Standard Keyway Practice
My question is about stand practice when cutting keyways. I know I can make it work, but I want to make it standard, if possible.
Key stock is square. So, using 1/8" key as an example and cutting a keyway in a shaft and a gear, what is the point of reference for the depth of the keyways on the shaft and on the hole in the gear? Seems to me that it would almost have to be at the center of the keyway on the surface of the shaft or hole. But I would like this confirmed.
If this is correct, where and how much additional allowance should be provided to allow easy assembly and interchangability?
Also, how much allowance is normally provided on the width of the keyways?
There is quite a lot of detail around this topic in Machinery's Handbook, including proper depths and tolerances. No easy answers though, there are too many factors.
Even after you digest the standards, there are plenty of instances of non-standard appplications, like gears that have precious little meat between the bore and the tooth roots might use an extra-deep slot in the shaft and a shallow slot in the gear.
This topic may be found in Machinery's handbook. The keyseat is in the shaft. The keyway is in the hub.
Generally speaking square keys are sized to the nearest nominat size to 1/4 the shaft diameter. Yhis is standaredized. Look it up in Machinery's Handbook or some other shop reference.
The deprh is measured at the chord meaning the tool is touched off at the diameter and then lowered by the chordal height. The tool is then fed to full deprh that is, 1/2 the height of the key.
The keyway in the hub is half the height of the key plus a clearance either 0.005" or 0.010: depending on key size but I forget where it breaks.
Keys havre to fit their seat snugly, that is metal to metal with no clearance. People whe were taught wrong and cling to their errors will tell you to endmill a keyseat with an on size cutter. Do this only if you want a crooked or oversized keyway. Endmills deflect to an astonishing amount. For that reason you cut a keyseat using an undersized cutter roughing out the middle and sideshifting to cut the width.
When measuring the keyway depth you measure from the side of the keyway on the shaft to the bottom of the keyway. If your measuring from the OD of the shaft there is a formula in Machinery Handbook to add that dimension to the half size of the key but don't depend on it for the right depth to cut to.
Most keyways in a shaft are half the height of the key plus .010". That is, for a 1/2" key the depth of the keyway would be .250"+.010"=.260". The same is true for a keyway in the pulley.
I always cut half of the depth, for example for a 1/2" key I would make a .125" for the first cut and then measure and make the next cut to finish to the .260" depth.
If you don't have a Machinery Handbook buy one.
Be aware that keystock that's supposed to be a particular dimension, like 1/4 inch or 3/16 can actually be oversize by three to five thousanths of an inch. For some reason the plated keystock that is sold as "Precision" brand is maddeningly oversized, sometimes by .005. So be sure and mic your keystock. I don't know what the average person does when they buy this brand, which is sold by MSC and J and L. I've ground quite a bit of it, until I gave up and found other brands of keystock that are on size. I guess they make it assuming you're replacing key stock in wallowed out keyways.
I use to wonder why all my prints had keys cut with woodruff key cutters, but it's for the reason Forrest stated. You Don't get an onsize keyseat cut with an on size mill.
I never liked the name "keyseater" for the Morrisons and Mitts&Merrills used on so many gears and sprockets. Now I know why-- those were key WAYS they were cutting!
The keyseat is in the shaft. The keyway is in the hub.
Thanks, Brian, for sharing what you've been finding on keystock. I thought it was just me running into this!
It's true that keystock does vary in size and I don't like plated keystock at all. It really depends on the fit you need as to the process you use to cut the keyway in the shaft.
A woodruff cutter or slot cutter will cut a better fit than a two flute endmill for sure but a tight fit is not always needed or wanted. When your using a broach to cut the keyway in the pulley or sleeve the width is controlled by the broach and the depth by the shims. You then make the key fit the broached keyway.
Since the broached slot determines the width of the key and the key should be a slip fit in the broached keyway then you have to use a method to cut the keyway in the shaft to produce the fit you want.
It's easier to make the fit in the shaft once you have a fit in the broached part and both should be a snug slip fit. I don't like sloppy or tight keyways in the pulley or shaft.