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#61

1, sounds the same to me?
2. depends on how you plan to round over edges. If you overhang then you can heat the plastic and round it over the metla plate. Otherwise you will be using a mechanical method like a bullnose bit on a trim router to round over the edge.
3. no tests have been done to say one is better than the other. Look at Bar’s initial videos, he uses the routers’ base which is 6" or so in diameter. Engraving warped wood? then a small sled might be better because it follows the contours/ warped areas better.

Remember the router doesn’t need to be in middle of round sled either.
4. the more the marrier. tell the laser company to keep the round center hole cut out and that way people can use it to center any router they try to make / modify to fit. having 6mm central hole will help too.


#62
  1. I like threaded standoffs in general, but I’m not sure I understand how they’re being used in this application. Is there a picture/drawing? Rigidity and dimensional accuracy are the concerns.

  2. One vote for overlaping edge.

  3. I like the stability of a large sled, but that doesn’t need to be all metal. If you choose a smaller diameter I’ll probably put an 18" disc of 3mm ply under it (see #2 above). It seems like the diameter greatly affects the overall weight and the size of the added weights; how much difference are we talking about?

  4. If your rig can accommodate that Bosch router, it should be possible to handle just about anything with a bit of ingenious jiggery.


#63

I think a small metal sled will keep it economical as well. bigger circular sleds = more circular waste scraps and higher much higher shipping. People can always buy a cutting board at Walmart and bolt it to the metal sled, just laser cut 3 or 4 mounting holes to accommodate this.


#64

Standoff may not be the best way to describe what I’m thinking.

Think a section of threaded rod inserted into the sled. The brackets would sit down on them and be tightened with lock nuts. Pretty much the same mounting method as bolts going through the sled except these will just be threaded into the sled.


#65

Sorry I’m being so dense - if this means the bracket is clamped to the sled by the nut, that sounds good to me. If the bracket stands above the sled on top of the standoff, I would be concerned about the accuracy.


#66

I would love for them to be threaded into the sled, but if the sled is thin
enough, a countersunk bolt head would be better.

you will want one bolt under the link that you can really tighten (to keep the
bolt from moving) and then one above the link that can be loose enough for the
link to move.

David Lang


#67

Not sure what you mean by link.


#68

I have noticed an increased drag (sled turns) with going from ~15" to ~18" sled. The sled can only be balanced on one height of the sheet (preferably middle), the ‘pressure point’ moves away from the bit with a bigger sled increasing the ‘arm’. While a turning sled should not be an issue with a centered router, it looks the drag adds a little chain sag on the chain that feeds out. I am going smaller than 18" again.


#69

the arms (as opposed to the standoff’s)

David Lang


#70

I have noticed an increased drag (sled turns) with going from ~15" to ~18" sled.

Thinking about this, the metal sled only needs to be big enough to hold the
linkage and the router. you can have either plastic or wood under it to slide
on, and the dimensions of the plastic/wood are not critical, so people can try
different things.

It’s the mounting hole positions for the moving parts that are critical.

The sled can only be balanced on one height of the sheet (preferably middle),
the ‘pressure point’ moves away from the bit with a bigger sled increasing the
‘arm’.

I still don’t see this.

David Lang


#71

Friction force =u*Normal Force

so technically surface area does not matter. However it does matter because there are splinters or tear out that cause the sled to physically plow over them.
the bigger the sled the larger the normal force is spread out and the easier for a bump to throw the sled off course.

He’s talking about the torque arm generated. a 6" sled has a 3" torque arm, a 18" sled has a 9" torque arm when the sled hits a bump and causes it to move.


#72

Taking time on things is not a bad thing. You have more skills to doubt me.
The sled tilts if you look at X axis from the side.
I have seen Z-Towers built high that they can compete with Mordor.
On the top sheet (middle) this sled will have a higher tendency to turn back then at the bottom of the sheet.
The connection point chain>sled moves since triangular, with brackets the connection point on the sled was fixed. The biggest difference I saw was how the sled changed behaviour on height differences.

Edit: Edited


#73

If you are looking at the sled from the side (along the X axis), the chain attachment should be at (or near) the balance point in the Z direction, and that should not change as you move around the workpiece. It will change as the Z axis goes in and out, but we can’t (reasonably) fix that [1]

in the Y direction, the CG will be well below the bit, but the chains should always be pointing at the bit, so that is where the force should be applied. If the bit is not at the center of rotation of the triangulation kit (so the bits don’t point at the bit), then as the sled moves around, it will rotate, and the amount it rotates will vary based on cutting force, friction, chain tension, etc.

I still don’t see how the sled is ‘balanced’ in one area of the workspace but not in others.

David Lang

[1] I could imaging a setup where the weights moved up as the router moved down, but that’s a lot of added complexity :slight_smile:


#74

I may be missing something (it happens), so I’m really trying to understand this, not dismiss it.


#75

Imagine for a moment that the sled is suspended in air by the two chains. Consider the instance where the forces from the two chains are perfectly opposite - 180 degrees apart. In this condition, the forces from chains will not affect the angle that the weight distribution of the sled causes. Now consider when both chains pull from the same direction, where they are parallel, in the middle of travel. Will the sled hang at a different angle? It seems to me it will.


#76

so as the sled hits tearout (or whatever), the sled wants to then rotate,
lifting the rest of the sled off the workpiece.

In that situation, the smaller the sled, the easier it is to tip over. The more
it tips, the less surface there is in contact, and so the harder it is to ride
over the tearout.

the lower the weight is on the sled (in the Z direction), the lower the chains
end up being, and the harder it is to tip.

In both cases, the angle between the chains and the line from where the chains
would intercept the bit and the edge of the sled is smaller.

The bigger the sled, the more area of thw workpiece is covered, which means that
the effect of cuts is smaller (as you go over cuts, the force that would
normally rest against the material that is missing gets distributed over the
rest of the area), but that you are also going over more splinters/tearout that
can catch on things.

cutting on OSB with an OSB sled would be about the worst case, but people have
noted that even with plywood, if you have the plywood grain in the same
direction as the workpiece grain it’s more likely to catch than if you rotate
the sled 90 degrees

Have I wandered off to a different problem with this thinking?

David Lang


#77

If the sled is unbalanced (in the Z direction), then the amount of tilt will
vary a bit, but if the sled is balanced, it will be balanced in both cases.

I don’t think that it’s a matter of it being balanced in one place and
unbalanced in others, but rather a matter of it being unbalanced in all cases,
and the resulting tilt will vary based on the two chain angles.

David Lang


#78

if you are using a metal sled that has a fixed weight of about 10 lbs the difference in height between a 1/8" thick sled that is 18" diameter vs a 5/16" sled that is 12" diameter is only 3/16" .


#79

if you are using a metal sled that has a fixed weight of about 10 lbs the
difference in height between a 1/8" thick sled that is 18" diameter vs a 5/16"
sled that is 12" diameter is only 3/16" .

say that the CG is 3" above the bottom of the sled.

with a 6" diameter sled, the angle from the chain down the the line from the CG
to the edge of the sled is 45 degrees.
with a 10" sled the angle is 30 degrees
with a 12" sled the angle is 25 degrees
with a 15" sled the angle is 21 degrees
with a 18" sled the angle is 17 degrees

if the CG is 1.5" above the bottom of the sled
6" dia = 25 degrees
12" dia = 16 degrees
15" dia = 11 degrees
18" dia = 9 degrees

the smaller the angle, the more of the chains forces will move the sled over the
obstruction and the less will try to have the edge dig it and tip.

Tipping has never been a big concern. I’ve never seen it even mentioned. .

we seldom have a sled visibly tip, but when you get to the edge, how close you
are to balanced affects how close you can get to the edge.

We have seen people who mount the chains to low (in the Z direction) find that
the sled lifts away from the workpiece.

but sleds that mount the chains at/near the balance point also seem to slide
more easily, and i suspect that if the chains are mounted too high, the sled is
trying to tip and dig the edge of the sled into the workpiece in the direction
of travel.

David Lang


#80

Yes all those angles sound about correct,
with a metal base the center of gravity will be close to 1.5 to 2" in height depending on router used.

A 6" sled is pretty small. to fit anything on. about the smallest practical sled is about 6"x12" x 0.5" which is about 9 lbs. And a 12" sled is the cheapest to ship.