New Extruder Design

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New Extruder Design

Postby bdring » Tue Sep 28, 2010 12:38 am

I finally got my extruder working reliably. While it works fine, I would still like to make some improvements. I don't like the PWM controlled DC motor. PWM control seems a little arbitrary and highly dependant on the load. I would rather be able to set the speed via RPM. I would also like to run the feed a lot faster. This will make changing the filament on a bowden cable system a lot easier.

I like some of the worm gear systems I have seen, so I decided to give one a try. I think this will work well with a stepper as a stepper should really be geared down.

I came up with this design. I have the parts on order. This uses the worm gear as the drive. There is a bearing that pushes the filament onto the gear. I am using a spring plunger as the method to push bearing. I will start with a traditional extruder then move to a bowden cable version and the MK5 heater.
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worm_extruder_cad1.JPG

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spring_plunger.JPG
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Re: New Extruder Design

Postby lasersafe1 » Tue Sep 28, 2010 9:59 pm

From the little bit of attention I have paid to these, I would also think steppers are better. So the gears force the filiment into a heating chamber and squeeze it out like a glue gun? I would have thought it would be better to use a screw inside the heating chamber to carry the product down for a higher tip pressure. This is how the big extruders run. If you don't spin the screw the nozzle is off and there is no latent pressure that could release at the wrong time. Just throwing out ideas... no desire to build one myself.
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Re: New Extruder Design

Postby bdring » Wed Sep 29, 2010 12:41 am

LS1, There are some screw type reprap extruders, but none that I know that have the screw in the nozzle. The software as a retract feature that lets you remove the pressure. It works pretty well.

I cut some parts tonight. I am still waiting a few more. The square nuts work pretty well. They have just enough pressure so they won't fall out.
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worm_parts1.JPG

worm_acrylic_parts.JPG
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Test Assembly

Postby bdring » Fri Oct 01, 2010 12:34 pm

I am still waiting on some 6mm bearings for the worm gear shaft, but I was able to test assemble just using some 6mm nuts as stand ins for the bearings. It went together well. There are a few minor changes I will make, but it should work fine as is. I was able to clamp a piece of ABS filament in and do a static pull test. I don't have a way to measure it yet, but it has a good strong grip.

The spring plunger works great. It allows fine adjustment, but quickly backs off for manual feeding. I can't run the motor until I get the bearings. I bought them through VXB, so it will be a few days.
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extr2.JPG
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Extruder Worm Drive Works.

Postby bdring » Fri Oct 08, 2010 3:53 am

I finally received the 6mm bearings for the worm gear shaft. They were on order from VXB with some other bearings, but they forgot to put the 6mm ones in the box, so it took two shipments.

I got it all assembled and it works pretty well. I tested it under Mach3 control. I will switch it over to RepRap after some testing. It appears to have quite a bit more power than my MakerBot gear motor and it can go very, very fast. Here is a video of it running and reversing at various speeds.

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Thrust Bearing

Postby bdring » Wed Oct 13, 2010 5:49 pm

A possible problem with worm drives is the load they place on the worm shaft. Every pound of force on the filament is also on the worm shaft. In this case it is the stepper motor. I know a lot of people have done this without problems, but the stepper bearings are going to wear a lot faster. I added a thrust bearing to the assembly. The worm is mounted against the bearing which will take the thrust load. The bearing is in blue and a holder in green is used to transfer the thrust to the motor housing rather than the shaft. It only works in the extrude direction, but there will only be a little load for short durations the other (retract) way.

thrust_bearing.JPG
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Re: New Extruder Design

Postby macona » Thu Oct 14, 2010 6:21 am

I doubt the bearing is necessary. Not at these loads. Most steppers can take at a minimum of 13lbs of axial force on the cheapest steppers to 350lbs on better ones that retain the front bearing.

I cant imagine 13lbs of force on that injector. The gear would eat through the filament before you would get that high.
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Re: New Extruder Design

Postby bdring » Thu Oct 14, 2010 11:24 am

I have actually seen steppers that basically say you can't put any more axial load on the stepper than the weight of the stepper.

I was attempting to measure the push/pull strength of the extruder, but I was getting slippage with the worm axially. I think I was getting at least 13lbs of force before the worm slipped. I never got any radial slip even without a flat on the shaft. Unfortunately my design failed to leave direct access to the set screw (oops...next rev). I tightened it as tight as I could though. It is a 1.5mm allen wrench that does not apply much torque. I thought the thrust bearing would address both issues of load on the stepper and worm slippage.

either way, it would not be over engineered without a thrust bearing :lol:
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Re: New Extruder Design

Postby macona » Fri Oct 15, 2010 3:53 am

I would say the those steppers are the ones with bronze bushing instead of ball bearing.

What you really have to watch out for is the shaft moving in. Lighter duty stepper do not retain the front bearing in the end bell. You can tell by pushing in on the shaft. If it moves its a lighter duty motor at about 13 lbs. If it does not move than there is a retaining ring and you can go a lot higher.
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Re: New Extruder Design

Postby Racer » Wed Mar 07, 2012 12:50 am

Do you have any more updates on this nice extruder project?
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