Monday, January 4, 2010

McWire After Build Issues

Some short comings of the McWire design have shown themselves after the build.

The first one concerns the method of attaching the drive screw to the Z sled. I seems the constant pressure on the tape covered coupling nut made the tape move over time. It telescoped out until the holding pressure in the conduit ran out. Then slipped out of the strap.

My solution was to build a bracket that presses the coupling nut against the Z sled, holding it in place. Overall this is a better way to go as you can adjust the height from the Z sled with a shim to get the drive screw parallel to the sled. This was a pain to do with the tape wound design.The X and Y have not been changed out but I will use the same solution if they become a problem.

The bracket material came from an L bracket that is used to hold drapery rods. A piece of 1/8"thick aluminum would do as well. It's held in place by 2 - #10 screws and tapped holes in the sled.

Another failure that has been occurring is breaking swing arms. They are not standing up to the task. I think it is the constant pressure of the spring and the size of the 5/16 hole plus counter sink relative to width of the swing arm. I have reduced the spring pressure as low as I fell comfortable with but failures are still occurring. I had 4 spares made when I ordered the laser cut parts and those are used up and now I am making parts to replace them.

I think the newly designed swing arms are working great and are easy to build. I used 3/16" press board because that’s what I had on hand. 1/4" thickness might be better.  1/8" aluminum would work nicely.

To make one of these :

  1. Use a template to mark the outline and hole locations. 
  2. Cut it out roughly with the outline markings and the drill 1/16" pilot holes for all 3 holes. (larger for aluminum)
  3. Drill the clearance hole (23/64") for the 5/16" bolt holding it to the sled.
  4. Mount the bearing with a small washer (M5) under it using a dry wall screw. 
  5. Screw in a #2 sheet metal screw to hold the spring.
  6. Dremel off the length the the screws protruding from the other side and grind them down just below the surface so the don't scratch the plastic on the sled.
  7. File the shape of the swing arm down to the outline marks. 


Conseils said...

How about applying some PVA adhesive to the wood to make it a bit tougher. 'Stronger than the wood its self'... if I remeber the advert. You could glue two together or add some cotton fabric to create a composite.

I saw the cracks in a swinging arm in your previous post. I wondered how long it would stay together. Is this the same arm?

I have a thought on your bracket, which is that if the alignment between the motor shaft axis and nut are in error then a number of things may happen if there is no compliance in the drive train. From putting force into the front of the stepper motor bearing to lifting the plate away from the PTFE bearings or cracking the perspex. Best case the coupling frets, at least that would be easy to replace. But I seem to recall that your coupling was quite stiff?

Jeff said...

I've had some luck with using a piece of thick-walled flexible tubing to hold the z-stage in place instead of the tape. I just used the same stuff that is holding the threaded rod to the motors, might give it a try if you need a quick fix in the future.

bjbsquared said...

@Conseils Reinforcing the wood is an option by I think it will do fine the way it is for now anyway. Time will tell. I think I would really like them to be aluminum but for now, the wood will do.

It most likely was that swing arm you saw. One of them was fine until i took pressure off and then it crumbled.

I see your point about the drive train stiffness. I have some down time while I wait for screw terminals to come in for the wiring so I might replace the PEX pipe with the tubing. I am able to turn each axis by hand using the 1/4" shaft so there is not really much drag. I think the tubing for the Plastruder will work fine.

Anonymous said...

There is a reason why the master design called for nails threw holes over screws.

Nails can move a little up and down so not cause virbrations threw the device to start cracking.

Changing some designs minorally cause major problems.

bjbsquared said...

@ Jeff - I see how that would work. The tubing would offer a bunch of friction and still be forgiving... I need to pick up some tubing.

Krafter said...

I recently finished my McWire and ran into the same Z axis issue. I ended up coming up with two solutions.

The first was to use some tubing the same way the electric tape was being used. This was supposed to be a temporary solution but its held great for two weeks so I might just stick with it.

The second was to use silicone to 'glue' the coupling nut to the clamp. This is also working well.

I did JB weld too but its too brittle so I haven't even tried it. I just know its not going to work well because it doesn't flex.

Great to see another McWire out there.

The big issue I am having right now is tuning Skeinforge. There are no presets I see for a McWire so nothing is turning out quite right when I extrude plastic.

Chris Kraft

bjbsquared said...

@Krafter - Thanks for the great info. I still have to hook up the electronics do the firmware stuff and then I will be tuning Skeinforge as well. Hopefully you'll figure it out before I get there ;)

eeparkerj said...

I've been working on skeinforge settings for a bit and I'll post what I'm using sometime this week. The main thing is to get your flow rate calibrated as the McWire moves pretty slow vs a belt driven machine. It tends to end up with extrusions thicker than you expect.

Here is a mcwire machine.xml for ReplicatorG if you use 1/4-20 threaded rod:


Krafter said...

Thanks for the machine.xml settings @eeparkerj. That is very close to what I have so I at least feel like I have that part right. I even had the same max values 300mm/m although I've boosted them in an effort to get my machine moving quicker.

Right now my main issue is I get too much plastic coming out.

I've slowed the rate on the extruder down to 185 and I still get too much. I also changed the extrusion diameter over thickness and set the feedrate (mm/s) as high as I can go without having the steppers stall.

I must be missing something obvious. Maybe later this evening I can put pictures of my failed attempts up for review.

-- Chris