edge as a datum?

[Madhu454]

Hi All,

assume a machined square block, the 4 side wals of theblock is having a taper of 5 +/_ 0,1 deg. the top surface of the block has a tapped hole. to locate that hole, as usual the bottom surface is selected as primary datum feature. can we select the 2 edges (formed due to taper) as secondary and tertiary datums. ? pratically it is possible to acheive a repeatable DRF which provides origin for measurement..

if the above said method is wrong can anyone suggest me how to locate such a hole

but even i have not seen any drawings which uses edges as datum feature. even if we see the definition of datum it says it can be point, axis or plane. instead of using the term axis if they use line then i think above method can be used to define edges as datums. but again one more question comes to mind, is edge is a feature? if not, then it can not be a datum feature also. also i dont know y the edges are excluded from thr definitions of the feature. does it not a physical portion of the part?

can any one suggest how to locate such a hole?
y edges cannot be used as datums even though if we are able to acheive a repeatable DRF?
y edge cannot be a feature? y it is excluded from the definition of feature?

 

[pmarc]

Madhu454,
I do not see a reason why edges can't be assigned as datum features. You can easily do it by specifying datum target lines.

You have to remember that datum is something perfect that does not exist in reality and is derived from a datum feature. Therefore usually it is point, axis or plane. Datum feature (in this case edge) has nothing to do with datum definition - it is an imperfect physical portion of a part.

Even if you use datum target lines as secondary and tertiary datum features you will end up with datum reference frame consisting of 3 mutually perpendicular datum planes.

 

[drawoh]

Madhu454,

I would very cautious about applying datums, or ±[ ]dimensions to an edge of a non-orthogonal face.

Your machinist is going to take off the sharp edge. Unless he does this by applying a very precise radius, you do not know where your orthogonal tool will pick up the edge.

I strongly recommend that you use the angled surfaces as datums. Basic dimensions from the resulting theoretical edge, are valid. Are there any other holes you can use?

Perhaps you can use the tapped hole as the datum, and use that to control the edges.

JHG

 

[SpaciouS]

Madhu454,

Yes you can use edges as a secondary and/or tertiary datum.

All you need to define a secondary datum is a minimum of 2 points and for a tertiary datum, a minimum of 1 point. You can establish both of these requirements with an edge.

You will not need to use angled basic dimensions to locate your hole, even though the part is tapered, you can just dimension the hole the same way as if the part where square. The inspection gauge will still be 3 mutually perpendicular surfaces and your part will drop in on the primary surface, slide to the secondary surface until contacting at least 2 points on one edge and again slide until the final point of contact is made on the other edge.

A good example for you to look at is in the ASME Y14.5M-1994 standard, page 54, FIG4.4.


Thanks,
Sean

 

[drawoh]

SpaciouS,

Figure 4-4 illustrates my point, not yours. The datum is the angled face, not the edge. The basic dimension is from the theoretical point. The radius of the corner does not affect the measurements.

JHG

 

[SpaciouS]

drawoh,

I'm not sure what you mean by not mine. All I was talking about was dimensioning the HOLE. For that reason my point is valid when looking at FIG4-4 (150 basic dimension). You are right however about making the face a datum, if you so choose, this is a better practice. I simply answered the question that was asked, "can we select the 2 edges (formed due to taper) as secondary and tertiary datums?" The answer is yes. Am I missing something? I don't think I mentioned anywhere that your answer was incorrect...


Thanks,
Sean

 

[MechNorth]

Madhu, either way is technically legal, but which way you go depends on the function of the part. If the edge is going to limit the movement of the part wrt a mating part, then the edge is valid as a datum feature. What Drawoh indicated is important though, if the machinist puts a handling chamfer or rad on that edge, your results would change; as a result, the hole must be located wrt to that finished edge (whatever it is). To ensure that you get the edge (whether sharp, chamfered or radiused), modify the datum reference at the tangent plane (T).

Jim Sykes, P.Eng, GDTP-S
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