Need Help! Need advice on a design for a 30in capacity pitch lapping machine


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Thread: Need advice on a design for a 30in capacity pitch lapping machine

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    Member noctcaelador's Avatar
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    Default Need advice on a design for a 30in capacity pitch lapping machine

    Hello,

    This is my first post here. I'm designing my first machine and I could really use some advice. It's an unusual type of machine and not one that is usually computer-controlled. It's a telescope mirror grinding and polishing machine for the amateur astronomy market. This website explains how a few designs of traditional machine work: Document sans nom

    There are other types of lapping machines, and the main difference here is that the tool is on top, and the part you're lapping (the mirror) is on the bottom. The mirror spins and the lapping tool is pulled across the surface, slowly polishing away at the glass. The desired surface is not a flat one either, so the tolerances aren't in reference to a plane, they're in reference to a curve, more specifically a paraboloid. Those are the main differences.

    Anyways, I basically want to do a sanity check. Is this design as problematic as I think it might be? You should be able to see in the attached images that there is a linear rail and carriage that moves back and forth over the mirror. It pulls with it the spindle (the vertical post), and the post would sit down in a notch in the top of the tool (not shown in the pictures). The tool is a disk that sits on the mirror, with pitch on the bottom, and a polishing compound is put on the mirror. As the mirror spins under the tool, the friction between the tool and the mirror is shown by the blue arrow in the second photo. As you can see, the force is tangent to the rotation. That's the issue. Becuase the force is going to cause a lot of twisting action on the linear rail. The force could be up around 200lbs at the bottom of the spindle/post. But I want to design it to withstand more.

    My main question is this: is it possible to build a linear rail to withstand this?

    I'm designing this machine around PBC Linear's rails (https://www.pbclinear.com/Products/A...Slide-Assembly).

    It seems like if I went with their Roller Pillow Blocks it could work (https://www.pbclinear.com/Products/R...r-Pillow-Block).

    I know this is probably a little vague and rambling, I'm just looking for help.

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    Member mactec54's Avatar
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    Default Re: Need advice on a design for a 30in capacity pitch lapping machine

    Quote Originally Posted by noctcaelador View Post
    Hello,

    This is my first post here. I'm designing my first machine and I could really use some advice. It's an unusual type of machine and not one that is usually computer-controlled. It's a telescope mirror grinding and polishing machine for the amateur astronomy market. This website explains how a few designs of traditional machine work: Document sans nom

    There are other types of lapping machines, and the main difference here is that the tool is on top, and the part you're lapping (the mirror) is on the bottom. The mirror spins and the lapping tool is pulled across the surface, slowly polishing away at the glass. The desired surface is not a flat one either, so the tolerances aren't in reference to a plane, they're in reference to a curve, more specifically a paraboloid. Those are the main differences.

    Anyways, I basically want to do a sanity check. Is this design as problematic as I think it might be? You should be able to see in the attached images that there is a linear rail and carriage that moves back and forth over the mirror. It pulls with it the spindle (the vertical post), and the post would sit down in a notch in the top of the tool (not shown in the pictures). The tool is a disk that sits on the mirror, with pitch on the bottom, and a polishing compound is put on the mirror. As the mirror spins under the tool, the friction between the tool and the mirror is shown by the blue arrow in the second photo. As you can see, the force is tangent to the rotation. That's the issue. Becuase the force is going to cause a lot of twisting action on the linear rail. The force could be up around 200lbs at the bottom of the spindle/post. But I want to design it to withstand more.

    My main question is this: is it possible to build a linear rail to withstand this?

    I'm designing this machine around PBC Linear's rails (https://www.pbclinear.com/Products/A...Slide-Assembly).

    It seems like if I went with their Roller Pillow Blocks it could work (https://www.pbclinear.com/Products/R...r-Pillow-Block).

    I know this is probably a little vague and rambling, I'm just looking for help.
    That is a ridiculous price for these linear packages, these also normally have clearance in the linear bearings, which I don't think you would want to have for your machine build.

    Mactec54


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    Default Re: Need advice on a design for a 30in capacity pitch lapping machine

    Hi,
    the reason mirrors are hand lapped is that a machine cannot come within a bulls roar of the accuracy required. A good machine might have an accuracy of 1um, but
    for 1/4 wavelength diffraction limited mirror you'll need 140nm at the focus or 70nm at the glass, and that is many MANY times the accuracy of even the best machines.
    To be commercially successful you want to do better than that, at at least 1/10th wavelength on the glass or 55nm. If you want to do mirrors for holography the
    accuracy goes up by twenty fold, say 2-3nm!

    Hand lapping works as well as it does because the strokes are quasi-random, and its the statistical averaging process that allows such fine figures.

    The machines that replicate the hand lapping procedure have at least two axes that allow the machine to 'jiggle' the lap in a random fashion. Otherwise the mirror will take on
    a figure that is statistically correlated to the rotating table underneath and the linear upper axis.

    I've done a Dall-Kirkham 6 inch and a Newtonian 8 inch....and I swore never again! Man did it take some doing! I'd love to do a Ritchey-Chretien but the prospect of figuring
    a hyperbolic mirror by hand just puts me off.

    Craig



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    Default Re: Need advice on a design for a 30in capacity pitch lapping machine

    Thanks! I didn't realize they were so expensive in comparison. By "clearance" do you mean that there's space between the linear bearing and the shaft? Cause that shouldn't matter too much, but I'll definitely look elsewhere for linear rails



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    Default Re: Need advice on a design for a 30in capacity pitch lapping machine

    I've refigured over 20 mirrors by hand and made 5 mirrors from scratch by hand and machine. They ranged in size from 6in. to 24in. So I understand the concept of mirror making. The position of the tool doesn't need to be known with the same precision as with the mirror's surface, because you're not removing the desired amount of glass in a single pass of the tool, there are thousands of passes. And I will be doing regular optical tests, so I will know if the machine is not doing what I want it to. So I would have many opportunities to adjust. But I agree that randomness is a big concern. The motion back and forth was going to be defined by the sum of two sine waves, which would give it Zeiss-like motion, but only in two dimensions. The more I think about it, the more I see reason to add a third, vertical axis to that carriage that would, as you say, jiggle the tool around. Basically, give it an offset spindle so that the tool makes small circles. So I agree, and I'm designing it with all that in mind. There are also people who make excellent optics with Elgin-Waineo type machines, they just have to change the settings regularly.

    But my question is really more about designing linear rails to take the necessary loads.



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    Default Re: Need advice on a design for a 30in capacity pitch lapping machine

    Hi,
    maybe have a look at some of the cheaper CNC router tables.

    They traditionally have two orthogonal axes X and Y and a vertical axis Z. If you were to replace the typical fixed planar bed with a rotating platform then you could use
    trajectory planning to drive the X and Y axes to replicate ALL or ANY of the machine movements you linked to.

    Craig



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    Default Re: Need advice on a design for a 30in capacity pitch lapping machine

    Hi
    the problem I encountered, mind you I was learning, and from a book at that, was that I found myself working in a particular manner that meant that I was unintentionally
    introducing a figure into the mirror whereas the true statistical norm would have been spherical.

    Just as a matter of interest I see that there is a variation of one of the older machines inspired by Ritchey....do you suppose that is how he achieved a hyperbolic figure?

    Craig



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    Default Re: Need advice on a design for a 30in capacity pitch lapping machine

    Hi,
    I note that you are looking to use round rails and cylindrical bearings?

    They certainly work but are not really stiff enough for CNC use. All the top CNC machines use square rails with four re-recirculating ball circuits in each car. They are vastly VASTLY superior
    to round rails when it comes to restraining loads.

    Craig



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    Default Re: Need advice on a design for a 30in capacity pitch lapping machine

    Craig,

    Reading back over my comment, I'm sorry if I was rude. I realize I come across as a newbie, and I am when it comes to CNC, so I understand the impulse to educate, and thank you for that.

    Ah, I didn't know that about round rails vs square, that's really good to know! I've also since had the idea to add a pivot/bearing on the carriage that is parallel with the rail, and the spindle/post would be attached to the pivot instead of directly to the carriage. That would alleviate the carriage of any twisting force, but still allow the spindle/post to pivot up and down with the curve of the mirror. So now I only need a linear rail that takes a few hundred pounds (I'd like to build it to withstand 600) of force going directly into the carriage, with no twisting force. I'm guessing most profile rails will handle that ok?

    I'm mostly not considering a second linear axis (like you described) because it adds unnecessary complexity and cost. I'd like to only pay for the motion I need, I guess. And it will be random enough if I have another rotary axis that makes the tool go in small circles.

    When you were making your mirror, and it was moving away from a sphere, were you using a full-sized tool? And was this during the grinding or polishing phase? It's fairly easy to deviate from a sphere, it actually takes a bit of effort to keep it a sphere as you work. If you use a full-size tool and hang it over the edge a little, and make W strokes, depending on where the center of that W is it will cause the mirror either to flatten or to deepen. So it's all about finding the balancing point where it stays spherical.

    In the same vein, it's not too difficult to make a hyperbolic surface, it takes more work than a parabola because it's farther from being a sphere, but it's possible. When I was making mirrors I liked to go past parabolic to make it slightly hyperbolic, and then work it back towards the perfect parabola using full-size W strokes with a sub-diameter lap. The sole reason for doing that was to end the figuring process with a stroke that smoothes things over, which W strokes are good at.

    Do you think that something like this (https://tinyurl.com/9svtcce9) would take 600 lbs going directly into the carriage, perpendicular to the direction of travel? I can't tell what the base plate/housing is made of. Would I have to worry if it was aluminum for example?



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    Default Re: Need advice on a design for a 30in capacity pitch lapping machine

    Hi,
    the link you posted is to a company that I have bought products from on quite a number of occasions over the years. Buying either 'new old stock' or even second hand
    will allow you to get seriously good components at a fraction of what you pay new. Highly recommended.

    To be honest I think the idea of having just one linear axis and one rotary axis will not result in a sufficiently diverse motion that would preclude impressing a poor figure into
    your mirror. Whether a second axis is required or not, the problem you describe is the torque induced by the spindle which is of course attached to the lap via some sort of swivel
    must be still be considered.

    Simple mechanics dictates that the further the rails are apart the less force on each of the four cars is required to offset the torque of the load. If you want to suitably restrain
    a load of 600lb applied at the spindle say 6 inches from the plane of the rails then I would suggest you make the distance between the rails somewhere in the region
    of 6 inches to maybe as much as 10 inches. The further apart the rails the less significant is any local flexure of the cars or the rails.

    Even if you can secure good rails and cars that torque must be borne by the gantry. What's the point of spending big dollars on rails/cars only to have the gantry
    twist?. I'd suggest a length of 150mm x 150mm x 10mm RHS as the gantry. Such sections are used widely in construction and a 1m piece could probably had for a box of beer!
    Bang for your buck, or rather rigidity for a given sum, still favors steel by a large margin.

    My new build mill has 20mm HSR (heavy duty rails/cars) made by THK, a leading manufacturer of linear motion products. They are mounted 225mm apart on cast iron
    beds. I would expect them to restrain a 2.5kN force applied 150mm above the plane of the rails, for a torque of 375Nm, with well less than 5um deflection. This would be a
    fairly normal cutting load for my machine.

    I think therefore that if you mounted 20mm square, or even 25mm square rails on a piece of 150 x 150 x10 RHS you would have a gantry that withstand the forces you
    intend with only modest flexure.

    Craig



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    Default Re: Need advice on a design for a 30in capacity pitch lapping machine

    Hi,

    When you were making your mirror, and it was moving away from a sphere, were you using a full-sized tool? And was this during the grinding or polishing phase? It's fairly easy to deviate from a sphere, it actually takes a bit of effort to keep it a sphere as you work. If you use a full-size tool and hang it over the edge a little, and make W strokes, depending on where the center of that W is it will cause the mirror either to flatten or to deepen. So it's all about finding the balancing point where it stays spherical.
    The first mirror I made, or really my uncle made while I pissed him off!!!, a 6inch Dall-Kirkham was made with two 6 inch glass blanks. They were hand ground to approximately
    spherical and the convex one formed the basis of the lap...so a full sized tool. This was when I was 11 years old...so a long time ago now.

    The second mirror I made was in my first year at University. The grinding and polishing occurred in the same manner, ie a full sized tool. The advantage was I was able
    to use the optical lab facilities of the Physics department. They had a huge granite slab, something like 2m by 4m and 300mm thick. There was a nice Zeeman stabilized
    helium laser for illumination and some nice optical parts to do the (repeated) Foucault tests.

    I found that it was easy to overshoot or undershoot the required curvature largely because of my lapping technique. It meant many trips to the lab to measure how I cocked it up
    this time! I got it to diffraction limited eventually but it took many more hours than it should have.

    This mirror was f4, but in truth who cares if the focal length was 32 inch or whether it was 32.5 inch, so long as it focused properly. The part that put me off doing
    a Ritchey-Chretien was that if the design called for 32 inch focal length you had to be spot on otherwise all the anastigmatic advantages disappear, so a 32.5 inch focal
    length would be a fail. Holy crap.

    Craig



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    Default Re: Need advice on a design for a 30in capacity pitch lapping machine

    One thing that you Really want to be able to do for a Mirror Figuring machine
    is to be able to Stroke off Center...Currently you are Stroking Center over Center.

    You want to be able to Offset the Stroke in Varying distances away from the Center
    of Optical Axis to control your surface...

    I have made several machines that do this manually...and have thought about doing
    a CNC version many times while watching the machine work away and Manually
    Changing the Offset and Stroke length while Figuring.

    Best Regards,

    Preston



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    Default Re: Need advice on a design for a 30in capacity pitch lapping machine

    Machines work just fine. You don't have to do it by hand AT ALL. The final result needs to be ACCURATE. The randomization makes it possible to get accurate results by hand by averaging the flaws down until they are within range over TIME. A machine can get the same result faster. The Elgin design is the most common because it is the simplest and gets perfect results if setup right.



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Need advice on a design for a 30in capacity pitch lapping machine

Need advice on a design for a 30in capacity pitch lapping machine