How to measure start of sweep from woodruff cutter?

[Rustybolt11]

Our shop cuts slots for square keys on shafting. We use woodruff cutters.
Our dwgs specifiy where the end of the keyway flat ends and the rest is just sweep left by the
cutter.
Is there a simple way to measure where the cut bottom flat portion ends and the sweep begins?
Trying to hold +/-.005" from the end of the shaft.
We've let a few go too short and the key sticks out of the hub for our customer.

 

[guythatbrews]

If you've got good eyes slide a .010 scale along the keyseat (not keyway ) until you see daylight underneath. A magnifier sure helps.

Or mill a piece of keystock thinner so it slides freely in the keyseat. Put it in the keyseat and measure over shaft with a micrometer. Lock the mike at this setting. Place the mike close to the runout. Push the key until it stops. You've hit the ramp. Maintain a sharp edge on the keystock. You can also use a steel ball but you need 3 or 4 hands.

Or stand the shaft vertical. Zero a test indicator in a height gauge on the bottom of the keyseat. Run the ball off the end of the keyseat. Carefully move the indicator back onto the keyseat until you reach your indicator zero and zero the height gauge. Tram the indicator to the ramp and note the length on the height gauge.

 

[ThunderPants]

I would use a contracer (if you have one that has the stroke needed to go the length of your slots).

 

[ballen]

If you do a lot of these, I would make a fixture containing two dial indicators. The first dial indicator rides on the keyseat. The second dial indicator registers the end of the shaft. The fixture sits on a workbench or in a vise. You slide in the shaft until the first dial indicator starts to move, indicating that you've hit the end of the keyseat. Then you consult the second dial indicator to see how far you pushed in the shaft.

The reason that this is hard to measure is that when the keyseat starts to deviate from being straight, it has the same circular shape as your woodruff cutter. Because the circle it cuts is tangent to the straight line defined by the keyseat, the deviation Y from straight (what dial indicator #1 above shows) is quadratic (X * X) in the distance X from the place where the straight line ends. If your woodruff cutter is radius R, dial indicator #1 is set to read zero in the straight part of the keyseat, and dial indicator #2 is set to read zero where the straight part of the keyseat ends, then Y = 0.5 * X * X/ R. Suppose your woodruff cutter has a diameter of 1/2 inch, so that R = 0.25". Then here's a table

X (axial distance from where keyseat stops being straight)	Y (radial distance of curved keyseat from straight line)
0.001"	0.000 002"
0.002"	0.000 008"
0.003"	0.000 018"
0.005"	0.000 050"
0.010"	0.000 200"
0.020"	0.000 800"
0.022"	0.001"

So if you make a fixture like this (or a jig that you can hold in your hand and put on a shaft while it's still in the mill) use a 0.0001" indicator for the dial indicator that rides in the keyseat, and a 0.001" dial indicator for the one that indicates the end of the shaft.

Alternative approach: have the customer to use a shorter key, or to add a feature on the other part which traps the key 1/8" before the keyslot ends.

 

[EmGo]

Have readouts ? Just touch off the end, move half the diameter of the cutter in and you're centered. That's zero. Now go the distance of the key plus maybe .010" up to .025" to be safe.

If no readouts, travel dial indicator works fine too.

 

[guythatbrews]

If you do a lot of these, I would make a fixture containing two dial indicators. The first dial indicator rides on the keyseat. The second dial indicator registers the end of the shaft. The fixture sits on a workbench or in a vise. You slide in the shaft until the first dial indicator starts to move, indicating that you've hit the end of the keyseat. Then you consult the second dial indicator to see how far you pushed in the shaft.

The reason that this is hard to measure is that when the keyseat starts to deviate from being straight, it has the same circular shape as your woodruff cutter. Because the circle it cuts is tangent to the straight line defined by the keyseat, the deviation Y from straight (what dial indicator #1 above shows) is quadratic (X * X) in the distance X from the place where the straight line ends. If your woodruff cutter is radius R, dial indicator #1 is set to read zero in the straight part of the keyseat, and dial indicator #2 is set to read zero where the straight part of the keyseat ends, then Y = 0.5 * X * X/ R. Suppose your woodruff cutter has a diameter of 1/2 inch, so that R = 0.25". Then here's a table

X (axial distance from where keyseat stops being straight)	Y (radial distance of curved keyseat from straight line)
0.001"	0.000 002"
0.002"	0.000 008"
0.003"	0.000 018"
0.005"	0.000 050"
0.010"	0.000 200"
0.020"	0.000 800"
0.022"	0.001"

So if you make a fixture like this (or a jig that you can hold in your hand and put on a shaft while it's still in the mill) use a 0.0001" indicator for the dial indicator that rides in the keyseat, and a 0.001" dial indicator for the one that indicates the end of the shaft.

Alternative approach: have the customer to use a shorter key, or to add a feature on the other part which traps the key 1/8" before the keyslot ends.

You are correct Bruce. Good catch.

These numbers are indeed contracer territory, and way overkill for the application.

So to answer to the OP's question it is impractical to measure cutter runout location to +- .005. With numbers this small even the surface finish will have a big effect on the results.

The practical method may be just run a scale along the bottom until you see dsylight. The good news is the key can ride up the ramp until it hits the keyway in the hub. With .010 clearance over the key and a .5 dia cutter that distance is .070. For a 1" cutter it is almost .100.

Rustybolt while it is not difficult to create this feature and hold +- .005, practical limitations prevent you from inspecting this feature to +-.005. Maybe the best you can do is be very sure of setup and the program. Stop off the same end of the shaft as the keyseat so shaft length doesn't affect keyway length. For inspection use the scale method to confirm your program as best you can, just to eliminate outright blunders like the program is wrong by 1/4". Let the clearance over the key take up the slack.

 

[EmGo]

You could probably calculate it pretty close by knowing the diameter of the cutter, the depth of the slot, and the breakout dimension of the arc. All of those would be easier to measure than the point at which the arc starts to leave the straight bottom.

 

[guythatbrews]

You could probably calculate it pretty close by knowing the diameter of the cutter, the depth of the slot, and the breakout dimension of the arc. All of those would be easier to measure than the point at which the arc starts to leave the straight bottom.

Of course! Depth mic from the end of the shaft to runout. With care you can measure the runout within .001 with a common tool. This is probably the best you can hope for.