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Beam Gears and chain selection

Technical Details
#1

Out of curiousity, I was wondering:

  1. Why were 10 tooth gears chosen for the top beam?
  2. Why was the small chain chosen for hanging the sled?

Aside from the obvious: the chain fits the sprockets and the sprockets fit the chain, I am curious why we don’t use bike chain. I do a modest amount of biking and 8 speed chain is the widest and most durable of the bike chains and is the cheapest. It would open up the ability to use 11/12/13/14 tooth gears on the top beam which could theoretically increase the upper feed rate limit rather than jumping from 10 tooth to 25 tooth with the chain currently on the system. My disclaimer is that I’m asking because I want to know the constraints as to why things are the way they are and if there is any room for improvement. Some things are fixed and won’t change and others are tradition and would change if someone decided to try. Anybody care to tell the story?

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#2

the sprockets are the smallest available, bike chain would require larger
sprockets.

that would require bigger motors to get the same force available to move the
sled.

that would require more powerful chips on the motor controller

these changes would drive up the costs

The maslow is very much in the ‘just barely good enough to get the job done’
category.

and you would still be limited by the force of gravity to move the sled away
from the motors. (this is a severe limit in the bottom corners with stock
dimensions)

David Lang

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#3

I wasn’t around early on so don’t know the specifics as to why the combination was picked, but there’s nothing that prevents using other chains/sprockets that you can fit on the motor… as long as it doesn’t stretch or break.

#4

I recently read a comment about going from 10 teeth to 25 teeth to increase speed, but the motors may not have the torque to push it. My thought was why not go to 12 or 14 tooth sprocket and take a % increase rather than an multiple increase? It makes sense that to drive costs down, the minimum was chosen. To move to a more efficient (this means higher feed rate to me) setup, what would be a reasonable approach? Is it expected that more weight would be needed to have a higher feed rate?

#5

the motors already struggle with the existing weight in the top center. going to
a higher/wider top beam can increase the force available in the bottom corners
while decreasing the force needed in the top center (play around with the
spreadsheet) but it’s not enough of a difference to make a larger sprocket
worthwhile.

currently we can’t run the machine at the full speed that the current motors and
sprockets can support because we don’t have acceleration planning in the
firmware, so it thinks that it can go from a stop to full speed instantly, and
the fact that it can’t causes errors.

David Lang

2 Likes
#6

I have some 13T gears that I have yet to test.
My theory is you need a heavier sled to lesson acceleration issues.
Which requires a taller motor to workspace distance to reduce forces.

for example if you use 40" tall beam and 35 lb sled and restrict cutting width to 6 ft vs 8 ft (99% of projects I’ve ever seen) all the numbers are better than stock values

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most people probably have 8 ft ceilings so it is not practical to go much higher than a 40" beam and in reality you might only be able to get around 35" high beam.

so in conclusion I think it’s possible for a modest speed increase but it requires a taller frame, and heavier sled with reduced work area of 6 ft vs 8 ft.

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#7

Lets say you wanted to increase to an 18T gear which is 1.5" diameter vs stock 10T gear of about 1" diameter. I believe that would increase torque by 50% so max forces on top center would need to be much lower.
Example below with 60" motor height might work, max force of 24 x 150% is about 36 max which is close enough to the original max force of 33, but people do not have ceilings that high.

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#8

From all th responses, this is my take-away:

“If we get acceleration to work in the firmware, that may open up some options for speed enhancements.”

I’ll look at the spreadsheets again for force comparison. Right now it isn’t practical for me to have 12’ beam, let alone a 14’ one. I could go higher with the beam from the work piece, but I’m already at 22", so it should be fine. Fun fact: I can put a 10’ 2x4 in my civic, but I don’t think I can go longer (passenger floorboard to closed trunk).

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#9

note that going higher without going wider will make the bottom corners even
worse.

going wider without going higher will make the top center worse.

if you go both higher and wider, you can get improvements in both areas.

David Lang

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#10

the other variable is the work space area. everyone assumes 4x8’ is the holy grail, but if you only cut projects 4x4 or smaller then optimize the layout for that size. for example increasing the bar height helps in top center and doesn’t adversely effect 4x4 lower corners.

#11

My frame started higher with a 12’ beam (1044.66923mm) and I was getting Holey errors between 2.8 to 3.5%. Multiple calibrations sometimes made the error worse.

I moved the workspace up 2 or 3 feet and now my holey errors are less than 1%

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#12

there is one of my problems i am in brazil, and here these parts do not exist, so i have to import but the tax is 90% on the value of the product plus shipping. I wanted to adapt the chains and gear to the ones on the bike and be able to assemble my cnc here but due to only 10 teeth I stopped my project. is it possible to get around this?

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#13

you can set the number of teeth in the software, so theoretically, yes, though I don’t believe anyone has done it with bike chain yet, but several of us have thought about it. You can get an 11 or 12 tooth bike sprocket from a rear cassette (smallest ring). The issue will be getting the right motor to have enough power to run it, but this guy did it. Maybe @powieff or @MarcinanBarbarzynca can help you get some ideas on where to source materials.

#14

What size is the chain supplied? Does the software compensate for sprocket diameter or is it calculating on that chain pitch?
If not already I want to look at using metric chain.

#15

the chain is ANSI #25 roller chain 6.35mm per link (1/4")

the software should compensate for different chain size and sprocket size, go
ahead an use metric chain, if we don’t handle it well, we will fix the software
to handle it properly.

David Lang

3 Likes
#16

If you go up in sprocket size, won’t you decrease the resolution(resolution has a big effect on of the machine? For 1 degree of rotation from the motor and you get x amount of linear movement from the chain. A bigger sprocket would mean more linear movement. Maybe the 10 tooth was chosen because it is the smallest sprocket that is easily available. I would prefer to keep the resolution above any gains in speed.

#17

at 8000 steps per revolution, the system has more than enough resolution. the concern is: does the motor have enough torque and are the gears durable enough to withstand the higher forces for a faster drivetrain? A larger gear means more torque on internal gears for the same hanging sled weight. The rest is math. Right now the motors have a movement resolution of just under 0.05 degrees, which to me is overkill, but that is one of the benefits of having the encoders on the motor shaft, not the final gear-reduced drive shaft.

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#18

most of the maslow was guesswork and ‘that works’ with everything then
built/speced to ‘just barely good enough’ in the interest of driving the price
down :slight_smile:

that said, the problem with larger sprockets isn’t that it moves more (the
resolution on the encoders is something close to 50x what’s needed), that’s an
advantge, the problem is that the motor has a limited amount of torque and the
larger sprocket results in less force available. The default setup arguably
overstresses the motor along the top, especially in the top center.

play around with the spreadsheet and look at the max tension/force numbers. If
you make the sprocket twice as big, you need to have either more powerful
motors, or make that max force half as much.

if you go with more powerful 12v motors, they will draw more current, so the
stock motor board will burn up and the power supply will run out of ummph. In
theory the board should be good to run 24v, which will give you higher speeds
and more force, but I don’t know of anyone who has done this in practice.

David Lang

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#19

Interesting. I haven’t looked at the spreadsheet yet, or really even looked at the motor specs (I couldn’t really find them). I’ve only just ordered my first kit. Pretty excited to get it up and running.

#20

I think this is the key issue. The increase in the torque required is pretty significant so larger motors might be needed to drive larger sprockets.