How do you tell how fast your Lathe is turning?


Lucky1406

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I just picked up a Delta 46-612. Great find actually, when I went to look at it, everything was there, but it would only turn at one speed. Bought it for $150. Got it home and I came to find out that the wire the moves the reeves drive wasn't connected. One trip to the store for a washer and screw and 45 cents later I had a fully working lathe(now worth around $1000). Not a bad deal(feel kind of bad for the guy though). Well, I decide to upgrade some of the belts to get rid of the vibration(they were worn). I installed some link belts and now its amazingly smooth, I can pace a nickel on edge on the bed while running, and it won't move or fall over(SWEET!). But because of the difference in belt lengths, The range of speed changed. Depending on which belt I lengthen or shorten with the reeves drive, I can change the range from slow/fast, to med-slow/really fast. But I'd like to find out what the actually revolutions are. Does anyone know how to tell how fast it's spinning? I'd like to get somewhere near the factory settings, though I may go with a slower bottom end(for large pieces).

Thanks,

Nick

As I was typing this, I thought of an idea, I could turn a large diameter circle, then trace a pencil along the face while it is spinning towards the center. If I traced it slowly for, lets say 5 seconds) and count the lines it makes on the face, I should be able to tell how many revolutions it makes during those 5 seconds, then I can just multiply it out. Does anyone else have any ideas?

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A tachometer will surely tell you how fast the lathe is going. I would tend to set the lathe up for the slow-fast you mentioned above. I tend to turn more by feel than depending on a certain speed. Have you googled for the normal operating speeds for this lathe? This will at least give you an educated guess as to where you are in each range.

Roger

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As I was typing this, I thought of an idea, I could turn a large diameter circle, then trace a pencil along the face while it is spinning towards the center. If I traced it slowly for, lets say 5 seconds) and count the lines it makes on the face, I should be able to tell how many revolutions it makes during those 5 seconds, then I can just multiply it out.

This is a really clever, low-tech idea. It might be easier to do the same sort of thing by running your pencil down the length of a long cylinder rather than on the face of a large diameter plate.

Does anyone else have any ideas?

As Roger mentioned, a tachometer would be the obvious, high-tech solution.

If you have a camera where you can control the shutter speed, you might also be able to put some sort of mark on a spinning plate, take a picture of it, and then calculate the speed of the lathe by noting how far the mark moves during the time the shutter is open.

Example: Suppose the shutter speed is 1/100 of a second, and that the mark moves through 1/4 of a revolution during that time. That means that the mark would make a complete revolution in 4/100 of a second, or 25 revolutions in one second, or 1500 revolutions in a minute (RPM).

Hmm. That almost sounds like it would work!

-- Russ

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If you have a camera where you can control the shutter speed, you might also be able to put some sort of mark on a spinning plate, take a picture of it, and then calculate the speed of the lathe by noting how far the mark moves during the time the shutter is open.

This sounded like so much fun that I tried it. Turns out it sort of works. I had a hard time getting a picture where the mark was sufficiently distinct to tell for sure how much it rotated during the exposure. Some photoshoppery helped a little bit. I've attached the best pictures that I got (after the photoshop work). The two on the left were taken at a shutter speed of 1/60 sec, the two on the right at 1/125 sec. Based on those numbers, and the angles I measured (with a protractor), the lathe was running at about 1300 RPM during the experiment. Based on the (known) speed of the motor and the sizes of the pulleys involved, I should have gotten something closer to 1100 RPM. Could be that I didn't measure the pulley diameters accurately, of that the belt isn't seated all the way down in the pulleys, or that the shutter speed on my camera isn't accurate, or that I goofed up when measuring the angles, or <insert your own excuse here>.

That little $11.00 tach looks pretty good at this point!

-- Russ

post-685-0-88860500-1318558022_thumb.jpg

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Just a thought, how do you know that $11 tachometer is accurate? What do you check it against?

I like the turn by feel method personally. If my lathe is boucing, its too fast. If my tool is bouncing, can I speed up the lathe and keep it stable? I used to worry about the the precise speed it was turning, but now I just guess at what the rpm's are while I'm turning if someone asks. Close enough is close enough for me.

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Just a thought, how do you know that $11 tachometer is accurate? What do you check it against?

In my case, I checked it against a drill press that had its own digital readout for the RPMs it was turning at. The little $11 tachometer was consistently within a few RPMs. So it certainly was accurate enough for what I needed it to do.

I agree — turn by feel works well, and it's what I do for the most part. I like Bill Grumbine's line about lathe speed: "If your lathe isn't shaking, and you're not shaking, it's not too fast."

But it's also good to have a ballpark idea as to what RPM your lathe is turning at, so that when you read other people's experiences, you have some way of knowing what they are doing.

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