Geometry of Sewing Machine Feed Cam

[Gymnast]

If you are a bit interested in geometry, I think you will like this short theoretical video about a widely used cam in sewing machines:

I have tried to find some background information about it, but it was not that easy. Therefore I made this video.

In general you will see this triangular type of cam with a fork follower (Constant-breath follower). At some other places you will see a normal excentric with a circular shape used with a bearing. But this triangular cam is very often used to control the horizontal stroke of the feed dogs. The cam may also be used to control the vertical stroke and the zig zag on a few sewing machines. It is special by keeping the fork still (no movemen) two times of 60 degrees of the rotation and the fork is moved two times of about 120 degrees.

[chuck123wapati]

very similar to the cam in an engine it also moves the feed dogs up and down doesn't it? Shouldn't the cam red drawing be pivoting at the center as it is an offset lobe on a round shaft? Just asking as I cant remember I took a sewing machine apart a year or two ago to build jigsaw. Very intricate machines, I was amazed at the mechanics.

[Gymnast]

2 hours ago, chuck123wapati said:

very similar to the cam in an engine it also moves the feed dogs up and down doesn't it? Shouldn't the cam red drawing be pivoting at the center as it is an offset lobe on a round shaft? Just asking as I cant remember I took a sewing machine apart a year or two ago to build jigsaw. Very intricate machines, I was amazed at the mechanics.

Regarding an engine, I think you will normaly think of the cams on the cam shaft, that activate the valves. The followers to the engine valves are spring loaded. The geometry of this cam described here are much different and are not spring loaded. In the years 1960-1980 a lot of domestic machines were made with spring loaded cams to control zig zag and feed dogs. Some of these cam wheels turned 12 times slower than the main shaft.

If you turn this triangular geometry around the middle, you will not get the stops of movement of the follower in each end of the stroke. You can try it by cutting out a piece of carton or paper and nail it in the middle.

Edited January 19, 2020 by Gymnast

[chuck123wapati]

5 minutes ago, Gymnast said:

Regarding an engine, I think you will normaly think of the cams on the cam shaft, that activate the valves. The followers to the engine valves are spring loaded. The geometry of this cam described here are much different and are not spring loaded. In the years 1960-1980 a lot of domestic machines were made with spring loaded cams to control zig zag and feed dogs. Some of these cam wheels turned 12 times slower than the main shaft.

If you turn this triangular geometry around the middle, you will not get the stops of movement of the follower in each end of the stroke. You can try it by cutting out a piece of carton or paper and nail it in the middle.

interesting, that's why I asked now i gotta go see what is the brand of your machine. The cam in an engine also turns at a different speed from the main crank via the timing gear, yes its use is just to go up and down and at much higher speed hence the springs, or hydraulic lifters, to keep the lifters in place on the cam. 

[chuck123wapati]

ok I went and looked, lol, at three of my machines noting these are all domestic sewing machines, two newer models and one really old one . I have taken a few pics to show the cam you are talking about also the bottom end. on All three the rod and cam provided about a 16th to maybe a 1/4 rotation of the shaft that directly moves the feed dogs. In the old royal it had nothing to do with the dogs but instead ran the shuttle back and forth, the main connecting rod ran the dogs. in the Ken more it provided back and forth movement while the main connecting rod ran the up and down movement of the dogs. the other machine I just photoed so you could see plainly the cam, it doesn't have any lower mechanism. As you can see It is a triangular cam placed off center from its main shaft almost at the point of the lobe but not quite, very similar to your simulation. The stops come from  the lobe shape going over top dead center, the flat part of the triangles,  just as the cam of an engine does. At the bottom the shaft is just going back and forth where as in an engine the motion is used up and down . The difference of a cam on a shaft as compared to an offset shaft, such as the main connecting rod, is that the cam can be tweaked to provide a momentary pause in a continuously moving assembly as well as different amounts of lift at different times, the offset shaft can only do one thing up, down, in, out, etc. Very cool and interesting thing to think about this morning.

Edited January 19, 2020 by chuck123wapati

[Silky]

That is known as a Reuleaux Triangle.

https://en.wikipedia.org/wiki/Reuleaux_triangle

I made some of these with a CNC mill while experimenting.

Cheers.

[Gymnast]

1 hour ago, Silky said:

That is known as a Reuleaux Triangle.

https://en.wikipedia.org/wiki/Reuleaux_triangle

I made some of these with a CNC mill while experimenting.

Cheers.

Thank you for this reference. I actually did look for information on internet and found some mechanical engineering books that showed this cam. Unfortunately they did not made a reference to some of the original descriptions like Euler or Reuleaux. But it is obvious, that some of the early mathematicians should have looked at this. When I look the in this wikipedia article, it is not that clear, that the rounded corner triangles can be considered Reuleaux triangles too.

It is a funny shape, and I guess it could make some fun for children to learn about,

[Silky]

This is one I made that worked beautifully in a 2" fork.

When precisely fitted, there is no backlash so it runs silently and quite fast ... as in a sewing machine.

Lube is critical as well as the heat treatment of the mating surfaces.

 

[Gymnast]

@Silky I guess it should be exciting to make it from scratch like you do.

Now that you know the name to be Reuleaux Triangle then you may know this other funny cam or mechanism. I have called it Rotoscillo from the US Free Sewing Machine Company, that used in about 110 years ago. I saw it used in a Köhler sewing machine from 1950ies. It may origin from some older work. I link to a couple of videos showing it:

 

Look at 6:05 in video.

 

[Silky]

Thanks for posting that ! Very interesting indeed. I LOVE the mechanics demonstrated from so long ago. They likely used sticks, pins and "trial and error" to design with, more so than math and computers. Amazing !

Thanks again,

[Gymnast]

I also find these old, special and simple mechanical designs intriguing.

This Rotoscillo makes use of larger oiled internal surfaces to transfer the load, and I think it should be better than a round cam against a plane surface.

The German Köhler engineers may have been inspired by the Free sewing machine, because both machines use the Rotoscillo, but they do also  use rods and bearings to produce the needle bar motion and motion of take-up lever. Other long shuttle machines typically use special machined cams for that. The needle bar makes two dips each stitch. Later on in the video about the Free machine you can see this mechanic. This video show the similar mechanics in the Köhler machine:
 

 

Edited August 12, 2021 by Gymnast

[DonInReno]

Thanks for those videos - for some reason I missed the original video in this post, but it was very interesting seeing it now!

[Gymnast]

On 8/17/2021 at 2:09 PM, DonInReno said:

Thanks for those videos - for some reason I missed the original video in this post, but it was very interesting seeing it now!

This is a mathematician with some interesting points about these Reuleaux Triangles - so if you like to see this for fun: