Effects of slop between hobbed bolt and bearing

Why are we using m8x22 ball bearings for the extruders and then putting a 8mm "hobbed bolt" through it that is smaller than 8mm? If you could buy bolts that actualy measure 8mm on the OD this would work fine but most if not all bolts are purposefully made about .25mm smaller on the OD. This leaves a bunch of slop between the bearing and the bolt. Maybe this is not enough slop to cause problems, i dont know. But it surely causes issues when the filament feed direction is reversed doesnt it? Ive built about 3 or 4 printers and each of them seem to have the same problems with the extruder retracting filament and then not laying down enough plastic for a moment when it prints the next line. All of them have used the wades style extruder.

Is this a big enough issue to try and fix or am i chasing the wrong problem?

You won't like bowden extruders then, there is plenty of slop there, but I get by.

The shank of a bolt is always meant to be slighlty smaller than the bolt diameter. Perhaps instead of using a bolt one could use an 8mm rod which will be exactly 8mm diameter and should fit snugly in the bearing.

You may then need to cut a thread at one end to be able to secure the drive gear with nuts. You would need to use a mild steel rod rather than the hardened rods such as used for the carriage as that would probably be too hard for the die to cut into.

Alternatively, if you do use a hardened rod you could just use another method to secure the gear, such as a key and a clamp. The key slot shouldn't be too difficult to cut with a dremmel using a diamond edged cutting disk, perhaps the same as used for hobbing.

If it were beneficial i can easily make bolts with an acurate OD. I manage a machining company and own a few machines myself. In fact i machined my own hobbed bolts but i have just used a bolt and cut the grooves into it. It wouldnt take much to machine a thread on both ends of a shaft and then weld a nut onto one end to drive the bolt from the gear, same way that is currently done. Im just not sure if there is anything to be gained. Guess theres only one way to know for sure.

Do you use Slic3r? There is that command to extrude extra "_mm" in the tab that controls retraction.

Between the tolerances on the bore diameter on the bearing (as much a 0.1 in low cost untis) and whatever rod is used the fit may be problematic. An 8mm thrtead cut is not 8mm after it's cut. There are thousands of printers with this design making good prints. There may be an issue that with that particular assembly or calibration but as a whole the design is adequate for low cost applications.

I offered people the choice of knurled surfaces where the bearing meet to ensure an accurate fit, but it seems that it is not needed, and no one wants to pay for it. Slop is negligible because of the other bearing limiting its distance of travel. In fact I'd be more worried about the distance of travel from the plastic flexing, which also is not an issue because of the bearing holder on both sides of the extruder.

and just an fyi, an 8mm bolt is between 7.9 and 7.94mm.

Edited 1 time(s). Last edit at 08/29/2013 08:19PM by jamesdanielv.

You don't need to fix the bolt in place if you are using fishbone gears to drive the bolt. I have ben running with this: [forums.reprap.org] hobbet bolt for quite some time.

It is made from stainless steel, thoug tool steel would probably be better, but it works. The nut is a standard M8 welded on, and milled 12mm, so that it runs true.

If you could fint a bolt with a shaft long enough, I think it could be made the normal way with a tap.

Tolerances on bolts and nuts are pretty lax, i bought a bunch of 8mm bolts that actually measure 7.75mm on the straight shaft. I suppose if you were to buy some certified bolts you could get more consistent diameters.

Instead of making a whole new hobbed bolt, why not tape around the bolt you have to take the slack out? That should give you an idea of what improvements it might make, if any. Some of that aluminum duct tape might work really well for that... just a suggestion. Let us know what you find.

The inner race of the bearing will be pressed against the bolt by the pressure from the
pinch bearing so the chances of anything slopping about or rattling are pretty remote.

Hobbed bolts are bolts that drive the plastic filament through the hot end. It is an incredibly difficult thing to make when you need hundreds to be exactly the same. Development these bolts take considerable time.