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Turning a threaded part around and threaded into the next toolholder

xring

Plastic
Joined
Nov 27, 2006
Location
CT-USA
Our CNC Department makes what we call chucks, parts that are threaded on one end and have a boss to locate them in their final resting place. They are then turned around held by a solid diameter, and then a hole drilled into the non-threaded end. The problem is the drilled hole runs out from .0005 which is acceptable to .005 which is unacceptable.

Is there a way to turn a threaded part around and thread it into a threaded work holder that had been trued up so that a hole doesn't run out when drilled? Sounds like a lot to ask for but can this be done so we don't have to drill the holes as a secondary operation. Thanks
 

DDoug

Diamond
Joined
Oct 18, 2005
Location
NW Pa
Our CNC Department makes what we call chucks, parts that are threaded on one end and have a boss to locate them in their final resting place. They are then turned around held by a solid diameter, and then a hole drilled into the non-threaded end. The problem is the drilled hole runs out from .0005 which is acceptable to .005 which is unacceptable.

Is there a way to turn a threaded part around and thread it into a threaded work holder that had been trued up so that a hole doesn't run out when drilled? Sounds like a lot to ask for but can this be done so we don't have to drill the holes as a secondary operation. Thanks
Add a cut on the OD, concentric with those threads, made in the initial chucking, if you want to run concentric after re-chucking for the 2nd OP.
 

prime_mover

Plastic
Joined
Jul 3, 2017
It should be relatively easy to achieve better consistency during the 2nd op. Like DDoug said, clamp on a machined feature which is concentric with the thread. Changing the order of the operations might also be a solution. The threads themselves could be clamped on with soft jaws if they are coarse enough and with enough surface area. The part could be threaded into a fixture for the 2nd OP, centered by the chamfer or the thread OD or other OD or ID feature. Someone in your cnc department should be able to think of several solutions more or less instantly.
 
Last edited:

sfriedberg

Diamond
Joined
Oct 14, 2010
Location
Oregon, USA
Locating on threads is a losing game.
If the boss is on the same side as the threads, hold it by the boss. (That's what DDoug is suggesting, I think.) If you can't do that, and assuming that you turn the OD for the threaded part and it doesn't raise burrs, try holding the thread major diameter in a plain (unthreaded) collet.
 

guythatbrews

Cast Iron
Joined
Dec 14, 2017
Location
MO, USA
We make many, many parts where the only way to hold them on 2nd op is by the threads. Soft jaws and collets. Works fine. You just can't go at the second op hammer and tongs.

Somethings off if you've got .005 TIR problems. Maybe it's your cnc department. It's cumbersome to screw a part into a fixture for the second op. Sometimes it may be the best way but not often. As others have said be sure the dia you are chucking for second op runs true with 1st op work. Make sure drill isn't walking on second op and second op chucking repeats TIR. This should be routine work and easy to achieve.
 

prime_mover

Plastic
Joined
Jul 3, 2017
Maybe the drill is not on center. Maybe it's been poorly resharpened by hand. Maybe they aren't center/spot drilling.
 

xring

Plastic
Joined
Nov 27, 2006
Location
CT-USA
Our CNC department does good work for the most part. This part in question is threaded, a boss turned on it behind the threads, and the OD turned, all in the same operation. The boss is Ø.562 x .127 long and the drilled hole in question is Ø.062. Admittedly I don't know exactly what they are doing but I've heard all the buzzwords about the collets from them. Tooling is top-notch, and drills are carbide. Turning this around for the 2nd operation, how much run-out can be expected knowing that everything was turned concentric in the 1st operation, on the major diameter which is larger than the threads and boss?
 

DDoug

Diamond
Joined
Oct 18, 2005
Location
NW Pa
Our CNC department does good work for the most part. This part in question is threaded, a boss turned on it behind the threads, and the OD turned, all in the same operation. The boss is Ø.562 x .127 long and the drilled hole in question is Ø.062. Admittedly I don't know exactly what they are doing but I've heard all the buzzwords about the collets from them. Tooling is top-notch, and drills are carbide. Turning this around for the 2nd operation, how much run-out can be expected knowing that everything was turned concentric in the 1st operation, on the major diameter which is larger than the threads and boss?
At this point, it's time to post a fully dimensioned & toleranced print.
"Arm Waving" won't cut it.
 

guythatbrews

Cast Iron
Joined
Dec 14, 2017
Location
MO, USA
the major diameter which is larger than the threads and boss?
I don't understand this ststement.

Minimum runout to be expected will come mostly from collet runout. The drilled hole itself I'd expect to run out easily less than .001.

Things to check:
1. Runout and repeatability of part in collet. Drill alignment.
2. Confirm zero tit on face to be drilled before drilling
3. With the drill being small spot drill and confirm spot doesn't run out.

In short leave nothing unchecked. Systematically check every operation. You can get away with less than a good starting point with a bigger drill but small drills not so forgiving.

Sounds like some hard head is overlooking something and refuses to search for the problem. This shouldn't be that tough.
 

Sandoz

Plastic
Joined
Aug 17, 2021
I'm not a native english speaker so this might be a dumb question, but in which direction does the runout happen?
 

TeachMePlease

Diamond
Joined
Feb 11, 2014
Location
FL
So If I'm reading that right, the boss is actually an undercut behind the threads?

Why not grab on this?
 








 
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