thread gaging irregularity

[Philcott]

Hi Friends,

 

I am making a part with an external thread. All measures well. Major diameter, pitch diameter, minor diameter's all good. When I use the ring gages the go goes, the no-go no-go's and all is good. Here is where things get a bit weird. I now mill two flats on the thread and all of a sudden the no-go, goes!!

 

Here is a screen shot of the part.

 

 

My question. Does anyone know why this happens? My guess is when the ring gage is made, they grind the gage threads eccentrically thereby having, perhaps, three contact points that are ground to the thread pitch diameter and the rest is relieved. This might be done to allow space for any grit and crap that gets in the ring to sit and prevent galling on the part. If so, when I rotate the ring, there is space for one part of the thread to move into the relieved area and allowing the no-go to go on. What do you think. Ever run into this? It's a great trick to play on QC.

 

Phil

[JG]

Just a thought, maybe the no-go was not going on due to the "functional size" of the thread, did you measure them with a full form thread roll?

 

Jg

[Mike Whitten CAD/CAM Contractors]

Phil,

 

This is a very good question!

 

I am not sure of the answer but am very interested in this one.

 

You most likely have thought of all of this but just in case.

 

-What type of material is it?

 

-Is it possible that the part was smashed when milling the flats. Thereby reducing the PD.

 

-It would certainly seem that it must have something to do with the thread form.

 

- If you measure the PD with wires or a pitch mic before milling the flats, how round is it? Where does it lie in the tolerance band. IOW. is it on the very low? If the part is not round and you mill off the high points, this will do it.

 

-Does the PD measure the same with wires or a pitch mic after milling the flats when check the same area of the part?

 

- You may want to call an engineer at the manufacturer of the gage. My guess is that they would be easy to get a hold of. That may have the answer that you need.

 

 

Mike

[Philcott]

JG,

 

I did not. The only gage we have measures pitch diameter but I don't understand how a full form roll gage would tell me anything different.

 

Phil

[JG]

PhilCott,

Full Form rolls measure the cumlative error in the thread, its been my experience that when threading and measuring with both full form and

single element rolls (side by side) the full form rolls show a slightly larger thread and climb sooner than the PD rolls.

 

Jg

[Mike Whitten CAD/CAM Contractors]

Phil,

 

One more thing. My guess is if the pitch was off slightly on the thread, it may still gage OK until cutting the flats. It would then be loose.

 

BTW, I am a mill man, so.....

 

Mike

[Philcott]

Michael,

 

Good questions. Made me have a second look at some of the data. Here are the results.

 

 

-What type of material is it?

 

303 S.S.

 

-Is it possible that the part was smashed when milling the flats. Thereby reducing the PD.

 

I doubt it. It's pretty tough stuff.

 

-It would certainly seem that it must have something to do with the thread form.

 

I think it is more likely in the shape of the gage at manufacture.

 

- If you measure the PD with wires or a pitch mic before milling the flats, how round is it? Where does it lie in the tolerance. IOW. is it on the very low? If the part is not round and you mill off the high points, this will do it.

 

P-dia. .3468/.343 measures .3451 all around with no hi/low spots.

 

 

-Does the PD measure the same with wires or a pitch mic after milling the flats when check the same area of the part?

 

PD stays the same so there is no movement due to milling.

 

- You may want to call an engineer at the manufacturer of the gage. My guess is that they would be easy to get a hold of. That may have the answer that you need.

 

I did that and could hear them scratching their heads over the phone. After talking to the fourth person, I was asked for my phone number so they could have the president get back to me. I am looking forward to the call.

 

Phil

[Philcott]

Michael.

 

Good point about the pitch. I have not measured it just assumed it was right cause it is right in the program. I'll go see.

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Pitch is fine.

 

Phil

[Bruce Caulley]

I might just be a surface area contact issue. It would be easier to wind a thread into a nogo if there was only 20% of the part in contact.

 

I would try this if I had a small torque wrench in in/pds or n/m. Measur the torque it takes to stop the thread in the nogo before milling. Measure it after and see if the torque has decreased by as mach as the relative surface area lost.

 

Think of it like a cluch in a car. small cluch + big load = slip.

 

HTH

Bruce

[Mike Whitten CAD/CAM Contractors]

Phil,

 

Oouch! This is killing me now.

 

Post the answer when you get it.

 

Thanks,

Mike

[Mike Whitten CAD/CAM Contractors]

I wonder if you make the PD on the very high if the gage will be good?

 

Mike

[P_Scott]

Phil,

 

I think Bruce is on to something here. It stands to reason that if you had a full thread, there would be more surface area for the gage to contact. More contact = more friction. The adverse effect would be by eliminating surface area (machining flats) you are also eliminating contact area and reducing friction. Maybe what's needed is to machine the flats first and adjust for the no-go gage after threading. This is good stuff. Let us know what the pres. of that gage company says, if he even gets back with you.

[Philcott]

Bruce,

 

After milling, the no go slips on without any resistance at all. There would be nothing to measure with a torque wrench.

 

As far as running at the top of the tolerance. I would have to go above the upper limit for it to snug up.

 

OK. Just to add more credience to my eccentric ID theory. I just drilled and tapped a piece of brass and made a nut to test on the part. There is no noticable difference in the fit before or after milling the flats. I therefore conclude the thread that is left after milling is not being distorted in any way and that the contact area of the gage must not be round but eccentric.

 

Phil

 

I am still waiting on a call from the gage manufacturer.

[Philcott]

OK.

 

Here if the reason as explained to me by Bob (who did return my call) at Alameta Thread Gage company in California.

 

Bruce was on the right track in regards to the contact area. Bob said the gages are ground very close to the correct PD and then fitted on to a setting standard guided by a number of gage setting rules. Once this is done there would be a number of areas of contact between the standard and the gage. Lets say there were 25 contact points. When I machine the flats on my part I am removing, let's say, 16 of these points. With only 9 points left to contact the part, the gage will feel loose because there is now room for the gage to slop around.

 

Moral of the story.

 

Thread ring gages will not give you an accurate pitch diameter comparison if the part being checked does not have it's full circumference.

 

Phil

[JazzBass]

I Learn something new every day.

[CAM-mando]

I think it is the effect of functional gaging designed for a full thread. Because you have removed so much of the thread you have reduced the "effective" pitch diameter seen by the functional gage. Think of a Diamond Pin vs. a Full Diameter Round Pin.

 

oops ... I see you got an answer I am too slow

[Mike Whitten CAD/CAM Contractors]

Phil,

 

I am mo smarter now .

 

Thanks,

Mike

[Philcott]

Wait till Monday and I will tell you about what I learned the difference between european and north American metric thread sizes. Did you know there was a difference? I didn't. I thought metric was metric. Not so my friends.

 

Aw hell, why wait till monday.

 

Europe works to ISO metric specs while North America works to ANSI metric. I have not looked into the size differential yet (no time) but will follow up on monday unless someone else beats me to it.

 

Off to take the youngest and a buddy snowboarding. Road trip. Ye-ha. There'll be some sippycups a spillin' tonight. Road trip Ye-ha.

 

Phil