Finding Center of Gravity for the Sled Ring

Hi all,
I’m having some issues that I think are related to the center of gravity of my sled being off. The latest issue is test cuts that are shallower at the top than at the bottom, and chain skips.

I’ve searched the instructions and forums for more details on how to accurately position the ring so that the center of gravity is right, so I apologize if it’s been answered and I just haven’t found it.

I found this instruction from MetalMaslow:
Metalmaslow

Sep '18

Tie a piece of string to the ring and hold it up in the air
Adjust the height until it is balanced perfectly and then raise it a tiny bit so it is just above the center of gravity

Someone asked “Does this refer to the entire sled” and someone answered yes.

I’m having trouble picturing this. Do I remove the chains? Where on the sled do I attach the string (or rope, b/c string doesn’t seem like it would hold the sled with router and bricks)? Is the sled hanging with the big round circle parallel to the floor or perpendicular to the floor? What does “balanced” look like? How does hanging it in midair get the right balance when during use it’s against the work surface which is tilted at 15 degrees?

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Is not the best advise. Better:
Move your sled to the centre of the sheet. With 2 fingertips on the chain near the ring, pull the sled away until it hangs free. Jude the angle of tilt. Edit: You want your sled to be vertical (or the top a pinch tilted to the sheet). If you raise the ring, the top will go towards the sheet.

Once the sled is balanced, then take the distance from the chains to the sheet near the sled. You want to move your motors in or out, so the sprocket teeth match the distance you measured on the sled. This will stop chain skipping on the sled-chain side.

Edit: Where are my manners
Welcome to the Forum!

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Thank you so much! That’s very helpful. And thank you for the welcome, I’ve been lurking for a couple of months and finally got up the nerve to ask a question:)

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@Metalmaslow
How do you feel about editing your post? Personally, i feel it’s a wrong advice, but am open for discussion.
My theory (i’m trying to convince @dlang till date) is, that since triangular we have a moving CG on the sled as the ‘chain attachment points’ are moving, depending on how high the motors are and where we are on the sheet. 2 strings (non slipping), in the middle of the ring part the bearings are moving, would work, perhaps. A string at the top of the ring is not going to give you the right balance.
I hope this pic will make it clear.

@RSM

Glad you did it! You’ll find lots of amazing people here, and all are helpful and kind.

Kind regards, Gero

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the balance point in the z-axis should not change as a result of the movement of the chains around the ring. The ring should be on the same plane as the vertical plane through the balance point (ie, with the sled vertical, there is a point at which the whole sled will balance that is located in the positive z direction from the base of the sled which contacts the work surface). If you adjust the ring slightly above this plane (positive z from the plane) then you will cause the sled to tip toward the work surface, which is, I believe, what we are looking for.

The change in balance point that will happen is in response to the change in the router/spindle height as it moves through the z-axis. I am not sure anyone ever suggested a point at which to put the router when determining the balance point of the sled, but perhaps it should be a consideration. Ideally it would be balanced when the bit hit the work surface, and then would tip toward the work surface with continuing movement into the negative z axis. That initial point of hitting the work surface would be dependent on the bit length, though.

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The change of CG with movement of Z is what i have left out. The reason on why i went on "all weight close to the sled’.
This was only on CG with positions of the chains.
Hope i can construct a extension for the top beam to take a video from the side to show that the tilt changes, moving from the bottom to the top.
Agreed that i’ve not proven it, and till then it stays a theory.

I balanced my sled by hanging it on a piece of angle-iron (under the top ring
bracket) and found that the position of the router had a fairly minor effect on
my sled’s balance when I moved it in and out an inch or so.

not no effect, but a difference of less than a 1/4" in the balance point.

David Lang

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And you don’t have a lower Z-depth at the top compared to the bottom?
Wow, need to try that.

Opposite, you could have a ‘bottom-loaded’ sled, same as if the ring is to low. Dragging effect is visual on how the sled turns.

a lower Z depth at the top probably means that under the higher tension in the
top center, you are pulling the sled away from the workpiece

look at the chains from the side, are they parallel to the workpiece or are the
motors out further than the ring? If they are not fairly close to lined up, you
can have chain skip problems, as well as this sort of pulling away problem.

You can be at the right mounting point on the sled, but have the motors too far
out. If so, that’s an easy fix. get a 4x8 sheet of foam insulation and put it on
the machine between your workpiece and the frame to space the workpiece out
until it’s level (you will need to add support at the bottom of the machine to
keep the workpiece from falling off)

David Lang

So I don’t know if this makes sense, but I used these clamps to kind of hold the sled away from the work surface so I could make adjustments to the center of gravity, it seems kinda sorta along the lines of what you guys were recommending.

Once I did that, I removed the clamps, then measured the distance of the chains from the work surface at the sled and at the motor, and found that there was almost 2" difference. I made adjustments to my frame to correct that, and I also adjusted the tilt of my frame to be close to 15 degrees

After all of that, I had two last chain skips, but then no more, even when I repeated the same maneuvers that had caused the skips the first time.

So I was feeling good, the sled was moving where it was supposed to move, then the clip that holds onto the router body (and moves up and down with the z axis motor) spontaneously came loose, the router body had nothing anchoring it, the bit dug too deeply into the wood, and then the bit fell out before I could turn the router off and hit stop.

I’m assuming that the bit falling out was connected to the failure of the clip. I’ve read enough in the forums that I wasn’t surprised by this failure, and I’ve seen a lot of z axis solutions, so I’m going to try my hand at one of those tomorrow.

Thanks again everyone for helping me trouble shoot and taking time to explain things.

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I’m having trouble parsing this.

Are you saying to manually rotate the motors in Ground Control?

Or are you saying to loosen the screws attaching the bracket to the top beam and make the sprocket in line with distance observed?

Or is it something completely different.

Edit: I’ve blurred out my incorrect interpretation so that anyone skimming the thread doesn’t get the wrong take away.

Exactly that. Once the sled is ‘balanced’ , adjust the motor mounts so the chains are parallel to your worksheet. This way the chains will not come in from an side angle to the sprocket, when you get close to the top corners.

The easiest way I’ve found to do this is to lower the sled to the bottom-center of the sheet and measure where the chains cross the sheet and where they attach to the sled. you should end up with four measurements that are all the same.

I am no person to be solid on claiming truth (as a warning).
I do post comments though that can be taken or ignored.

Just imagine that you are moving the sled to top of the sheet in the middle (X) and drill holes under where the chains are, on the most outside of the sled.
Now imaging you move the sled to the bottom and drill holes under the chains there as well.
Connecting 2 strings to the upper holes, hanging the sled free hand, do you assume that the tilt angle towards the sheet is the same as if you put put the sting in the lower holes?
I balance the sled at X0/Y0 till i’m proven wrong.
My single personal view is backed by extensive testing, but never confirmed, caution is advised.
That i balance my sled at X0/Y0, means nothing.

Kind regards, Gero

and I think that to balance it to tip forward is wrong because you are pulling
above the center of gravity (which will cause it to tip earlier at the edges,
and pull the sled harder against the workpiece, increasing friction)

I believe that you should balance it to hang as straight as you can (recognizing
that as the router moves in and out it will vary a bit) so that you are pulling
as close to the CG as you can.

David Lang