Tolerance

[raminee]

Hello All,

I am very new to Mechanical Engineering and/or product design. So glad to see this forum.

Here is my question:

Every time I discuss my products with manufacturing/molding houses they ask for the Tolerance.

My question is simply what is Tolerance ?

Is it the smallest dimension in my design ?
or
Is it the margin of error like +/- 0.1%
or
what ????

If possible please give an example with your explanation.

Many thanks.

Raminee

 

[Erich]

Every dimension on a mechanical part has a tolerance. It is the range of size that is acceptable to you the designer of the part. Any dimension that is beyond the tolerance is a reject part. Your suppliers are asking you for the definition of a good part. The size of the tolerance on any feature is determined by the function of the part.

 

[raminee]

Thank you Erich for your reply to my Tolerance question.

But how does one come up with such a Tolerance figure ?
Is it something the CAD tools could provide ?
Or something that a good product/ME designer would know because of their knowledge/experience ?

Surely this is very much also based on the capabilities of the mold making and/or manufacturing part houses. Is that not so ?

Also, is this Tolerance figure given in a value or percentage form ?
Say I have a 5 mm length and I want it to be within 0.1 mm of 5 mm. So do I provide +/- 0.1 mm or 2% as the answer ?

Thanks

 

[Erich]

The tolerance is usually a +- number on a drawing. Not a percentage.
Determining the value of the tolerance IS the value that a ME provides. You know what your design is trying to accomplish. You know how the parts need to fit together to work as you intend. Not enough clearance and the parts will not fit together or move as desired. Too much clearance and the part wobble about and do not move as precisely as your design requires. Those are the considerations of what size the tolerance needs to be. Once you know those numbers you can move on to the second part of design.
Choosing a manufacturing process to make your parts. Each process has capabilities for producing parts within a range of tolerances. Knowing what your design requires you find a process that can meet your needs.
Sometimes the process is already chosen for you. LIke injection molding. Maybe because that is the only process that can make part in high enough quantity at a price that is low enough to make your product successful in the market. The you are stuck with tolerances that can be achieved by the process and you have to figure out designs that can achieve your objective within those limits. Thats what separates the good designers from the CAD modelers.