I am updating an old drawing. how would I apply gdt for the holes they want to be .060 tir
Pick one hole for datum and apply GDT to the other, similar as the one below (for total run out)
Exactly what Christian said. I would also consider changing that bolt circle from a radius to a diameter as well but that could just be my preference.
The PDF presented does not indicate there are two holes as there is no depth call-out. The assumption when design checking that this 2X is an error. However. 2X or not:
Where are the
I'm not sure if you guys understand, this is a sheet metal hopper, the holes are on each side, they want the 4 .25 holes on one side to be .060 tir with the .25 holes on the other side. can I use the 1.28 bc as a datum?
Attached is some GDT reference info for you to use in the future. The color chart is particularly useful I think.
I just noticed that in the original dwg they have the top holes on one side marked A and the lower holes on the other side marked B
The holes you have marked are the Ø0.25 holes when I think you want to mark the Ø1.78 holes instead.
Thanks for sharing
I guess they want to mount a shaft/pin between the 2 holes so they want to control the runout between them. You can use the center line between the 2 holes for a datum (best) or you can use either hole to be a datum (hard to inspect)
Here is my suggestion:
another thing I see in the front view is that they have a centerline thru the center of the 1.78 holes. is the concentricity from the center line?
It's just for a position of the .25 holes and has nothing to do with TIR
Look at the color chart I attached above to see what concentricity means. Pic below for reference.
Be careful to use the concentrcity since 2 sections might be concentric but overall might be run out
I'm sure they just want the holes on one side to be in line with the other side
see if my dwg will do that, also how do I position the datum a so it means the center line of the 1.78 hole not the surface
Below is how I would do it. I know it is not perfect as I did it in MS Paint
you might be correct but from the PDF file, seems to me that they ref. to the .25 holes, not 1.78 holes
You can save the PDF file as bitmap file and embed into a SW dwg so you can add the symbols easily
I have not had a lot of GDT training and not taken any classes since High School (many years ago before we even USED computers) but what I remember and what I have been taught by machinists and highly educated mechanical engineers, is to dimension features on the view that shows the features (as you have done) - not to look through a part and try to dimension the features on a different side or view.
I don't know what the "Right" answer is regarding this but you should be able to compare your drawing view to the same relative part view and be able to measure or inspect any feature that is on that side. Exceptions would be a hole or feature that exits that side that was fully defined on another side so it would be redundant on this face (example - a hole drilled from the other side that was fully defined on the other side and as to position, depth, or any required GDT and is just exiting on this face).
The holes in question in this Hopper drawing are separate holes on separate surfaces so they should be defined separately as you have done. If they need to be Concentric or have a limited runout then that should be figured into the dimension tolerances or shown referencing a datum or similar.
If I remember, I will bring up this example with a GD&T Trainer/Guru in my user group (Thomas Allsup ) and see what he would do.
chris I like your solution but the 1.78 holes are just clearance, the 4 .25 holes are the what we are trying to control, heres what I did, what do you think
I believe in order to measure run out of those you would actually need something to measure. Since a bolt circle is not a physical feature it might be tough to measure. You might be better controlling them with a positional tolerance instead but lets see what the other here think about it as I could just be dead wrong.
You want to modify the dwg the way you think correctly or just want to translate from the note on the dwg to GDT symbol?
Practically, how the guys on the floor inspect it?
From the PDF File, seems to me that they want to control the runout the holes on both sides and they might mount a shaft/pin which they can use for inspection?
yes I want to translate their silly note to modern GDT
Actually, the note totally makes sense to me if they want to mount something between the holes
Here is how you add a datum to to center of the 1.78 hole
chris, I'm not trying to figure out what concentric means
I am far from an expert, and I am surprised someone more knowledgeable than me hasn't said something. My understanding is Concentricity is difficult to inspect and rarely called for, unless you have a really good reason to control median points. The symbols that folks have been adding are for total runout, which is different from runout and concentricity. My guess is that none of these would be appropriate for a sheet metal part. Position is the callout you will want.
Scott, your aren't being helped by the initial drawing. You are going to have to do some reverse engineering and figure out what this part is supposed to do.
I have to add that threads like this one is exactly why so many companies and engineers are reluctant to move to GD&T. It just leaves to many people scratching their heads as to what it means or what the "correct" way of dimensioning a part is. I have been following this thread in order to improve my knowledge and honestly it seems to me that the clearest way would be to extend the bolt hole center lines to show they match and then add a note. If designers and drafters who are supposed to be trained in creating prints have this much trouble figuring out the "correct" way to dimension a hole how can we expect the poor guy on the floor to know what we are trying to convey?
Just my two cents from someone who has had very little training in GD&T.
"It just leaves to many people scratching their heads as to what it means or what the "correct" way of dimensioning a part is"
I'm new to GD&T as well, and there are a few points I believe are true, but I would like some confirmation from someone that works with this regularly:
- Is it true that engineering knowledge is required to understand the function of the part in context, so the order in which the tolerances or datums are applied is relevant?
- Is it also true that one person might generate a different spec than another, based on their experience?
- Isn't the purpose of GD&T intended to remove subjective analysis, so the part will be inspected the same way regardless of who does the inspection?
My understanding would be that "in theory" statements 1 & 2 are incorrect and 3 is correct. With that said, look at how much effort is being placed in figuring out the correct way to tolerance these holes. Do you expect the shop to have a better understanding than the guys here on the forum? Will they put any where near as much effort into understanding the print?
Its kind of confusing to see diff. answer from this post because, I guess, this part was desinged by someone else and we provide help based on some assumptions from the PDF file. if we know exactly how this part is made and mounted, then the answer would be ended up the same
Be definition, GDT specifies how far actual surfaces are permitted to vary from the perfect geometry implied by the dwg. So the designer will view the part in the assembly and decide the Max and Mim conditions for the part functioned properly
Jim already answered to your question so #3: GDT can be used for both making and inspecting the part
Edit: if I was the one to design this part I'd would want
1) Both side plates are parallel or both perpendicular to the base
2) holes on both sides perpendicular to the side plates
3) if I mount a shaft thru 2 holes, then 2 holes must be concentric in a certain amount
The first 2 seem to be straight forward, but I have to think about how this part is inspected for #3. Usually an indicator is used in the machine shop for inpsection so I use TIR to make sure the shaft is not rotated out of the center line between the 2 holes. I can generally provide concentric GDT (still correct) but the guys in machine shop might have no idea how to inspect it
"Be definition, GDT specifies how far actual surfaces are permitted to vary from the perfect geometry implied by the dwg. So the designer will view the part in the assembly and decide the Max and Mim conditions for the part functioned properly"
-This would suggest that it is impossible to provide GD&T for a lone part with no context ...?
-How can the designer know the Max and Min conditions for the part to function properly ... it would be necessary to have engineering input to determine this?
For example, I reverse engineer a lot parts from scans. I don't have a context for the parts, and if I did, I am not a mechanical engineer so I don't believe I would be qualified to indicate what the tolerance should be.
Please see the below image with solid lines for perfect geometry and dash lines for actual surfaces
1) without straight GDT, the part can be bowed like the the shape of dash lines even though the dimension is still correct. If you are willing to spend $1000 for a smart phone, you sure don't want a bowed shape iPhone
2) In the dwg, you add tolerance so the shaft can be sliding fitted with the bearing surface (solid lines); in reality, the shaft can be bent and not be sliding fitted into the bearing even thought the dimension is still correct. so you need to determine an allowable amount of bend so the shaft can still be fitted into the bearing as shown in the 2nd image - This is what you need of perpendicular GDT
Christian, this is exactly my point.
If one is not an engineer and know the context/use of the part, the GD&T spec that this person might use could be completely arbitrary and, even worse, incorrect.
Not really a need of an engineer. A drafter with enough experience can do this
And believe it or not, good tool makers can also find out what missing from the dwg if you let them build the prototype for you
The company I contract for occasionally has requests to provide GD&T with drawings of the parts I reverse engineer from scans. My position has been that, although I can certainly dimension the part, I cannot provide GD&T.
If I were to know how the part is used I might be able to assign the specs in a meaningful way; however, the tolerancing has many considerations that I may not be aware of.
A little known secret is that engineers aren't taught those tolerances in school either. We have to get them from the same place you would , Machinery's Handbook or experience (read: doing it wrong first).
True. there are many things you learn from the job, not from school - SW class can be found at a ITT, not from university
it might take you 10 yrs or more to get out of the college if all the stand practice is included
Normally, the designer, engineer, drafter are Designing (Forward Engineering) the part and know what it needs to do and how it needs to fit so they know what GD&T to use to make everything fit and work as they intended.
In the case of this drawing, the original was not completed by the original team (or that's the way it sounds to me since they were old - incomplete drawings according to Scott).
I think you are looking at if from a Reverse engineering standpoint - in which case, you would not know the original intent unless you had all the details, assemblies and information.
In this condition it is very difficult to know precisely what the original intent was since we do not have all that information. So in a reverse engineering world you can only make an educated guess as to the way to implement GD&T unless you have all the information.
Thanks Rick, Jim, Christian
I did go a bit OT here, but have been looking for some general info related to how GD&T can be specified in cases where the usage is unknown and there is not an engineer to consult with.
No problem ! just post a question here and we can provide help as much as we can
There are books and other information about GD&T but it can be overwhelming at times.
I am no expert and have no formal training in it but have purchased are read some books and documents - but my company doesn't use it in drawings much so I don't get to use or know it well.
If you want some training or information you can look here:
Thomas Allsup is a GD&T training and this is his company.
He will be at SWW2017 this year but I did not see his GD&T class in the schedule the last time I looked (about a month ago), but if you are going there and get a chance to meet him he should be able to answer some questions.
Just adding to Rick's suggestion
It'd be a good practice to consult with the guys from the machine shop how to inspect the GDT
Thanks for that info Rick. I also downloaded the Ford GD&T Pocket guide a while back that has some good info. I just tried to find a link again to share for those that have not seen it, but everything I see requires signing up for an account, although a few indicate no charge.
The document I have is named gdnt_pocket Ford.pdf.
You can attach here or simply put on Dropbox (if possible)
I could, although I'm not aware if there are any distribution limits. I don't have the original link where I got it. I looked for a public link for it but could not find one. I did find a few that required signing up for a free account:
Ford GD&T Pocket Guide
Thank you Chris that should come in useful.
I purchased a PDF book "Geometric Dimensioning and Tolerancing - Applications, Analysis & Measurement [per ASME Y14.5-2009]" a few years ago that is good - but I had to purchase it. They were running a special of some sort at the time so I didn't pay full price but it isn't cheap ~$130.
It is very detailed (about 575 pages long).
We were trying to improve our drawings to use better definitions at the time, but never did - so I haven't done much with it. - Maybe someday it will be needed by me.
The book you purchased is one of the best on the subject. I’ve taken GD&T training from James Meadows and it was well worth the cost.
To add to what Jim Steinmeyer and Christian Chu stated, If you look at the drawing you will see that it is just a portion of the actual drawing and is missing a lot of detail and dimensions.
For instance, there is no defined X dimension for where the holes in question are located on the part.
The drawing would need all dimensions to be a proper drawing. There would also (possibly) be notations to indicate the designers intent or the use of the final part. The designer is responsible to make the points you raised be clear enough to the fabricator so that the part can be made accurately.
In the case of this part, as it is shown to us, was an incomplete drawing because it did not provide an absolute definition of the tolerances, GD&T or specific alignments needed.
Scott Cole said he was updating an old drawing - so I would imagine it was never completed or he removed details so he would not be showing the critical details for anyone to copy. Scott can correct me if I am wrong, but I think he is trying to best define the part so there will be no questions in the future, but that he is also not 100% sure of what is the best way.
This is a good candidate for true position with a circular tolerance zone.
James Riddell wrote: This is a good candidate for true position with a circular tolerance zone.
James Riddell wrote:
Exactly what I was thinking.
that sounds good, what would it look like on the drawing?
See if this helps you out at all. (http://www.gdandtbasics.com/true-position/)
that is talking about each hole, I need to control the four hole pattern
Put the TP on the BC in that case.
I'd make the "base" a datum and one of the sides the second datum. Use true position on each with a diameter of 0.030 RFS to both A & B datums (in that order).
However, do you really need that tight of a tolerance?
GD&T is best used to define engineering intent and allow all non-essential dimensions as much 'room' as possible - it can significantly reduce manufacturing costs and eliminate ambiguity of exactly what is important. Most people over-use it and thereby increase costs w/o any functional improvements.
all I want to do is make sure the four .25 hole pattern on one side is within .060 to the other side
One of the sides that has the holes needs to be a datum. One of the 1.78 holes needs to be the next datum. One of the patterns needs a true position callout to the the first two, and receives a datum callout. The 1.78 hole on the other side gets a true position callout referencing the first 2 datums. The second pattern gets a true position callout referencing the 3.
I suspect the position of the pattern relative to the to the 1.78 hole matters, and matters more than the position of the patterns relative to each other. In this case, the second 1.78 hole is a datum, and the second pattern gets the required position callout to the hole, and then gets a second position callout to the first pattern. GD&T allows you to make the tight callout to the center hole, and then a much looser callout to the other pattern.
Post up a screenshot of what this thing is supposed to do, and a drawing file in a format that can be marked up.
IMHO, that makes things much more complex and difficult to inspect.
If I were looking at what Scott is saying, my interpretation is that the 1.78 hole is already a clearance and therefore not as important. He's saying that the mount holes are driving the position requirement (right Scott?).
yes jim that is correct, I just want to control the two hole patterns
like this matt, I'm not sure how tight the tolerance should be
I think that is getting closer. B should have some perpendicularity callout to A since no other datum is present to control its orientation, but what is desired is clear enough. The positional tolerances need a diameter symbol. The 45 deg dimension should probably be basic, but it would need a datum to reference. I also think the second 1.78 hole should be a datum (Say D), and the second pattern gets the same .01 diameter tolerance zone relative to A and D but gets the looser .06 dia tolerance relative to the first pattern (defined by ADC, I think).
I would guess that the tolerancing really applies at MMC. If so, that can effectively loosen the tolerances and you will still get parts that go together. If this is a production part, you can in theory have a set of go-no go gages to quickly inspect the part.
I am far from an expert, so I keep hoping someone who knows what they are talking about jumps in.
OK REV 5
I think that is a lot better than what you started with.
You need a diameter symbol for the second pattern tolerance. You also need a tighter tolerance callout to A and D (but not C). I am guessing that there is some kind of flange mounting to the part. The tighter tolerance controls the pattern to the center hole, while the .060 is the much looser control for the orientation to C. So you will end up with 2 Feature Control Frames for the second pattern, the first to ADC for orientation, and second to A and D for position to D. The concept is in ASME 14.5 section 5.4
Thinking more about this, the entire initial 5 hole pattern could be called out as a datum (B, say), and the entire second pattern gets the loose orientation tolerance to A and B, and then the same tolerance to A that the first pattern has. If you aren't ready to put your fist through your monitor, I think this is the best way- but I don't know how this part is really supposed to work. If you do this, then the 45 deg dimension becomes basic.
I am also guessing that the tolerancing really applies at MMC, which is a useful concept to get familiar with.
It may be appropriate to point out that number in the tolerance is the diameter of the tolerance zone, so .06 means no more than .03 away.
This exercise is making me appreciate how many of my own parts I make.
Just a wild guess but I suspect that the goal is to obtain something like the attached view.
The bolt pattern that both bearings are located by needs to be within a tight tolerance for the shaft to be in the bearings without a bind. as the OP has stated the center hole is large enough that even if it is a little off the shaft will clear. Again as with my earlier post, a drawing is an attempt to clearly convey the required dimensions and tolerances to the shop floor to get the part made correctly. I am afraid that since it has taken this many attempts to get the "correct" GD&T tolerance information on the drawing the shop will look at and have no clue what is required. Then one of two things will happen, either a diligent welder will hunt up the engineer/drafter, or more likely, he will shrug his shoulders toss the print aside and weld it as he feels like.
Here where we don't know GD&T and hire just anyone with a pulse that can strike an arc so our prints have to be dumb-ed down. I would create a detail view of the hole pattern at location A and provide tolerances in that view. Then using the projected views that you have I would extend the center lines and include a note indicating that the bolt holes MUST be aligned to the desired tolerance. Crude, yes. Archaic, probably. But the shop would get the idea and make the parts reasonably close to what was desired.
I know that isn't what you asked about but I have been yelled at far to many times because the stupid engineers are unable to make clear prints that the shop can easily follow.
this hopper has a bearing on one side with a feed screw, the other side is just a spout that's why the .060 range
Scott, I have had some years experience with GD&T and this looks to be way overkill. You do have some problems with it even so. Your 0.060 tol is rectangular, making it circular will improve alignment. Keeping the frames assuming RFS is usually a better way to go but it requires a little more inspection time/calculation. Your QC will have a difficult time establishing your BC datums - this means added cost/time. By calling out TP on the right BC to the opposite hole you are defeating the purpose of keeping both BCs within 0.060 of each other.
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