Repeat Reading Gauge and Checking Surface Plate Calibration 

THANKYOU for getting down in the weeds. I'm one of these people that would want a full brake down of what ever topic is being covered. I know you barely scratched the surface of the topic but you explained the underlying theory really well, and I appreciate that. I did not know what an arc second was before this. I heard the term and seen measurements that used it, but never knew what it really was.

I'll be using that for the first time on Monday. You're a great mentor. Thanks for the video.

I have no intention of ever using this information but enjoy learning about things i never thought of before

The explanation of arc seconds was spectacularly useful. I got a really good qualitative sense of how small the increments are you are dealing with. Thanks for the trip to the weeds! Now I understand why guys doing this work step away from the plate to measure and wait for the heat from their hands to dissipate in the tools. Thanks for this exploration of some basic metrology.

Just scratching the surface. Lol. Awsome video. Thankyou.

Hello Keith, Great video... I enjoyed the time in the weeds... I definitely learnt a lot... Thank you. Take care. Paul,,

Years ago i ground gears for helicopter transmissions , inspector checked our plate every monday morning . Aprox 16 by 18 inches in size , was unusual for plate to pass three mondays in a row. He used a repeat reading gauge of a slightly different design , same principle . Our use was very localized , so wear was fast .

Fascinating and impressive the level of degree of accuracy that is obtainable.

great tool to have in your shop

At a mill where I did some work, there was a table made from cast iron leftover from ww2 which had been hand scraped back then when they made artillery shells during the war. When I got to see this 8’x10’ table it had been totally ruined by being used as a welding table. I nearly cried!

In rifle shooting moa or min of angle is used which is roughly is 1 in. Actual is (1.047) At a 100 yards. This translates to 10 in. at a thousand yards. Quality sights either scope or irons are set with up to 1/8 min adjustments which will move the point of impact 1.25 in at a thousand yards.

It is absolutely stunning how easily you have explained a so complicated concept. Really enjoyed it. Keep going Keith!!!

Excellent video. Very informative, as so many of your videos are. Always look forward to your Monday and Friday postings.

Keith, this is a FANTASTIC video. Thank you for taking the time to do it. Please continue to make videos like this when you have the work to justify it. It is videos like this that make us all smarter. Thank you for taking the time to teach!

I'm less than 100 miles north of Vermont Photonics and never knew they existed. They are located in a very fitting place, an area known as "Precision Valley", which used to be home to many machine tool manufacturers...GearShaper and Jones and Lamson among them.

Keith, you are an excellent teacher. Thank you.

Boy Keith, You've got a great site! I find everything you present interesting. I'm a retired millwright/maintenance supervisor and you've explained a lot of questions that I've had in the past! Thanks!

This is a fascinating topic, thanks for spending some time on it

Best visualization of what an arc second looks like. Awesome!

So few people have any appreciation for how important a surface plate is, it is the starting point for all precise measurements. I used to love watching those guys come in and lap our surface plates, they used to sling those cast iron laps back and forth across our large tables and I thought for sure one would go right off the edge, but they would redirect it at the last second and send it off in another direction spinning and swirling around at a good clip, it was poetry in motion.

I have to say by the expression on your face and smile from ear to ear you most certainly have a real nice example of a repeat-o-meter. Also, I have to thank you Sir for all your work and dedication to the machining arts. God bless and peace too. V

Wow, that's a really good gauge you've got there! Very sensitive!

Where I used to work before retirement, we had an environmentally controlled surface plate room. All our surface measuring tools were calibrated on this MASSIVE granite surface plate. They also had an arm system that would check points on parts and give readouts to .0001". they had an intercom at the door and electronic locks to prevent you from screwing up a measurement. (Yes this was NASA)

I am nerd and enjoyed this video. Thanks Keith!

Great info, do not apologize.

We used lasers in our metrology lab where I worked at an aircraft helicopter company as a Metrologist. Our air temp and humidity was critical in our lab as well. I think we were at 20C, 68°F+/- 1° and 50% +/- 1% humidity, if I remember correctly. So we always worked at a nice cool temperature. The kicker was the tools were calibrated correctly to our standards then when the workers used the tools out on the floor where it was very hot some days, they had to take the temperature into account defending what they were measuring. The key is how accurate do you really need something. If you measure it with a micrometer mark it with a piece of chalk and cut it with a hatchet? Great job on the school lesson too. It is something many young students should know regarding arch seconds and measuring.

Really fascinating video! Getting down in the weeds, as you put it, was very educational Keith. Thank you

Hi Keith - thanks for the video - a really good explanation of arc seconds.

Thanks Professor Rucker!

Don't apologise. That was fascinating! Thank you. Can't wait to see the lapping process

Thanks for this teaching moment and had no idea that stuff can be measured that accurately!!

Love it! As a cal tech, I'd say you knocked it out of the park.

Thank you sir for the excellent presentation and interesting information...

Excellent video...great explanation re the geometry and math...don't worry about being "in the weeds" many of us enjoy the in-depth content...I was astounded when I looked at the label on my surface plate and found calibration in millionths of an inch and wondering how that was possible...you just explained it in the the clearest terms....thanks

I just calculated how much meter on the surface of the earth, a arc second is. Its slightly more than 3 m !!😮 Showing how small a arc second is. Mindblowing small. Grtz from the netherlands Johny geerts

Challenge is a great boost to learning. Making something as near perfect as possible is a challenge as we have to learn how to do it. You just showed us what it takes to make that imperfect piece of stone nearer perfect than when it came from the quarry! Maybe you could make a series of videos that take us from crib to maturity of one of those stones? Thanks, Greg.

This is why toolrooms are temperature controlled and why I got shouted at as an apprentice for leaving the door open.

Most of the time arc-seconds and small parts thereof are used is in astronomy because of the vast distances of space between stars, planets, moons, and asteroids. When you started talking about the repeat-o-meter, I thought of the videos of Tom Lipton about building one. I am glad you got the one he helped design.

Great job Keith

Good explanation on the whiteboard....! Puts it all into perspective.....

When Keith started talking about the repeat-o-meter, my first though was "Hope he contacted Tom Lipton about this, he made one!" (forgot it was SMW who made it). Well, I wasn't disapoint! :D

Really interesting discussion on Arc Seconds. Thanks Keith!!

Keith, I spent about 5 years of my career working in an electrical standards lab and got involved in measuring standard resistors to 1/10th of a part per million. I also have 2 standard resistors sitting above my laptop on a shelf. I understand precision measurements so I was not at all bored with your surface plate discussion.

Nerd! Not quite there yet but on the journey to being one. Thanks for sharing.

I loved your explanation, Keith. Interestingly, the accuracy you describe started ith 1800's.

Getting in the weeds from time to time is good. I guess I'm a nerd LOL!

Keith, thanks for another great video. Trivial item: Fully Collimated light is an impossible ideal much like a perfectly flat surface plate ( even a single bump smaller than atom would disqualify it). Gas tube Laser light sources can be very well collimated, but still far from perfect. NASA uses a highly collimated Laser to bounce off the reflector left on the Moon by Apollo 11, their beam leaves the Earth at about 3 meters in diameter and is about 2 kilometers in diameter when it reaches the moon, if it were actually Collimated it would not increase in size with distance. Your lenses and light source yield a somewhat collimated light source, quite adequate for a short distance, if it were fully collimated the eyepiece image would be razor sharp and about as bright as the light source. It would also be usable when the mirror was miles away. P.S. I had to enable the comment sections "Enhanced" Spell checker to stop it from flagging Collimate and it's derivatives as spelling errors.

Thanks for sharing!

A handy reminder is that 1 arc second is an angle of 1/206264.8 rad. That means an arc second is 1 mm seen from a distance of 206 m , or, in imperial units: 1/32 inch seen from a distance of 179 yards or 1/10,000 inch seen from 20 inches away. Also, using the mirror doubles the reading, effectively doubling the sensitivity.

Thanks for a very informaative video. Excellent explaination of the columator and the repeatometer.

14:23 Professor Kitteh is checking yer math. :)

thank you Keith

I recently learned that second (as a time unit) is actually short for 'minuta secunda'. So divide something in (60) small parts you get minutes (as in miniatures). Than do that a 2nd time you get seconds. (and yes I watched this video till the end :) )

Gday Kieth, I watched right to the end and throughly enjoyed every minute, very interesting, The level of accuracy is absolutely mind blowing, I’m afraid that all the mathematics is way to complicated for me to process and I really wish I had of taken maths in school much more seriously, live and learn mate, thanks for explaining how all this works, cheers. Matty

Keith you can be as in the weeds as you want. I learnt a lot from that, some of reminded me that I was taught it some 50 years ago at school 😁 now I know why they kept saying I should've paid more attention. Thanks

Birds of a feather for sure Keith! Keep up the great content, I always love watching your videos!

Interesting session. Enjoyed the flow of information. Thanks.

Excellent introductory presentation.

Id follow you into the weeds anytime Keith, great video as always. your style, your voice, I bet you are a favourite teacher to people. Love yr work, Im not even a machinist lol. I love to learn about it tho. Maybe one day Ill get a wee lathe or mill.

Like to get out in the weeds. As an engineer, I have the general concepts but seeing it explained more in application and not theory is pretty neat. Thanks Keith!

THANK YOU...for sharing. Very interesting.

Thanks for the explanation 👍👍👍

1 minute of angle when projected on the surface of the earth from the center of the earth is one nautical mile. (1852 meters, 6076 feet, or 1.151 statute mile.) wikipedia

14:27 Maryann sticks her head up!

For some stupid reason, I find this stuff interesting. I want to to do a three plate lapping with some cast iron like Oxtool did. But always remember, "God curved the space-time continuum for a reason." A straight line is nothing but a circle that has an infinite radius.

Thanks Keith, not only interesting but informative as well. I knew I watched this for a reason 😉

Flatness is a fleeting goal. I ran lapping machines for a number of years and used a 6” diameter optical flat with a collimated light source in order to keep the tables flat ( which was a constantly moving target) the best we could hope for while lapping hydrostatic pump and motor valve plates on a commercial basis was (as specified to me at the time ) about one lightwave. If memory serves that equals about one millionth of an inch as measured by the optical flat that we had at the time. That was over forty years ago so my memory is a little suspect.

Well done and well said!

arc seconds are also used by astronomers to measure the spacing between two stars that are either orbiting around each other or are visually very close, and for measuring the apparent diameter of planets at different parts of their orbitsfor example the moon as seen from earth is approximately 30 arc minutes apparent size, mars varies between 3.5 arc seconds (at its furthest point in its orbit to 25 arc seconds at its closest point, currently mars is about 179 million kilometers away and appears to be 7.8 arc seconds

I have a Mahr Comparamess gauge, measures +/- 0.005" at full scale, 0.0001" divisions. I got mine 'cheap' (relatively) at local surplus store that used to deal with NASA although contact tip was missing. I don't watch anything CNC so missed the build, going to look up links.. Now, all I need is the rest of the body and a surface plate to measure.

I watched the entire video and found it fascinating. I know this wouldn't be practical but I kept thinking about the slates on my pool table. I've got the table as level as possible, but I know there has got to be low spots in the slates. When a ball rolls quickly across a certain spot, it will roll straight. If it rolls slowly to a stop near a certain spot it rolls off. I know it wouldn't be practical to have the 4 1/2 by 9 ft. pool table slates lapped but it would be interesting to see how much better the table would play if it could be done.

Watched them lap in our granite tables at SSRL/SLAC to AA quite an art. The Metrology Department came in and checked the finish using the same tools you showed but then went to a laser measuring device and electronic level. When I asked why they were verifying both the laser and electronic level calibration.

GREAT LEARNING EXPERIENCE, GREAT VIDEO, [ NOW LET'S GO TO WORK... ]

Great video. Ill never need to do work to this level of accuracy however it was very interesting all the same. Thanks for sharing

Good explanation arc seconds.

Interesting!

Nerds make the world go 'round! Thanks for the interesting video!

The thickness of the plate is part of the grading. The plate will warp under use so the higher the grade the thicker the plate needs to be. Many people seem to gloss over this fact. If a plate fits the flatness to be a grade AA but only the thickness for a grade B, its a grade B.

Keith, great math class. Thank you!

Seems like I saw Robin make a repeat-o-meter?? Thanks for the explanations.

Looks like the software from Vermont Photonics was written in LabVIEW, a test and measurement software development environment from National Instruments in Austin Texas. I developed in LabVIEW for many years.

Use your cell phone camera to look down the eye piece! That will usually work. (Need to center the camera right exactly in the optical axis of the eyepiece)

So the secret sauce is in the 5" distance. That's a serious "multiplier". I suspect the reason of the relative high cost of these instruments is in the calibration and certification of each unit. Seems like the calibration process must be interesting.

As another aspiring precision nerd, I would have loved to have been at the January class.... Maybe next year!

Enjoyed thinking about the math thanks

This video just reminded me why I became a truck driver. Whew.

very interesting....you're mapping out your surface plate using the Cartesian coordinate system (using the X-Y axis in rectangular notation) so you can measure the relative flatness at different locations in the Z axis using polar notation. BTW: gunsmiths and precision marksmen also use the polar system of notation, although with much less precision. A precision rifle should shoot better than 1 minute of angle, or 1 inch at 100 yards. 3/4 or half a minute would be better.

Some plate lapping in n the future? If you need a plate for a lap in could build you a pattern.

Great explanation!👍🏻

Another enjoyable video about both the reasoning and practical implementation of machining concepts. I have to say though, it was a bit off seeing you label the degrees on a circle like that. Generally in math they start 0° on the horizontal line to the right, and it goes around counter-clockwise.

32:44 I would avoid using the term "scratch the surface" near a surface plate...

Completely over my head but still very interesting.

Great stuff. Thanks!

180° 45' 32" close to Mali in the southwest Saharan Dessert

This reminds me of that old saw about the difference between a physicist and a philosopher. A Philosopher studies less and less about more and more until he knows noting about everything. The Physicist studies more and more about less and less until he knows everything about nothing. Glad I live in a rough measurement reality/ At least for now. LOL

That was awesome, Thank you

Thank you

I remember when we had our surface plates resurfaced. They lapped them in with diamond dust. They used an instrument similar to your gauge- although that was about 30 years ago. Not sure what they use today.

Good morning Keith

Keith I will send a set of dimension drawings and photos you can post on vintagemachinery.org of the repeat meter I designed and built to calibrate the surface plate I made from a 24x36x5" granite slab form the local cemetery monument company.