hobbed bolt bent?


I was having a lot of trouble with my extruder tool kit on my TAZ 3.1
Main issue has been slipping of the small herringbone gear from the extruder motor shaft

Recently, Ray from your Lulzbot shop and I have corresponded and exchanged nearly 70 mails in 2 conversations on these issues. So we covered everything and couldn’t come up with the solution why my transmission faults the small gear to slipping.

The whole system of two gears was kind of lent to one side a fraction, that is to say it was not perpendicular to the plastic parts that carry the entire construction.

To cut the story short, in the images you will see how the hobbed bolt is not cut out well.
That is my most recent discovery, because I would’ve never guessed that this could be the issue

I would kindly ask you to tell me if my hobbed bolt could be the reason for my problems?

Best regards,

Hobbed bolts are usually made by placing a bolt into some sort of machining jig and cutting the ridges at a certain rotation. Bolts tend to vary from manufacturer to manufacturer, but within production runs they are usually pretty similar. It would take a great deal of force to bend that large of a bolt, but you could easily check it for straightness by placing it against a straight edge and observing any variance.

If it was bent, the effect you would see would depend on the location of the bend. A bend right at the head of the bolt would cause a gear wobble in and out from the face of the extruder. It’s possible that is the issue, but it’s much less likely than any of the other causes of gear slippage.

The small gear has a set screw in it. That setscrew can eaily back out and fail to engage on the shaft. The plastic of the small gear itself that holds the nut that the set screw sets into can crack and split. etc. I’d look there first.

Hi there,
Firstly, thank you for the reply.

All the possible issues with the small gear, such as the set screw or the M3 nut issues have been covered in correspondence with support team. I’ve changed several small gears, and even used the threadlock glue for the set screw
…it just won’t hold after a while

My guess is that the hobbed bolt actually is bent at the root near the head. So the movement around it gets irregular

I am not suggesting that the screw could somehow get bent from usage, because that hard steel cannot be bent in that way
Maybe it was just made like that

So here is the video of the check up:

It clearly wobbles.
Do you think it could be the cause of my problem of the small gear slipping? The way the large gear transfers force unequally


The gear itself looks warped to me.

That is because the tip of the gear has a delaminated layer… the body of the gear completely flat

From your earlier photos and the video, it seems like the head of the bolt is not perpendicular to the shaft. Very strange.

You are quite right!
That’s what I have been trying to show with those images. Head sides are not perpendicular somehow
That couldn’t have happen during printing? I mean, it takes a lot of force to scew or bend steel…

I am just puzzled, if the bolt was wrong from the start, how come I didn’t get my problems earlier.

I heard about the existence of spring/give with some extruders… maybe I could find a design and try to incorporate it somehow

We have checked 3 batches of hobbed bolts and roughly 70% are the same as yours. This is perfectly normal and has never been the source of any extruder issue when assembled properly. Thanks!

Thank you huffray,

So far, I’ve assembled my extruder tool kit for about 10 times or so, and getting pretty good with it.
I’ve changed several gears, etc… the instruction for the assembly in the OHAI kit for the tool head are quite simple and well explained.
Only thing I am left with is to change the hobbed bolt, and maybe some other printable parts on my extruder tool head



A new question.
Here is an image of my slic3r settings for extruder:

Pay attention to the Z lift setting I am using, in combination with “retract on layer change”.
In the explanation for this retraction option it says that it does a retraction move on every change of Z parameter.
So if that includes Z lift as well, could I have been having too much of retraction?
So much that it overloads gears with force, and then causes my small gear to slip after a while?

So simply put,

Can highly frequent retraction cause issues with the gears? Because retraction is the fastest move concerning gears …