using V-blocks for checking roundness

can anyone please tell me the reason why V-blocks are used to check the roundness of a cylinder? thanks

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Because you can't check roundness across only 2 points (like with a set of mikes) The V block makes contact in 2 places, and an indicator is the 3rd.
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Naturally one point is when using a granite table and height measuring instrument. Say that of a 48" gear or such. Mikes are not much use that large - but when mounted in V blocks - then the height instrument is very useful - in measuring many points.
Martin Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH & Endowment Member NRA Second Amendment Task Force Charter Founder
Dave Lyon wrote:

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On Mon, 09 Jan 2006 22:08:06 -0600, "Martin H. Eastburn"

Odd that this should come up..I picked up a pair of 4" Starrett V blocks saturday at a yard sale.
I wonder if they are worth the $0.50USD I paid for em? Cast iron scrap weight...minty but for a very small amount of surface sure on the bottoms. Cleaned off with a rag and some Kroil.
Got a Rigid 3" pipe cutter too. That was $5.
And the power hacksaw..got to replace a burnt out yard light....
Gunner

The aim of untold millions is to be free to do exactly as they choose and for someone else to pay when things go wrong.
In the past few decades, a peculiar and distinctive psychology has emerged in England. Gone are the civility, sturdy independence, and admirable stoicism that carried the English through the war years . It has been replaced by a constant whine of excuses, complaints, and special pleading. The collapse of the British character has been as swift and complete as the collapse of British power.
Theodore Dalrymple,
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amitsoni wrote:

Checking for out-of-roundness can be simple, or complex, depending on the actual shape of the cylindrical work piece.
If the work piece is oval, ie. has ONE major and ONE minor diameter, a micrometer or caliper will show out-of-roundness, and how much.
If the work piece surface consists of lobes, typically 3 or 5 may be encountered, the situation is more complicated. In this case a two-point measurement (by micrometer or caliper) may NOT show any out-of-roundness! The WHY requires a graphical demonstration since verbal description is tedious.
Simply put, the lobes all have a constant radius drawn from the opposing side vertex; that's why the # of lobes is odd. A two-point measurement will simply measure a constant dia. under these circumstances.
Placing this part on Vee blocks and placing a dial indicator point on top, while rotating the part, will provide an INDICATION that the part surface is out-of-round. It will NOT show by how much.
To actually measure the amount of out-of-roundness would require a Vee block with an included angle of 120 degrees for parts with a 3-lobed surface, and an included angle of 72 degrees for a 5-lobed surface. In fact you can purchase special micrometers with anvils of this shape to measure directly.
Surfaces as described above are encountered on centreless ground parts where the machine has been improperly set-up. I understand that even well-adjusted machines may produce a very slight amount of lobing.
Now then, how do these conditions create problems in the real world?
Some years ago a college student came to me with an R-8 endmill holder and two 3/8" dia. endmills. One endmill shank measured .3748" dia. and fitted into the holder. The other endmill shank measured .3746" dia. and would NOT fit into the holder. Needless to say my student was just a little perturbed about this! Checking the shank with Vee block and indicator showed the first shank to be round (it had centre holes at each end); the second shank showed out-of-roundness.
Even though a two-point micrometer measurement showed its dia. to be significantly smaller than the mating hole, the shank would not fit! This is the real problem with this condition. If you were to draw this on paper or CAD at 10X full size you would immediately see that the high point of the lobes fall outside the "measured" diameter as made with a two-point instrument. (This is quite difficult to imagine; if I hadn't experienced it myself a number of times throughout my career I wouln't believe it either without a drawing.)
To summarize: lobed surface "cylindical" parts have an EFFECTIVE outside diameter that is greater than a two-point measurement would indicate.
Hope this helps.
Wolfgang
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snip---

Not in my experience. All that is necessary for a part to cancel lobing is to NOT run on center. You can see a change in parts with one pass light through the machine, hitting only the highs. You have to balance tractive effort against the amount above center, which, in theory, should yield the best condition for grinding, particularly for short pieces. If parts are run too far above center, they're inclined to lose contact with the regulating wheel.
Very nice report, by the way.
Harold
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Harold,
Thanks.
Although I have overhauled a Landis centreless grinder decades ago, I have never operated one. Consequently my verbiage on operations is what I either read or obtained through discussion with machine operators.
All the rest is my personal experience.
As you stated:"You have to balance tractive effort........". How critical is the "above centre" adjustment? Is the "sweet spot" for cylindricity easily determined or is it largely suck-it-and-see? Just curious.
Regards,
Wolfgang
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Damned shame, Wolfgang. ! They are truly one of the "magic--gee wiz" machines. If you have a precision grinding background, I think you'd really enjoy the experience. I know I did. It's nothing short of amazing how you can feed some ugly, heat treated parts in one end and have them come out the other in very little time, perfectly sized and shiny. Wonderful machines, and fast.
Consequently my verbiage on operations is

All of my centerless experience came from running #2 Cincinnati grinders, aside from once when I leased time on a machine to finish grind and order of steering pins for the B-52 bomber. The machine in question was foreign made, I think Italy, but I'm not sure.
It's been years since I last touched a centerless, but I recall that we ran roughly 5/16"---3/8" above center. I don't know that there's really a sweet spot-----not as long as you stay a reasonable distance from center and still have traction. Size isn't a factor, in fact, if you change size, so long as you keep the center height constant, it doesn't demand adjustment of the guides or changing the angle of the grinding wheel. You can minimize setup time if you keep center height constant. If a large variety of diameters are ground, it can be a problem to maintain the proper blade height, which would usually be the only reason to change grinding height. Mind you, I'm talking running above center, not below.
We ran one item that was beyond the capacity of the machine at our disposal. The #2 was limited to 3" diameter, but we often ran some short steel heat sinks that were 3-1/2" diameter (but only 3" long) and required centerless grinding. They couldn't be run between centers because of a machined configuration through the sinks. If the grinding wheel wasn't new, there was enough travel to permit running the part, but even with our shortest blade, we had to run above center far enough that traction became an issue. I recall reporting for work on my shift to find the centerless grinder with a large divot out of the grinding wheel. The other shift had the misfortune to have a part start chattering, at which time the wheel picked it up and spun it up. It left the grinder, straight up, falling back between the wheels sideways. Slammed the grinding wheel to a dead stop instantly, without breaking the wheel. Mussellman, the day operator, was reputed to have undergone a change or color from the occasion, turning quite ashen from the experience.
Be well,
Harold
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