Deep drilling GD&T

[00Viggen]

Question regarding GD&T for deep drilling / boring drawing. What is the common practice for tying down an internal bore, taking into account straightness, cylindricity, and 'wander' from start to finish of the bore?

00viggen

 

[ctopher]

Assuming for a metal machined part. How deep and do you need to hold the tol the whole depth, or part of it?

Chris
SolidWorks 06 4.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 10-27-06)

 

[00Viggen]

Yes, it is a metal part. The bore is through the entire part. About 12 feet long total.

As the OD of the initial stock bar will be turned to a finished diameter, I have usually specified a drift of 0.001" per 1.0" depth. As the OD is sufficiently oversized, the total 'wander' of say 0.144" is not outlandish.

My feeling is to define cylindricity on the bore itself, and then perpendicularity to one of the ends.

The finished diameter (after turning the OD) would then be controlled by way of concentricity to the original internal bore (difficult to check though)...

 

[Heckler]

What are your processes? Are you boring the hole then honing or burnishing to finished diameter? What diameter and depth? I've seen really precise deep bore holes but they have been multiple processes to achieve the critical tolerances.

Best Regards,

Heckler
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[00Viggen]

The OD of the stock material is 3.5"
The ID of the bore (though entire length) is 2.125"
Depth of bore (thru' all) is 12 feet.

Hole will be bored, stock OD will be turned down to finished diameter 2.875".

As there is sufficient stock OD to work with, it should be relatively straight forward to bore the hole, then use the bore's axis as the datum to turn the OD. As long as the bore does not wander excessively, and remains straight (or cylindrical) relative to itself along all 12 feet.

 

[ctopher]

Depending how the part is used.
You can use projected tol or total runout.
If the part is assembled to mating parts only on the ends you can call out a tighter tol on the ends.
You can use the I.D. as the datum, that is will it will be chucked for turning the O.D.?

Chris
SolidWorks 06 4.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 10-27-06)

 

[ringman]

Almost sounds like you might want to just control the mininum wall thickness of the tube.

What is the function of the tube?

 

[00Viggen]

Never did get a definite answer on this one... Any further thoughts or comments?

All I am really after is the tolerancing required for the first step in the deep boring operation, which is to drill the hole straight though the bar. All the finishing is handled by another drawing. I just want to have a drawing that is easily understood and can be inspected with minimal difficulty for the first boring op.

My thoughts have been somewhat mixed, including: perpendicularity of a cylindrical tolerance zone (specifying an allowable deviation of the axis per unit length) to the entry or exit plane. Also cylindricity appeared to be somewhat sensible, but prohibitive to inspect.

I can see runout being useful on the finishing drawing, but not necessarily of any use on the initial operation as a means of putting a bound on the allowable drift.

Someone out there must do deep hole borings for a living and have this done to a fine art!

 

[KENAT]

The tolerancing should normally be driven by the function of the finished part not the process to be used to make it. Obviously you need to make sure the tolerances you ask for can be met but hopefully you get the point.

As such without knowing the effect on the finished part & assy it can be difficult to answer a question like this.

If all you really want to do is maintain a minimum wall thickness for later machining why not consider positional?

 

[ringman]

If minimum thickness is all that is required, which appears to be the case, specify that and forget all the other 'stuff'.