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Correct way to call out a hole inside a recessed feature?

Springy

Plastic
Joined
Oct 22, 2021
Tried googling and didn't come up with much on this. Say you have a flat-bottomed bore that's 1.000" diameter, 0.750 deep from a flat top surface which is used as Datum A. Inside that bore, you have another feature, which is a 0.250" drilled hole (eccentric to the bore), that is 1.250" deep from Datum A (or 0.500 deep when measured from the bottom of the bore).

Is the correct way to call out the depth of the 0.250" hole:
1. In a "top-down" view of the bore (ie you are looking directly into the bore) with a callout of "∅0.250 down 1.250"
2. In a "top-down" view of the bore with a callout of "∅0.250 down 0.500" (since the top of the 0.250 hole is at the bottom of the bore)
3. Use a section view to describe it specifically using linear dimensions
4. Other

I ask because in certain cases, either 1 or 2 could be considered ambiguous unless there is a drawing standard that says you would always use one or the other.
 
Why wouldn't you give the depth relative to the datum plane? (Option 1) That's how it would be measured.

Three reasons I can think of:
1. Because in some cases the bore would be large enough in diameter and the drilled hole small enough in diameter, that measuring down from the datum plane would be very difficult (not necessarily feasible to directly measure - we don't assume these prints are going to places with CMMs that can get into tight/deep holes)
2. If the hole has a tolerance relative to the bottom of the bore rather than the datum plane (in which case you could argue that it should be another datum plane)
3. When you're programming it, you only need to have the tool feeding from the bottom of the bore, not the datum plane - however in most cases I'd say this is not a factor that should affect how the part is drawn, and if your WCS is set at the datum plane, any movements below Z0. are going to be inside the stock boundaries.

Possibly the answer is that the callout should specifically reference a specific datum plane? Eg "0.250 down 1.250 from Datum A"?
 
The way I would want to see it if there is no model to go with the print is... 1.000 down 0.750 then 0.250 down 1.250 from Datum A. Then a section cut showing refence dimensions and the axis of the two holes showing the eccentric dimension.
 
ANSI Y14.5-2018 says "The depth dimension of a blind hole is the depth of the full diameter from the outer surface of the part. Where the dimension is not clear, as from a curved surface, the depth should be dimensioned using dimension and extension lines rather than notation.

I take this to read, Unless otherwise dimensioned the depth is given from the outer surface regardless of the relative size of the features. In this case the callout must point to the diameter of the largest hole feature (where that hole meets the surface). The figure 4-36 "Holes with Multiple Counterbores" bears this out as well.

It would be poor practice, but if the callout points to the smaller feature then it could be assumed that the depth is given from the surface at the junction of the hole and surface where the callout points. If the callout includes the dimensions for the larger hole feature then callout is illegal.

Cheers

Ken
 
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The way I would want to see it if there is no model to go with the print is... 1.000 down 0.750 then 0.250 down 1.250 from Datum A. Then a section cut showing refence dimensions and the axis of the two holes showing the eccentric dimension.

Thanks. There is a model in this case but we want to get the print right regardless.

ANSI Y14.5-2018 says "The depth dimension of a blind hole is the depth of the full diameter from the outer surface of the part. Where the dimension is not clear, as from a curved surface, the depth should be dimensioned using dimension and extension lines rather than notation.

I take this to read, Unless otherwise dimensioned the depth is given from the outer surface regardless of the relative size of the features. In this case the callout must point to the diameter of the largest hole feature (where that hole meets the surface). The figure 4-36 "Holes with Multiple Counterbores" bears this out as well.

It would be poor practice, but if the callout points to the smaller feature then it could be assumed that the depth is given from the surface at the junction of the hole and surface where the callout points. If the callout includes the dimensions for the larger hole feature then callout is illegal.

Cheers

Ken

Thanks, this is exactly the kind of info I was looking for!
 








 
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