Internal threading problems 1

[chiph]

Hello,
I have a problem where I have to cut a 48mm internal R.H. thread into a blind hole @ 3 inches deep.
I propose to cut the thread in reverse with a left hand boring bar and feed out of the hole. I have enough room to relieve the bore at the bottom of the hole and I think it will be easier to watch the threads progress as I work "inside out".
My question is this. Is there a carbide threading insert I can use on a boring bar that will work in this orientation?
Thanks in advance for any thoughts or suggestions.

 

[ornerynorsk]

It's easy enough to set up a travel dial so you have an indication of where you need to disengage the halfnut. You can also run backwards as you stated, you just need the right tooling setup.

 

[chiph]

thanks for the thought but, it's a metric pitch thread so i can't disengage the half-nut lever[i think].

 

[Nate101]

Chiph

This should work fine. I've never cut an ID thread out, but the CNC programing manuals say it can be done. You didn't say if this is going to be done on a CNC or engine lathe. The process is just about the same anyway. As far as I know there isn't any threading inserts that will fit in a boring bar. There is for ID groover bars though. For a three inch long thread I'd go with a Vardex style wiper threading insert. This will wipe the burr off the top of the thread. Several companies make this style of insert and bar. If the burr isn't an issue, then use the ID groover bar, the inserts are cheaper.

 

[ornerynorsk]

There are a hundred varieties of internal boring bars that accept thread inserts, just for your info.

 

[Nate101]

Chiph

If your leadscrew can be set for the metric pitch then the halfnut should be fine.

 

[Nate101]

Ornerynorsk

Could you give me an example, so I can stand corected?

 

[chiph]

thanks guys but, as far as i know, unless the lathe has a metric pitch leadscrew the half-nut lever cannot be disengaged once the metric thread has been started. even with a thread dial indicator.
i've looked at valenite and carmex threading inserts and holders and it appears as though the insert is held in the bar in such a way that the insert has support only when feeding into the headstock spindle-- not away from the headstock as i propose to do.

 

[unclesyd]

Take a look at this website. We have used their tooling numerous times.
You might have to use both Urls.

http://www.manchestertools.com/support/threading/pages/002.html

http://www.manchestertools.com

 

[Nate101]

Chiph
I talked to one of our toolmakers with the most manual threading experience and he suggested leaving the halfnut engaged for the most acurate thread, with multiple passes. Back off the carriage and reverse the spindle to get the tool repositioned for each pass, of course.
If their threaders are anything like their deep grooving tooling, it'll be good stuff.

 

[BillPSU]

What type of machine are you trying to thread this part on? CNC lathe, manual lathe, CNC mill, or manual mill?
The idea of threading moving away from the head stock would require you to reversed the spindle and change the orientation of the threading tool to across centerline of the lathe or turning the tool upside down meaning a left hand threading tool. This application would probably be the only application you could use a left handed threading bar unless you cut left handed threads. Slide lag for a CNC lathe may cause you inaccuracy for some unknown distance when threading causing imperfect bottom threads.
Depending on what your customer needs, shouldering to the bottom of the hole or minimum thread engagement. I would recommend cutting a square groove at the bottom of the hole and threading into the groove. The threading tool will naturally cut a groove in the ID so the customer will have to accept at least that groove. If on a CNC mill you could thread mill the thread or buy a bottom tap with 1 1/2 threads of chamfer.
Good Luck

 

[chiph]

It's being done on a manual[engine] lathe.
I believe it can be successfully done by turning the headstock in reverse with the toolbit face-up on the far[away] side of the headstock spindle and feeding out from the bottom of the hole.
The reason for doing this is as follows;
1.I won't be working blind into the hole. I will be able to see the toolbit track the full length of the thread and check for problems on the way.
2.It is a metric thread being cut on a machine with a thread per inch cut leadscrew [as most lathes are]. This means I cannot disengage the half-nut lever after each pass. All I can do is stop the headstock, back-off the crossfeed for the toolbit to clear the threads, reverse headstock rotation[which will also reverse carriage feed] and carefully return to my starting point of my thread and start my next[deeper] pass.
If I was to do this with "normal" feed and rotation[forward and in], I would stand a good chance of not stopping the machine at the right moment and have the toolbit crash into the bottom of the blind hole[remember, I can't disengage the half-nut and stop my forward motion].
Reverse and out thread generation gives added leeway and I will be able to jog the headstock and carriage back into the bore to the starting point of my thread and start my next pass.
If anyone has suggestions of a better way, PLEASE feel free to offer.
As for the boring bar and threading insert, I can off-hand grind a single-point threading tool from high-speed steel but, I believe it would be time saving to use an insert.
unclesyd directed me to an excellent website and I will talk to them the first of the week but, in the meantime, I'm open to any suggestions. thanks to you all.

 

[EdDanzer]

The Manchester tools will do threading in either direction. Your biggest problem is stopping in the bottom of the hole. Since it is a metric thread you will have to keep the half nut locked in after the first pass. Some lathes have a higher speed reverse than forward, so starting in the inside will be a disadvantage. No mater your choice, stopping at the bottom will be the problem.
If you plan on making more than one, subcontracting this out might be less expensive than tooling up and spending a couple of hours per part.
We have used mfgqoute.com for quantities over 20 parts, but have CNC machines for the smaller lot quantities. With out knowing more about the part our cost to do 1 part would be about $120.00. The Manchester bar with a few inserts will cost over $200.00.

Good luck with your choices, Ed Danzer