keyway tolerance

[ecdez]

So, I broached a keyway into a part and all went well until I tried to assemble everything. The key was very tight and had to be pressed on. I've never done a keyway before so I don't know if this is normal or not but it seemed unusual to me. Could be the inexperience talking though.

I decided to measure the parts to see what the difference was. The key is .0015 larger than the broach. Is this normal?

 

[Rbeckett]

I normally lap the key to slide in and out of the keyway smoothly. I also make sure that the pulley or whatever fits over the broached area is a nice smooth fit. If I have to pound or beat it on then I will have to pound harder to get it off the next time I need to get access. So in the interest of less damage I take the extra few to make sure everything slides together nicely. I have pulled and broken a ton of pylleys and sheaves in my days as a small engine tech and there was no reason other than poor fit up and lack of protection. Some of those pulleys were way way expensive and some were very hard to get even from the OEM parts supply system, sears being the worst offender in that area. So my vote is file it smooth and slide it together by hand.
Bob

 

[ecdez]

I was thinking it should be a slip fit also. I have been working on engines for a long time and never had to force a pulley over a key.

Would I just be better to find a broach that's actually the correct size or are they all a little undersized?


Or is my key oversized? It's supposed to be 3/16" which is .1875. Is this normal?

 

[ecdez]

However your measuring it with a Digital Caliper sideways if there was any bow in the Key or the broach your reading will be large. Micrometer will make a far more accurate measurement!

Good point, hadn't considered that.





The key is tight in the shaft too. In my frustration, I tried to sit the broach in the shaft and it fit fine. I got the key from Fastenal so I assumed it would be reasonable quality, but as stated the plating may be throwing it off. I assumed they accounted for this in manufacture and undersized the key so that when it was coated, it would be the correct dimension.



Funny how things don't make sense when you think about them, but after you say them out loud (or type them out), it seems so obvious. Maybe I'll pick up a plain key and see how it fits.

 

[November X-ray]

Just because you sourced from Fastenal does not mean it will be a higher quality material than most any other chain store. Have you measured the keystock both directions of the square? As stated you could lap the keystock or take a file and rub it lightly till it just begins to fit and maybe use a really fine grit sandpaper to finish it.

 

[Tony Wells]

What is happening is that commercial key stock purchased from hardware stores, etc., is that it is zinc plated over size. It will measure quite bit over nominal size. When you buy a keyway broach, you should buy a nominal size and you will get in this case, a 0.1875 +0.002/-0.000 slot. The key stock will need to be stripped. You can do this with paper if you have the patience and don't mind sore fingers, or if you happen to have some battery acid, you can remove the zinc in a few minutes. It should be a close slip fit in the shaft/pulley.