We are looking for an coupler to go from the 8 mm motor shaft to a 1/2" drive shaft.
We will be using it for a ball thrower so both need to have a key to provide a positive lock.
We have limited machining capabilities but do not have the capability to cut key ways.
Does anyone know where an adaptor like this can be purchased?
Find a piece of 3/4 round steel. Drill it with an 8mm drill through then from one end half way through with a .5 in drill. Make this thing about 1.5 to 2 in long. Drill and tap for four 1/4-20 set screws. Grind flats on your shafts where the set screws contact them and you will not need the keyway.
Bruce
I think its going to find something with those criteria:
You may be able to make something simple, like stated above, with a set screw, or something simpler than a keyway, but it is not likely you will be able to buy a part off the shelf to meet all those specs.
SDI-SI has several couplings available:
https://sdp-si.com/eStore/
Though none of them are metric-english couplers, nore do any have a 0.315 - 0.5" coupling combination. You may be able to get away with a 0.25" to 0.5" coupler by boring out the 0.25" hole to 8mm. Hope this helps.
8mm is only about .0024 larger than 5/16". Get a 5/16" to 1/2" keyed adapter, and it’ll be a tight press fit into the CIM shaft. If it doesn’t fit at all, drill out the 5/16" side with a O drill (.316).
Use set screws or roll pins for the CIM motor side, unless you can find an imperial-to-metric key.
We found ourselves in the exact same situation. We ended up using 2 Andymark hubs. It’s kind of a bulky and expensive solution, but we didn’t really have machining available, and we already had the hubs laying around.
Here is a fantastic 8mm to .25" coupler:
We use these and have never had an issue. Very tough and reliable.
There are many styles and sizes to choose from: Hereis a budget version (Plastic instead of AL).
This vendor (Automation Direct) is a FIRST supporter and KOP supplier.
(And they ship VERY fast …)
For 25 bucks I would hope so!!! That doesn’t even include shipping!
If you have a lathe and drill press, you can make a very positive coupler. Make the 8mm to 1/2 inch coupler of mild steel, 1 1/2 to 2 inches long. Cross drill with a 1/8" diameter drill, and press in a 1/8 hardened pin. Make the pin a little shorter than the coupler diameter, (recommend a minimum of 7/8 stock) so you can peen over the top of the hole above the pin to ensure pin retention. We have used this on 5/16 CIM motor shaft to DeWalt transmission adapter with no failure. This should see less load than that. 
Perhaps something like this would work? A 1/2" to 5/16" flexible coupling:
http://www1.mscdirect.com/CGI/NNSRIT?PMCTLG=00&PMAKA=8802536
Or find a steel bushing in the hardware drawer section of Lowes or a place like that. You may find one that will slip over the 8mm shaft and, with luck, will be about 1/2" outside. You might be able to split it and get it to clamp firmly in a straight 1/2" coupler (with enough Loctite).
It’s not ugly… its unique.
That’s what I keep telling our robot at least…