Tunalover
Mechanical
- Mar 28, 2002
- 1,179
Folks-
There is something that has bothered me for a LONG time and I hope that maybe you could help me understand it better.
At our company, with many of our purchased RF electrical components we have no control over tolerances and sizes of the component mounting holes. Instead this data is determined by the vendor who typically has little or no knowledge of GD&T and the fixed and floating fastener fit formulas. Thus, we often have to accommodate odd (often unfriendly!) hole sizes and tolerances.
Imagine a component being a simple rectangular plate with, say, four clearance holes passing through it. If you imagine also that this component is mounted to a flat sheet metal surface with four clearance holes (of different diameter and in-pattern tolerance than the component) and you have the picture. The holes in the sheetmetal are positioned and spaced to line up with the mounting holes in the component.
Many sources give the following floating fastener formula:
H1+H2=2F+T1+T2
where
H1=the clearance hole diameter in Part 1 at MMC,
H2=the clearance hole diameter in Part 2 at MMC,
F=the fastener major diameter at MMC,
T1=the feature-relating positional tolerance of the holes in Part 1 at MMC, and
T2=the feature-relating positional tolerance of the holes in Part 2 at MMC.
Let the component be Part 1 and the sheet metal mounting surface be Part 2.
The component (Part 1) has a clearance hole such that H1=.104. This is too small for a No. 4 screw, so we choose a No. 2 with F=.086. The vendor holds the positions of the holes to +/-.005 relative to each other. Therefore T1=.014. Plugging this all into the formula we get the following formula relating H2 to T2:
H2=.082+T2.
What does this say? Does this mean that, say, if T2=.002 then H2=.084? But .084<.086 meaning the screw won't clear the hole!
Obviously I'm missing something. Also I can't find a reference that can explain this. What additional conditions (besides H1>F and H2>F) or forumlas apply in this case to explain the above formula?
Thanks in advance!
Tunalover
There is something that has bothered me for a LONG time and I hope that maybe you could help me understand it better.
At our company, with many of our purchased RF electrical components we have no control over tolerances and sizes of the component mounting holes. Instead this data is determined by the vendor who typically has little or no knowledge of GD&T and the fixed and floating fastener fit formulas. Thus, we often have to accommodate odd (often unfriendly!) hole sizes and tolerances.
Imagine a component being a simple rectangular plate with, say, four clearance holes passing through it. If you imagine also that this component is mounted to a flat sheet metal surface with four clearance holes (of different diameter and in-pattern tolerance than the component) and you have the picture. The holes in the sheetmetal are positioned and spaced to line up with the mounting holes in the component.
Many sources give the following floating fastener formula:
H1+H2=2F+T1+T2
where
H1=the clearance hole diameter in Part 1 at MMC,
H2=the clearance hole diameter in Part 2 at MMC,
F=the fastener major diameter at MMC,
T1=the feature-relating positional tolerance of the holes in Part 1 at MMC, and
T2=the feature-relating positional tolerance of the holes in Part 2 at MMC.
Let the component be Part 1 and the sheet metal mounting surface be Part 2.
The component (Part 1) has a clearance hole such that H1=.104. This is too small for a No. 4 screw, so we choose a No. 2 with F=.086. The vendor holds the positions of the holes to +/-.005 relative to each other. Therefore T1=.014. Plugging this all into the formula we get the following formula relating H2 to T2:
H2=.082+T2.
What does this say? Does this mean that, say, if T2=.002 then H2=.084? But .084<.086 meaning the screw won't clear the hole!
Obviously I'm missing something. Also I can't find a reference that can explain this. What additional conditions (besides H1>F and H2>F) or forumlas apply in this case to explain the above formula?
Thanks in advance!
Tunalover
