Slide rule restoration - Page 1
Forum section: electronics / general

author: Analog Kid
author_rank: Super Contributor
timestamp: May 24, 2025, 11:56:06 pm
content: I finally got a round tuit, as they say, and restored my old slide rule. I don't know exactly how long I've had this (couple decades at least) nor where I got it.The body of the rule was fine, but the top and bottom pieces of the cursor were, as they say, perished (see photo of remnants below). Not sure exactly what the material was, some kind of "ivoroid" maybe, but it was literally crumbling to dust. There were signs that the whole thing had gotten wet and corroded at one point, as the screws for the metal frames went into threaded sleeves that were well rusted, but surprisingly the frames themselves were in perfect shape, no corrosion whatever. (The body of the rule is celluloid over mahogany, in near-perfect condition.)I thought I could use the original screws but some of them were also crumbling away. Too bad, as they fit the frame holes perfectly and were slotted rather than Phillips.)Anyhow, I ended up cutting a couple pieces of styrene stock for the replacement pieces, leftovers from my previous lifetime as a model-railroad modeler (love styrene! such a nice material to work with). I had to laminate a couple pieces to match thickness of the rule body. In the upper piece I fashioned a small spring out of a clock spring, drilled in the center and held to the styrene with a tiny nail, filed more or less flush with the spring, to exert tension on the cursor. I built a small jig, basically a chunk of wood with 4 pushpins to squeeze both styrene pieces against the rule so I could place the metal frames and mark their holes for drilling.One thing that made this whole project possible was the really nice assortment of teeny-tiny screws I got from Temu (before I severed my connection to them); I found just the right size screws, flat-head Phillips machine screws. I found they fit nice and snugly in a #61 drilled hole. I was going to put a drop of super glue on them but found that wasn't necessary.Drilled the holes, cut the ends and rounded them, sanded them smooooth (down to 1500 grit), screwed on the metal frames, and Bob's my uncle. (I had to fit a couple of thick paper shims under one of the frames to make it clear the face of the rule, and now it slides very smoothly.)Now I just have to relearn how to use the damned thing. Very nice old piece of equipment to have around the shop. (It's a Keuffel & Esser 4080-3 log-log model.)

author: David Hess
author_rank: Super Contributor
timestamp: May 25, 2025, 07:21:32 am
content: Quote from: Analog Kid on May 24, 2025, 11:56:06 pmCelluloid plastic (nitrocellulose) replaced ivory in the late 1800s, so I suspect that is what the cursor frame was made of. I think that would explain how the material degraded.

author: Analog Kid
author_rank: Super Contributor
timestamp: May 25, 2025, 07:49:39 am
content: Quote from: David Hess on May 25, 2025, 07:21:32 amWell, the body is definitely celluloid over wood (I looked it up online), so why didn't that deteriorate as well?I think the cursor frame was a different material.According tothis pagethis rule was made from 1939 to 1947.

author: tggzzz
author_rank: Super Contributor
timestamp: May 25, 2025, 01:59:58 pm
content: Quote from: David Hess on May 25, 2025, 07:21:32 amCelluloid plastic wasn't invented that early; perhaps you mean the late 19th century.

author: Gregg
author_rank: Super Contributor
timestamp: May 25, 2025, 04:22:39 pm
content: I still have a Keuffel & Esser Deci-Lon slide rule I purchased new in the 1960s and it came with a very nice hard bound manual. (Those days of real manuals are long gone.) I see some of these manuals are available on Amazon if you are interested.My slide rule is all plastic but the cursor looks identical to yours. If I were to make new cursor slides, I would probably use Delrin.

author: David Hess
author_rank: Super Contributor
timestamp: May 25, 2025, 04:44:29 pm
content: Quote from: tggzzz on May 25, 2025, 01:59:58 pmI thought it was invented during WW1, but all of the references I found referred to replacing ivory billiard balls before 1900.

author: jpanhalt
author_rank: Super Contributor
timestamp: May 25, 2025, 06:37:07 pm
content: My K&E looks similar and was purchased new in about 1962 (my second year at uni). Before that, I made do with an aluminum Pickett. My K&E had the model and patent information on the edge, rather than face. Model # is 4081-3 "copyright 1947."The rails and slide are a laminate of facing (a white plastic looking material), mahogany, facing material, mahogany and facing. The slide is mahogany with facing on each side. It has remained straight and usable to this day. The cursor looks fine. We used talcum powder as lubricant.Did you happen to store yours anywhere near cyanoacrylic glue (even if unopened) or an accelerator for it?

author: Analog Kid
author_rank: Super Contributor
timestamp: May 25, 2025, 06:44:55 pm
content: Quote from: jpanhalt on May 25, 2025, 06:37:07 pmI didn't, and I suspect that didn't happen to whoever owned it previously either.Looks to me like moisture was the culprit, as the remains of the spring were badly rusted, as well as the (steel?) screw inserts.Anyhow, styrene is as good a replacement material here as I could hope for. Holds the screws and slides nice and easily.

author: David Hess
author_rank: Super Contributor
timestamp: May 26, 2025, 06:13:32 pm
content: As I surmised, celluloid plastic (nitrocellulose):The most common problem is the "K&E Rotting Cursor Syndrome," known as "KERCS." (You know it's a problem if there's an acronym for it.) Many rules made prior to ~1950 had celluloid cursor rails that didn't hold up well over time and would eventually crumble to pieces. This begins with the outgassing of the nitric acid from the celluloid, which rusts the cursor spring and compromises the spring's anchor going through the rail. Eventually, the spring breaks away and the celluloid rails become brittle as the camphor plasticizer is also forced out of the celluloid.*https://www.allaboutastro.com/all-about-ke-rules.htmlI have many slide rules, but only one K&E mahogany slide rule and I think its cursor is acrylic plastic. Its serial number and features seem to indicate 1961 production so the most recent of this design.

author: jpanhalt
author_rank: Super Contributor
timestamp: May 26, 2025, 06:46:13 pm
content: My 4081-3 appears to have serial number 250135; although, it is not labeled as such. It was purchased in 1962, which appears to be the end of the run for that model.

author: Analog Kid
author_rank: Super Contributor
timestamp: May 26, 2025, 09:18:05 pm
content: Quote from: David Hess on May 26, 2025, 06:13:32 pmStill a bit confused here: isn't the entire rule covered with that same material (celluloid)? If so, why doesn't it deteriorate like the cursor rails? Is it because the coverings are thinner? or are they actually a different (but similar) material?

author: David Hess
author_rank: Super Contributor
timestamp: May 27, 2025, 02:59:20 am
content: Quote from: Analog Kid on May 26, 2025, 09:18:05 pmI think that is the case, however I suspect the celluloid plastic for the two parts is processed differently because one is molded and the other is used as a lamination. Also, thick parts will not outgas as easily allowing them to collect degradation products inside which further accelerate their degradation.

author: Analog Kid
author_rank: Super Contributor
timestamp: May 27, 2025, 08:50:41 pm
content: That sounds plausible.Another question: does anyone know how slide rules were made?Specifically I'm wondering how the scales were "etched". I put that in quotes because I'm not sure that's the right term for how those markings were made. (I know some rules were printed, by silkscreen or other methods, but I'm not interested in those.)Several possibilities come to mind:The scale laminations were somehow stamped or molded, which brings up the question: how were the dies or molds made?The scales were engine divided, but then how were the figures (numerals, etc.) made? (Not really sure just what "engine dividing" is anyhow.)The scales were cut by some other method.Would love to see the machines that made the scales.I've searched online with no success. Unfortunately there was no YouToob back when these things were made ...

author: David Hess
author_rank: Super Contributor
timestamp: May 27, 2025, 08:56:53 pm
content: Quote from: Analog Kid on May 27, 2025, 08:50:41 pmI suspect stamped, and the stamping die was made with photo etching.That leaves how the scales were colored.

author: TimFox
author_rank: Super Contributor
timestamp: May 27, 2025, 09:00:32 pm
content: Some slide-rule histories point out which (expensive) rules were "machine ruled".I believe that equipment was also used to calibrate liquid-in-glass thermometers and mechanic's rules with linear increments.Diffraction gratings were made by machine ruling long before CNC machining originated.I don't know how to generate logarithmic scales, but I believe the linear rulers could divide a measured length into equal increments.

author: Analog Kid
author_rank: Super Contributor
timestamp: May 27, 2025, 09:07:56 pm
content: Quote from: David Hess on May 27, 2025, 08:56:53 pmWell, that last part at least is easy: after stamping/etching/whatever, the laminations were inked/painted, probably at least most of the excess wiped/squeegeed off, then after drying the remainder was removed by lightly sanding.