measureements for dowel holes...

[GlennW]

I have never seen a Diamond Pin used in a permanent application on engines or machines. Their purpose is for ease of quick assembly/disassembly in repetitive applications such as fixturing.

[art.h]

Using two round dowels in two round holes is automatically creating reduntant location problems. Both items are trying to locate in X axis and Y axis and acting as two absolute zeros. This can be done but it will require extreme accuracy. Most mills are not capable of this accuracy, no matter what your DRO says-- that is why jig grinders, EDMs are used. I've seen punch press dies destroyed by internal stres after a few cycles when someone ignored this issue. Use one dowel in a properly located hole this locates X/Y location then a Diamond pin in a properly located second hole with the true Dia. located in the Y axis of the part, When your mating part is fitted it will be be located X/Y location by the round dowel and the Y axis only by the diamond pin, the relieved sides of the diamond pin have no part in the location. Less internal stress and suitable tolerences to satisfy mill capabilities. X axis should be the greater dim.
When designing parts each individual part should only have one absolute zero 0,0,0 and each axis XYZ??? located only once.
If you are planning ease of assembly//disassembly aluminum pins not a good idea as dowels unless you have liberal clearances on the holes and then all of the above is not relevant.

[Richard_W]

Using two round dowels in two round holes is automatically creating reduntant location problems. Both items are trying to locate in X axis and Y axis and acting as two absolute zeros. This can be done but it will require extreme accuracy. Most mills are not capable of this accuracy, no matter what your DRO says-- that is why jig grinders, EDMs are used.

I respectfully disagree. For the situation at hand the degree of accuracy you describe isn't necessary. Also I have built many fixtures using a Lagun mill and a DRO and have had the pins just go together nicely with .0003 to .0005 clearance on the pins.

I've seen punch press dies destroyed by internal stress after a few cycles when someone ignored this issue. Use one dowel in a properly located hole this locates X/Y location then a Diamond pin in a properly located second hole with the true Dia. located in the Y axis of the part, When your mating part is fitted it will be be located X/Y location by the round dowel and the Y axis only by the diamond pin, the relieved sides of the diamond pin have no part in the location. Less internal stress and suitable tolerances to satisfy mill capabilities. X axis should be the greater dim.
When designing parts each individual part should only have one absolute zero 0,0,0 and each axis XYZ??? located only once.
If you are planning ease of assembly//disassembly aluminum pins not a good idea as dowels unless you have liberal clearances on the holes and then all of the above is not relevant.

Aluminum wouldn't be my choice either, but the electrolysis of two different materials may be a factor for this application. I have seen car heads lined up from the factory with nothing more than a couple of short lengths of tubing used for head to block alignment. I think your last sentence just might be the correct answer. The natural tendency is to make things to a higher degree of accuracy than necessary. Then again it is sometimes hard to know when good enough is perfect!

Richard W.

[David Powell]

I back up Richard's comments. Nowadays , with reliable DR0s, good quality reamers and drill bits you can use dowel pins to ensure locations are accurate enough for very many applications. However, once you start talking about engines, be they steam or gasoline, you have to consider the problems of expansion and contraction and corrosion. That said, for example, If I were bolting an aluminium head on a cast iron block I would NOT use dowel pins by choice for location. If I had to do so I would allow a few thous clearance , even though that would mean the location would no longer be ' exact', to ensure nothing nasty happened when things get hot! regards David Powell.