In a threaded rod, are the threads counted in the diameter?

E.g if you have a 3/16 inch threaded rod, will the diameter of the rod exceed the 3/16 inch, or is the rod 3/16 inch regardless of the threading?

sort by: active | newest | oldest
jeffrey42834 years ago
No it will not exceed 3/16. That is what's called the major diameter which is the max outside diameter of the rod or anything threaded. For instance a thread called out as 1/4-20 has a 1/4 inch outside diameter and 20 threads per inch.
astroboy907 (author)  jeffrey42834 years ago
So if I bought a nylon gear with a .187 bore size ( .005 off of a 3/16), it would pretty much just slip over the rod? Not what I was looking for... hmm I was planning on threading it somehow on the rod for better stability. I guess I might just have to JB weld it and put some nuts around it to lock it on..
That would depend on how tight the tolerance of the gear and the rod are. If neither are very high precision then you won't lnow till you get them. If you don't mind me asking what is the gear rod combo for? And where were you getting them from?
astroboy907 (author)  jeffrey42834 years ago
The gear rod is for a Pan/Tilt device I am trying to make (some what like this)

I am trying to get it so that the stepper motor i have driving the thing will not have to be constantly on (reduce power consumption , as well as reduce the gearing ratio.

The gears are from SDP/SI (the gear in question), and are standard nylon/acetal gears. They appear in the CAD files to have a spot for a set screw, but I doubt it would be tapped. I dont have the $$ to buy $20 brass/steel gears, so I was going to go with the cheaper option. 

The 40:1 gear that I linked to would pretty much fit over the rod. Maybe it would self tap a little, but not a whole lot..

I can get a 30:1 gear with an internal bore of 3/16. The hub diameter on this would be about 0.6 inches, so I could drill it and self tap or just tap it if I had to... 

My biggest concern is time restraints.. I have only a few weeks to build this so have to order the parts this week, and get all the hardware done the next. There is a pan and tilt device like this (eMotimo), and I might be able to get their source code (Arduino uno compatible), and hack it a bit to run this, until I write my own custom code...
astroboy907 (author)  astroboy9074 years ago
An 8-32 rod I believe is slightly larger that .1875, so do you think I could self thread that gear onto it? Is nylon soft enough??
You would want a 10-32 threaded rod with a major diameter of .190" which is larger than your bore. From the link you posted it looks like the gear is actually acetal resin which is both a machinable plastic and can be tapped. I'm not sure that you could just force the rods threading cleanly and straight enough to prevent binding into your gears bore though. You can pick up a tapping wrench for about $6 and a 10-32 hand tap for about $3 i think.
astroboy907 (author)  jeffrey42834 years ago
how about nylon? Just asking in case I switch gear ratios...

And could you define "binding"? Is that just the gear locking on to the rod after a few threads, and sticking there?

I asked my Automated Manufacturing teacher and he says he has a 10-32 tap... Would I just tap it, or do i have to drill it first?
astroboy907 (author)  astroboy9074 years ago
I can also get the gear with a "brass insert" (not sure if that is the set screw or an acetal gear with a brass center...)

If I got a brass one, would it still be easy to tap or would I need lots more machining...
You can tap nylon but its not as durable as acetal resin. When I say binding I mean the rod will catch threading into the gear and when you force it loose it'll probably tear up the threads you just made. If there is an option for a brass insert for a set screw that's the way I'd go just get a rod that gets a nice slip fit into the gear (slightly smaller than the gear bore) and use a nylon tipped set screw. It'll give you plenty of grip and shouldn't slip.
astroboy907 (author)  astroboy9074 years ago
Sorry- 10-32 is the correct one :)
No, it won't. Your 3/16" threaded rod has an OD of 0.1875". Your nylon gear has a bore (ID) of 0.1870 in. That means the rod will jam. However, since the rod is steel and the gear is nylon, if you clamp it well enough, you should be able to use the rod as a "self-threading die" and twist it into the gear.
+1 to both kelsey and jeffrey.

3/16 starts as solid slightly-larger-than 3/16 rod and then gets the threads machined, hopefully bringing it down to the correct OD. Cheap 'all-thread' or 'readyrod' is notoriously imprecise so be ready to make adjustments to the gear. If possible, get a gear that has a set-screw and file a part of the rod flat to give a really good interference hold.