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

"amitsoni" wrote in message
ups.com...
can anyone please tell me the reason why V-blocks are used to check the
roundness of a cylinder?
thanks


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.

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:
"amitsoni" wrote in message
ups.com...

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



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.



----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

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

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

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




Dave Lyon wrote:
"amitsoni" wrote in message
ups.com...

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



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.



----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

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,

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

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

wrote in message
oups.com...
snip---

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.

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

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

wrote in message
ups.com...
Harold,

Thanks.

Although I have overhauled a Landis centreless grinder decades ago, I
have never operated one.

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
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


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