Reamed hole not round?

[R1chJC]

Hi All,

I have 17mm dowel with a H7/g6 fit. However the parts do not want to play ball, I can't even get the dowel started.

The manufacturing engineers of the supplier tell me the hole is drilled and reamed with a 17H7 reamer and checked with a bore gauge.

The dowel is ground to g6.

So any ideas whats happening? My suspicion is the hole is not correct, the dowel seems about right when checked with a mic. I don't have access to anything to check the bore really.

Is it possible the hole is out-of-round or thee is some other kind of form error? No geometric controls exist on the drawing other than the H7 size requirement.

Any pointers?

Thanks.

 

[hendersdc]

Not a lot to work off of if you can't measure the bore. What does the start of the hole look like, are there any burs or rolled edges? Is there a lead in chamfer?

 

[R1chJC]

HI,
No lead in chamfer i'm afraid. No obvious defects that I can see.

I suppose I could ask them to check roundness of the hole?

 

[berkshire]

check to see if the hole has a sinusoidal wave around the periphery, this is a fairly rare, but known defect with reaming. The hole will measure true at any two points on the diameter , but will not accept a go plug gage.
B.E.

You are judged not by what you know, but by what you can do.

 

[3DDave]

In a similar situation a sledge hammer started the pin just fine. Finishing was more difficult, what with the head of the pin being all mushroomed. Some assembly techs are persistent and no amount of difficulty will give them pause.

Look at Cerrosafe to duplicate the geometry of the hole. "Cerrosafe shrinks during the first 30 minutes of cooling and then at the end of an hour, is EXACTLY chamber size."

http://www.brownells.com/gunsmith-t...rosafe-reg-chamber-casting-alloy-prod384.aspx

Until you know what is wrong you won't know what to do to fix it.

 

[ScottyUK]

Any chance of thermal expansion of either part being a contributor to the problem?

 

[R1chJC]

Hi all,

Thanks for the replies.

Could be some thermal issues, but we'd be talking only small variations in room temps as far as I can see. I wouldn't expect that to be enough to significantly effects fits - happy to be told otherwise though.

 

[btrueblood]

The ground pin may also be multi-lobed, we had some come in recently with that defect. You need a 3-point contact device to pick up the size variation for 3-lobed parts. Or use optical techniques (section the cerrocast form, and inspect on optical comparator).

 

[Compositepro]

AS a minimum the hole and dowel should be deburred, which is, in effect a chamfer. I have often found it very difficult to get a shaft started into a tightly toleranced hole without first chamferring the hole and shaft. Without chamfers it is almost impossible to initially line-up the hole and shaft with sufficient accuracy. Also, no chamfer means that the parts have knife edges, which are easily damaged, and also easily cause damage.

 

[BrianE22]

Is the surface above the hole square to the hole (spotfaced or machined)? Getting it started straight might be the problem. Can you build a fixture that will hold the pin parallel and concentric to the hole to help get it started? We've used 3D printed plastics to make simple fixtures like that.

 

[AidanMc]

Is it just an assembly problem? Could the pin be frozen or cooled and the part containing the hole heated? We use nitrogen and heat in order to assemble some of our bearings...

Aidan McAllister
Metallurgical Engineer

 

[R1chJC]

Is it just an assembly problem? Could the pin be frozen or cooled and the part containing the hole heated? We use nitrogen and heat in order to assemble some of our bearings]

I did consider that but for a H7/g6 (clearance fit) I shouldn't have to go to such lengths!

I'll take another look at trying to offer the dowel up to the hole in a more accurate fashion.

AS a minimum the hole and dowel should be deburred, which is, in effect a chamfer. I have often found it very difficult to get a shaft started into a tightly toleranced hole without first chamferring the hole and shaft. Without chamfers it is almost impossible to initially line-up the hole and shaft with sufficient accuracy. Also, no chamfer means that the parts have knife edges, which are easily damaged, and also easily cause damage.]

Sorry I wasn't being clear with my terminology, there is a lead in chamfer on the dowel, nothing on the hole.