Slots as Datums
Slots as Datums
Hello guys,
Question: I have a series of 5 slots side by side (width-wise) cut into a large flat plate. The slots are symmetrical about the center slot, which is the largest.of the 5. There are a total of three different slot sizes. I want to locate slot two with respect to slot four with a position tolerance, and slots 1 and 3 with respect to slot 5 with a different position tolerance. In other words, i want slot four and slot five to be Datum slots. What is the best way to indicate this On the drawing, and how would a slot be used practically as the datum during measurement?
Working to ISO. Thanks!





RE: Slots as Datums
I should mention that these slots have radii at each end, in other words they are essentially elongated bolt holes. Would the centerplane(s) of the slots make good datums?
Thanks!
RE: Slots as Datums
Draw a print, describe the function of the slots, and "have-a-go" at dimensioning it. Then share it with us.
Paul
RE: Slots as Datums
Paul,
I am afraid it won't be that easy to attach something after Reply Window "look" change. Unless you know how to do it...
Jieve,
The answer for the first part of your question is simple: if slot #2 needs to be controlled to #4, assign width of #4 as a datum feature and refer to it in FCF's controlling location (position or symmetry) of #2. The same can be done for #1 and #3 relative to width of #5.
Widths of elongated holes as datum features are quite common choice in industry. They often serve for constraining last remaining rotational degree of freedom of a part. But they can be used in control of symmetrical relationship between features too. I am not sure what you need to know when asking: "how would a slot be used practically as the datum during measurement?". Are you looking for more details about different methods of establishing datum plane from slot's width (e.g. hard gaging, open setups)? Or is it something else you would like to know?
RE: Slots as Datums
Slots are certainly valid datums, other than that a picture would really help.
Frank
RE: Slots as Datums
Hmmm, I prepared a pdf describing the situation a bit more cleanly but I can't figure out how to attach it to my post, likely what Pmarc mentioned above. Can anyone tell me how to do this?
Also Pmarc, my question about "practical slot measurement" was just a question of how the slot was used as a datum during measurement, in other words do they insert some type of gauge pin into one end of the slot, do they make a certain part that fits the slot nearly exactly and then measure from there, etc. I understood that holes used as datums generally use gauge pins to establish the centerpoint. I was wondering how this is done with slots.
Thanks.
RE: Slots as Datums
Is it maybe only my naive mathematical, non-shop, experience background speaking, but, can’t they measure the slot for conformance to size and then measure to the edge of the slot and add half the size?
Frank
RE: Slots as Datums
This is simplified approach and can work under certain circumstances. However in general this is not the best choice since size (width) of the slot is not measured relative to datum reference frame referenced in positional callout. In other words your method may not work for example when actual perpendicularity error of slot's median plane is significant, unless you do what you suggest in couple cross sections along a depth of the slot.
RE: Slots as Datums
RE: Slots as Datums
Can One of you guys explain how to add my attachment ? It's wont let me browse my documents to Post my PDF and I'd really like you guys to take a look at the drawings.
Thanks!
RE: Slots as Datums
I am attaching a pdf which hopefully will give you an idea of what I am trying to do. Your feedback would be much appreciated. The assembly is relatively simple, it consists of 10 parts.
1) 4x M8 Hex Cap Screws (not shown)
2) 1x M10 Hex Cap Screw (not shown)
3) Rectangular Body
4) 2x 20mm diameter posts with threaded holes in them (not shown)
5) Angle
6) Baseplate with Slots
The 2x 20mm posts are inserted into the 20mm holes in the rectangular body. The rectangular body sits on the baseplate and is screwed down to the baseplate by 2 M8 screws. These M8 screws, the 2 posts, and an M10 Screw pass through the slots. The slots provide adjustment capability for the assembly. One M8 Screw inserts through each clearance hole in the angle and screw into the ends of the posts. The angle should be able to move up and down when assembled, and the entire assembly should be able to move back to front (via slots). The locations of the 20mm holes relative to each other are very important, as proper clearance is necessary for proper gliding of the posts. The location of the center M10 threaded hole in the rectangular body is relatively unimportant. However, the 2 posts and 3 screws pass through the slots.
The slotted plate drawing (shown somewhat small relative to the other parts, sorry about this, I am adding a second link to the underside of the slots after assembly, the clearance on each side of the posts as seen in that second pic is 1mm) has a hole table with the dimensions of the slot. Since all of these parts need to pass through the slots, would it be better to simple link all of the hole tolerance zones in the rectangular body together as a group on the drawing with a dual position FCF? And do the same with the slots? While the way I've done it works for the posts, I'm not entirely sure about the slot positions relative to the holes in the mating part.
Also, I would appreciate any clean-up tips you guys could give, in the case there is something on the drawings that could be indicated better.
Thanks.
Drawings
Slots Assembly
RE: Slots as Datums
Thanks.
RE: Slots as Datums
RE: Slots as Datums
Thanks!
Mostly Full Assembly
RE: Slots as Datums
Would you pls try to upload your sketch again, file upload works now.
SeasonLee
RE: Slots as Datums
If necessary I can add them in separate posts, but I'm not sure if I can add more than one attachment in a single post.
Thanks.
RE: Slots as Datums
I agree that my suggestion is very simplistic, isn't checking a few points on a CMM, also simplistic for surface flatness? Aren't compromises made all over for various reasons? My impression has been it was all generally based on: "are we close enough for some reasonable level of confidence", "not is it perfect"? No?
Frank
RE: Slots as Datums
Mayby using "&" symbol in file name is not such a good idea.
(Learned the hard way
RE: Slots as Datums
I have nothing against making things simplified - especially in CMM metrology, which for me is an art of making compromises between theory and real-life situations - but I always try to stand against something which is in conflict with the "physics" of geometrical tolerancing.
That is why for instance I hate CMM reports extremely often treating hole's axis as a single point with no perpendicularity deviation. Would it really require so much effort, time and money to measure hole's center at least at two different planes to see how approximate axis behaves?
RE: Slots as Datums
I agree if I had my way, I would like to insist on it, the whole purpose of the face as primary is to extract some idea of perpendicularity. I have always assumed it was just being taken for granted as "the process naturally produces". My actual experience tells me it is so taken for granted that it has actually become forgotten that the requirement is there altogether.
Frank
RE: Slots as Datums
Can you explain your use of the word "physics" in more laymans terms, please?
Frank
RE: Slots as Datums
Drawings
RE: Slots as Datums
RE: Slots as Datums
RE: Slots as Datums
I will use two examples to clarify my point:
1. Already described issue with CMM verification and reporting of positional tolerance of a hole/pin: It is simply incorrect to boil down to 2-dimensional case a tolerance which is 3-dimensional by "physical", or maybe I should say by "geometrical", nature (has certain depth). Of course there are situations when such oversimplification can work without risking too much, but in general such approach does not take any axis orientation error into account. So if you imagine a hole with severe actual perpendicularity error to primary datum plane, do the reported coordinates of hole's center [x,y] measured at one plane only (usually located somewhere near to the top of the hole, assuming bottom is the datum plane) really give you something? Do you think such info is useful to assess hole's ability to mate with counterpart's pin?
2. Your example of surface flatness error verification on a CMM: As your collected data will be a set of relatively small finite number of probed points, the only conflict with theoretical approach will be in the amount of points checked, but the "physical" nature of verified tolerance will not change - you will still be probing a surface to check one of its geometrical properties, just like you would do if you had a possibility to probe absolutely every portion of this feature.
Does this explanation make sense?
RE: Slots as Datums
Yes it does.
Thank you,
Frank
RE: Slots as Datums
Our in house shop that is making my parts does not have a CMM machine and in my discussions with the machinists, the syndrome that Frank pointed out above seems to be prevalent - "the process naturally produces". I've also been asked more than a couple times the question "how are we supposed to influence that?" and "how are we supposed to measure that without a measurement lab?" regarding some of my GD&T callouts. Albeit this is a mostly "academic" setting and the guys machining the parts are in their 3rd year of machinist training, based on a lot of your posts here I'm thinking this is a real world problem. I'm trying to impose on the students the importance of GD&T in design, as they are all working on a dual study program to become design engineers. While they're pretty good at turning out good parts with practice and they do have decent understanding of GD&T, it bothers me that I can't always answer the "how to measure" questions. However, thanks to this board, I can give examples of why I spec what I spec and I can often see a light go on with some of them during the discussions.
BTW if you guys have time to kill, it may be a lot to ask, but I would be extremely appreciative if you could just take a quick look at the drawings I posted and tell me how you would recommend dealing with the slots in the question I posted earlier and if there are any ways I can simplify or make the drawings better. I'm posting the drawing of the shafts/posts that I didn't include before so you have all of the drawings of all parts in the assembly.
Thanks!
RE: Slots as Datums
Frank
RE: Slots as Datums
Thanks.
RE: Slots as Datums
Powerhound, GDTP S-0731
Engineering Technician
Inventor 2010
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RE: Slots as Datums
Frank
RE: Slots as Datums
Rather than calling this throwing out datums and hoping i have enough, I'd call it realizing that my original reasoning may be incorrect because I didn't have a füll understanding of what would happen during assembly. It seemed to me in this case that the priority is parallelism with the axis, although i didnt realize that there was a difference in interpretation when specifying which datum comes First in this case.
RE: Slots as Datums
Sorry, I did not mean to pick on you. It is something I see and have heard that description used for it before. That was all I was trying to point out. I can actually see other reasons like maintaining a simultaneous requirment on multiple features, also.
As you see it depends on how it functions, if it only mounts against a face or is it actually piloted in a long bore, function matters.
Frank
RE: Slots as Datums
As Frank said, as you have the drawing shown, B as a secondary datum adds no value, thus A is the only datum necessary. If, however, you need the hole to be perpendicular to the datum B surface, then it should be called out first to control perp, then A as a secondary controls the location.
Powerhound, GDTP S-0731
Engineering Technician
Inventor 2010
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Slots as Datums
When I get this question in class, I bring up the mousehole and the electrical box in the wall. When checking the location of the electrical box, where do you start? Typically people will say "bottom of the wall, directly below the electrical box". If the electrical box is directly above the mouse hole in the base of the wall, would you not measure from the top of the mousehole then? Yes, if there is a +/- dimension used. If a datum reference frame has been called out, you measure from the datum simulator instead. In this silly example, the floor would be the datum simulator because it effectively mimics the net bottom of the wall which would be the datum feature.
Jim Sykes, P.Eng, GDTP-S
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