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Scissors Lift Hand cranked wood model

I was searching the internet for interesting hand cranked wood models to build, and saw this simulation of a scissors lift driven by a worm gear.  You can watch it using this link.



Free Linkage.Exe simulation program for Linkages

I decided to try using this free simulation program to come up with the design characteristics of a hand cranked wood model.  I started with two links each 12 inches long, which should be about right size for a hand-cranked model.


One confusing thing in the Linkage.exe program is how to get a link with 2 end connections and a middle connection.  I finally figured out you must select the triangle linkage to start with. You then get 1 of the 3 links about center and use that.


Video of Simulation in Linkage.exe

I played around with the stroke of the linear actuator, and a 4 inch stroke gave 6.25 inches of lift on the platform.   I thought a 4 inch worm gear length would be ok to make, and the 6.25 inches of lift would give it some action.


I have a 3/4" wood gear cutting set and it gives 6 teeth per inch.  Four inches of worm gear stroke will take 4x6 = 24 turns of the lead screw.  this is too slow for a kids hand cranked model.  If I double the speed, it will only take 12 turns which should be better. If I use a 3 inch gear on the worm gear driven by a 6 inch gear, this should be ok.


I will start the lower position with the arms 15 degrees up from the horizontal. I think it will be too complicated to get the arms down to zero degrees with the clearances required.



Sketchup

After I got the concept roughed out in sketchup, I stroked the worm gear 4 inches to see how much platform lift I got........and a miracle happened......I got 6.25 inches of platform lift, the same result I got with the Linkage.exe program !!


Advancing the Design

Threading Wood

In the 1980s, I bought a threading kit.  The tap for the female thread worked fine..............but the die for the external thread did not work worth a darn!!


I tried using boiled linseed oil as a lubricant, but I still got excessively chipped threads on the OD.


I googled it yesterday, and it said to soak wood anywhere from a day to 7 days in a fluid to prepare it for external threading. Fluid choices included:


-raw linseed oil [do not use boiled linseed oil because it acts as a sealant.

-mineral oil

-mixture of raw linseed oil and mineral spirits


I went ahead and cut two 12" long pieces of birch and oak dowel, and soaked them in mineral oil.


Purchasing 3/4" threaded wood dowel

I googled it and found a company selling one foot long theaded pieces, so I bought 2 of them.........in case my own threading goes bad.


The 2 threaded dowels I bought, shown above, are made perfectly.  Unfortunately, my nut that I made is too tight to fit on them.............and I already sanded the OD of the thread using my Dremel with a 1/2" diameter drum sander !!  I don't think I can use these on my project :(


1st Wood Threading Experiment

I made 1 red oak and 1 birch shaft ..........and used the lathe to turn down 1 end to 1/2" diameter.  I soaked both pieces for 24 hours in mineral oil.

On the birch piece, I did not push in as hard as I could when I cut the first 1 inch of thread, and noticed some crumbling of the threads.   I then pushed as hard as I could while turning the die, and the rest of the threads came out 98% ok.


The red oak shaft did not go well.  The threads were not bad after I cut them, but the nut wiped them out.

For the nut, I used a piece of red oak. I drilled a 5/8" pilot hole, lubricated with mineral oil, and cut the thread.  It seemed to be ok as best as I could see.  The nut did not turn easily on the birch shaft, so I used the Dremel with a 1/2" diameter drum sander to sand down the threads a little.   The nut turned almost effortlessly on the birch after this.


Longer soak time

The birch shaft was almost perfect after 24 hours of soaking in mineral oil.  I should go ahead and soak another blank for 7 days to see what happens.   I don't think longer soaking time will  help the red oak.


On my scissors lift design, the nut is 3 inches long.  I might want to try a couple of options like thread for all 3 inches, maybe thread of 2 inches long with 3/4" hole 1/2" deep on both ends, and maybe just 3/4" of thread in center.


Red oak seemed to work ok on the nut I made yesterday.  I don't have any flat stock of birch.  I could try walnut.  I could drill the 5/8" pilot hole and soak the nuts in mineral oil also before tapping.


YouTube video on how to make wood threads

I found this good video and he uses maple wood with 7 days mineral oil soak time for both bolt and nut.


Maple

I ordered a couple of maple dowels from lee valley.  I think I will try maple on the block portion of the scissor lift model and soak it also.



Main Threaded Maple Block

I wanted to make this block first, so I could it start soaking in the mineral oil now.  I glued up 4 pieces of Menard's 3/4 thick maple to make it.


I had a 3.5" wide piece of maple, so I made the boards 3.5 vs 3.0 on my sketchup drawing........so I changed my Sketchup drawing to as-built condition.


I thought it would be very difficult to dye the block red after it had soaked in mineral oil for 7 days, so I went ahead and dyed it. I will oven dry it, so the dye is less likely to mix with the mineral oil.  I will do 2 coats of red dye with oven drying.


I thought about going ahead and putting on the polyurethane so no chance of red dye mixing with mineral oil, but then the mineral oil could not travel from the ends up the grains, or tubes in the maple.............so no poly at this time.


On the Youtube video of making the big threads, the guy cleaned the surface of the mineral oil soaked surface with acetone so glue would stick to it.  I will clean mine with spirits or turpentine before I put on the poly.


Blue Dye

About 7 years ago, 2013, when I bought my selection of water-based dyes, they did not offer a blue color.   Now, blue is offered, so I bought a bottle.......and may try it out on this project.


Updated Sketchup Design

making thick ends on scissor linkages

Process:


1. Scroll saw out linkage from 3/8" thick maple

2. Drill holes using Forstner bits

3. Scroll saw out 1.25" dia maple discs from 3/8" stock, do not drill

4. Glue on 1 maple disc to linkage, oven dry at 120F

5. Forstner bit drill 1 maple disc added

6. Repeat process until all discs are attached and drilled

7. Sand ends using 3/4" bore 60 grit sanding drum on drill press

8. sand whole linkage with 220 grit using drill press drum sander

Mineral Oil Soaking Pan

Model starting to take form

I made a temporary pine base board to hold everything while I assemble it.  When ready, I will place the assembly on the finished oak base, and mark out where to scroll saw the openings.


Mortise Machine

A couple years ago, I bought a used Delta mortising machine.  I have used it with a 1/4" bit.  I bought a new 3/8" bit, but never tried it to see if it fit.   I tried it out and it fit fine.


I designed the posts to be 3/8" by 3/8" so they would fit in a square mortised hole.


I printed out the pattern from Sketchup, and white Elmer's glued it onto the top platform, the went to the mortising machine.  I want the string or rope to be low enough to keep the barrels from sliding off, which it is.


Unexpected Problem at trial fit-up

Since the rough bore for the threaded block is 5/8", I got a 5/8" birch dowel and turned down the end to 1/2" so it would fit into my bearing.


First issue I noted was the upper platform was so heavy it was pushing our my RH support, even though I had the RH support clamped to a bottom horizontal dummy piece, as tight as I could clamp it !!!!!!


The upper platform (1/2 thick base with 2 horizontal pieces 3/4" thick) of white oak was much heavier than I expected !!!!!!!!!!!


I need to reduce its weight, or replace the platform with a lighter wood.


Also the top of the platform should be about 11 inches, and it 14.25 !!


After some investigation, I found I had placed the LH verticals at the wrong spot. I was able to move them to the right position.


On many of my larger hand-cranked models, I have used a temporary base with clamps to the components. This is the first model where some of the verticals have quite a bit of lateral force because of the 2 scissors linkage, that is why they are moving when I activate the linkage.  When the verticals are set into the 3/4" oak base, they should resist the lateral force fine.





Demonstrating How Hard to lift scissors with oak top platform

I made this video to show how difficult it is to move the scissors with the white oak top platform.


Plan B

I decided to make the top platform and 2 horizontal supports out of common pine, instead of white oak........to reduce weight.


I went ahead and decided to make the bottom white oak base, since it helps to keep the vertical supports take the lateral force.


Another issue found at the trial assembly, was that the top platform tips down towards the front.  I need to check the design and see if I need to  shift the 2 horizontal grooved bars below the platform.  I found I needed to move the 2 horizontal supports by 3/16" closer to the front of the model.



Force Calculation

Boy, had to blow out the cobwebs because I have not done a Free Body Force Diagram for 43 years at the U of Illinois !!


The force required to operate the scissors lift goes up dramatically the lower the starting angle, which is why I saw trouble with the heavy white oak platform.


Easy to operate scissors lift with no platform

I made this video to show how easy it operates with no upper platform.


Once I got the pine platform made (except for the posts), it worked much better than the heavy white oak platform.  I also moved the left hand support 1/4" to the left to slightly increase the starting angle, which dramatically lowers the force per the Excel spreadsheet above. This video shows the better operation.




OOOPS

On the RH vertical support, I accidentally had it our of position by 90 degrees and scroll sawed the wrong hole.  I glued back in the piece I had scroll sawed out, and then cut the right hole. Looks ok.


Model done except for wood threaded screw

Tapping threads in big wood block

After I soaked the block in mineral oil for 7 days, I tapped the female thread today.  My process was:


1. Put big block in big vise

2. Periodically lubricate the tap using artist's brush and mineral oil from green plastic cup

3. Tap for about 1 inch, back tap out, removed thread chips with pick, then tap some more


It did not take excessive force to turn the tap, and the threads cut nicely. Per the photo below, they came out very nice.



Soaking special order 3/4" maple dowels in Mineral Oil for 7 days

I thought the trapped air in the wood cells could escape more easily and get replaced by mineral oil, if the dowels were soaked vertically versus horizontally.  So, I made a 1.5 inch PVC pipe and glued the cap on the bottom..........plus made a wood stand so it would not fall over.


-I put a piece of scrap wood in the top, to press the 2 dowels below the top of the mineral oil..........otherwise the top sticks out of the oil as the dowels float.  I used blue masking tape to temporarily hold them in place.


-I also made 2 pieces 7 inches long each, to use as test pieces for the die. I may need to adjust the cutter depth to get the male and female pieces to match well...........and I can use these pieces to do it.


Mineral Soaking

The youtube video showing making 2.5 inch by 5 TPI threads on maple, said you can tell when piece is oil saturated, because it no longer floats and it sinks.  He recommended 7 days of soak time for his 2.5 inch dowels.


On my 3/4" maple dowels, I noticed about day #5, the 2 vertical pieces no longer floated and had sank to the bottom. I did not check the 2  short horizontal test pieces until day #6, and they no longer floated either.  So maybe for the smaller 3/4" dowel, to reduce the 7 day wait time, stop soaking when the pieces no longer float?


Test threads after 7 days soak time

I soaked 2 short test pieces for 7 days.

On the first piece, I quit pressing down while turning, and got bad threads.  I had made a pencil mark on the bit, and I experimented with the bit a little further engaged, and less engaged by a hair. Did not seem to make a lot of difference.   I was still not getting perfect threads, with some chipping.

I wondered if maybe my bit was dull?   I pulled it out, and it had some rust on the shiny cutting surface.  Using the Arkansas stone that came with the threading kit, and some cutting oil, I sharpened both of the shiny faces.  I got perfect results for about 2 inches of thread, which at that point, I had used up test piece #1.

I also used my Crescent adjustable wrench as a crude micrometer, and found the threaded portion was slightly smaller in OD than the 3/4" uncut portion of the dowel.  I think this the expected result, since the 3/4" is the pitch diameter.   I also checked with the nut, and it seemed to work ok.   Time to try test piece #2 with my sharpened bit, and pushing down all the time.

Eureka..........good threads on Test Piece #2 !!

Threading 1st real piece

I soaked 2 blanks for 7 days in mineral oil.

I went ahead and threaded the first blank the whole distance it needed to be threaded (I put a pencil mark where the threads needed to stop).  With the blade sharp, there is no grinding noise while threading, and it takes less force to turn the threader box.  The chips came out well and are neat curly shaped.


The threads were beautiful when I got done.  I checked the fit into the big block where they get used on the model, and they were too tight. So I then:


1. Moved  the cutter towards the center of the dowel, still too tight.

2. Moved cutter again and rethreaded, better but still too tight.

3. Kissed the female threads in the block with the 1/2" drum sander on the dremel.

4. Wrapped piece of sandpaper around male thread and stroked it vertically.

5. Added Johnson's paste wax to the threads, still mineral oil on them, so the 2 fluids mixed together

6. Fit just about right


When I rethreaded, I got much smaller chips than I thought. I don't know if the cutter was rubbing more than cutting with the mineral soaked wood.


Fine tuning before assembly

I cleaned both the female and male wood threads with paint thinner.  I then sanded the OD of the male threads again.  I coated both threaded pieces in Johnson's paste wax, then use an adapter on my drill to "exercise" the threads. Time to assemble using Piece #1 and see what happens !!



Broke spacer between 2 vertical uprights

I made it from 1.25" birch dowel..........but I forgot that later I had to drill a hole thru it for a 1/4" dowel.   When I tried drilling it in place, it split......so made new one from oak and drilled it on drill press.......worked ok.


Initial Results

Whole assembly worked pretty well.  I tried to dowel the top gear shaft as tight as possible to take more wobble out when you crank the gear.  I applied Johnson Wax to all parts as I did the final assembly.


The 3/8" end caps did not work well on the pin on the scissors.  They don't give must depth to glue to the dowel.......ending up making longer one on my lather.


The verticals move quite a bit, and  you can move the vertical at the end of the threaded rod if you crank too hard against it. I could add a horizontal stabilizer later if I need to.  The extra brace is shown in pink in the drawing below........


1st Video of finished assembly

I usually make a Youtube video of these types of projects.  Although the mechanism worked ok, when I watched the video, I noticed the excessive wobble in the top gear that is cranked.  You can watch this video using this link.


I learned on past projects that a 3/4" thick cantilevered gear works ok if mounted on a 3/8" shaft...............but if you mount it to a 3/4" shaft, there is excessive wobble in the gear. My choices to reduce the wobble are:


1. Cut a new piece of wood and glue it to the axle vertical mounting piece, increasing the thickness from 3/4 to 1.5".

2. Make a threaded bolt, tighten the nut to the right spot, then pin it using a 1/4" dowel pin.


Since I had a short threaded test piece left over, I decided to try the threaded nut approach.






Threaded nut worked fine and took out most of the gear wobble!


Decided to add extra horizontal support

This new support stiffened up the 3 vertical risers.  It also made a better end stop, because that vertical was tipping when the block hit it.  I did install 1 screw from the bottom up into the new piece, to stiffen everything up.


The first vertical is not exactly lined up with the other 2.  If I tighten up the new piece, it causes binding of the rotating shaft.  I backed off the screws a little, and everything went back to ok.  In retrospect, I should have installed this horizontal board at the beginning to hold all 3 in perfect alignment to each other.


Updated Model Design

Sketchup Warehouse

I uploaded my design to the Warehouse, where you can see, or download the Sketchup file for free using this link.


Thread Box

I recut the threads several times, each time moving the cutter closer to the dowel.


I had a left-over piece of 3/4" dowel, so I decided to thread it and turn it into a bolt.  But when I went to thread it, it would not thread !!   I pulled off the thread box, and the cutter was trying to take too big of a cut !!


So I guess this answers the question, "Should I have had the cutter set deeper the first time I made the scissors lift screw?"........and the answer is no.


Many years ago, I marked the factor depth setting with a pencil.  It was hard to see, so I filed over the pencil mark on the cutter so it was easier to see in the future.


Finished Model

YouTube Video of model in action

You can click on this link and watch the model in action.


Closing Thoughts on this Project

This was a fun, but challenging project. I'm sure glad I don't keep track of  how many man-hours I spend on a project :)


I was able to simulate and develop the design using the Linkage.EXE program, which saved time compared to doing it all in Sketchup.


The hardest problem was learning how to cut nice wood threads.  The secret of soaking the dowels for 7 days in Mineral Oil proved to be the secret of success.


I was also surprised how much horizontal force is required to lift the platform at a small starting angle !!


Hopefully, the grandkids [and some adults] will enjoy playing with this model.