Using Dowel Pins for locatimg stacked parts.

[Tom Griffin]

OK, I'll play...False.

Normally one hole in the mating parts is under the nominal size of the dowel for a press fit and one hole is slightly oversize for a slip fit. This makes for good alignment and allows the parts to be disassembled without removing the pins.

When I worked in the die-making industry we would drill both holes undersize and at the same time, have the parts hardened and then hone one of the holes for a slip fit.

Tom

 

[Tony Wells]

Precision ground dowel pins are toleranced as: +0.0002/-0.0000, standard practice is to ream or bore to nominal size +0.0000/-0.0003. That allows for an interference fit of up to 0.0005 tight. Clearance for mating parts in an assembly is a function of the positioning tolerance of the location, and can change depending on the number of dowel pins specified. Typical is the use of 2 pins on small parts, with a clearance of 0.001/0.0015 on diameter, but may vary depending on the tolerance and method of dimensioning for the hole location. This could get into a discussion of Geometric Dimensioning and Tolerancing. But that's an entirely new thread.



And Bill, you don't have enough zero's in your first example of reamed hole size. Post #3 you have the size correct, but should say -0.0002/0.0003. You would have a hard time with that much press.

 

[Chazz]

Dowel Pins

#1 Exactly what are Dowel Pins? - Basically, 3 types, or varients (I think),

Type 1 - Joining - More common in wood-working as a simple method of joining two pieces of wood together, similar to 'biscuit joinery', this method of joinery is not so common in 'metal' work.

Type 2 - Locating - The simplest example, big-block V-8 engine, it's kinda important to relocate you heads to your block after a gasket change, the studs still alow the head to 'wiggle-wobble' around, the 'Locating-Dowels' on the block allow a consistent, accurate realignment of the mating parts on a consistent basis. One more example would be locating pins or dowels in a 'Press Die' or Plastic injection mold.

Type 3 - Locking - These are the 'Tapered' variety where you purchase special 'Tapered' drill bits, tapered dowel reamers, and finally, your tapered dowels. (I'm not sure if these are employed as much thses days), but Generally speaking, they serve as either a 'perminent' fixture (ie, you don't plan on EVER taking it apart) or to act as a 'Shear' pin. Example - I've seen a small bench-top-lathe that used a tapered-dowel pin to couple the Lead-screw to the gear train, the theory being, if you have a 'Head Crash' or such, the dowel\shear pins will break first. (As per proper machine design)

#2 How are they used for alignment? - See Type 2 above

#3 How are they mounted? - What are your engineering needs?
They can be press-fit, lock-tighted, epoxied, welded, is it somthing you need to assemble\disassemble a lot? or you only get a one time construct? (Think Space shuttle, Voyager...)

#4 How many do you actually need? - Kinda goes back to #3.
Minimum, (for hobby machining and-you-feel-you-need-it) IMHO, 2, one on opposite corners.

#5 Anything else you can add to make it a good discussion. (Tricks etc.)
I've found 01 Drill Rod (Silver Steel) makes excellent 'Dowel Pins', it usually comes in an 'Anealed' state (soft & workable), then depending on your application, you can leave it 'soft' such as a shear pin, or harden it.

HTH

Cheers,
Chazz

 

[12bolts]

Type 3 - Locking - These are the 'Tapered' variety where you purchase special 'Tapered' drill bits, tapered dowel reamers, and finally, your tapered dowels. (I'm not sure if these are employed as much thses days)....

Still employed in industry. Used in a variety of applications. Not common on locating matching parts together, but more often used to pin something, (eg butterfly disc), to the rotating shaft. Makes for easier assembly of critical parts alignment. As the tapered pin is inserted it self aligns the two parts. Also in dissasembly, once the pin is moved a small amount, it becomes free in its hole and is usually not required to be punched out by its full length. Not usually relied on as a shear pin.

Cheers Phil