Extruder problem

I have a problem with my extruder If the head is a distance from the bed (say 20mm) the extruder operates perfectly However if I try to print and the head is close to the bed the extruder start to skip bacwards and forwards I dont see why but it is almost as if there is a feed from the z axis sensor I am using Reprap firmware 1.09,

I decided to build a revised extruder with a spring loaded feed but exactly the same happens

Hi Dodger

Is your nozzle too close to the bed for the first layer? It may be that the nozzle can't extrude, as it's being 'blocked' by the bed, so the extruder skips. The nozzle should be the height of the first layer off the bed, eg around 0.3mm up. Make sure you get your first layer correct. See: [reprappro.com] and [reprappro.com]

Ian
RepRapPro tech support

Hi, Ian's right, but there is a little more to this for you to understand. What is happening is the initial height of the Extruder isn't right.
Basically, the glass (or tape if your using it) is blocking off the extruder nozzle. The Extrude is blocking the feeder and it can't push the plastic throu, so it jumps....
What you have to do is get the height right initially (this can be compounded by the temperature of the extruder as well BTW...[I use a temp of 215 or 225 for the hot end for the first layer] ie your extruder needs to be warm enough to make the Plastic filament the consistency of tooth paste basically).
Ok so how to set the height....
This is easier if you don't have filament oozing out of the nozzle.... Just lower the nozzle until you can't see light between the nozzle tip, and the glass/tape. Raise it 0.1mm and if you can see light, then lower it again by 0.1mm....
Then issue the G92 z0 command to the printer, to set this height to ZERO.... Slic3r sets the height of the nozzle ABOVE the glass for the initial layer.

3D printing is tricky as hell when you start off, so don't despair.... . Normally the reason why you are having problems getting a print to start is Bed / nozzle height, and having the bed out of level.
On thingyverse you will find mine and others designs for DTI mounts (Dial Test Indicators). These are really helpful in setting up the bed.
We all think glass is flat, and the X axis assembly is straight and true, until we mount a DTI and realize that it rarely is....
Hang in there.... It's worth the frustration, and soon you will spot what's wrong very quickly.

DC42 has done some cracking work with IR and sonic height sensors to pick up the Z0 height.... Personally I prefer to use the tilting method with my BigBlue fan duct, picking it up with a microswitch.

When starting off, or progressing.... Try 215/225 for the hot end and 57 for the bed temp. If the first layer is failing, then the initial height is out, or if it starts great in one area, but fails in another the bed is out.
I use the 3 point triangular Ally sub bed. Level the back first, then level the front. If you have a DTI use it to check the bed is level. After it's level near the mounting screws then check the middle.
Chances are either the glass has a bulge, or your X axis arm is out. This can be trued nicely using shims between the Xaxis arm and the stiffener.
Good luck, and don't give up.
Kim...

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