It is a common symbol that references how flat a surface is regardless of any other datums or features. It comes in useful if a feature is to be defined on a drawing that needs to be uniformly flat without tightening any other dimensions on the drawing. The flatness tolerance references two parallel planes parallel to the surface that it is called out on that define a zone where the entire reference surface must lie. Flatness tolerance is always less than the dimensional tolerance associated with it.
Flatness is can be measured using a height gauge run across the surface of the part if only the reference feature is held parallel.
You are trying making sure that any point along the surface does not go above or below the tolerance zone. This is a 3D measurement so points must be measured across the length and width of the part to ensure the entire surface is in tolerance. Flatness cannot be measured by simply placing the part on a granite slab and running a height gauge or microheight over it.
This would be measuring parallelism instead as you are fixing the bottom of the part as a datum.
Profile of a Line
Flatness is the 3D version of surface straightness — Instead of the tolerance zone between two lines ; the tolerance zone exists between two planes. When you want to constrain the amount of waviness or variation in a surface without tightening the dimensional tolerance of said surface. Usually, flatness is used to give a surface an even amount of wear or for sealing properly with a mating part.
Commonly used on a fixture that must mate flush with another part without wobbling, but where orientation is not important. If you want to make sure that a tabletop is perfectly flat, if you did not have a flatness callout, you would have to constrain the height of the table very tightly to make sure that the entire surface is straight.
With flatness, you can allow the table to be flat without constraining the tabletop thickness very tight. Flatness is not the same as parallelism.
Parallelism uses a datum to control a surface while flatness does not. Think of a table with two missing legs at an angle to the floor. The tabletop may be within flatness tolerance but would not be parallel to the floor.
So you can have a bad Flatness while be compliant with a good dimension example of a twisted metal plate — the thickness is steady but it can be unflat as rectified when assembled.
Flatness can be applied to an entire feature in the case of a sheet metal plate or to just the surface of a part. When applied to a surface of a part the flatness must always be less than the dimensional tolerance. When applied to a feature of size however, it can actually be larger. This is because rule 1 does not apply to flatness and straightness when applied to an entire feature.
Thanks for pointing this out! In fact it is recommended to be half of the tolerance of size.
Does not only apply to position. Regarding tolerance stacks, is the flatness added to the dimensional tolerance, or is the flatness included in the dimensional tolerance?
The way you have it drawn, it looks like the table could be even taller than the dimensional tolerance when including the flatness. This means your form can never allow your part to go over its size dimension without certain exceptions You part must always fit within the envelope that the max or min size dimensions create.
This means in the table example, if the size of the height was at the max your form would need to be near perfect. The Datum surface is identified as -A-, and it has a flatness tolerance of 0.
The opposite surface has a parallelism callout of 0. What dimension supersedes, or which one has the priority? Can a parallel callout be larger than the flatness?
The confusing part is the 0. It makes for an interesting measuring technique. These are known as refinements. Your flatness is limited to 0. The paralellism is simply stating how far off the orientation of the surface can be.
The geometric tolerance all depends on what is functionally required. For certain application such as a bracket, a higher flatness would be fine.
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But for a more precise application such as a sealing surface you may need a tighter tolerance. Can flatness be used on multiple coincided surfaces? If the four surfaces could be seen as segments of a big surface, we could get away with only one notation.
If you call out flatness for multiple surfaces your are only controlling the flatness of each surface to itself i. In the example you give, I could machine the legs of the table to within. What I suspect you want to do is restrict this wobble and the way to that is with profile of a surface control.
The number of surfaces to be controlled is called out next to the feature control frame and the tolerance zone is applied unilaterally away from the true profile. All elements of all controlled surfaces have to lie within the specified tolerance zone. Thus, form flatness and coplanarity are controlled.
Gd t position tolerance pdf printer
Could you not use a common zone modifier in this application? This would require the 4 legs all to be within the same tolerance envelope regardless of height. I wish I could offer further insight, but as of current we are not offering guidance on the ISO standard. Are you asking what should the tolerance be? The tolerance that should be on the print is the loosest tolerance that still allows your part to function as intended.
CZ is the common zone modifier where the entire feature that is broken up is to be treated as one surface this is only the ISO standard. For the first one, you would only measure the flatness of the individual feature that the control frame is on. For the second you need to surface map all of the common zone features and determine if the flatness of that acting as one feature is within the tolerance zone.
I was taught over forty years ago that the correct way to check flatness is to zero your surface to itself on three jacks, surface facing up and indicate over the surface.
I was taught that using fixed stands checks parallelism not flatness. This was the method used at Hughes Aircraft in my day. Will someone please explain in a clear concise way the difference between these two methods?
I believe we are talking the same thing. The way you were instructed to check flatness is correct. I think the image that is tripping you up is the one showing the dial gage checking the surface of a part as it rests on top of a table.
A better image here would be either the part resting on 3 spherical tipped pins and measuring beneath or having the dial indicator protrude from a hole in the table measuring the part surface as it rests on the table.
Sorry for the late reply on this.
I hate to give you advice and have it be wrong. I really think you should ask the customer for a clearer explanation. Take this to mean that a sufficient number of data points are taken with a probe on a DMM to adequately define a feature. Is it okay to put the Datum in flatness tolerance? What does it mean? I saw some drawing in my company.
The datum is simply placed on the surface that the flatness tolerance is also called on. Other than also having flatness called on that surface, there is no other difference. Calling flatness on a datum surface can make sure that the datum is uniform and flat enough. Remember though — the is not flatness relative to the datum — it is simply both a datum, and a flatness callout.
Hello, I have a drawing in which a 6 hole pattern has a positional tolerance of 0. However the datums have a profile tolerance of 0.
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Could this be possible? Or due to the tight positional tolerance on the holes the datum surfcaces should also have a flatness surface requirement of 0. That will make the part quite expensive. There is no rule preventing a datum from having a profile tolerance it really depends on how you have things set up.
Dimensioning and Tolerancing Principles for Gages and Fixtures
Keep in mind the datums indicate how the part is to be set up for inspection. As is your case you would have a mutually orthogonal DRF, so your set up would have the broad flat surface contact the inspection equipment first, then the secondary datum and lastly the tertiary.
From this intersection of three planes you would base all of your measurements from. This is because your pattern still has to be basically located to the datums.
An in-depth study of the principles of GD&T
You would essentially be adding material to the far side of the part. So flatness is a little bit odd. There are a couple of old school methods for doing this inspection.
Probably the easiest one would be to have a suitable inspection surface with a hole in it large enough for the probe of a dial indicator.
Place your part on the inspection surface over the dial indicator and set the bezel to 0. Then move the part 1 mm while keeping an eye on the indicator. If at any point your full indicator movement exceeds.
Not saying what you have is wrong, just a little different. Hey i just ran into an issue that i cant seem to find any definitive answer on, I was wondering if you have any insight. Ok i have a part with a. Since my part is bowed, and two jacks are on the high side , and the other jack is on the low side my measurement is the amount the part is bowed X 2, When really the part is flat within.