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Airy Points / Bessell Points

SIP6A

Titanium
Joined
May 29, 2003
Location
Temperance, Michigan
I just got a copy of Graham T. Smiths book Machine Tool Metrology

People on here are always talking about Airy points as where you support a surface plate. Turns out that isn't correct. Airy points are where you support an end standard so that the end faces are parallel.

To properly support a surface plate you use Bessell points. The Bessel points will support an evenly-loaded bar or plate at the point of minimum deflection.

Airy points are 1/ square root of 3 = 0.577L

Bessell points are given as 0.554L The text doesn't say how that number was derived.

This is an interesting book there is also a biography of George Schlesinger

Interesting stuff
 
I just got a copy of Graham T. Smiths book Machine Tool Metrology

People on here are always talking about Airy points as where you support a surface plate. Turns out that isn't correct. Airy points are where you support an end standard so that the end faces are parallel.

To properly support a surface plate you use Bessell points. The Bessel points will support an evenly-loaded bar or plate at the point of minimum deflection.

Airy points are 1/ square root of 3 = 0.577L

Bessell points are given as 0.554L The text doesn't say how that number was derived.

This is an interesting book there is also a biography of George Schlesinger

Interesting stuff

I am not sure anymore if even Bessel points are THE "correct" one
Ie see references here:
Airy points - Wikipedia
 
The correct points to support a surface plate are the same one it was manufactured and certified on, or you want it as good as can be - as is.
For inspection / certification / correction I think it would be a suitable approach to follow existing standard federal standard GGG-P463 (if memory serves me right).
 
If your test equipment is good enough to measure a difference between the flatness of a plate supported at .577 and .554, you're playing at a different level than me.
 
The Mitutoyo catalog has a diagram comparing Bessell and Airy points. Agreed there is not much of a difference.

I wonder how Airy became common place terminology?

I just got a copy of Graham T. Smiths book Machine Tool Metrology

People on here are always talking about Airy points as where you support a surface plate. Turns out that isn't correct. Airy points are where you support an end standard so that the end faces are parallel.

To properly support a surface plate you use Bessell points. The Bessel points will support an evenly-loaded bar or plate at the point of minimum deflection.

Airy points are 1/ square root of 3 = 0.577L

Bessell points are given as 0.554L The text doesn't say how that number was derived.

This is an interesting book there is also a biography of George Schlesinger

Interesting stuff
 
If your test equipment is good enough to measure a difference between the flatness of a plate supported at .577 and .554, you're playing at a different level than me.

Might not be you but I want me plate dead flat when I place my coffee cup on it. Or cigarette.
 
If your test equipment is good enough to measure a difference between the flatness of a plate supported at .577 and .554, you're playing at a different level than me.

Could be. But I use the recommend points out of pure laziness.

I don't want to do the math for either. I don't even want to be bothered that it exists to be looked-up online.

Happy to take the "freebie", place the damned bumpers, go do something else that is several years overdue.

I did say "lazy", not "scholarly"?

Why go on as to how many Angels can dance on the head of a pin when Angels are chronically on backorder, anyway?
 
If your test equipment is good enough to measure a difference between the flatness of a plate supported at .577 and .554, you're playing at a different level than me.

I'd add, most mere mortals position them with a piece of chalk and a tape measure from underneath. If not like they are jig borered off a Moore or a SIP. And they are normally on a welded / fabricated frame.

Regards.
 
Lumberjack is correct in saying that you want to support the plate at the same locations it was supported when manufactured. If it has been certified, you want to use those locations. If it has been resurfaced or lapped, you want to continue to use the support locations used at that time.

Hopefully the locations are based on the standards, and not what was easy to weld with the material at hand.

If the plate never has any significant load, it does not make much difference where those points are.
When the support location matters is when there is a significant load placed on the plate. The Bessell points will support that increased load with the least deflection.

You COULD make a 3x5 plate supported by points at two adjacent corners with the third point in the middle of the plate just over 1/2 the plate length. Parts light in weight could be inspected anyplace on the plate. The results of inspecting a heavy part will depend on where it is on the plate. If inspected on the unsupported portion of the plate, the plate is deflected by the weight of the part and is out of tolerance.
 
I just got a copy of Graham T. Smiths book Machine Tool Metrology

People on here are always talking about Airy points as where you support a surface plate. Turns out that isn't correct. Airy points are where you support an end standard so that the end faces are parallel.

To properly support a surface plate you use Bessell points. The Bessel points will support an evenly-loaded bar or plate at the point of minimum deflection.

Airy points are 1/ square root of 3 = 0.577L

Bessell points are given as 0.554L The text doesn't say how that number was derived.

This is an interesting book there is also a biography of George Schlesinger

Interesting stuff

.
.
99.99999% of time i do not work on simple rectangular objects. when you deal with complex shapes the weight or density varies so you do not normally support part in Airy points based on size so much as weight distribution.
....in general make a best guess and bring supports in from ends a bit the exact amount is hard to determine exactly.
.
i have often seen big parts weighing over 10 tons when i removed 1/2 ton of chips and coolant that are on and in part the center of the part springs up of course. .0005" over 10 feet is perfectly normal.
.
sometimes i get inspection reports telling exactly how a part is off in flatness, perpendicular, twisted, etc. i can use the data to either
.
1) jack up corner of part .001" and machine it so when inspected it will be in tolerance
.
2) or move a pad location the part sits on closer to Airy points determined by inspections results. often a best guess is a foot or 2 off and pad needs to be moved based on inspection results
.
names like "Airy" dont mean much. i have worked in other countries where a billion people call stuff other names. who's to say what is correct name. i wouldnt get too concern with terminology
.
the other thing is of course large metal parts like castings have different hardness and other properties in different locations of part. i have often seen harder sections typically in certain locations usually due to part thickness in that section and the rate that part section cooled. the different metal properties in different locations of large parts can make them distort not just from gravity but from different sections pushing and pulling on other sections. similar to cold rolled steel. if you mill the skin off one side a longer cold rolled piece will often bend like a banana.
.
2nd picture shows a casting on its side that typically warps curved shape and needs multiple finish cuts to get within .0005". that part warpage in that case is not caused by weight but internal stress in the metal
.
airy is 2 dimensional part sitting on 2 supports
bessel is 3 dimensional part sitting on 3 supports
or at least i believe thats the difference and i might call both airy points. if you talk to average operator and mention airy or bessel points good chance they never heard of either name but most understand the principle of not supporting a long flimsy part on its ends
 

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These points, along with others, can be found in Wikipedia.

Sent from my XT1053 using Tapatalk
 
I use the Bessel points when checking straight edges after grinding. I use .559L as this formula shows.

Maybe you should switch to "points for minimum sag, 0.5536" instead Bessel since straight edge is not a length gauge :crazy:
 
unless you got simple parts there rarely is a exact formula do to weight distribution and part shape thickness varying in spots
.
as column picture shows its a triangle shape and has ribs and bosses or pads in spots. plus it can be full of internal metal stress that pulls part curved when part on its side but when part on its back the warpage curvature and gravity sag can cancel out.
.
rarely is it a simple thing compensating exactly for gravity sag
 

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Maybe you should switch to "points for minimum sag, 0.5536" instead Bessel since straight edge is not a length gauge :crazy:

Jayzusss.. what a huge delta !!! Talk about shaving a gnat's flatworm's virus'es arse-hairs.

Folk have to EAT, too.. some s**t would never get back into USE to earn the client HIS crust, grind shop plays f**k-f**k too damned carefully and long, what with Lunar tides, Solar tides, road traffic, thunderstorms, temperature, Pre-Menstrual Stress of the Customer's mistress's Sister's cat's fan club, and all them other "variables".

Shall I load the 12-bore with double-ought? SSG's? Or just #1 buck? Or use a 16-lb sledge hammer to keep the noise down?

Target is them pesky "sugar" ants... it's America, after all!

:D
 
Jayzusss.. what a huge delta !!! Talk about shaving a gnat's flatworm's virus'es arse-hairs.

:D
My defense is that at least "it wasn't me" who started originally nit-picking over terminology ;)

Besides I have spent my time by calculating even sillier things like how much atmospheric air pressure variations compress and effect gauge block dimensions! :D:willy_nilly:
 
My defense is that at least "it wasn't me" who started originally nit-picking over terminology ;)

Besides I have spent my time by calculating even sillier things like how much atmospheric air pressure variations compress and effect gauge block dimensions! :D:willy_nilly:

I am actually grateful for that. And a brazillion similar things, the world around.

Human race's primary advantage, specialization can be.

So long as the specialists can avoid being driven bugf**k by the minutiae of it, anyway.

Risky occupation for an ethnic Finn, no? Asian blanket-sharer helps offset that risk.

Don't Ask US How We Know That.

:D
 
as column picture shows its a triangle shape and has ribs and bosses or pads in spots. plus it can be full of internal metal stress that pulls part curved when part on its side but when part on its back the warpage curvature and gravity sag can cancel out. rarely is it a simple thing compensating exactly for gravity sag
Wouldn't the matter of the fact be, that's a Gleason 400 GH column? And your just skimming Turcite? It appears it already has Linear ways fitted, Would you dare suggest they have the sag?

206519d1503582883-airy-points-bessell-points-400gh_column_79000021_turcite_3rdop_b.jpg


I don't see any gravity sag???? Wouldn't you just be bullshitting yet again?? Wouldn't that be why "you" claim machines don't need scraping in. Pretty sure assembler's will hand tune your fuckups

Big old 25 year old Toshiba with a heritage 15M You should check the grid shift. That's really important. Make's a huge amount of difference were you plonk that job.
 
These points, along with others, can be found in Wikipedia.

Sent from my XT1053 using Tapatalk
Why do I get the feeling, your a Tapatalk, Wikipedia kind of guy?
What does your XT1053 say about that? Any chance you could actually attach a human brain thought?

Bonus points for being from Aland Islands.
 
Why do I get the feeling, your a Tapatalk, Wikipedia kind of guy?
What does your XT1053 say about that? Any chance you could actually attach a human brain thought?

Bonus points for being from Aland Islands.
I have a feeling I'm not getting your hopefully humorous references?

I don't look at Wikipedia much, and question their credibility on the specifics.

XTxxxx is my phone model

I use Tapatalk because PM sucks on mobile. Tapatalk sucks slightly less.

What brain thought is needed? Just pointing out that there are other points on Wikipedia, such as the points where the center of a supported beam is at at minimal +- deflection from the supported points.

[Insert human brain thought 😉]



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