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QU-BD Design Improvements

The QU-BD extruder was designed to be a low cost replacement to the MakerBot MK7. In general the quality of the workmanship is good, but a lot of people have had troubles getting reliable performance out of them as designed. This page is a consolidation of all the improvements people have suggested. Even with the problems noted on this page, I would still buy another one. The price is great, the modifications are cheap and the result is a reliable high performance extruder.

The extruder works a lot better with ABS than PLA. PLA appears to climb up the nozzle and lock up when it cools. Smaller retracts can help, but not for everyone.

The files are posted below and on Thingiverse.

Drive Gear

The QU-BD drive gear attempted to replicate the success of the MakerGear drive gear, but altered it to add a groove. That groove reduced the sharp points and the gear has measurably less grip than other gears. The set screw has very few usable threads, so it tends to lose its grip on the shaft.

I suggest replacing it with a MakerBot style gear that is available from MakerBot and eBay sources.



MakerBot MK7 gear

Other Options

Flip the drive gear

There is a post about having success by just flipping the gear and using un-grooved portion.

Hob it with a tap

Check out this post on how to hob it with an M3 tap.

Filament Idler

The QU-BD system uses a set screw to push the filament against the drive gear. This has three basic problems.

  • There is no way to know how tight to make the screw.
  • It does not adjust for inconsistent filament diameters.
  • It adds friction to the moving filament making it harder to move.

There are two good solutions posted to fix this. They both use a spring tensioned idler bearing. An idler bearing provides a wide, flat rolling surface. The spring gives a consistent tension and self adjusts for changes in the filament.

The first solution is this one posted by whosawhatsis on Thingiverse.

The second one is by me. My solution requires a low profile button head cap screw, but might be easier to print on some printers because the tolerance requirements might be lower.

The screw I used is a button head cap screw 4mm x 10mm long. They are available various places. I bought mine from McMaster Carr (p/n 92095A190). Other low profile head screws may work.

The bearing is a 4mm I.D. x 13mm O.D. x 5mm thick. The standard designation i a 624 bearing. It is available at VXB or McMaster Carr. You should use a thin washer under the bearing so only the inner race contacts the bearing and the outer race doe not rub. You could print something like the washer, but I was afraid that might crush.

I got the spring from Home Depot. It is the middle sized one from the pack. (3/8” - 1-1/8” x 0.041)

You may need to clean out the holes with a drill. The mounting holes should pass the mounting screws smoothly so the lever can rotate freely. The bearing screw should self tap into the hole with about a 3mm dia drill.

The lever is slightly thinner than the block so there is a little gap to allow rotation. Fully tighten the block, but make sure the lever will rotate when tightening it's screw.

Assembly Instructions

There are standard and reverse versions based on which side you want to mount the heater block.

STL Files


The nozzle photos at the QU-BD store show a flat on the tip, but the delivered nozzles go to a point. These are extremely sensitive to damage. Many people have have to get replacement nozzles from damage in shipping. Be very careful when using them or you will damage them. They are difficult to impossible to repair.


Photo from store.

All fine nozzles will eventually clog. This is due to impurities in the filament. Use good filament and keep it clean. Use acetone for ABS to dissolve out clogs. PLA is more difficult to dissolve. Some people use drain cleaner to do it. The nozzle is compatible with MakerBot MK7 and other eBay nozzles, so it is often easier to just replace them.


The goal of a good extruder is to heat the nozzle, but not to allow the heat to migrate up the filament. If it does the filament gets soft and does not push as well and is more likely to be stripped by the drive gear.

To improve the performance in this area, you can use heatsink grease between the heatsink and the block that supports the nozzle. The motor can also get hot and conduct heat to the drive gear. You can use grease to conduct from the motor to the block.

Feed Tube

There is a lot of chatter about the roughness of the inside of the feed tube. When I push in a new piece of filament I can feel this roughness. This could probably cause a number of problems from friction to dust. I have not altered my feed tube and it runs fine.

I have also heard rare problems with leakage. I have not had these problems yet.

If you have altered/improved your feed tube, let me know about it and I will add that information here.


QU-BD has sells both power resistors and heater cartridges for their heater blocks. I have only used the cartridge and am very happy with it.

I used a little heatsink grease when installing it to get more even and quicker heating.

Some people have reported very loose fits.

Edit: It appears the people with loose fit were trying to use blocks drilled for resistors with cartridges.


The thermistor is very close to the 100k thermistor (Epcos B57560G0107F000) which is built into most firmwares. I use that, but if you want to get totally anal about the values, you can download a tables from the downloads tab on the QU-BD store page for the thermistor.

Per Qu-BD instructions, you attach the thermistor with Kapton tape and secure the leads with a screw. The screw eventually caused an intermittent short on my extruder. This led Repetier to think it was very cold. It turned on the cartridge full blast until it reached over 450 degrees (according to Repetier once the short opened again).

Another problem that can occur is the thermistor losing thermal contact with the block. That will cause a temperature runaway too. MakerBot adds an interlock switch to prevent this. I know several people who have had a thermal runaway, so it is not an issue to be taken lightly. No damage was done except for a little toasting of the insulation, but smoke was billowing off….scary.

I used the tape and a little thermal epoxy (Arctic Silver) I got from Fry's right on the bead. I then taped the leads to the heater leads for strain relief. The epoxy can easily be chipped away if you need to replace the thermistor.

ord_bot/qu-bd_extruder_improvements.txt · Last modified: 2013/03/19 09:15 by bdring