Limits and Fits

[mrmitche]

I'm dimensioning a bunch of 2D drawings. Some of the dimensions require tolerances where parts fit together (we are using H7/g6 locational clearance). This part I understand for the most part from reading Machinery's Handbook. But another designer added a symmetrical tolerance to the hole centre location of +-0.005 mm. How did he get this value? Is there any other good resource for learning about tolerances?

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[JNieman]

H7/g6 are ANSI standard tolerances on the diameters of the shaft-and-hole only. They constrain size only. Not location.

You still have to locate the parts based upon their intended function. The tolerance will be up to the designer to dictate. Do you have control over the manufacturing process? If so, I've seen some design documents specify that holes are to be "match drilled at assembly" or variations of that phrase. This basically gives you a much freer locational tolerance but applies a process that hopefully guarantees a much tighter in-line condition, as would be appropriate for a locating feature such as a dowel pin and bushing.

However, if you're using a slotted bushing and round bushing with your pins, maybe you can dimension and tolerance them separately with +/- 0.2mm and things will simply mate up satisfactorily depending on the desired assembled condition. Or maybe things are tight and the geometry is not forgiving, and +/-0.005mm is all you can get away with.

I don't know what your features are - I speak of bushings only for illustrative purposes to convey how you would have to approach things differently.

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NX8.0, Solidworks 2014, AutoCAD, Enovia V5

 

[mrmitche]

Okay, thank you. When I asked he made it seem like this was some value I should have known. I'll just make notes of the tolerances the company commonly uses and go with those in the future.

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[drawoh]

mrmitche,

I understand that there is a symmetry specification in ASMY Y14.5, but I have no idea of why I would use it. Maybe your designer is being silly. Symmetric about what?

If your H7/g6[ ]fit is being used to locate something, almost certainly, you need to apply positional tolerances somewhere. A positional tolerance plus symmetry may be a way of showing that one datum is more critical than the other one.

--
JHG

 

[JNieman]

I apologize, I completely missed the word "symmetry" when I read your original post, mrmitche. I have no idea what the reason for that tolerance would be from your post. I'm equally as confused as drawoh seems.

_________________________________________
NX8.0, Solidworks 2014, AutoCAD, Enovia V5

 

[KENAT]

NO

If you want to be doing it even remotely properly you normally need to be determining what is the loosest tolerance that will ensure part function (most typically that the mating parts will fit together)*. For instance ASME Y14.5M-1994 gives some equations for making sure hole patterns match up in appendix B.

Secondly you need to ensure that the required tolerance you've calculated are within the process capability of how the part can be made and can be achieved at acceptable cost.

Just blindly copying other designs, or over relying on the tolerance block or on some industry standard of process capability is not doing it properly.



* Often the tolerance necessary for function/fit are very loose, in this case it's not necessarily worth doing in depth tolerance analysis - just pick a number well within process capability and carry on with your day.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[drawoh]

mrmitche,

Let's actually answer your question.

You need a copy of the standards. I strongly recommend getting a copy of ASME Y14.5-2009 Dimensioning and Tolerancing. GD&T is a language, not a procedure. You need to understand what you are telling people.

Most if not all of what KENAT is saying, above, is common sense. Your part has to work. I don't see the specific combination of h7/g6 anywhere in my Machinery's Handbook, but it is valid, and it sounds like an accurate sliding fit. If that is what you need, you are doing it right.

Symmetry specifies geometry in 2D, not 3D. If you are centreing something, you need either true position, or concentricity.

--
JHG

 

[mrmitche]

Drawoh, maybe I described it wrong. I took at look at ASME Y14.5 on the symmetrical tolerances, this is not what I was referring to. I simply meant the tolerance type in SolidWorks was symmetrical (instead of basic, bilateral, etc.). So the hole diameter is dimensioned with H7 and then the hole position is dimensioned 20.00 +- 0.005. What I understand is that if you need a sliding fit, using ANSI Hole Basis Metric that tells you to use tolerance combination H7/g6. Is the value of +-0.005 related to this, or is it some other separate tolerance so that the hole location lines up with holes in the adjacent parts (plates in this case). If it is a tolerance irrelevant to the fit and simply for alignment of the holes, then does the value just come from experience and knowledge of our machinist's capabilities?

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[mrmitche]

Here is some additional information on the parts if it helps. There are two plates in an mold with an insert passing through one (Plate 1) and into the next (Plate 2). The insert passes through Plate 1 with the sliding fit using the hole tolerance H7 and the insert's shaft tolerance g6. This hole is 18.0 mm, but the hole on the adjacent plate is 19.0 mm. The general tolerances on our drawings for 0.0 are +-0.1. Both holes are 20.0 mm from the centre of the plates. Only the hole through on Plate 1 has the additional tolerance of +-0.005. The insert's shaft is 18.0 mm.

So Plate 2 doesn't require a more accurate tolerance on the hole location because it has 0.5mm clearance around the insert. But Plate 1 is guiding the shaft with the sliding fit, so the hole location needs additional accuracy?

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[JNieman]

Actually, a +/- tolerance value is bilateral. Using 'symmetric' in that case will obfuscate things as we've just seen.

_________________________________________
NX8.0, Solidworks 2014, AutoCAD, Enovia V5

 

[drawoh]

mrmitche,

20.00[±].005 controls the position of your hole. If this is millimeters, this is very accurate, and possibly not fabricatable.

The specification Ø18[ ]H7 is a valid thing to do on a drawing, as per ASME[ ]Y14.5. You can work out the actual tolerance by reading the Machinery's Handbook. Your machinist can work out the tolerance by reading the Machinery's Handbook. Your inspector can work out the tolerance by reading the Machinery's Handbook. I like to call up fit classes on my assembly drawings, where it functions as design information. I want numbers on my drawings. Does your CAD's tolerancing feature understand this? I know SolidWorks does.

--
JHG

 

[mrmitche]

If I understand what you are saying correctly, then yes. When I insert a dimension in SolidWorks, then under Tolerance/Precision selection "Fit with tolerance" and choose H7, it puts the tolerance number values beside H7 on the drawing. The +-0.005 was manually input for the position, which is why I was unclear how that value was determined. Some of the products we mold require very accurate tolerances, is it possible that +-0.005 comes from knowledge of the accuracy the customer requires?

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[drawoh]

mrmitche,

There is a process for working out the size tolerances and the positional tolerances required to make clearance holes clear bolts and screws. I have written up on this, and posted it to my personal website. None of this applies to H7/g6 holes, as they provide insufficient clearance to clear multiple features. Common practice with dowel pins is to provide one round hole, and one slot.

From my Machinery's Handbook, I see that your Ø18[ ]H7 hole is in fact Ø18.018/18. This is what you should show on your drawing. The H7 bit is design information that the fabricator and inspector do not need to know.

The 20mm dimension appears to locate your hole. Since your fit is very accurate, this locates the mating piece in your assembly. Read KENAT's remarks above. If you want your parts accurately located, you are doing it right. I am not sure your fabricator can locate a hole to [±].005mm.

--
JHG

 

[dtmbiz]

mrmitche,

My mind visualizes your descriptions of two parts located by pins with a sliding fit. Unless the 20mm dimension from surface (side) to the hole is related functionally, so that this +/-.005mm tolerance is required (at best it would be quite expensive if possible to fab as mentioned above); or is it that the hole should be defined as a datum feature used in a DRF which would be used to locate the side surface relationship? If so then the +/- could be more liberal.

Sure would be nice to see both part dimensions. Minds do wander


JNieman,
True bilaterial is +/- tolerance however both values do not need to be the same (e.g. -0.1 / +0.3).
Maybe Solidworks uses "symmetric" as a crummy way of indicating an equal bilateral tolerance?