Slots as Datums

[Jieve]

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!

 

[Jieve]

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!

 

[PaulJackson]

Draw a print, describe the function of the slots, and "have-a-go" at dimensioning it. Then share it with us.
Paul

 

[pmarc]

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?

 

[fsincox]

Slots are certainly valid datums, other than that a picture would really help.
Frank

 

[Jieve]

Guys, thanks for the replies.

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.

 

[fsincox]

pmarc,
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

 

[pmarc]

Frank,
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.

 

[pmarc]

I recall second part of last sentence from my latest post. There is no "unless...". It simply won't work.

 

[Jieve]

Pmarc or fsincox,

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!

 

[Jieve]

Guys, thanks for the replies.

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

 

[Jieve]

For some reason it seems that my links aren't working because the colon after the https isn't being added. To open them, just right click and open in another tab/window and add the colon to the address.

Thanks.

 

[pmarc]

As I was able to open the links, I have to admit that have some troubles in visualizing your assembly. Maybe it is just my temporary lack of imagination, but is there a chance to see a picture with those components in assembled condition?

 

[Jieve]

Here is the assembly I described, but without the plate with the slots. The two outer screws on the bottom of the assembly pass through the slots in the plate shown in previous picture.

Thanks!

Mostly Full Assembly

 

[SeasonLee]

Jieve

Would you pls try to upload your sketch again, file upload works now.

SeasonLee

 

[Jieve]

Here are the drawings. The other documents/pics can be accessed simply by right clicking on the above links, opening them in another tab/window, then when the error screen shows up, add the colon : after the https in the address.
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.

 

[fsincox]

pmarc,
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

 

[CheckerHater]

Picture doesn't load.
Mayby using "&" symbol in file name is not such a good idea.
(Learned the hard way )

 

[pmarc]

Frank,
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?

 

[fsincox]

Pmarc,
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