RPM gauge/sensor for lathe/mill

M

mac1911

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I have a 3-1 smithy 1220 and was wondering if I should get some sort if rpm gauge/sensor to actually see what the belt positions are really providing for rpm?
Will a inexpensive non contact tach be fine? I see inexpensive magnetic style that would need mounting. Looks like a pick up is just glued onto the spindle.
I also have thought about DRO but I think that money would be better spent on a future piece of equipment.
 
If it is just belt positions they are not changing so there should be information somewhere as to what those speeds are.

No gauge is needed unless there is a variable speed motor.
 
I and many others have used a lathe for a half century and more without an rpm indicator. A Cadillac said, it doesn't add much useful information for a motor without variable speed. You can get a good indication by measuring pulley diameters and the rated motor speed. Close enough for most purposes.
 
Agreed, the absolute speed just isn't that important for most hobby type machining. The cutting tool geometry and sharpness matter much more
The tach units I've seen are not too expensive
You glue a small magnet on the spindle and there is a pickup and display unit. You need also a small 9-12 volt dc power supply or wall wart.
 
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Agreed, without variable speed it's not really that helpful. If you want to know the cheap ones off Amazon will give you a real time display but just running the machine will tell you what you really need to know.

Use the RPM chart to calculate SFM for your workpiece and learn how to grind good HSS tools.

John
 
I have a magnetic rpm/tach on my Atlas TH42. Magnet on the spindle and a simple sheet metal bracket holds the sensor.

I use the rpm reading to figure out my SFM, which is an important value to know when turning.

I also have a Baldor 3/4 hp motor and a Danfoss speed control. But I still use the pullies and back gears to get my workpiece in the right range and then fine tune with the speed control.

In theory, you should already know the rpm of the spindle based on the pulley chart for your lathe and motor RPM. Knowing SFM from that point is just a simple calculation (or a charted value if you prefer). The variable speed control is great to get the value as close as you can and also gives you the ability to change the SFM "on the fly" if turning a large piece down to a smaller piece.

I find I mostly vary the SFM "on the fly" when parting. Regular turning is mostly a "set it and forget it" value. But in my experience, keeping the SFM as close to where I need it helps me avoid chatter when parting. Probably more necessary for me because I'm parting on an Atlas, which is not exactly known for bedrock granite rigidity....
 
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markba633csi said:
There's a reason the nickname for Atlas is "noodle"
:)
Click to expand...
Its not in my shop. Not the most rigid piece for sure, but certainly not a “noodle”…even more so when you take into consideration its essentially a home hobby machine.
 
RJSakowski said:
I and many others have used a lathe for a half century and more without an rpm indicator. A Cadillac said, it doesn't add much useful information for a motor without variable speed. You can get a good indication by measuring pulley diameters and the rated motor speed. Close enough for most purposes.
Click to expand...

Your ear can tell you if the speeds and feed are right.

No need for some sort of gauge.
 
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