Datum Targets

what we're going to look at today is a

variation on creating datum reference

frames and this is based on a concept

called datum targets with datum targets

we're going to specify a region or

specific location and to understand how

to use these we'll have to start off

with understanding how to construct our

fundamental three plane datum reference

frame that we've seen before based upon

a set of datum targets and then how

would we go about establishing the datum

reference frame as we did with the

regular datums that we've seen

previously well first of all you might

ask why do I need datum targets when

we've seen datum surfaces and datum

features serve our purpose quite well in

establishing a datum reference ring well

sometimes we have specific processes

such as forging sand casting weldments

as well as others that create irregular

surfaces and when that occurs it might

be difficult to establish a datum

feature based upon those irregular

surfaces so what we could do is go for a

specific region an area or a line

something that could form a line such as

an edge or in a specific point on the

surface that we know is dependable if

we're going to do that we might also

identify where a locating pin might

appear perhaps in a fixture or in some

type of inspection process and if we're

going to do that then we have to give it

a position and a size so before we can

use our datum targets we need to

understand our three plane reference

frame so when we constructed from three

planar surfaces a reference frame in the

previous discussion what we were doing

is geometrically

driving a plane and as you know the

general equation for a plane looks

something like this

ax plus B y plus cz plus T equals 0 now

if I look at the coefficients of a B and

C they represent this direction vector

of the plane in other words it tells you

how the plane is oriented in space D is

going to tell us something about the

location of the plane with respect to

the origin so I have the orientation of

the plane and I have the location with

respect to the origin of course if

they're normalized such that this is a

unit vector then we think of ABC as

being the unit normal vector well

getting back to our primary datum if we

consider a primary datum plane in this

case we can determine this normal vector

to that plane by taking the cross

product as you know if I have two

vectors here representing three points

if I cross these two vectors I'll end up

with a vector perpendicular to those so

V 1 let's say B 1 is that vector V 2 is

this vector I cross these two and as you

recall this is how we would take the

cross product the determinate with the Y

Z coordinates z x coordinates and XY

coordinates of course K is the unit

vector representing the z axis J the

inter vector representing the y axis I

the unit vector representing the x axis

and now I've got my vector and P if I

calculate that with actual values for

the points now I've got this vector

corresponding to the normal to the plane

and then to solve for D in our original

equation ax plus B y plus cz plus 2

equals 0 all I have to do is select one

of the three points and substitute it

into the equation and solve for D so now

I have my primary plane

also note that it's completely

unrestricted because it is the primary

datum there is no influence from any

other features in terms of establishing

that datum reference but now when I go

to the secondary plane now I do have a

constraint that has to be taken into

account based upon border so what we'll

do is we'll take the cross product of

any vector in that plane and the primary

normal vector so here you can see this

constraint of the primary datum on the

secondary datum so here is NP that we

already calculated that and here is any

vector in the secondary plane by any

vector we really mean any two points so

I've got two points here describing this

vector v3 and if I cross that with NP

then I'm going to get a vector and s

perpendicular to that plane and of

course we calculate the cross product in

the same way we also calculate D in the

same way we substitute either one of

these points to determine D and now I've

got a secondary plane that is

perpendicular to the first plane and

then finally the tertiary plane here we

have to take into account the primary

and secondary constraints on the

tertiary so for the tertiary all we're

going to have to do here is take the

cross product again but just use the

primary and secondary normal vectors in

other words we don't have to know

anything about the third plane to find

out its orientation so we already know

NP we already know n s and therefore NT

will be perpendicular to these two

vectors so what do I need this well

anything on the surface of this third

plane what what do I really need well we

need a point and why do we need this

point we need the point to determine D


so in our ax plus b1 plus cz Plus D

equals zero equation we still need to

know the value of D this will give us

the values of a B and C because it

corresponds to the normal vector to the

plane now I've got three planes

constructed to form my datum reference

frame well how could I use datum targets

in that context to construct that three

plane system let's take a look at how we

would observe the datum targets in the

design specification first and then

we'll see how we'll put them together to

construct that three plane system we're

going to identify regions on the design

or points lines and if you see a symbol

like this it's telling you you have a

datum target on the top of the circle if

there is a number there it's going to

represent the target size on the bottom

we see our datum identifier as before

and then we could have multiple targets

to establish that datum and so we have a

number and there could be multiple datum

targets for one datum and we'll see why

a little bit later you might also see

this X here indicating a point symbol

attached to the datum target symbol so

let's look at a very simple example here

we've got a surface and we want to

specify regions or point or line for a

point typically what you're going to see

is a location on the surface so we've

got a specific location on the surface

that will correspond to our point and

then edges cross-sections axes we could

specify as a line so here you can see we

have an edge corresponding to the edge

of that surface that we want to use as a

datum or we could use a small region and

the size of that region is

described as we said before in the top

here we see it's circular with a

diameter of 0.3 well if we use datum

targets we can combine them in many

different ways and here are some

examples of what you might see so for

the primary datum there's a variety of

ways in which we could establish that

datum reference frame based upon the

three plain system as we saw in our

equation we could just use three points

or we could use a line in one point or

an entire region or three regions

representing three components of the

plane or two regions and again this is

not an exhaustive list but it gives you

an idea of how they might be combined

for the secondary datum we might take to

datum target points or a target line and

finally for the tertiary we might take

one datum target a point or region again

telling us something about where that

tertiary plane might be so let's look at

an example on an actual specification

here we've got a as you can see a

position tolerance that is being used to

control the location of this

through-hole here and now we have to

think about the datum reference frame as

specified here ABC well first we note

that a looks to be a typical datum it is

the bottom of the plate here

corresponding to this surface so we can

construct that and establish it using a

surface plate as we did before but now

when we get to our datums b and c we

also see datum targets now note that

well the reason why you've got these

data Maidan fires here is to identify

the features in this case two surfaces

that we're going to construct datums for

the addition of the datum targets gives

you more detailed instruction

as to how that datum will be established

so for instance for B we see two points

and those points show up both the top

view and the side view here and it gives

us a basic dimension telling us where

those points are so now I can determine

these points with respect to a distance

and here again a distance here again a

distance from here and then from the

side you can see that we want to point

in the mid plane and that's indicated by

basic dimension here we also do that for

B so we're being very specific about the

points we're choosing to establish first

of all the secondary datum reference and

that's going to be done with these two

points and we saw previously when we

constructed the three planes that's all

we need are two points for the secondary

and then for C we only have one datum

target and we saw why when we

constructed the tertiary plane we only

need one point to determine the value of

D in ax plus B y plus cz D plus the

equation so you can see we can get much

more specific about where we're going to

establish our datum reference and again

that goes to specifics about what we

expect the surface of these parts to

look like after our manufacturing

process so at this point you should

understand when you have a datum target

what it looks like and also how to

identify it in terms of its symbol the

target number I can have one or more

target numbers they are going to be

combined together to form a datum

depending upon the nature of that datum

so in summary it's not frequently used

but you might see it and when it does

appear usually you're thinking about in

a regular surface

the datum targets can be combined to

create a datum reference frame as we saw

based upon this notion of three points

for the primary two points for the

secondary one point for the tertiary and

one or more datum targets can be

combined to create that datum in a

feature control frame