"Setting up Between Centers"

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SteveHGraham
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"Setting up Between Centers"

Post by SteveHGraham »

In one of his videos, Keith Fenner takes a shaft off his lathe and puts it on a device that has two centers. He rotates it and checks it at different points with an indicator. Can someone explain the purpose?

If I were trying to make a uniform shaft, I would level the lathe, put the shaft in the 4-jaw, indicate the crap out of it, drill a hole in the far end, and use a live center. Then I would turn it, indicate it on the turned part, and do the other end. Either that, or I would center-drill both ends and use a dog plate to do the whole job between centers, forcing it to run concentrically with the holes. Apparently these approaches don't work.

I'm taking it on faith that the video appears below.

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John Evans
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Re: "Setting up Between Centers"

Post by John Evans »

Steve: That tool is a "bench center" ,you can check for lobed parts also tapers ,eccentricities etc. Gee ! I :shock: just realized I don't have one !!!! Guess Monday when I go down to my dealer friends place to do some work on a BP mill he has I need to buy one. :roll:
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SteveHGraham
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Re: "Setting up Between Centers"

Post by SteveHGraham »

Why did he need it? Why wasn't the lathe good enough?

You definitely need to buy one.
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BadDog
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Re: "Setting up Between Centers"

Post by BadDog »

My understanding is that the bench centers provide a higher degree of precision reference point. If you did it on the lathe, you would be reading the quality of your head stock bearings, live center, accuracy of setup, and perhaps influenced by things like bed wear or even loose gibs and/or wear letting the saddle move a bit. I've never had enough concern for the things I make on a lathe to need it, but I got a a few small bench centers when found for a good deal. The smallest is a Taft Pierce bar centers, and a big heavy block of iron with 2 high precision centers (forget the brand, came out of Honeywell) for larger items. So far the main thing I recall using them for is checking a huge lot of commercial arbors I picked up at auction, and that was really just an exercise in curiosity. And I have used my lathe like a bench center to check and confirm what I thought was a bent shaft, but that would have been WAY too big for my little 24" plate and centers.
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Harold_V
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Re: "Setting up Between Centers"

Post by Harold_V »

The best way to understand this is to spend some time on a cylindrical grinder, which readily exposes the problems with lathe turned parts. Such parts may have been machined with a live center, with a tip that is not concentric with the bearing centerline. It's not all that uncommon.

Parts machined when held in a chuck with a live center at the opposite end are often not concentric with the center, due to tailstock oscillation, the result of a center hole not being on true center, and the rigid chucking resisting the center's attempt to cause the part to run true.

Centers in a part do not guarantee roundness, nor concentricity. If the centers aren't lapped (there's machines made for that purpose), parts are subject to minor irregularities, in spite of the fact that they may be ground between centers. Add to that the possibility of centers that may or may not run on true center (some grinders allow for the headstock center to rotate), there's always a small risk of problems that can be introduced to finished parts. In this instance, the center hole would be eccentric, although the finish grinds may all be concentric with one another. True so long as the part is not flipped, so each end is ground in a different setup. In that case, each side would have eccentricity with each other. Such conditions are all pretty much disclosed by inspection between centers, using a set such as has been discussed.

Harold
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SteveHGraham
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Re: "Setting up Between Centers"

Post by SteveHGraham »

So you spend thousands of dollars on a precision tool, and then to make it work, you have to buy a REAL precision tool. And somewhere, there must be a REAL REAL precision tool to fix the inaccuracies introduced by the REAL precision tool.

How do you know when you need a bench center?
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John Hasler
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Re: "Setting up Between Centers"

Post by John Hasler »

> And somewhere, there must be a REAL REAL precision tool to fix the inaccuracies introduced by the REAL precision tool.

That one's at the BIPM in France.
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SteveHGraham
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Re: "Setting up Between Centers"

Post by SteveHGraham »

Here's another interesting thing in that video: when he has to clean out the spaces between flanges, he turns the part in the lathe over and over so he can keep using the same right-hand tool. I would have alternated with a left-hand tool.
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WesHowe
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Re: "Setting up Between Centers"

Post by WesHowe »

I thought the same thing. However, there must be a method to the madness, because anyone that can straighten an 18' long shaft within 0.002" using a torch and a little water has to know something.

Or the answer could be simpler, maybe he doesn't have a LH tool that uses the right insert.
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SteveHGraham
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Re: "Setting up Between Centers"

Post by SteveHGraham »

In the comments, he says he doesn't have a left-handed tool with the same insert, but you don't need the same insert to rough.
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BadDog
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Re: "Setting up Between Centers"

Post by BadDog »

As to when you have enough real REAL precision, the answer is simple. When you can be confident that your parts meet the precision/tolerance you require. For most of the things I make, calipers or even a 6" or 12" rule is plenty good enough. For my more precision parts, a micrometer or indicator (and/or DRO) is more than enough. I've not yet needed gauge blocks, DTI and height gauge, or bench centers to qualify any of my lathe work; much less needing my reference tooling certified. But you never know, and I hate being caught with no options. Some day... :roll:
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Harold_V
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Re: "Setting up Between Centers"

Post by Harold_V »

SteveHGraham wrote:Here's another interesting thing in that video: when he has to clean out the spaces between flanges, he turns the part in the lathe over and over so he can keep using the same right-hand tool. I would have alternated with a left-hand tool.
While his reason may not pertain to what I'm going to say, I think you'll discover that cutting towards the tailstock offers problems that you may not experience when cutting towards the headstock. On several occasions I've experienced chatter that was difficult to control, for example.

Harold
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