Depth of a Counterbored Hole 1

[superptrucker]

If the depth of a counterbored hole in the top face of a plate is directly toleranced, what is the proper way to measure the depth? Is it to a tangent plane established by the top face of the plate or would it be more localized e.g. measuring the depth with the tail end of a caliper? According to the standard, for countersunk holes, the depth is determined by the creation of the countersunk diameter so even if the plate was warped the depth of a number of countersunk holes would follow suit. Fig. 1-40 shows countersink on a Curved Surface. Since the purpose is usually to sink a fastener into the plate, I'm wondering if something similar applies to counterbored holes. I guess the same could be asked for drilled, counterdrilled holes, and spotface depth also.

 

[drawoh]

superptrucker,

Countersinks really ought to be specified by diameter and angle. Diameter is easily inspected, and it controls how the flat head screw sits. The correct depth is affected by the diameter of the clearance hole. This will make for an interesting feature control frame.

Figure[ ]1[‑]40 in what standard? If the countersink is in a curved surface, I would produce a section view what shows clearly what I wanted measured.

If you are interpreting someone else's drawings, you will have to use calipres, or a stainless steel scale. Hopefully, the tolerance is not accurate. Perhaps you can call the drafter and ask them to fix the drawing. There is a time and place to stare blankly, stupidly and uncomprehendingly at people. This may be it.

--
JHG

 

[superptrucker]

Sorry I'm referring to Fig 1-40 of ASME Y14.5-2009. For countersinks it is intuitive that controlling the diameter would automatically control the depth, and that for a severely warped plate, the countersunk depth would naturally follow the warped surface so that in practice, all the countersunk screws would sit flush or slightly below or whatever the intent is.

I'm curious if the same concept applies to drilled, counterdrilled, counterbored, and spotfaces that just have a depth callout.

I noticed that 1.8.10 mentions "The depth dimension of a blind hole is the depth of the full diameter from the outer surface of the part." I guess what I'm wondering is if the outer surface of the part is considered just the area immediately surrounding the hole or if it's something else.

 

[Burunduk]

superptrucker,
ASME Y14.5 doesn't define how the depth tolerance of a counterbore should be interpreted. If the depth uses a dimension line with two arrowheads* and is directly toleranced (i.e. limit dimensioning / plus-minus) you are free to interpret the requirement in whatever reasonable way you like. If you want to use the caliper, use it. If you want to establish a tangent plane to a planar top face and measure from it, you can do that too. The purpose of the requirement for a counterbore is usually to sink the head of the fastener so that the top face of the fastener head is at the same height or slightly below the face of the part. So considering only the local area around the counterbore as the measurement zero reference (i.e. using a caliper) should be good enough.

* The only case where the directly toleranced depth dimension has only one interpretation is when a circle (dimension origin symbol) replaces the arrow at the side of the planar face. Then the measurement must be from the tangent plane of the entire surface.

 

[drawoh]

superptrucker,

Figure[ ]1[‑]40 is connected to paragraph[ ]1.8.13, which states that the diameter you specify applies to what is described as the minor diameter of the countersink. Do you really trust everybody to recognise and understand this? Do they know/believe that you understand it? Do a section view and make things clear.

If the hole is counterbored, the diameter applies to parallel surfaces that are not affected by the curved top face. The only problem is when the curved face gets down below the counterbore diameter. When things get weird, do a section view and make things clear.

--
JHG

 

[superptrucker]

drawoh, that's a good point about using section views to make things clear about how the depth is to be interpreted when the surface is curved or otherwise not flat.

Burunduk, I wonder if there are a pattern of counterbored holes if adding "INDIVIDUALLY" to the multiple instance callout would assist with interpretation that the depth should apply locally in order to follow the not-so-flat surface of the part. Anyway it's interesting that the standard doesn't define how counterbore depth should be interpreted. Good point on the dimension origin symbol.

 

[Burunduk]

superptrucker,
Where specifically did you think of applying the INDIVIDUALLY notation? In its standardized use, it is associated with a geometric tolerance and/or datum feature symbol to indicate that a requirement or definition which by default would apply to a pattern, should apply on an individual basis (dismantling the basic relationship between the tolerance zones of the features or between the datum feature simulators). It wouldn't tell anything about how the depth dimension should be measured.

 

[Tmoose]

Our parts often have counterbored holes for fasteners in castings. The thickness of the material between the bottom of the c'bore and the mounting face is sometimes the important dimension, NOT the "depth" of the c'bore.

Ditto for spot faces, that sometimes really are counterbores I guess.

 

[Dean Watts]

The counterbore depth can be measured from any reference point on the surface of the part that the feature is cut into. Counterbore depth specified as a directly toleranced dimension is just like all other +/- location controls. It is ambiguous. The direction of measurement and reference point are not rigorously defined.

The ambiguity is not usually significant relative to the amount of depth tolerance specified, so counterbore depth is usually OK to specify this way. If the depth or remaining material is a more critical characteristic them profile of a surface using a relevant surface as a datum feature is best.

Dean

http://www.validate-3d.com

 

[drawoh]

Dean Watts,

If you specify the depth of a countersink, the geometry depends on the angle, the depth, and the diameter of the clearance hole. If you specify the diameter of the countersink, the geometry is controlled by the angle and the diameter, only. The diameter of the hole is irrelevant. The diameter is easily measured.

--
JHG

 

[Burunduk]

drawoh, I suppose that by the easily measured countersink diameter you mean the diameter of the countersink edge at the side of the flat surface (the other diameter is the hole diameter). What do find to be the preffered method of measuring that diameter?

 

[3DDave]

For countersinks into flat surfaces one can use a countersink gage. In CNC work a precision cylinder/ring gauge is used during tool touch-off to locate the axial location of the countersink apex if non-CNC qualified countersinks are used. On CNC qualified countersinks a flat is made on the "sharp" end with a known distance to the countersink apex as provided by the maker or .

What is amusing is that the apex is the common feature between the screw and the countersink, particularly for holes where there is a counterbore to the countersink and no diameter controls the screw installed depth. This apex can be determined for installation on rough or angled or otherwise irregular surrounding surfaces and relates directly to the machine motion to create the countersink. In spite of this industry concentrates on the sometimes difficult to measure "diameter" with which no feature interacts on the grounds that it is convenient.

Most amusing? The expensive countersink gages convert depth to a "diameter" without actually finding the intersection. Similarly amusing is a CMM will find the diameter by finding the apex relative to the surrounding surface in order to find that diameter.

 

[Kedu]

3DDave and all,

Speaking about countersinks how do you think they should be specified to provide an unambigous requirement? Otherwise stated, what is your opinion about how to define them correctly ?

 

[Dean Watts]

drawoh,

Why do you bring up countersinks? Countersinks and counterbores are two very different types of features.

Dean

http://www.validate-3d.com

 

[Kedu]

Dean,

I am not drawoh, but I would guess countersinks have unclear definition per Y14.5 hence how do you recommend those to be clearly / unambiguously defined?

 

[Dean Watts]

Hi Kedu,

For countersinks, the angle spec is often just to specify which tool to use to create the countersink and the diameter is measured to see if it is within tolerance. The diameter can usually be specified sufficiently large to ensure that the entire head of the fastener will be below the surface. The diameter isn't a functional feature, as 3DDave pointed out, but it works well enough in the vast majority of applications and it's and easy low cost thing to measure.

Some (few though, I believe) will also measure the angle of the countersink, but to really control the feature in more critical applications I think surface profile would be best. Using profile would be very seldom needed for a countersink, the same as using profile for the depth of a counterbore. Usually the there is enough design margin to allow the ambiguous +/- depth spec for a counterbore or the indirect diameter spec for a countersink to work well enough.

I think this is similar to chamfer and radius specs. For more critical applications for which a simple but ambiguous +/- radius or chamfer spec will not be adequate we need to fall back to using profile.

Dean

http://www.validate-3d.com

 

[Burunduk]

Usually the there is enough design margin to allow the ambiguous +/- depth spec for a counterbore or the indirect diameter spec for a countersink to work well enough.

Same for the diameter of a spotface?

 

[Dean Watts]

Hi Burnduk,

Since Y14.5 specifies that the diameter of a spotface is diameter of the planar surface excluding any fillet radius in the corner, we have an odd difference between that spec and the
diameter of a counterbore. The diameter of a counterbore applies to the cylindrical wall of the counterbore. This has always bothered me because the function of a counterbore is normally to accommodate a fastener, just as it is for a spotface. One type of feature is just deeper than the other. For both a spotface and a counterbore, the diameter of the planar surface must be sufficient to accommodate the diameter of the washer or the head of the fastener. If the diameter of a counterbore is measured within spec, but a large corner radius at the bottom of the counterbore makes the diameter of the planar surface too small to accommodate the fastener, then the specs have failed to ensure proper function.

Y14.5 says that the fillet radius MAY be specified for both a counterbore and a spotface, but the radius spec is then included on all the spotface figures and left out for all the counterbore figures. I think the corner fillet radius is actually more important to specify for counterbores than it is for spotfaces. This is because the diameter spec of a spotface does ensure an adequate seating surface for the mating fastener but the diameter spec of a counterbore, as it is defined by Y14.5, does not.

The diameter of the cylindrical surface of a counterbore will be measured the same as any cylindrical surface. The fillet radius of a counterbore or a spotface will be more difficult to measure and without a profile applied to the radius there is some ambiguity. While it's worthwhile to measure the diameter of a counterbore's cylindrical surface to ensure that it is not oversize (removing too much material from the part), if there is any question about whether sufficient margin between the washer or head diameter of the fastener and the diameter of the planar surface at the base of the counterbore I would also want it measured as for a spotface, where probably a vision system is used to measure the diameter of the planar surface at the difficult to identify tangent line with the fillet radius. As with all measurements there is uncertainty to deal with, but this measurement may be very important to ensure that the fastener will seat properly.

I think it would be an improvement if Y14.5 specified that both the cylindrical surface diameter and the planar surface diameter should/must be specified for counterbores. Combining the cylindrical surface diameter and the fillet radius spec provides only indirect control of an important functional consideration. As with the diameter spec of a countersink though, we all usually get away with the indirect control.

This took way to many words to attempt to pin down .

Dean

http://www.validate-3d.com

 

[Kedu]

For countersinks, the angle spec is often just to specify which tool to use to create the countersink and the diameter is measured to see if it is within tolerance. The diameter can usually be specified sufficiently large to ensure that the entire head of the fastener will be below the surface. The diameter isn't a functional feature, as 3DDave pointed out, but it works well enough in the vast majority of applications and it's and easy low cost thing to measure.

Some (few though, I believe) will also measure the angle of the countersink, but to really control the feature in more critical applications I think surface profile would be best. Using profile would be very seldom needed for a countersink, the same as using profile for the depth of a counterbore. Usually the there is enough design margin to allow the ambiguous +/- depth spec for a counterbore or the indirect diameter spec for a countersink to work well enough.

I think this is similar to chamfer and radius specs. For more critical applications for which a simple but ambiguous +/- radius or chamfer spec will not be adequate we need to fall back to using profile.

Dean
www.validate-3d.com

Dean,
I see your point, but I am asking from the coaxliality between countersinks and the applicable holes point of view?
If they are not coaxial (within some tolerances) maybe additional stress cound be present into the fasterner after get assembled. Additional stress means additional fatigue / stress concentration for the parts/ screws.

How do you propose to circumvent that without "killing" the countersinks with feature control frames ? Will notes being suffice?
What other options do you think could be deployed?

 

[Dean Watts]

Hi Kedu,

The coaxiality issue with countersinks is a valid concern, especially if the process used to create the countersink allows this type of error.

If coaxiality of the countersink is a concern, then I think the only effective alternative is to "kill" (as you say ) the countersinks with profile of a surface.

If the mating surface of the part is planar and normal to the holes then it could be the primary datum feature for the profile tolerances. The secondary could be each respective hole, with "INDIVIDUALLY" specified by the datum feature symbol and the feature control frame. If the surface the countersinks are cut into is planar, that may make a better primary datum feature, because then the depth of the countersinks would then more reliably be ensured to keep the heads of the fasteners flush or below the surface, as is usually needed.

We could then consider each countersink to be well controlled relative to the hole it serves, or maybe overly controlled, or maybe "dead" due to the effects of feature control frame application

For most cases, the countersink tool pilots well enough on the hole though, so this may not be such a concern, at least for a floating fasteners type of application.

For a pattern of fixed fasteners though, the cylindrical holes hole becomes a clearance feature and the location of the countersinks is critical. This is especially true when relatively short, large diameter fasteners are used. For those cases the profile of a surface applied to the countersinks would not use each individual hole for the secondary datum feature. No secondary datum feature could then be best, with best fitting of the data to the pattern of countersinks as specified by basic dimensions, handing the location of each feature within the pattern. With the threaded holes in the mating part controlled by position at RFS with a similar datum reference frame, the screws should all tighten nicely without a bunch of tilting of screws and misalignment within the pattern of countersinks.

Dean

http://www.validate-3d.com