Hole Pattern Tolerance Analysis 1

[joshuabass]

I have limited experience in tolerance analysis so I was hoping someone here could help. I have two parts that fit together using pins and holes. One part has 4 holes in a square pattern. The other part has 4 pins (or dowel rods) in the same pattern. We actually want a gap between the pins and holes.

Note that the pins are on an existing part that is dimensioned from pin to pin. We would like to design the new part with dimensions from hole to hole.

So the things to be concerned with are:

[ul]
[li]Hole to centerline spacing[/li]
[li]Hole diameter tolerance[/li]
[li]Pin to pin spacing[/li]
[li]Pin diameter tolerance[/li]
[li]Minimum allowed gap between pins and holes[/li]
[/ul]

I am not sure how to approach this mathematically. In the past I would use CAD to lay out the design and just tweak the number to work, but I am more interested in how I would set up a spreadsheet to accomplish this.

See attachment for reference image.

 

[CheckerHater]

Are you using GD&T at your establishment?

 

[powerhound]

There are more things to be concerned with than your list, such as form and orientation. You will certainly need a modified virtual condition calculation.

What is the minimum and maximum space required between the pin and the wall and what size and tolerance are the existing pins?

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[pmarc]

In order to answer to your question, it would be good to know how the holes and the pins are dimensioned and toleranced on a single part drawing, and what is the minimum gap between them allowed.

But in general, for the simplest possible case what you need is to compare sizes of virtual conditions (VC) of pins with sizes of virtual conditions of holes - of course assuming that you are using GD&T. The assembly will be possible, if VC of holes are not smaller than VC of pins.

To translate it into plain words, I think the easiest way is to use numbers:

For a part with pattern of 4 holes:
- size of each hole as defined on the print is for example 10+/-0.1, or in other words 9.9-10.1;
- each hole is located with the use of positional tolerance with relation to some datums, for example |pos|dia. 0.1(M)|A|B|C|;
- true position of the pattern with relation to datums and spacing within the pattern are defined by basic dimensions.

For a part with pattern of 4 pins:
- size of each pin as defined on the print is for example 9.6+/-0.1, or in other words 9.5-9.7;
- each pin is located with the use of positional tolerance with relation to some datums, for example |pos|dia. 0.1(M)|A|B|C|;
- true position of the pattern with relation to datums and spacing within the pattern are defined by basic dimensions (the basic dimensions are exactly the same as for pattern of 4 holes).

The size of 4 virtual conditions of holes is determined by taking the smallest possible size of the hole (which is MMC size) and substracting positional tolerance at MMC from it.
This means that: VC_hole = 9.9-0.1 = 9.8
The four virtual conditions are perfectly oriented and located with relation to datums A|B|C as defined by basic dimensions.

The size of 4 virtual conditions of pins is determined by taking the largest possible size of the pin (which is MMC size) and adding positional tolerance at MMC to it.
This means that: VC_pin = 9.7+0.1 = 9.8
The four virtual conditions are perfectly oriented and located with relation to datums A|B|C as defined by basic dimensions.

When VC_hole = VC_pin, this means that assembly is possible, but for each pair of corresponding hole and pin no gap between them may occur in certain areas.

If you want to assure that there is always a certain gap in the assembly, the equation for virtual conditions has to look like this:
VC_hole = VC_pin+gap
This means that you have to play with your size and positional tolerances for both patterns in order to achieve desired effect.

 

[joshuabass]

I am new at this company, and while I see some drawings with GD&T, most of them do not use it, so I will say that most likely this component will not use it either. This is a welded part, not machined if that makes a difference.

The component with "pins" is something that already exists and we are simply building a component to interface with those pins. This component was made by another company entirely, but I do have the drawings for it which shows a +/- 4MM tolerance for the spacing and diameters. We would like a 6MM minimum gap between the pins and holes. The maximum gap is not a concern, other than we cannot make the holes much larger than they are.

 

[drawoh]

joshuabass,

I have notes online on how to do this, including various plus minus dimension configurations.

HTML:

http://home.eol.ca/~hgibson/tolerances

PDF:

http://home.eol.ca/~hgibson/tolerances.pdf

Read my section on implied 90[°][ ]angles. We have discussed this issue extensively here, and we have not agreed on an interpretation.

--
JHG

 

[pmarc]

drawoh,
Are you able to share a link to a thread where this topic was discussed in details? I am really curious what others said about it.

 

[drawoh]

pmarc,

Here is my original thread: thread1103-261904.

Here is another nice discussion: thread1103-337980.

I only searched for the stuff I participated in.

​

--
JHG

 

[pmarc]

Thank you, drawoh.