Why only one of two mating parts are threaded?

[marvincwl]

I'm unsure as to why only one of two mating parts are threaded?

The top part is usually a clearance hole and the thread is only applied to the bottom part. Why is this so?

 

[Jobrag]

Sorry this doesn't make much sense can you provide a drawing?

 

[potatoecannon]

If you have a thread running through both pieces, the threads must line up when it is tightened.
This is due to the continuous thread on the fastener (bolt or screw).

If the parts aren't clamped together when you tapp the holes, there will be a gap (less than 1 thread spacing) when assembled. If you try to overtighten to close the gap, you will strip threads.

By tapping one piece, it limits the clamping forces to the surface area under the fastener head & the threads on ONE piece (the far one, wrt the fastener head), which ensures the pieces will mate properly (& eliminates the need for a nut).

 

[marvincwl]

Hi Jobrag,

This is what I meant. I do not get why it must be the second case (lower drawing).

Why can't both mating parts (top&bottom) be threaded?

 

[marvincwl]

Thanks potatoecannon.

 

[enigma]

Another thing to consider is bolts usually go in a bolt pattern of two or more.

The positional tolerance of the holes can fairly easily be +/- .005 to .010 inches. The mating part then has slightly oversized holes to accommodate the pattern's "slop" as well as the "slop" of the oversized holes. If they were both threaded, the only way to get them to fit together would be to either spend a fortune to get +/- .0002ish on both parts or match drill. The same effect could be attained by using alignment pins, so its never done.

 

[Travis_King]

It's used to bring the bottom part tight to the top part. As they've said, if both parts are threaded, you can't really get it tighter than the combined bolt patterns will allow (You can't turn the bolt and tighten just the bottom plate).

However, if you've got a hole just large enough for the threads to pass through in the top plate, and a threaded hole in the bottom plate, when you tighten the bottom plate up it will squeeze the top plate, and you can get a very tight clamp.

 

[marvincwl]

thanks a lot everyone