dowel & threaded holes "nominal size selection" (m6 tapped - 6mm dowel or m6 tapped - 1

[Vah1D]

Hi all,

If you are to add two dowel holes to two plates that are going to be clamped together with M6 screws, what size of dowel you would use? 5mm? 6mm? 8mm? and Why?(assuming they are not under any shear force).

Not just from functional point of view, but also manufacturing, assembly? One of the people who was kind of a mentor to me said generally same size or a size below, but I never asked why?

Also why we generally put dowels inbound (between) of tapped holes?


Thanks!!

 

[BrianPetersen]

There are questions to which the answer is "Because". Or "Flip a coin." Or "Because we felt like it." Or "Because that's what fits." Or "Because this is the way the mechanic doesn't end up cussing and swearing at me."

Do what makes sense in the application that you are working on.

In the world of mechanical gizmos and gadgets that I am familiar with - Motorcycle engines and transmissions - the most common design is to use a roll pin concentric with a couple of the bolts that secure the assembly together with those bolts passing through the center of the roll pin. Obviously the roll pin is sized such that its inside diameter is a clearance diameter for the bolt (usually M6, can be M8 for cylinder heads and cylinder blocks and the like) and the outside diameter is whatever it is beyond that.

In some cases, it is necessary to pick and choose which bolts to apply this roll pin to, based on the ability to drill and tap that particular hole deeper without hitting something and have the bolt head projecting out further, in order to clear the length of the roll pin on both mating parts.

Try to get it so that a future mechanic has a hope of getting the remains of a broken or seized pin out without breaking the whole thing or drilling through expensive parts to get to the backside of it.

 

[drawoh]

Vah1D,

If you are tightening your bolts down hard, the dowels make your assembly repeatable, but absolutely nothing else. The bolts do all the work. Tiny dowels will be knocked loose. Otherwise, the dowel size is not critical. The same size as the bolt or screw is an excellent, crude rule of thumb that lets you worry about important stuff.

--
JHG

 

[Vah1D]

Thanks Brian and Drawoh, your answers were helpful.

@Drawoh, Was my second question clear/valid? I've seen dowel holes put inbound tapped holes almost all the times, but I never understood why. Only thing I can think of (in case of 2 screw and 2 dowels), if you put screws as far as possible (which means you probably have to place the dowels in between) you probably have better clamping, or better spread of loading path of the screws, but I don't know if this is true or not.

Thanks,
Vahid

 

[drawoh]

Vah1D,

Again, the bolts are doing all the work. They need to be positioned optimally.

If your angular repeatability requirement is marginally within the capabilities of your dowels, you would place them outside the bolts to achieve the largest possible radius. I don't sit around worrying about this stuff unless I have a problem.

--
JHG