How to Prevent External Thread Damage?

[Andrews]

We are manufacturing a no. of threaded components for diffferent automotive and non-automotive applications.The threads range from M5 to M30 with a variety of pitches and also 1/8" x 27 PTF threads.These threads are generated by thread rolling operation . Lately we find that thread damages have increased in our components .What are the areas we should look into to avoid thread damages in external threads?

 

[JRKH]

Re: Thread Damages

Andrews said:

We are manufacturing a no. of threaded components for diffferent automotive and non-automotive applications.The threads range from M5 to M30 with a variety of pitches and also 1/8" x 27 PTF threads.These threads are generated by thread rolling operation . Lately we find that thread damages have increased in our components .What are the areas we should look into to avoid thread damages in external threads?

You state that damages are increasing lately so my first inclination is to look at some change in process.
I would begin by looking at material handling.
Is the damage more prevelent in certain sizes/pitches?
How are the parts manufactured? Is it by Screw Machine? Are the parts dropped from the machine into a pan? How often is the pan emptied and samples checked?
How are the parts transported? Are they laid carefully in pans or dumped?
Has there been a change in operators?

Just some thoughts from an old lathe hand. I ran turret lathe and we used thread rollers extensively. Rolled threads, in my experience, are somewhat more robust than cut threads (depending on how fine the pitch is). Once the machine is set and running good parts rollers are subject to very little wear. But they can become chipped if ran over a diameter slightly too large. If this is the case you may be getting some malformed root diameters or bad lead ins. In addition too small a lead angle on the blank will cause the first thread to roll out over the end of the part. This might gage OK at the machine but extremely suseptable to damage, so look at the lead in angles on the blanks.

Hope this helps
James

 

[Andrews]

Hello JRKH,
Thank you for the response to this thread.

First I would like to reply to your questions:
1) We are facing problems with M10 thread and above , and pitches 1mm or less.

2) Threads are manufactured either on semi-automatic screw machines or on thread rolling machines.

3) Parts fall into steel pans filled with oil

4) Parts are emptied once in an hour after inspection.

5) Parts are transported in steel trays that are moved to stores on trolleys and then transported to subsequent operations in steel trays or polybags.

6)Parts are dumped when bulk quantities are involved i.e most of the time because we deal with quantities in the range of 10,000 to 50,000 nos per batch.

7)There has been no. change in operators

Based on your query, I THINK you suspect handling of parts to be the cause for the thread damage.We were under the impression that threads made by thread rolling operation are stronger.For your information, the parts we deal with weigh between 10 to 15 gms per part.We usually provide a chamfer of 20° to 25° (with respect to the horizontal) on the axis of the part.

 

[JRKH]

Andrews,

Based on what you have told me I cannot find a problem. The lead angel SHOULD be sufficient. Dropping them in a pan of oil SHOULD prevent damage etc. However I still suspect either material handling, or some process change as the culprit.
I would increase inspections of parts at the machine, in particular the blank dimensions before rolling to see if there is any change in lead angle size, or blank dia. You may need to go ti a 100% inspection to track this one down. I would also follow several batches through the material handling - moving, packing, installation etc. to see if anything shows up.

Good Luck

James

 

[D.Scott]

Andrews

We did a very extensive study to determine the cause of thread damage once the customer's part arrived at our plant.

We did 2000 piece random inspections on incoming bins followed by 2000 piece inspections at each process module.

We found the following areas of concern:

1) Degreasing the incoming parts - parts were dumped into a "basket" which was rotated in cleaning solution to remove oil, then dumped into our material bins.

2) Bins were then dumped into the feed boxes which fed the vibrator bowl keeping the process lines fed.

3) After coating and drying, the parts were dropped approximately 4 foot into the customer bin from the oven.

Nicks were found in the threads at each of these modules.

The customer was contacted and we arranged for parts to be sent in "dry" which cut out the degreasing portion. We sent our process bins to the customer and they shipped their parts in our bins which eliminated the bin transfer. We lined our feed towers with plastic and installed a "chute" to minimize the fall into the tower from the bin. The vibrating bowl and feed track were padded. Finally, a chute was placed at the end of the oven to stop parts falling into the shipping bins.

We spent a lot of time and money trying to eliminate thread damage, but a follow up showed there was always "some". We reduced it to where the customer could live with it, but when you are handling thousands of parts at a time, they are always susceptible to damage. The point is, a lot of it can be eliminated, but because of the size of the parts you are handling, you are not going to prevent all damage.

If you eliminate the handling damage, you will probably eliminate 95% of your problem.

Good luck.

Dave

 

[Sam]

Andrews,
Just some additional thoughts.
- What type of thread damage is evident?
- At what point in the process do you find the defect?
- Is the damage on every piece?
- Could the gaging process cause the damage?
- Have you changed inspectors?
- Has the inspector become more aware (picky) in his observations?

 

[Andrews]

change material

Will changing to a harder material help? Has anyone taken such an action and solved the problem? At present we are using en1A material.

 

[JRKH]

Although I am not a metalurgist, my guess would be probably not.
My first concern has to do with increased costs due to reduced tool life, particularly in the thread rollers. Harder material may cause more problems than it solves in reduced insert life, more trouble controlling rolling diameters and consequent chipping of rollers and premature failure of same.
Since the process of thread rolling deforms the material rather than cutting it, there is an inherent toughening of the threads due to work hardening. I suspect that going to a harder material will NOT yield a significant increase in toughness of the threads. By toughness I mean resistance to damage due to material handling.

Again, I am not a materials person, just an old lathe hand who rolled thousands of threads between 1/4"-28 and 2"-12 dia's on 1040 and 1144 steel and 303 to416 SST.
Maybe someone else can provide better info for you on materials.

I would not suggest changing materials until making a thorough examination of the process. Perhaps in the manner described by Dave Scott above. Once you have eliminated or minimized other failure areas you can experiment and evaluate a possible change in material if failures remain prohibitive.

Based on your earlier questions and responses I note the following:

1) "Lately we find that thread damages have increased in our components ."
This indicates to me that the process was working fine for some period, then began to deteriorate. Therefore something must have changed.

2) "We are facing problems with M10 thread and above , and pitches 1mm or less."
So the problem is with larger diameters (thus heavier) and finer threads (more suseptible to damage). I do not believe a material change would significantly effect this.


What steps have you taken thus far to narrow down the problem? Do you know:
What type of damage is observed? (i.e. torn threads, flattened threads etc.?)
Where is the damage located? (i.e. on the crest of the threads, always near the start of the thread, on the flanks and root of the thread etc.?)
What is the reject rate at the various inspection points?

Let us know what you are currently finding out and maybe we can help you narrow it down some more.

 

[Jen Kirley]

So the problem is with larger diameters (thus heavier) and finer threads (more suseptible to damage). I do not believe a material change would significantly effect this.

What steps have you taken thus far to narrow down the problem? Do you know:
What type of damage is observed? (i.e. torn threads, flattened threads etc.?)
Where is the damage located? (i.e. on the crest of the threads, always near the start of the thread, on the flanks and root of the thread etc.?)
What is the reject rate at the various inspection points?

Let us know what you are currently finding out and maybe we can help you narrow it down some more.

I like the course this discussion has taken thus far.

Changing to a harder material is more likely to bring value if needed when the parts are in use, but for this purpose changing the process is more likely to have the desired value effect.

This looks like a good place for a Cause-And-Effect diagram. I also think that sampling for defects at each process step is a good idea. I would be interested to learn if different process steps produce different types of damage in different sized parts. If that's the case, the problem could be getting muddled by two or more problems happening at once.

So, to sum it up, check each manufacturing stage for types of damage, perhaps make a P-chart (which can correlate defect rates to different operators, flow rates, material efects such as oil heating up and thinning, environmental impacts etc.) and construct a fishbone diagram. My instinct is to do this for each of these process steps. From that brainstorming process (ask the operators to help) you can get clues of what to do to minimize the parts striking each other or surfaces that might be marring them.