Whats the Weak Link in the design?

based on the reports I’ve read, the problem appears to be a metal quality issue more than anything, it appears to be hit and miss.

without better numbers of total motors sold and some changes to GC (to incorporate a permanent log of minutes run per motor, and chart the load on the motor controller (1x 2x and 3x simultaneous movement/mins) it will be harder to pin down quickly what kind of failure rates/conditions actually exist.

re motor mounts, @blurfl, the motor mounts appear robust enough, assuming they’re secured well I’ve not seen much to think that they’ll pry themselves loose or bend.

2 Likes

Just feeling I need to clear some things about my previous comments in different posts.

  1. Mentioning that ‘my motor mounts’ bent/twisted was not the L-shape motor brackets that come with the kit. It was the dirty build with C-shape 2mm steel arms I used for 14 months that gave up with the TLE5206.

1a) It was a glitch in the software/config during the ‘pull tight’ where the 3 seconds did not kick in and the left motor kept pulling until I pulled the plug.
1b) With early versions of FW/GC (2017) I’ve had a series of buffer overflows and who knows what that made GC trying to send the sled into orbit (both motors pulling at ‘full’ force on the sled). I have fried 2 1/2 Maslow motor shields in the process. My C-shape motor arms have been abused quite allot.

  1. During the first ‘pull tight’ with the TLE5206 I also noticed a huge flex in the top beam, that went unnoticed before. I’ve tried to make it rock solid now and still have a flex of 2mm measuring the chain length during calibration.

  2. I am now hearing 4 or 5 ‘beeps’ during the ‘pull tight’. Still need to figure out from where this is coming. However something is stalling and letting me know by sound. I’ve never heard that before. It does not shredder my gears.

  3. The Weak Link in the past was with no doubt the motor shield that would fry short and perhaps occasionally take the 5A original power supply with it. With the TLE5206 the shield is not the weak link any more.

The second biggest ‘force’ that can be applied to a Maslow is the ‘pull tight’ during calibration.
This will show how good your top beam and your motor mounting is. Expect surprises.
The theoretical biggest ‘force’ is when due to a glitch both motors pull against each other on the sled. With brackets on a steel sled the stall should accrue. We have had wooden sleds been pulled apart.

With the triangular kits my expectation is:

  • The plywood kit could snap at one side and the router would swing sideways on the other chain
  • The ring kit would first deform/bend but might pull those tiny screws out of the sled resulting in a dropping router.
  • For the metal top-mount kit v1 I would expect the stall to show up, or it would break the connection link of the chain that is compromised by rotation under tension rubbing metal on metal.

Just my 2 Fils --> 2fils

4 Likes

I would not expect to break the connection link of the chain to the top mount, I would expect it to either stall or bend the arms (I haven’t tested it under those conditions, so I’m not sure which would happen)

@Gero, Your 2 fils are worth countless Dinars!
Hard lessons that will help everyone’s designs improve - Thank you very much for your reports!

Maybe one of the veterans here has a copy of the datasheet for this motor from ETOMN ?
I found one post that mentioned the part number ET-WGM58AE. Is this right?
If so, I may try to get more detailed specs from ETOMN.

Confirmed

Data Maslow git: https://github.com/MaslowCNC/Electronics/blob/master/Documentation/Data%20Sheets/ET-WGM58AE-1220.6(20171108003-H16035)%20Model%20(1).pdf

Data etonm http://www.etonm.com/products_detail/productId=89.html

Edit one more (this might end up at Bar so use with care, contact supplier first). Buy ET WGM58AE with 7PPR encoder 12v in China on Alibaba.com

3 Likes

Thanks, @Gero - saved me a bunch of hunting!

From the discussions about @mrhall4’s Gearbox failure - need replacement , it may actually be that having the gearbox as the mechanical failure point is acceptable… assuming we can source the internal gears.

Hopefully I can ensure it never comes to that - I’m currently thinking that, instead of my earlier microswitch cutoff (or in addition to it), putting a monitored load cell between the Top beam and the motor mount is the way to go. When chain load exceeds, say, 100lbs it hits pause / throws an alarm.
This adds circuit complexity, but keeps the failure mode non-destructive.

Besides, i’ve been having trouble figuring out how to spec the motor mount screws to shear within a predictable pressure range anyway. (my background is Electrical, not Mechanical Eng.)

Random question: are you wanting to have the machine run autonomously?

I am sure there are people on both sides of the fence but after watching my machine plunge a bit all the way into a board and continue for another 10 minutes I have been seriously lucky to not have a fire (just lots of smoldering) I would never plan to leave the machine alone for almost any length of time. There are too many variables.

3 Likes

Use a lathe to core drill the fastener until you get the failure at the rate you want. You will have a lot of work to spec it out. Totally achievable.

is it wort it?

thank you

1 Like

@ChrisB, I don’t aim to run it un-attended, but I definitely want to reduce the reliance on my response speed to mitigate catastrophe!

@Bee , I think it would only be worth it if I was simply putting in different machine screws…

Maybe not so much if it takes me weeks to figure out the specs!
(Although it seems like basic engineering stuff I should learn about anyway…)

Great idea with the coring of the screws!

Absolutely understandable and I think you will find that with the stock setup, your response time does not have to be anything extraordinary since it is probably moving about 20in/min or there abouts. I know some (myself included) are looking at changing sprockets/motors to increase this but as of yet I have not had the time.

Part of this is figuring out what you want the failure mode to be.

As bad as it is to burn out a controller board, having a chain come loose and
have the (running) router start swinging around the shop on the other chain
seems like a much more dangerous thing to have happen.

The forces here aren’t that big (66 pounds of force on each motor), it should be
pretty easy to make it so that the worst thing that happens is that the system
stalls, and nothing bends or breaks.

Having a bolt that breaks under force is good when not having the bolt break
will cause something else to break instead, but when the movement is slow like
on the maslow, having the motors stall is better than having anything break.

David Lang

2 Likes

Agreed!
I previously got the impression that the motor would never stall without breaking itself, or something else.

2 Likes

Late replier…

I don’t see anybody suggesting a fuse inline with the motor. Measure or look up the stalled motor current and use a slow blow fuse around that value, maybe? It might take some testing with the pull-tight part of calibration to verify that’ll work

1 Like

I seem to have burnt the left motor after the first calibration with TLE5206.
The strange thing is that I was able to cut some parts after the calibration, last weekend and this weekend the motor doesn´t work anymore (motor test actually passes but the motor moves very slowly).

I did notice much more deformation on the frame when tensioning the chain. Something that didn’t happen with the maslow shield.

I’m not using standard motors. Maybe the motor is the weakest link in my case.

I definitely need to try something like the fuse solution.

This is the same observation I had when switching to the TLE5206.
My guess is that the original shield or the power supply never get the motor to stall like the TLE.
What is the A out of your power supply?

6 Amps
Isn’t there a way to electrically limit the motor torque?

no, you can adjust the voltage you send to the motor, and you have the encoder
to tell you how far the motor moves, but you have no way of measuring the torque
the motor generates

different motors (even in the same model) will produce different amounts of
torque for the same power.

but the problem is that the motors draw more current when they stall than when
they are running, and the board has no way to measure the current that’s being
drawn. If too much current is drawn, the chips get too hot and are destroyed.

the new boards use a chip that self-regulates when they get hot.

David Lang

1 Like

Could 2 of the aux ports be converted to current sense ports for the motors? Using something like this connected in series to the motors should allow the arduino to sense the current draw of the motor.

If they work as expected, maybe the next revision of the board could even have these chips already hooked up for current sensing to the primary motor ports.

1 Like

those chips require an analog input to read, the arduino only has one analog
input (and it doesn’t go to the aux pins), so those chips couldn’t be used.

Yes, something could be done in a future board, but we already have a second
design that has controller chips that will self-limit so they don’t burn out. If
you have that, what benefit do you get from also sensing the current?

David Lang