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Old 05-24-2017, 07:38 AM
Kerry Pinkerton's Avatar
Kerry Pinkerton Kerry Pinkerton is offline
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Default Thoughts on scotch yoke machines

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Originally Posted by Michael Moore View Post
...I'd be interested in seeing some comments on the pros and cons of the Scotch yoke vs connecting rod for a home shop reciprocating machine if anyone wants to share them. The Scotch yoke seems like it would be limited to slower frequency cycles, but it also seems like it could offer a lot of bearing area that could be very reliable with the right bearing bronze and mating sliding surface.
Rather than clutter up Kent's thread, I started a new one specific to this question.

Clearly scotch yoke machines can be robust and long lasting. A few years back when we were discussing options for the group design thumbnail machine, there was a pretty good discussion on various mechanism options. I built several machines using different approaches both as a test bed and 'just because I could'.

One of those was a scotch yoke. I used a bearing offset in a rotating shaft and made a rectangular hole in a piece of 1/2 bar stock welded to the top of the quill. It worked fine and still does for the guy I sold the machine to. The problem for me is that, in spite of having two mills and two lathes, I'm not a machinist. There is a little 'slop' and that equates to noise.

I'm sure a real machinist could make one that would work fine. What I'm not sure of is if it will be easier/cheaper to make than the connecting rod system...or more efficient.

Some time later, my friend Barb Lawrence (Copperetta) was thinking about a lightweight reciprocating machine for her copper work and I came up with something I called the "Barbmax".

http://allmetalshaping.com/showthrea...hlight=barbmax

I believe the Liebert nibblers were also a scotch yoke design.
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Old 05-24-2017, 08:52 AM
Michael Moore Michael Moore is offline
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Thanks Kerry, I don't recall seeing the Barbmax.

I too am a HSM (home "scrap (both material and end result)" machinist) but I get lucky now and then.

I've got photos of Ben's "Shrinky" scotch yoke dedicated shrinker and I've liked the simplicity in his design.

From looking at the Pullmax patent documents it looks like there are minor differences through a cycle in the motion/leverages between a Scotch yoke (sinusoidal), a connecting rod and a Pullmax linkage. Those probably are a lot more important in an industrial machine nibbling .25" plate 40 hours a week than in a hobby reciprocating machine.

A comparison of the conrod and yoke movements, it looks like the yoke has somewhat lower maximum velocity and acceleration:

http://demonstrations.wolfram.com/Co...okeMechanisms/

Mostly I'm looking at "what is not too hard to make, quiet, robust and works well and appeals to me".

And of course, tool design is just an interesting thing to think about.

cheers,
Michael

Last edited by Michael Moore; 05-24-2017 at 09:01 AM.
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Old 05-24-2017, 09:20 AM
KAD KAD is offline
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At the beginning of internal combustion engine design there was a huge debate that raged for many years as to the best choice for this "type of mechanism" but eventually everyone settled on the crank/ connecting rod design as the "best overall decision".

Unless your machine has some really different requirements it can be a wise thing to look at what history says about a subject and tend to side with it as the "likely best approach".

Not always....but likely

The race is not always to the swiftest nor to the strongest but it's darn sure the way to bet.....
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Old 05-24-2017, 01:05 PM
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Quote:
Originally Posted by Michael Moore View Post
..

Mostly I'm looking at "what is not too hard to make, quiet, robust and works well and appeals to me".

And of course, tool design is just an interesting thing to think about.
I assume you've looked at the dedicated thumbnail machine threads? It's easy to scale up and it'd be hard to beat for ease of construction. I've even still have a few 1/8" eccentrics. Of course, if you want to have a tool lift you'll need to get into more complex designs.
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Old 05-24-2017, 01:05 PM
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Is 1725 strokes/minute suitable for you? The Heck Trace-a-punch is a scotch yoke machine and it is a direct drive 1 1/2 HP 1725 rpm motor. It is rated for nibbling 1/8" stainless steel all day long. It is a prototyping/production machine.
Kent's machine is a upscale version of the same general type of machine with some bells and whistles of adjustability and larger capacity.
A connecting rod machine is going to require a good bit more lubrication than a drip oiler. And a larger chunk of real estate to house a flailing connecting rod with oil slinging off of it. Just some things to consider.
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Old 05-24-2017, 03:38 PM
KAD KAD is offline
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Quote:
Originally Posted by mr.c View Post
Is 1725 strokes/minute suitable for you? The Heck Trace-a-punch is a scotch yoke machine and it is a direct drive 1 1/2 HP 1725 rpm motor. It is rated for nibbling 1/8" stainless steel all day long. It is a prototyping/production machine.
Kent's machine is a upscale version of the same general type of machine with some bells and whistles of adjustability and larger capacity.
A connecting rod machine is going to require a good bit more lubrication than a drip oiler. And a larger chunk of real estate to house a flailing connecting rod with oil slinging off of it. Just some things to consider.

If you use a steel connecting rod and a needle bearing like a two troke engine does then a couple of drops of oil is all that's needed and it'll take a huge amount of impact and not fail.

Aluminum rods and bushing inserts take lots more lubrication liek your talking about.
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Old 05-25-2017, 01:18 AM
crystallographic crystallographic is offline
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Default Overloads

When I see a Pullmax fail, it is in the overload condition, when the linkage rods bend. At least, if not that, then when getting the rods straightened it goes back to somewhat the original performance --- so it may be sumpin else --- but the cure can be an indication...? mebbe the rods ... ?

And infernal combustion engines can also lock up under certain loads and those conn rods then show suspicious signs of being less than straight ....? At least that has been my own 'sperience ....

However, the Scottish yoke just stops.

(Sorta like the Scotties do when threatened with bending ... )

... and then the motor kicks the heaters if you pause before hitting STOP. ....

Now, I'm probably one of those rare guys that overloads his gear - okay to that - but what will that machine do when I push the "overload" button ....?

I'd really rather the works just stopped on me.
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Old 05-25-2017, 10:32 AM
Marc Bourget Marc Bourget is offline
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[QUOTE=KAD;136952]If you use a steel connecting rod and a needle bearing like a two stroke engine does then a couple of drops of oil is all that's needed and it'll take a huge amount of impact and not fail.

Kirk,

I believe that approach requires experience to get the respective materials and heat treat correct.
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Old 05-25-2017, 11:23 AM
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After messing about with my walking beam machine for a number of years, and learning what it could do with different dies, I started hating the square tool shank I had used. It required minimal disassembly of the machine to change dies. Even at minimal, I hated that fact. Overall, I loved the walking beam machine though.

So I set out trying to come up with another tool mount system that made changing tools easier and eliminate the need to adjust them in and out and left to right. Height adjust was inevitable, so that stayed.

I came up with a simple mount system and wanted to try it on a different machine instead of cobbling up the walking beam machine. So I pursued a scotch yoke mechanism in a small machine to try things out. I had a Trace-A-Punch for a number of years, selling it off as I built these two machines.

Here's what I ended up with. It runs at 1200 spm off of a 1725 rpm 1 1/2 hp 56C motor. I used a vfd to find the sweet spot and allow 120V operation I have ran it as slow as 700 spm and as fast as 1800 spm. It walks about at much over 1200 and got me to working slower than I wanted to at the lower speeds. It shrinks well, but the die gap is critical, as Kent points out, the mechanism will stall out or lock up. And it locks up real tight.

The frame of the machine is pretty rigid too, helping out the locking up situation. #1 machine was made with 3/8" steel side plates, this one is 3/4" aluminum. But adjusted correctly and ran at the 1200 spm, it works well. I have maybe 100 hours between two machines and broke this one trying to shrink 16 gauge. It failed exactly where I thought it would, at the shoe. That might have been more due to the material I made the shoe from, and not the material I was testing. I generally don't shrink 16 gauge, so I've not tried it since. But on machine #1, I see some slop at the shoe. It's made from the same material as what failed on machine #2. I'll change that out at some point in the future.

I'm in the process of extending the throat with new side plates and building different dies for beading, offsetting and nibbling. So far, I like the little thing. It does well on either steel or aluminum. Doesn't take up much room and can sit on a shelf out of the way when not in use. The deeper throat machine won't be as accommodating space wise as this little one though and I think it will be on a higher stand and sit on the floor on casters.

Well, I can't seem to imbed the image in the post, so sorry for the attachment.


Mini Shaper Low Res.jpg
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Last edited by Steve Hamilton; 05-25-2017 at 12:39 PM.
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Old 05-25-2017, 11:51 AM
Michael Moore Michael Moore is offline
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Bob, that looks pretty cool! I'd like to see more detailed photos of it when you get a chance and if you are willing to share them.

FWIW, if I did the numbers right I think (for the other dimensions being the same) a 3/8" steel plate machine is significantly stiffer in the vertical axis and in torsion, but the 3/4" aluminum plate machine is stiffer in the horizontal axis. Steel is 3x stiffer than aluminum so if the only change is in the horizontal width of the side plates (same plate size except for thickness) then you might be better off with steel to prevent the arms from spreading apart vertically.

You could probably check that pretty quickly with a dial indicator and set the machine to negative clearance and use a torque wrench on the end of the shaft.

The Scotch yoke seems like it might give a more compact mechanism.

Here's a couple of photos Ben posted of the Shrinky tool.

user4387_pic2506_1287853289.jpg

shrinky2.jpg

cheers,
Michael

Last edited by Steve Hamilton; 05-25-2017 at 12:41 PM.
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