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Standard Keyway Practice

EPAIII

Diamond
Joined
Nov 23, 2003
Location
Beaumont, TX, USA
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.

Regards.

Finegrain
 
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.
 
The keyseat is in the shaft. The keyway is in the hub.

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! :)

Thanks, Brian, for sharing what you've been finding on keystock. I thought it was just me running into this!

Richard
 
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.
 
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.
The math doesn't make sense in the book. Let's say 2" shaft takes a .5 key. For the shaft it says make the height 1.718 (.282"). Ok that one makes sense it's .032 deeper for clearance. But the hub says measurement should be 2.223 (.223") that's .027 too shallow.
I just want to learn. Can you guys help?
 
I did not verify the numbers you quoted, but what you seem to be missing in your objection is that the keyway is not centered on the shaft dia, as others have already explained.
It is the height from the "shoulders" down. This is lower than the center of dia on the shaft, and also shallower than if centered on the dia on the bore in the gear. (draw it)

smt
 
Sorry, Forrest answered my question.
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.
I just didn't understand what a chord was.
 
For anyone interested a 2" shaft chordial (or whatever you call it) height is 1.968 - .25 depth = 1.968. That's the book number. No extra clearance. The hub is 1.968 + .25 = 2.218 book number (2.223) gives an extra .005 for clearance. Just like Forrest said.
 
The commercial sets are already sized for standard keys with bushings and shims provided.

Perhaps someone who owns a set could take a few measurements for you? It's been many years since I used them but I do remember it took a pretty serious press with proper alignment or the broach could be damaged by side forces.
 
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 always assumed keystock and generic keys were left oversize to allow for fitting due to not all keyways being cut to the exact same dimensions. I expect a replacement part from an equipment manufacturer to be a drop-in while generic keys often require filing and sanding or grinding.
 
Yeah I can't tell you the number of times I've seen guys take an on-size endmill and run it down a shaft for a keyway. *SMH* Those guys are probably the reason why they sell the keys oversized too.
 
Yeah I can't tell you the number of times I've seen guys take an on-size endmill and run it down a shaft for a keyway. *SMH* Those guys are probably the reason why they sell the keys oversized too.
Ha! I've been vindicated. I've always cut keyseats in shafts using three passes (one down the middle
about .015 short of full depth and then one down each side to finish). Always thought I was being overly
fussy, especially when all the Youtube videos of guys doing it in one pass started popping up. I'm mostly
self taught so it's good to know that my instincts were good from the start.

Cutting a keyway or keyseat a few thou deeper doesn't usually hurt anything but you want to get them as
snug as possible on the sides to resist wear...
 
Straight 2-flute cutters allow cutting a keyseat on-size in a single pass without the deflection issue. I cut hundreds of them. Never used commercially available keystock, it's intended to be filed to fit in the field. Always used cold rolled square (for MS shafts). Key should be a tap fit in the keyseat and stay there.
 
Straight 2-flute cutters allow cutting a keyseat on-size in a single pass without the deflection issue. I cut hundreds of them. Never used commercially available keystock, it's intended to be filed to fit in the field. Always used cold rolled square (for MS shafts). Key should be a tap fit in the keyseat and stay there.

Not in my experience. They do a little better, but certainly don't eliminate the issue.

Edit: missed the "straight" there on first read. While that might work, I have never actually seen one in person other than a router bit. Straight flute mills are pretty heavy bangers in my experience though. Might be I'd just rather make the couple extra passes.
 
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Straight 2-flute cutters allow cutting a keyseat on-size in a single pass without the deflection issue. I cut hundreds of them. Never used commercially available keystock, it's intended to be filed to fit in the field. Always used cold rolled square (for MS shafts). Key should be a tap fit in the keyseat and stay there.
I was taught by an old guy to make one pass with a 2 flute double end HSS end mill with an on size shank, if available. The cutter will be slightly smaller than the shank. Center over shaft, plunge until you get a full diameter circle on top of the shaft. Go to depth 1/2 the key plus whatever allowance is called for. It's always pretty close.

Nowadays I never have such a cutter so I make more passes. And likely there is more of a drawing requirement than of old, that is definite dimensions rather than "1/2 square keyway." Hard to hit a toleranced size reliably with one endmill pass.

Sometimes I use a 4 flute carbide endmill and feed lengthwise on finish depth then dead pass back out. The cutter pushes to the conventional side on the way in and recuts the opposite side on the way out. By varying the feed on the way in you can vary the amount of pushover, thus the keyseat width. Works great. BTW talking about small keyseat here, maybe 3/8 and under.

If your trying for one pass use a two flute same as a slot, since they don't deflect perpendicular to the feed direction.
 








 
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