Need help with external and internal threads please

[JamesYanco]

Morning everybody I am working on a small project and would like some thoughts and ideas on how best to get parts lined up. I hopefully uploaded an image of the part I am working on. The aluminum piece has external threads(3/4 by 16) and the bronze piece has internal threading. When I screw the 2 pieces together the bronze piece has an obvious slight deflection. My question is am I able to do anything to better get the 2 pieces aligned and save the items?

SouthBend 14 1/2 lathe(only had for about a month now)
I am not a machinist nor have I ever taken machining
Threading was done with taps and dies using the tailstock. I am by no means going to try using the lathe to thread, from what I have read that is above my present skill set.

Try not to beat me up to bad

 

[edwin dirnbeck]

It not good practice

Morning everybody I am working on a small project and would like some thoughts and ideas on how best to get parts lined up. I hopefully uploaded an image of the part I am working on. The aluminum piece has external threads(3/4 by 16) and the bronze piece has internal threading. When I screw the 2 pieces together the bronze piece has an obvious slight deflection. My question is am I able to do anything to better get the 2 pieces aligned and save the items?

SouthBend 14 1/2 lathe(only had for about a month now)
I am not a machinist nor have I ever taken machining
Threading was done with taps and dies using the tailstock. I am by no means going to try using the lathe to thread, from what I have read that is above my present skill set.

Try not to beat me up to bad ��

I have tried this many times ,there are too many things that cause error.Try to redesign the part so that the threads are loose and one part has a presision shoulder and the mating part has a corresponding pocket.For an example look at a good quality electric drill and see how the chuck is screwed on. I hope this helps Edwin Dirnbeck

 

[Mike C.]

Tap and die will never produce perfectly straight threads, if that's what you are after. Only way to do it is to single point thread the parts. Lathe threading isn't very hard to do, and do well. Just takes practice. Get some scrap material and cut a few. You'll catch on quickly. Since you already have the tap and die, best thing to do as a beginner is to thread with the lathe close as you can get and then run the tap and die across the threads to finish. That will line it up MUCH closer.

 

[Toms Wheels]

Mike said it, Tap and die are not that good at getting threads straight. I once had a SB 16, I believe the 14 had the same lead screw pitch of 6TPI, not the more common 8 TPI. Go online had download the booklet How To Run a Lathe. Also known as HTRL. Start with an easy number 24 TPI, get a 3/8 rod and nut. Thread the rod, check each thread using a nut. When you can do OD threads well, they go to internal.

Use a tap in a lathe, its best to have a spring loaded center to keep it straight. If it does not start straight, you will never get it straight.
On some internal threads I will single point the threads, but make a final pass with a tap.

 

[Forestgnome]

Not sure if I'm seeing the deflection you're talking about, but there should be an undercut on the threaded portion of the aluminum part at the shoulder, or a bevel on the brass part so there's nothing in the way of the two shoulders butting together.

 

[specfab]

All the above is good info. One thing to check is alignment between tailstock center and spindle center. If for some reason the tailstock is offset(if that is even possible on your lathe, I don't know South Bends well), this could contribute to misalignment of threading done with a tap, and cause other issues as well. Great info in the How to Run a Lathe book as noted above.

 

[jrmach]

Edwin is spot on

threads can be sloppy,but a guide in the design would be key

 

[billzweig]

As stated: best is to machine both threads. However one can get fairly good results using tap and dies in the late.

For the tap:
Support the tap female centre (inside the square drive) with the tail-stock dead centre and advance the tailstock to be in constant contact as tapping. For best results start with a taper tap continue with second and plug.

For die:

Use a Lathe Tailstock Die Holder and adjustable die. Start fully open (set screw on die in) and run once or twice more with the die progressively closed to the correct size.

 

[crossthread]

I would use this opportunity to learn to cut threads with your lathe. Sooner or later you will need to know. There couldn't be a better project for you to use. For one thing you aren't cutting these to some customers specs. No measuring over wires or anything like that. It is your project. If it screws together then you are golden. Plus you have both male and female parts. That is your gauge. If you were making some kind of widget that had to fit a male or female component somewhere else then there would be a need for gauges and whatnot. What I would do is cut the male (external) thread until it is sharp. Do about the same with your internal thread and use the male as a gauge. Sneak up on it and check the fit every cut. If your lathe doesn't have a thread gauge then just back off the tool and reverse the carriage. The hardest part will be keeping track of how much you backed off the tool so you can get it back to where it was and add a few thousandths for your next cut. Just count the turns and note it.

 

[steve45]

Lots of threading 'how-to' videos on YouTube.

The 'bronze' piece looks like brass to me. Bronze can be hard to cut threads in, as it's springy.

What brand of tap & die did you use? The ones that Lowe's & Home Depot sell really aren't very good.

 

[Mike C.]

" The hardest part will be keeping track of how much you backed off the tool so you can get it back to where it was and add a few thousandths for your next cut. Just count the turns and note it."

That's where the compound comes in. You zero the cross slide and feed in with the compound at 29 1/2 degrees. You only have to return the cross slide to zero, there is no number to remember. You advance feed with the compound, so it never moves until you adjust depth of cut again. Just remember to turn the compound 29 1/2 degrees the other way when cutting internal threads (handle will be on the back side of the lathe).

 

[EPAIII]

Yup! Single point thread them up to a few thousandths short of the final size and then use the die and tap. As for your present skill set, it will not take long to get the hang of threading with a lathe. And the die and tap will clean up your mistakes.

For the absolute best threads you would need to single point them all the way, but I doubt that that is necessary for your job.

A tip: When you single point threads it is desirable to go slow. Many lathes will not go slow enough. I made a hand crank for my SB-9. It attaches to the back of the spindle and is about 15 or 18 inches long. It is perfect for making a slow cut for learning, for internal threads, and for threading to a shoulder.

Mike said it, Tap and die are not that good at getting threads straight. I once had a SB 16, I believe the 14 had the same lead screw pitch of 6TPI, not the more common 8 TPI. Go online had download the booklet How To Run a Lathe. Also known as HTRL. Start with an easy number 24 TPI, get a 3/8 rod and nut. Thread the rod, check each thread using a nut. When you can do OD threads well, they go to internal.

Use a tap in a lathe, its best to have a spring loaded center to keep it straight. If it does not start straight, you will never get it straight.
On some internal threads I will single point the threads, but make a final pass with a tap.

 

[Anthony]

Learn to chase a thread. Once you become comfortable and proficient at chasing threads you will rarely use a die again. Good luck.

 

[rabtrfld]

When you have some time, you should try threading with the lathe, its really easy.

But the important thing is not to thread right up to the mating surfaces.
Undercut the male thread and bore out the female piece and let them
mate on flat surfaces. If you do that a tapped thread will probably be fine.

 

[rabtrfld]

I made a hand crank for my SB-9.

Cool... but ever turn the motor on by mistake with the handle attached?

 

[Spruewell]

It is possible to rigid tap in a manual lathe. I do it quite often. Set up a good chuck in your tail stock and drill the pilot, then put the tap in it (use a good one) and slow down to a couple hundred rpm, leave the tail stock free and run the tap in. Not locking down the tail stock will allow it to follow the thread pitch more easily. When you are approaching the desired thread depth, disengage the spindle let it come to a stop and then reverse it while you back out the tap with the tail stock. It is helpful to put a wrap of Teflon tape on the tap to mark your depth/stopping point as things can move pretty fast and you won't be able to watch the dials.

And I agree with the "single point roughing and then running a threading die to finish" technique.

Good luck!

 

[EPAIII]

Nope. And I don't plan to.

Cool... but ever turn the motor on by mistake with the handle attached?

 

[johnoder]

Perfect. See how that works? He got used to the idea of making machinery DO WHAT HE WANTS - you can too

I made a hand crank for my SB-9. It attaches to the back of the spindle and is about 15 or 18 inches long. It is perfect for making a slow cut for learning, for internal threads, and for threading to a shoulder.