Surface texture vs machining tolerance for turned parts 2

[bobde3]

I remember having a table of values of surface roughness or texture which may be expected based on the tolerance of a turned part. Does anyone have a table related to this subject?

 

[Adrian2]

Dear bobde3;

Surface Finish is more a function of the machining operation used . Machinerys handbook has a chart of surface finishes to be expected with different machining processes. Perhaps this is what you require.

Regards Adrian

 

[gbent]

bobde3:
I also recall seeing such a table. I think from a practical sense, the finish should be at least 10 times better than the total tolerance. For example, if a 100 microinch surface finish is acceptable, don't have a total tolerance less than .001 inch. The tolerance could always be greater than .001. A value of 50 to 100 is a more normal ratio of surface finish to dimension.

Remember the surface finish number is an average. On a cleanly cut turned surface with only a 10 to 1 ratio, it is possible a od measuring device would measure the part correct, while the bottom of the groove from the nose radius would be undersize (error of form).

 

[cranekiran]

Hi,

I may have come in late and Maybe My comment in this thread will go unnoticed but still I have to put my opinion here.

Tolerances are totally different from surface finish.I can have a mating parts with tolerances to press fit but I can maintain a glass finish in terms of surface roughness.On the other hand I can have tolerances for clearance fit but the surface roughness can be double delta.
The reason I am stressing on this point is I am surprised that how one can link tolerances which are given in the first place for Assembly references only with surface roughness which are given for special purposes for eg. bearing fitting,Wheel and Axle assly(interference fit),piston and cylinder assly etc.
Still, If I am wrong then I would like to have that table you mentioned.It will definitly be interesting to read it.

 

[vitrom]

In a way tolerances of a feature need to be based on some degree of surface finish. One can't expect a tolerance of 0.001" on a part that is cut with an oxi-acetylene torche.
How does one link the two (tolerance & finish) requirements is not so easy however the relation in my opinion is a key element in the economics of the part.
In general, due to the nature of the tools used in the manufacture of parts, surface finish and tolerance are related to some extent. I don't think that one can create a meanfull table with practical use from an applications point of view. But certainly a table can be generated for upper limit ratios by manufacturing method (ie. for turning a diameter X to a tolerance Y the max surface finish is Z).
Just my opinion. Never looked at such table.

Antonio Reis
01 209-834-1900

http://www.vitrom.com

 

[oth88]

I think there is an important issue here which is not always considered properly.

One experience I have had is with bearing seat on shafts. If you machine a shaft to the correct tolerance for a bearing seat by normal machining (ie without finish grinding), then the bearing will correctly to the shaft. If you then remove the bearing and recheck your shaft seat dimension you will find the dimension smaller and possibly undersize. Because it is designed to be an interference fit the bearing has the effect of burnishing the seat which improves the surface finish.

In this sense surface roughness does indeed have an effect on the tolerances chosen.

IMHO I think there should be guidelines for the surface roughness v tolerance required or in other words there should be a level at which it would be considered bad practice to simply machine a shaft without finish grinding.