What are some possible causes of crooked threads?

[bakerbj]

I have this machine that is threading tapping lock nuts at a defect rate of ~1800 PPM. The thread perpendicularity spec. is .005" and the majority (85%) of the parts fall within the lower half of that.

I've attached a picture of the machine, it has a loading station, chamfering, tapping, and unloading. The part can move approx. 1 mm when seated in the fixture, this space is needed for the loading operation. There is nothing holding the part down while the tapping operation occurs.

So, please take a look and let know what you think, I'm getting really tired of having crooked nuts. (sorry couldnt resist)

Other info:
1018 material
M36 x 1.5 threads
part is 5 mm thick
fixtures are flat and parallel to table

I'm thinking possibly:
Parts need to be held down while tapping?
Possible that a chip from chamfer station curls under the part tilting it?

Thanks

 

[limey1]

You may call it by another name, but are you using a 'finishing' tap? If you are, the part would tend to lift on one side when the tap begins to bite into the material.

 

[bakerbj]

Finishing tap? I would say yes. I was thinking that lifting upon the intial cut is something that could be going on and maybe adding some puematic mechanism to hold the part down while it's being tapped.

 

[bakerbj]

I was having the same problem at the chamfer station but fixed it with the spring loaded hold down devise (for lack of better term) in the back.

 

[limey1]

Do you have room to fit a guide?. Think of it as a sleeve that holds the part down, while the tap is kept centred as it starts to cut.

 

[ornerynorsk]

Sounds like your downfeed is not advancing fast enough at the initial "bite". You shouldn't need a hold-down, but you should really put some down pressure into the initial tap entry. If this is a lead-screw machine, put a part hold-down on your fixture.

At only 5mm thick, you shouldn't have to worry about stretching the thread as long as the feedrate adjusts to pitch once the thing digs in. Your first couple of threads are incomplete anyway, and the tap will clean them up nicely as it finishes through.

Good luck.

 

[bakerbj]

It is a lead-screw type machine so I'm currently designing a clamping devise to hold down the parts through the tapping operation.

 

[WoodyPE]

A picture of the defective thread would help.