Lathe - threaded hole/threaded pin, not concentric when bolted together!?

[The Aspirator]

I've made a bunch of threaded pins on my converted CNC lathe, M14x1.25 thread, and in another part I bored a 0.502" hole with an endmill (in the lathe) and used an internal threading insert to thread that hole to M14x1.25. But when I thread them together and spin the assembly in the lathe for a visual it's extremely wobbly!

Since everything was made on the lathe I assumed things would stay concentric enough for my needs (0.010" runout would be fine), but this is way way more than that. I'll go measure the runout now and take some pics and maybe a video to help describe it better, but I just wanted to get this thread posted and hopefully get some advice soon. Is this a common problem with threaded assemblies or am I doing something wrong? The thread fit is snug with very little wiggle when it's halfway threaded in. Once the head bottoms out it should be perfectly square, and centered! But apparently not.

Thanks guys!

John

[CalG]

Turned all over? Or are the "pins" chucked up stock?

[The Aspirator]

Good question, they're turned all over so it's not the stock inaccuracy. I took some pics and vids to explain it better, I'll get them edited and posted right now.

[307startup]

Your endmill isn't cutting on center, nor is your drill, nor is your tap.

You need to realign your tailstock and measure your offset by doing a few shallow cuts, using a dial indicator to get a better feel of what's going on.

You might need to chuck up on the part, leaving just the face exposed on the jaws, or use shorter tooling to minimize wandering.

[The Aspirator]

307startup, my endmill is cutting centered enough I think, I did a few dozen test cuts to get the threading insert dialed in, and the endmill is producing a reliable 0.507" hole and I see all 4 teeth are being worked (4 tooth center cutting Niagara). It's chucked into my toolpost and fed in with the steppers. I did have to re-align the angle of my tool post which made a HUGE difference and solved a lot of issues I was having with drilling that hole in the first place.

My "tap" (insert) is definitely centered because it's a single point cutting tool, those threads have to be concentric don't they? It's no different than turning the OD with a turning tool.

However you're right that my drill bit, for the 0.43" hole in the brass parts, is probably wandering a bit. However when I chuck up just the brass part and spin it, it's visually perfectly centered.

More info, the drilled hole in the big parts is plenty deeper than the threaded pins, so the pin is definitely seating completely on the "head".

I just measured the runout, I have 5 pistons and 5 aluminum pins to test and they went like this:
0.021", 0.011", 0.006", 0.033", 0.022".
All 5 pins were made back to back in the same run, and same goes for the pistons and threaded holes.

I measured a brass guide tube installed in the "top hat" and its' ID runout in the drilled hole is only 0.007", tolerable. For comparison, during the same chucking the 2" ID of the aluminum top hat was only 0.00025", so the whole unit was definitely centered, it's just when the part is threaded in that it becomes eccentric.

Also, the ID of the top hat is 2.000", the OD of the piston is 1.970", giving 0.030" diameter of wiggle room. I think that means I can have an eccentricity of 0.015", right? The o-ring is 1/8" thick and has enough squish to make up the difference, so that's fine, but I can't have the piston scuffing against the wall and binding!

Youtube video:
YouTube- Blowoff valve threading.mpg
(why won't you embed?)
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[The Aspirator]

I just threaded 4 more "top hats" and tested them out, some were bang on and others were just as wobbly as the pistons, what's going on?? I'm wondering if it has to do with the thread clearance, IE, the wiggle when it's halfway threaded in. Because this morning the first few I made had a lot of thread wiggle, then I tightened up the clearance by about 0.010" and the last one was so tight I couldn't thread it in by hand, but it was spot on centered!

Thoughts? I think I can mess with these 5 units to make them work, I'll just shave off a few thou from the OD of the piston because that's where it binds. However this is a production part and I plan on making 50-100 of them so I need to do this right. Any help would be greatly appreciated!

John

[christinandavid]

Use your tight pair as gauges for the rest.

DP

[Bubba]

It might also be that your using a three jaw chuck. Most of the time, it will not completely and accurately center the stock. In order to have all surfaces concentric, you would have to make the part in a single setup and not remove it from the chuck until completely finished! The other option is to use a minimum 4 jaw chuck and dial the stock in with each setup.
Another possibility (not probability as your lathe looks fairly new) is that the jaws of the chuck are bellmouthed allowing the stock to move off concentricity.
Just my .02

[CalG]

Threads themselves make for poor registration.

You may want to consider a redesign that includes some form of shoulder or seat. Not unlike an automotive wheel lug bolt.

Cheers

[brunog]

John,
if the the cylindrical snd theaded ends of the pins are concentric, and the theaded hole in the piston is also concentric to the pistons outside walls, then the threading is the issue, are you sure your threads are to specs. How do you measure your threads?

Have you tried to fit the brass part to your piston, are they concentric? If so, were the brass parts held differently than the pins. At what end did you start making your pins?
By the threaded end?

Best regards

Bruno

[WayneHill]

Maybe the scroll gear of the chuck is off. A solution is to bore a set of soft jaws for each setup size.

My $.01

[Delw]

The problem is the threads are too coarse,too shallow and not tight enough tolorance to the mating part and you have a shoulder for a lock down.
you need a minumum of a 3b and 3a thread, must be fine threads on top of that. then when thats all done the sholder that your tightening down on wont work..

you can try to change the design so there is a thread then a very tight tolorance od (id on mating part)and with a shoulder on the top or lock nut on the bottom, the slip fit od will keep the part from canting when you tighten them down somewhat.
the shoulder is whats causing your run out nothing else.
everytime you tighten it it Kants the threaded part do to the slop in the threads( ie pulls it to one side) there is nothing that can be done with the way you have it designed now.

2 ways to fix it redesign your part and recut your part with a treepan tool so you have one side as ONE piece. meaning the stem is part of the main piece.
or make your pieces like you are and lock tight them in and recut the stems. however once they lossen or move your mismatched again.

Hope that all made sence.

Delw