Quick question for machinists

[Brooklynrider]

How much bigger should the hole be than the bolt that is supposed to fit in it?

[Aesop Rock]

http://www.physics.ncsu.edu/pearl/Tap_Drill_Chart.html

Depends on so many things, just print this out and stick it to your wall.

[Brooklynrider]

Sorry, I'm having a hard time understanding all that. One bolt is 5.5mm while the other is 7.8mm. How much extra do you think I should drill?
I forgot to mention, the bolt is just supposed to slide through this hole. I am not tapping the hole.

[Eggit2]

You'd want to use the clearance hole size on that chart. 5.5mm is 7/32", I don't think 7.8 is anything common, but its close to 5/16". Where did you get those diameters from?

[Brooklynrider]

I know that I probably sound really dumb and I'm fully aware that I have no clue about machining. This is my first time making anything from scratch. My buddy's DH bike has an adjustable shock shuttle which must have been run loose and it developed a crack. Since the bike is a bit dated and the shock shuttle is unique to the bike, it is impossible to find a replacement. I thought I'd give it a shot and try to duplicate it. I've been using some very cheap calipers and a ruler so far. After getting all the measurements, I had a friend with a 3D printer print me a test version of the shuttle. The spacing between all the holes is perfect but the holes themselves seem to be a tad bit too small. The smaller bolts are just a tad bit too small (bolts went in but needed a bit of persuasion) but the larger top hole was noticeably too small. I think you are right, the bolts are in fact 7/32" and 5/16". Assuming those are the proper measurements, should I use the free fit or close fit measurement?

[Dr. Steve Brule]

I usually leave .2mm or so on smaller holes (under 8mm) and .3mm or .4mm on most things larger. Depends on the job though, sometimes you can need a bit more clearance, sometimes get away with minimal. I generally like to have as little as possible.

[G]

It depends on what you are asking it to do.

THere is a standard "clearance" hole size which will be given on charts as explained above, but sometimes you want it a closer fit so that a reciprocating load is less likely to wallow it out, or with a softer material you might want it looser to allow for a bit of deformation.

The reason that the 3d printed part came out with undersize holes is because of "stringing" on the curves, it works much better to print holes as "polyholes" rather than round holes. Google "polyholes" and "hydroraptor" for a fuller explanation.

Is this a Yeti?


G. 

[tecnic1]

I'll preface this by clarifying that I'm not a machinist, but they do occasionally let me put down specs long enough to design machined parts.  I also work in an industry where we often don't give a fuck about money.

But for a critical pinned joint like this, where even small amounts of play would not be acceptable, we would drill the hole intentionally undersize, and finish hone it until the actual bolt being used just fits.  In face we sometime specify the allowable force to insert the bolt to make sure someone doesn't get crazy with a hammer because they are ready to go home.

Doing so allows us to get the fit we want regardless of the tolerance stack-up.

[Brooklynrider]

I was told beforehand that the 3D model would have some distortion and it wasn't a totally accurate model of what I am making. I was just seeing if all the angles were correct and if the holes were spaced correctly. It's an older Morewood btw. Thanks for everyone's help.