Machining a Headlight Lens From Plexiglass

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Carrdo
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Machining a Headlight Lens From Plexiglass

Post by Carrdo » Mon Sep 18, 2017 1:40 pm

I jump from project to project all of the time and here is another (very small) one. Except, it took an hour to do the machining but weeks, months and years to do all of the needed preliminary work.

I have needed to make a number of locomotive headlight lenses for some of the locomotives here; a project which has been laying around for decades.

I chose to make them out of plexiglass and of course made the classic mistake of a beginner for if you use this material, one needs to buy cast plexiglass rod not the more common extruded plexiglass rod. If one tries to machine extruded rod, then there is a chance that the plexiglass will crack or "craze" with time. Of course, cast plexiglass rod is much more expensive and difficult to obtain.

Then, I had to make the ball turning tools for my lathe to machine the curvature on the lens. And since there are two lathes here I decided to make any and all of the ball turning tools fit both lathes. Ended up with two different sizes of ball turning tools (for both small and large radii) and a number of different adaptors - which is a project in itself but that is another story.

Then, I decided to make a number of grooved concentric circles (6) on the curved lens surface to make it look more like a fresnel lens. Just set your lathe's compound at 90 degrees to the lathe's crosslide, zero out both slides when the lathe grooving tool just grazes the innermost ring circle, feed the compound forward (0.010" in my case) to produce a groove, retract the compound then more the crosslide to the position of the next groove (0.060" in my case) then move the compound forward again until the grooving tool just grazes the next ring, set the compound micrometer dial to "0" again and repeat the above until all six grooves have been machined.

To finish polish the plexiglass material and remove any burrs, I used a dead smooth file, followed by some fine steel wool dipped in any oil, followed by a paper towel dipped in Brasso. The result, a crystal clear lens.

"Easy peasy "as Jack Bodenmann says but if you don't have any of the above, then you have a work program.
Attachments
1200 Parting Off the Headlight Lens.jpg
1202 The Completed Lens.jpg
1203 The Ball Turning Tool Used to Generate the Headlight Lens Curved Surface.jpg

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10KPete
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Re: Machining a Headlight Lens From Plexiglass

Post by 10KPete » Mon Sep 18, 2017 5:21 pm

Nice work! The radius tool is really great; it's the right size for the smaller stuff. I may 'borrow' the concept... :shock:

Pete
Just tryin'

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Carrdo
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Re: Machining a Headlight Lens From Plexiglass

Post by Carrdo » Mon Sep 18, 2017 6:43 pm

Hi Pete,

The ball turning tool itself is slightly modified from the plans originally supplied by Lee Hodgson of the 9 and 18 cylinder radial engine fame. I don't know whether they are still available or not as I got them from him about 40 years ago.

A couple of additional photos showing how the ball turning tool is centered and the tool in use. There is very little cutting force on the tool when machining plexiglass but I am always careful to take very light cuts when using it on any material.
Attachments
1204 Setting the Ball Turning Tool on Center.jpg
1205 Lens Forming.jpg

Glenn Brooks
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Re: Machining a Headlight Lens From Plexiglass

Post by Glenn Brooks » Mon Sep 18, 2017 10:44 pm

This is a very interesting thread, and great idea! A year or so ago, someone re-posted a similar thread from even longer ago, but used a different approach. I wish I could remember the URL, but unfortunately can't, and didn't bookmark.

Basically the earlier thread recalled how an old timer turned a die out of some round stock, then sandwiched colored plastic disks between the dies, then heated and gently pressed themdies and plastic disks for 30 minutes or so until the plastic formed itself into convex, stepped stepped shapes. Once cooled, wallah! Colored lens for RR lights.

Iam not knowledgeable about plastics, and certainly not a foundry type of person. But, sure sounded like a good idea. Might be a good compliment to this method.

Glenn
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Motive power : 1902 A.S.Campbell 4-4-0 American - 12 5/8" gauge, 1955 Ottaway 4-4-0 American 12" gauge

Ahaha, Retirement: the good life - drifting endlessly on a Sea of projects....

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BigDumbDinosaur
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Re: Machining a Headlight Lens From Plexiglass

Post by BigDumbDinosaur » Mon Sep 18, 2017 11:37 pm

Carrdo wrote:I chose to make them out of plexiglass and of course made the classic mistake of a beginner for if you use this material, one needs to buy cast plexiglass rod not the more common extruded plexiglass rod...Of course, cast plexiglass rod is much more expensive and difficult to obtain.
I needed cast acrylic rod (it should be noted that Plexiglas is a trademark for one brand of acrylic) and ordered it from here.
To finish polish the plexiglass material and remove any burrs, I used a dead smooth file, followed by some fine steel wool dipped in any oil, followed by a paper towel dipped in Brasso. The result, a crystal clear lens.
Automotive body rubbing compound works for polishing acrylic parts, requiring fewer steps.
————————————————————————————————
Science makes it known. Engineering makes it work.

rkcarguy
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Re: Machining a Headlight Lens From Plexiglass

Post by rkcarguy » Tue Sep 19, 2017 10:20 am

Nice work! I ended up cannibalizing one of those cheap black and yellow flashlights for the reflector and lens for my Baldwin S12 build. It's within 1/16" of being perfectly to scale and is the exact same style(chrome reflector behind a totally clear lens). Just have to make a bolted ring to go around its perimeter now.

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SteveM
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Re: Machining a Headlight Lens From Plexiglass

Post by SteveM » Tue Sep 19, 2017 7:24 pm

The 3M product for cleaning and polishing plastic headlights works well for clearing up plastic

Nice looking lens!

Steve

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Carrdo
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Re: Machining a Headlight Lens From Plexiglass

Post by Carrdo » Tue Sep 19, 2017 7:45 pm

Two final photos.
Attachments
1206 Grooving a Lens.jpg
1207 More Lenses for More Headlights.jpg

rkcarguy
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Re: Machining a Headlight Lens From Plexiglass

Post by rkcarguy » Wed Sep 20, 2017 11:16 am

The sparrow likes them!

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