Importance of Symmetry

Hello All,

I am new to the forum so forgive me if this isn't the correct area to post this discussion.

I have a crankcase and I am measuring an inline 4 crankbore. I want to know why symmetry is a crucial data point, when IMO you can achieve the same thing with True position and concentricity across all 4 bores. Concentricity is called out for the final bore to datum bore but then a tight SYM callout applies to all 4.

Does anyone have experience as to why SYM is crucial to callout?

The latest version of ASME Y14.5 has dropped symmetry as an option. Position is the preferred option to replace symmetry.

Sometimes (ok, maybe for than sometimes) Geometric Tolerancing is highly misunderstood when applying to drawings AND Y14.5 gets updated about every 10 years so you need to know what version of Y14.5 the drawing was drawn under.

Most of the changes are small, but as Miner above stated, symmetry was dropped in the latest version but if the drawing was made under Y14.5M-1982 it might be different then if it was drawn under Y14.5M-2018 as there were several versions between.

ShadiestWesley said:

Hello All,

I am new to the forum so forgive me if this isn't the correct area to post this discussion.

I have a crankcase and I am measuring an inline 4 crankbore. I want to know why symmetry is a crucial data point, when IMO you can achieve the same thing with True position and concentricity across all 4 bores. Concentricity is called out for the final bore to datum bore but then a tight SYM callout applies to all 4.

Does anyone have experience as to why SYM is crucial to callout?

Click to expand...

Hello Wesley,
Trying to work out how concentricity is applied in this instance? I'm assuming it's the bores on the end-faces of you crankcase that accommodate the main journals of the crankshaft? They share a common axis (Opposite each-other for a comparison of center-points).
I believe true position is the way to go, (mainly for the additional sqaureness requirement) what's important geometrically in your case?

• Flatness of your cylinder head location face
• Parallelism of your cylinder head location face to your crankshaft axis
• Top dead center i.e. an element of which is cylinder head location face to crankshaft axis
• Sqaureness of your bores / guides to your cylinder head face

The symmetry callout was to stress the importance of the bores relative to each-other, which is more important in this instance than their position relative to datums I believe. Even then on our drawings the bore centers (to each-other) and start positions (to datum) are all general machining tolerance ± 0.3 (mm). I have dealt more with 'V-twin' style crankcases. I'm going to take a stab at why this is and say things generally align themselves given room to do so i.e. crankshaft float. I believe the crankshaft position will be dictated by the top end of the assembly i.e. piston in guides (tight clearance), press fit pin etc. and then a crankshaft with float, obviously the conrod and crankshaft oil feed holes have to align...
Thinking about how else you could supplement (I won't say replace, drawing is the drawing) the symmetry.. Think about you crankshaft as just a line / common axis along a Y axis. Now we ideally want to achieve a common axis across the bores too (idk, side-loading of a full skirted piston isn't desirable, friction etc.) - imagine intersecting the bores with the cylinder head locating face and creating 4 points, constructing a line and calling the parallelism to the crankshaft axis? Sometimes these checks can be used by you as a reference on a report outside the drawing.

Ideally you would speak to the draughtsman / engineer who made the drawing and ask for clarification and / or a drawing revision, but given the standard referenced this likely isn't possible.

PS I believe in Y14.5M-2018 concentricity was dropped too:

1. For those working under previous versions of the standard, ensure concentricity is used only when runout or position controls are not sufficient.
   
   Concentricity
   My only comment on concentricity above is when trying to assess it on a granite table with a clock (DTI) you always end up with some form of runout (due to the inclusion of surface form i.e. circularity of features); Concentricity is strictly theoretical centers so not practical and we like equality of measurement lol
   
   And as Michael said, symmetry is also gone!
   
   PS Sorry for the rant, I really enjoyed thinking about your question.

Many thanks for all the responses. This is basically confirming my suspicions. I don't feel as though it should be on the drawing, I'll follow up with the design engineer and attempt to get some clarity regarding this feature callout and its importance.

I will add a little more context. This is a triple cylinder engine and a big boy at that! over 2.0L!

I'm advising in another factory and we seem to be getting capability issues. I'm trying to establish if it is manufacturing process capability or inspection. I don't have all datum system geometry available on the report (missing the joint face plane flatness) but the data I have shows good geometry. When a master part is measured we do get repeatable results (the worst error range is 8micron SYM for D4) in the latest SPC pack, however production variation is substantial (we just did a small scale R&R study using two operators 5 cycles. Program re-run without loading/unloading(program repeatability) then loading/unloading (reproducibility). This obviously leads me to believe it's just manufacturing capability but I want to be certain of results before going to management.

I am focusing on the crankbore, as this is the problematic feature. I would like to add a snip from the drawing but I don't know how, so can explain it in more detail here.

LH crank bore is datum D (D1) and I have bores D1-D4. Bore D1 has a Tpos of 0.1 back to standard datum system Y/Z which is the joint face (Y) and knockhole (Z) alignment. Parallelism 0.05/150 back to Y/Z and SYM of 0.04 back to Y.

Then on the opposing side of the case we have Datum E (For ref D4) this has a concentricity callout of 0.08 back to D1.

As people have stated concentricity isn't the correct constraint but is very common from a machinist mindset (runout vs total runout vs concentricity are basically the same thing to most outside of metrology). IMO with Tpos, Parallelism and Concentricity callouts applied the symmetry just adds conflict with regard to inspection.

This is a 2020 drawing so I assume it should be to the latest ASME spec

Any further information will help me in my discussion with the design team, so many thanks in advance for your time.

Kindest regards

Shadi

"LH crank bore is datum D (D1) and I have bores D1-D4. Bore D1 has a Tpos of 0.1 back to standard datum system Y/Z which is the joint face (Y) and knockhole (Z) alignment. Parallelism 0.05/150 back to Y/Z and SYM of 0.04 back to Y."

Adding to this slightly, obviously the SYM relates to all 4 bores not just D1 as it may be interpreted above. To me this is already conflicting with the concentricity callout of 0.08. I'm hoping to get some clarity from the design team this week, but they are notoriously slow