Rotary Laser centre finder

If you look him up Chris, you will find that he has been an extremely successful serial entrepreneur, founding (and selling) a number of companies.

**LINK**

I first took him to be running a University Lab (I know that at some US institutions like MIT - money is not a problem) but I suspect that the videos were actually shot in his own workshop. Probably still cheaper than a fleet of luxury/high performance cars though - and much more fun!

Regards,

IanT

Hi IanT,

I'm still, if not even more, jealous but i don't begrudge him a single iota. Wish he was my next door neighbour so I could do the machinery equivalent of borrowing a cup of sugar, but I fear I could not even afford to live in the same town let alone road. If had his brains and money I would certainly have an equivalent shop and not buy sports cars either.

chriStephens

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John,

For what it's worth [which probably isn't much] ... that third link of yours properly describes the shape that I had in mind when I wrote: "What I was hoping was that the circle of light would become emphatically ovoid when the ball was displaced from centre."

Wolfram's maths, and particularly the resulting diagram, do a much better job than my words! ... Given that we are especially good at recognising certain shapes [*], I wonder if there might be a visual "microswitch effect" when the alignment is correct and pure circle appears.

MichaelG.

[*] the Gestalt Effect

Edit: added footnote hyperlink

Edited By Michael Gilligan on 28/01/2014 08:34:53

Hi All

Hi Michael Gilligan
I think were on the same page when we thought about the geometry involved.

On Gesault

May I first digress, when we had a wholesale business it specialised in making custom items for the building and design trade, mainly furniture Bedding and elaborate drapery all made to customer order we supplied to retail and designers. If you are wondering how Engineering fits in it was designing and building special machinery for the business. Training staff to develop an eye for balance and proportion was quite difficult, We were lucky if we got drawings and a specification for an order, most of the time it was a sketch that we had to interpret. Many an hour was spent pondering over a ragged page trying to interpret it. Some staff got the design intent straight away while others struggled to visualise it in the finished form. I put it down to lack of experience in life. We were for ever training.

As always there are exceptions, From time to time certain individuals joined us that were exceptional I remember one young woman who with no previous experience joined us as an apprentice. her perception of form and proportion was remarkable. She worked in the drapery department making period window coverings like the ones you see in the film Gone with the wind. There were not many like her. We were lucky to met a few some people are gifted.

The underlying theory of the so called Gestalt effect relates to the brain viewing objects or concepts holistically. I think it does. Reading this page we see words not individual letters. The result is however limited (with some exceptions) to the sum of the experiences a particular individual has had. That is why we had trouble with training some individuals and not others. If you have not been exposed to good design you will have more trouble recognising it. If you do not have a working knowledge of the physical world with a smattering of history and philosophy (My academic wife would say a lot of history and philosophy!) and enthusiasm you will not do so well.

There has been a lot of work done in this field. This search leads to research on Eye perception.
**LINK**

Getting back to the Rotary Laser centre finder, As soon as I watched Dan Gelbarts Video It got me thinking, but it was not because of the aesthetic form of the patterns produced it was (why) the various patterns were produced. Thinking back to school geometry, which I rather disliked! Made me think of the intersecting cones and other shapes they tried to drum in,,,, Hmmm well at least I remembered some of it. Throw in a bit of engineering experience and the Idea for cone on a slightly different cone came out. And most of all a bit of stand back and why is this so thinking. Experience joined the dots.

Regards
john

Edited By John McNamara on 28/01/2014 12:55:41

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Mike,

You say:

"Square on and basic polish will make the spot described in earlier posting ."

Forgive me for responding in my best Pantomime voice:

... "Oh no it won't" ...

As I referenced earlier; the light exiting a circular Fibre Optic light guide with a square-on polished end will be a diverging cone with an included angle of [typically] 68 degrees. ... This is also the acceptance angle at the input end [assuming, in both cases, that the other medium is Air].

The light guide in the two pictures below is a 1mm diameter multi-fibre bundle, in a 2mm plastic sheath; and the ferrule is just under 3mm diameter. The bundle is Epoxy-potted and finely polished [they were made by Schott; so the quality is pretty good].

The light input for these pictures is a Green LASER pointer.

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First; the exit angle:

Now, the spot size, at a distance of 10mm

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As you will see; this is not a 1mm spot.

... If you want the small spot size, you will need additional optics. ... It's all "do-able" but can quickly get complex and expensive when compared to a Pound's worth of LASER Pointer.

MichaelG.

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P.S. Don't abandon us !

There are devices called quadrant photodetectors which are now very cheap and common, you can find one in the optical block of a CD/DVD player. As the name suggests the detector is four adjacent square detectors now made in the surface of one IC.. Lets call them a,b,c & d going around the centre. If you electrically add (a+b) but subtract (c+d) then you detect misalignment along one axis, adding (b+c) but subtracting (a+d) gives misalignment at 90 degrees to the first axis.

Using a small rotating beam painting a circle around the quadrant detector gives a constant brightness spot, Misalignment varies the proportion of the rotation that a quadrant is illuminated so we get a modulated signal which can be AC coupled to reduce steady state drift. So we could have two meter pointers, when both were reading no signal the circle would be rotating around the centre of the detector.

The quadrants are around 2mm square in some blocks, it is trivial to display microns with this arrangement.

Before anyone explodes off a high chair, this is a very well known set up with non-rotating sources. The Optical disc player uses a variation to recover height information . The player has a focusing lens driven by a coil actuator. The light from the lens is then sent through a round rod to create spherical abberation, the rod is at 45 degrees to the quadrants.

When the lens is focused on the disc surface the detector gets a round spot hitting all four quadrants equally. When the track is off then the detectors use the axis data to servo the lens to track the pits on the disc.

The focus is performed by adding (a+c) but subtracting (b+d), when the lens is too high the output is positive and proportional to the error, lens too low the out put negative. This is because of the cylinder lens abberation, out of focus the spot becomes eliptical, the major axis twists 90 degrees each side of focus.

This stuff was originally developed by Philips/Sony for the 12" video disc but was then recycled for CD & DVD players. The method of using servo's and quadrant detection is needed because you need micron accuracy to track the pits as they go around very fast on a disc that can go up and down several mm and also have the same kind of eccentricity. An all for ten quid for the player and electronics!.

You can use the rotating source and the quadrant detector to align co-axial shafts. A fixed beam can be rotated into a parallel rotating beam by rotating an inclined glass slide in the beam, that will scan the beam but it will not rotate a shaped beam. No fibers, no time domain measurements

Billy.

< condensed quote >

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John,

I'm guessing that your remark was just a good-natured nod to Billy, but [on the off-chance that you might consider making these commercially]; I think you would discuss royalties with Dan Gelbart ... his patent is dated 2007.

MichaelG.

Edited By Michael Gilligan on 29/01/2014 08:33:06

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Fascinating stuff, Billy

Inspired by your description, I found this very interesting [and practical] article.

MichaelG.

Very wise words, Wallace

I generally prefer to use ready-prepared light guides [obtained cheaply, as-and-when I find a bargain] but, for anyone interested in experimenting with small diameter "data" fibres ... this video is worth a look.

Warning: It's a lot of faffing-about, and the consumables are expensive.

MichaelG.

I've obviously missed something about the cones and the inclined rotating laser.

I see how the rotating laser indicates when the cone is not directly below the centre of rotation, but I don't see how you place the cone directly over the required datum point of the work piece.

I thought Mr. Gelbart's method of adjusting the laser circle diameter to pretty much coincide with the circumference of an existing hole, reducing the circle to a point to indicate an edge pretty much did it all. As did the edge "V"s for indicating the spot for cross drilling a cylinder.

I'm sure somebody can tell me what I missed about the cone.

Gary

The circle (cone) is created by the fact that the laser is rotating, ergo the cone must be concentric with the centre of rotation which in our case is the axis of the cutter spindle.

The cone is not important in itself, its the described circle that matters. You don't actually see a cone (unless you are in a smoke filled room!)

Also, you don't align the circle to the work, you move the work until the point you want is at the centre of the circle. His method of altering the circle diameter by simply altering the Z height means its easy to position the tool centrally in say a rectangular plate by using it two diameters to set the X and Y separately.

Ian P

Gary,

I think this slight detour in the discussion originated when Michael Williams mentioned Toolmaker's Buttons. John McNamara picked-up on this idea [excuse the pun] and determined that centering on a button would be easier if the button was conical ... presumably this would only be the top section, or it would be difficult to set the distances between them. Alternatively a single conical button [or, as I wondered, an Ickey Ball] could be used for positioning a master datum point.

Hope that makes sense, and that Michael and/or John will correct me if I have misrepresented them.

MichaelG.

Edited By Michael Gilligan on 29/01/2014 14:32:11

Edited By Michael Gilligan on 29/01/2014 14:32:55

Hi Garry

The cone was discussed a fair bit on the second page of this thread.
**LINK**

My thinking was that a small cone shaped object may increase the accuracy of the devise when locating on marks on the surface of a work piece where the laser strikes at a constant depth.

The cone provides an amplified slope to the horizontal of the laser unless the laser line (Circle) is cantered in a similar way Mr Gelbart shows V when locating over a cylinder.

As soon as I can get my hands on a decent pointer I am going to make one and test the theory out.

This is the first centre finder design I have seen that overcomes the zeroing in problem that laser pointers cantered in the collet or chuck have, and it does not matter if the laser spot is asymmetric when it rotates. Two sources of error eliminated.

Regards
John

Edited By John McNamara on 29/01/2014 14:41:40

Thanks Ian, but as I said in my original post, I understand all that. What I don't see is the use of the solid cone that was shown in the CAD drawings used to illustrate how the laser circle deviated from the horizontal if the cone wasn't concentric with the spindle.. There was discussion on whether a ball would perform any better than a cone - for what purpose? Certainly a ball would self centre in a hole, but would offer no more accuracy than the described laser circle.

Like I said, I don't understand the purpose of a real, solid, cone.

Gary

Thank you to John for pointing us to the Dan Gelbart series. I have watched all 18 and they are excellent. It's a delight listening to someone who understands the science behind why stuff happens.

His part 15 of 18 demonstrates the rotating laser centre finder and I was intrigued, as others in this thread have been. Never mind the theory that has been discussed, I decided to make one and try it out. It uses the commercial laser centre finders that have been discussed (here and ME?) before and fits into a 1/2" collet. I made the angle adjustable but it worked at the first try which is around 10deg from the vertical (20deg cone).

The image worked at around 600 RPM and the polarising cap had to be removed to see it well. I obtained lovely tilting circles on machined steel for upright bar and holes. Dan Gelbart is exactly right (and who wouldn't believe his every word) when he claims 50 micron alignment precision. In my language that is two thou 0.002" and that is my estimate of what's achievable.

I have been using the static, vertical laser to find centres and edges and with the eight thou spot size would have to try hard to get +- 0.004. Now, this device is a magic dream. I have put three photos below to show the device. I have also made a quick video and posted it on this link in case anyone has not seen the Gelbart original film.

Video Link

Who else has just made one?

Norm.

Edited By Norman Lorton on 29/01/2014 15:16:40

Edited By Norman Lorton on 29/01/2014 15:37:54

Hi Garry Wooding

Maybe this will assist.

Regards
john

Hi Norman Lorton

Well it looks like you get the first prize.... The first Rotary laser pointer off the production line

Nice Job and a good video.

Can you machine up a button cone and test it please?
One with a small angle difference please. I notice you have made the angle of yours adjustable.

Great to see it in action.

Regards
john

"Never mind the theory that has been discussed, I decided to make one and try it out. It uses the commercial laser centre finders that have been discussed (here and ME?) before and fits into a 1/2" collet. I made the angle adjustable but it worked at the first try which is around 10deg from the vertical (20deg cone)."

Thank you for that Norman, the most useful paragraph since page two.

I have ordered a couple of cheap laser "pens" from t' bay so I will crack on when they arrive. Going to make a couple and give them to my friends too.

I would be interested to hear if there is a source of (reasonably priced) good quality lasers as the cheaper models can have a variable quality.

graham.

Graham,

Which pens did you order please I quite fancy a play with one of these.

Paul