GD&T for centreline positioning? 1

[hobbs101]

Hi. I have a plain square plate of unknown exact size and I want to ensure that 4 holes on a 100x100 square pattern are positioned centrally and oriented correctly. How is this dimensioned/toleranced?

I've seen many drawings where plate centrelines will be marked and =100= dimensions are put between holes. Obviously this is dimensioning from virtual centrelines and is difficult to inspect.

How is it done 'properly' using GD&T?

Hobbs101
Mechanical Design Engineer

 

[hobbs101]

Yes, understood. I currently don't use MMC modifiers in any of my work. I need to learn about it and start using it if applicable in designs.

Thanks everyone for all the help.

Hobbs101
Mechanical Design Engineer

 

[mkcski]

hobbs101

You are most welcome. I would definitely start a move towards understanding MMC concepts and the MANY pluses of MMC to communicate design intent and the related manufacturing benefits.

 

[DeanAllen]

Looking at Jon Selby's sketch this is not quite right with what you want to do. You no longer use the == signs as the tolerance is sometimes difficult to work out.
The face of the plate should be the primary datum, datum A. This means the holes will be square to the face you are drilling into.
Datum B and C should be the length and width of the plate. Usually the longer side would then be the secondary datum. In this case it would not matter.
The 300mm dimensions (width & length) should be shown in brackets as these are references only as the plate sizes already exist.
Datum B & C need to be on the end of the 300mm dimensions, not to one side. This means the datums are the centrelines of the plate in both directions.
Draw the centrelines in in both directions.
Put box dimensions from the centrelines to the holes, 75mm both sides. (75mm dimension should appear 4 times)
Hole positional tolerance should be worked out from the min clearance of the bolt going through the clearance holes on both parts & positional tolerance applied on the other part. As this is a clearance hole the max metal modifier should be used in the positional tolerance box along with the dia 0,25 currently shown.
Assuming 2 plates are the same. 50mm hole smallest size of 49,75 and an M48 bolt then the positional tolerance would be dia 1,75 Max metal to datum A/B/C on both plates. If the other plate is tapped then the pos tol would be 1,75/2 on both plates. Do not use max metal on tapped holes, as the hole drives the pos.
Hope this helps