top slide, you are only cutting on ONE face of the tool. As posted by someone else, this " Old wives tale b--l s--t " method is a lot kinder to the cutting tool AND machine when cutting large pitch/any threads on smaller machinery. Other members have given a far more in depth ( ;D ) description than mine, but NEVER be afraid toask.
Regards Ian.

 

[kvom]

With the compound at 29.5, you align the tool with the gauge like this:

 

[NickG]

Thanks again, I can see this point, but you would apply the feed when the tool is clear of the work anyway, so you're not plunging into the workpiece. When you engage the half nuts the forces on the tool / workpiece are exactly the same?

 

[lathe nut]

Rick, hats off to you fellows, I work in the Oil Field, for several years when I was doing the pumping at well sites, would often think when we pumped from five to fifteen thousand PSI, sure hope the threads are right, 8R, Weco, or what ever we were pumping through if not we won't live to complain, sometimes they would drip but never had one blow out, we did depend on you fellows without fail, thanks, Lathe Nut

 

[Stan]

NickG: It would be so easy to show you on a lathe but hard to put into words. When you advance the tool straight in and start the carriage moving into the thread, the tool will be cutting an equal amount on both sides of the tool. Do you understand so far?

When you make your first scratch cut you have a spiral line going down the work. If you use the compound set at 30° to advance the following cuts, you never touch that first scratch cut again. Each time you advance the compound it moves the tool toward the work and toward the headstock It is a long time since I did math but I think the relationship would be the distance advanced into the work is the cosine of 30 times the amount you advance the compound handwheel.

If you now visualize the work when you have a nearly completed thread, the point of the tool is going deeper into the work a bit to the left of the previous cut and the left side of the tool is cutting metal. The right side of the tool is just going along for the ride.

The end result is the right side of the thread is at the point where you started but the bottom of the thread has moved toward the headstock. Why don't you try cutting a coarse thread on a piece of material and see what happens.

 

[NickG]

Stan,

Thanks for that. I think it was just me being a bit thick! I needed it spelling out step by step!

Just drawn a little sketch for myself and understand it fully now! I guess setting it at 29.5 degrees just lets it cut a small amount on the right hand tool flank to clean it up each time you advance the tool for the next cut. Will have to give it a go!

Thanks to everyone for trying to explain this, you've finally cracked it! ;D

 

[Circlip]

If you want to prove it for yourself without the BANG Nick,( this occurs when you put the last cut on,and is the sound JUST before you need to resharpen) get a piece of PVC or some other "Hard" plastic rod/bar to gain confidence threading. It might not be useful for a project, but it sure gives Friends and neighbors something to talk about when they see it on the coffee table. To explode another myth, It's best not to take the tool out of the tool post until you've finished, yes that's right BUT, if you need to, to resharpen, or regrind ( after a bang ) providing the tool is square in the toolpost and when winding the tool back in to line up with the job that you take up the backlash you find It's not difficult at all and takes far less time to do than read my explanation.
Regards Ian.

 

[Stan]

NickG: Glad you understand the principle. Single point threading is primarily a hand-eye coordination thing and practice is the only way to get good at it. Since I use DC drive, I can run as slow as I need for any particular job.

If it is critical that the thread end at a particular spot, I mount an indicator on the bed so that the carriage hits it about an inch before where I want to stop. I then watch the indicator to know when to disengage the halfnuts and back out. If you always stop at he same mark on the indicator, the thread will end one thead pitch past where it was on the first pass. If it is really critical, you can do the math so that you have each cut end short of the previous cut to eliminate any stress riser at the end of the thread.

Circlip has a good suggestion to practice on plastic. ABS sewer pipe is beautiful stuff to machine and it is big enough that you can watch the progress of the thread.

 

[NickG]

I did a few 1/4" x 32 bungs the other day for that boiler. I noticed that if I took the depth all the way to 27 thou , it was too much, but i guess that's not accounting for any radii, assuming a totally sharp crest and root. Just experimenting a little about 20 thou gave the best threadform. However, this could be down to my tool, it probably wasn't very well ground.

Nick

ps am going to practice with some larger pitches and try setting the compound slide over though