Change tolerances


#1

Hi all

what is the best tolerance to work under. I generally build models that are no bigger than 100mm square and my models have an absolute tolerance of 0.001mm, a relative tolerance of 1.0 percent and an angular tolerance of 0.1 percent. these generally work ok but sometimes curves that are projected to surfaces become very point heavy.
My question is, would changing the absolute tolerance to 0.010mm be ok

thanks in advance

Richie


(Rodrigo Bárcena) #2

Hi Richie,

In my own experience (product design, objects from 50mm to 3000mm), the small object template works fine, I have never had the need to alter the tolerances. Sometimes, problems with tolerances highlight an existing problem in some other aspect (too dense surfaces), inaccurate object snaps …

Projected curves are always interpolated, depending on the actual case, the curve might be too heavy, and you will need to rework it (rebuild, fit) to carry on working with it.

I guess you have already looked into this article? https://wiki.mcneel.com/rhino/faqtolerances


#3

High tolerances are not needed because existing manufacturing methods cannot print the models with high accuracy. The machines can cut alloys with high accuracy, but they cannot print alloys with high accuracy. If you make mechanical parts that must fit perfectly (e.g. bearings or gears), you define these tolerances with written dimensions. A craftsmen will ignore your model and he will set his cutting machine to follow your written dimensions. Swedish firm Arcam makes electron beam machines which print variety of strong alloys with +/– 0.4 mm accuracy. Other printing machines have lower accuracy. Parts made of polymers are elastic, so 0.5% accuracy is generally enough to ensure that they fit. If you design all fitting parts, give them extra space - better to make the fit too loose than too tight. Typical mold shrinkage of polymers is 2% - the polymers shrink when they solidify.

The angular tolerance is important when you make molds for polymers because it may be difficult to remove the part from the mold having wrong draft angle. Rhino DraftAngleAnalysis command is necessary to check the molds.

If the parts do not have to fit, the only reason to exceed 0.2 mm tolerance is to reduce friction (aerodynamic drag or hydrodynamic drag). In other words, the surfaces must be smooth rather than accurate. Rhino EMap and Zebra commands are best to estimate smoothness. A seam between two surfaces looks smooth when the continuity between the surfaces is G2 (curvature continuity) or better (G3 or G4). G2 corresponds with degree 3 polynomials. Some experts claim that high quality NURBS surfaces have degrees of 5 or higher, but they cannot prove it. High degree surfaces slow down Rhino and use more memory (RAM). Many mechanical CAD programs cannot import surfaces that have higher degrees than 3.

The smoothness is very important when the parts are made of monolithic bulk metallic glass. When the parts come out the mold, they have perfect mirror finish - they look like things that have been polished. If your model of the mold is not smooth, its flaws will be easy to see.