1.75 filament hobbed bolt

Hi, i want to manufacture a hobbed bolt for 1.75 filament. I know i could buy some, but i have access to a workshop, and thats faster.

What is a common thread diameter for the die? M2 od M1.6?

ps: I've found nothing by search, so excuse me if there are threads out there.

Try this one [www.reprap.org] on [www.reprap.org]

I've made the hobbed bolt with a M1.8 die. Works really great. The bite marks reach from one side of the filament to the other and for me its impossible to hold the filament with my fingers. I attached images.

Now i have another problem:
I want to have the possibility to print at a speed of 300 mm/s (printhead speed). So i calculated a feeding speed of 6.23 mm/s for the extruder. I tried to feed 6 mm/s. But sometimes the stepper looses a step or a filament dig out occures. The extruded filament has a diameter of 1.5 mm (!!!) at a nozzle hole diameter of 0.5 mm.

Is this print speed not possible with my hobbed bolt? Or do i have an error in my calculation?

Filament diameter: 1.75 mm
Filament cross sectional area: (1.75 mm / 2)² * Pi = 2.41 mm²
Nozzle hole diameter: 0.5 mm
Layer thickness: 0.1 mm
Speed: 300 mm/s

Volume per secound = 0.1 mm x 0.5 mm x 300 mm/s = 15 mm³/s
Feeding speed = 15 mm³/s / 2.41mm² = 6.23 mm/s

gear it down

Kysan 42BYGH243-5

1.5 coming out of a 0.5 nozzle? That's a lot of die swell!

You can gear it down to keep from losing steps (or feed it more electricity), but if you're grinding filament you're at the limit of your grip. That enormous die swell suggests that you should try a higher temperature. That will make it easier to push the filament through too. You may need to put more power into the heater to keep up with your aggressive speed target. Try again near your best achieved speed so far and monitor your extruder temperature to see if the heater can keep up with the amount of plastic flowing through. You may need a more powerful heater.

The need for speed!

@peach303
Your hob looks great.

Quote
Dale Dunn
if you're grinding filament you're at the limit of your grip. That enormous die swell suggests that you should try a higher temperature.

I agree.

Quote
peach303
Is this print speed not possible with my hobbed bolt? Or do i have an error in my calculation?

It's possible with your setup to extrude at 300 mm/s, will it make good parts, I don't know, and yes you have an error in your function.

Dale Dunn is correct, you are at the limit of your extruders feed rate because the temperature is to low.

This is evident due to the facts that you have missed steps, the filament is stripping, and the large extrudate diameter caused from the die swell.

To achieve your goal of 300 mm/s you will need to increase the temperature, which will reduce the extrudate diameter caused by the die swell.

Variables:
Filament diameter: 1.75 mm.
Filament cross sectional area: (1.75 mm / 2)² * Pi = 2.40 mm².
Extrudate diameter (realized): 1.50 mm.
Nozzle orifice diameter: 0.5 mm.
Layer thickness: 0.1 mm.
Print head speed: 300 mm/s.
Temperature: 000 C?.
Upper filament feed rate: 6.00 mm/s.
Material: PLA, ABS, ?

Filament feed rate given: 0.1 mm layer hight @ temp 000C at a rate of 300 mm/s:
Volume of a cylinder = pi r² h
Extrudate vol/s = 3.14 x (1.5mm/2)² x 0.1 mm h x 300 mm/s = 52.99 mm³/s

h = v/(pi r²)
Filament feed rate (h)/s = 52.99 mm³/(3.14 x (1.75 mm/2)²) =
Filament feed rate (h)/s = 52.99 mm³/2.40 mm² = 22.07 mm/s

Because you are occasionally stripping filament with a freed rate of 6.00 mm/s,
we will assume that a rate of 22.07 mm/s is not achievable with the current parameters, and that some thing needs to be adjusted or changed.

Your goal is rate of travel, so lets keep the filament diameter, temperature, and orifice diameter constant,
and adjust the extrudate diameter to a size that works with a feed rate of 6.00 mm/s, and then determine if the orifice diameter is up to the task.

Rearrange your formula, work in reverse, solve for extrudate diameter:
Filament feed rate (h)/s = ? mm³/2.40 mm² = 6.00 mm/s
v = mm/s x a
Extrudate vol/s = 6.00 mm/s x 2.40 mm² = 14.4 mm³/s

Extrudate vol/s = 3.14 x (?mm/2)² x 0.1 mm h x 300 mm/s = 14.4 mm³/s
3.14 x (?mm/2)² x 0.1 mm h x 300 mm/s = 14.4 mm³/s
(?mm/2)² = 14.4 mm³/s /( 3.14 x 0.1 mm h x 300 mm/s)
?mm/2 = sqrt(14.4 mm³/s /( 3.14 x 0.1 mm h x 300 mm/s))
Extrudate diameter = (sqrt(14.4 mm³/s /( 3.14 x 0.1 mm h x 300 mm/s))) x 2
Extrudate diameter = (sqrt(14.4 mm³/s /94.2)) x 2
Extrudate diameter = sqrt(.152866) x 2
Extrudate diameter = .39098 x 2
Extrudate diameter = 0.782 mm

An extrudate diameter of 0.782 mm is achievable with a Dia. 0.50 mm orifice.
So now you need to increase the temperature to help increase the flow rate.

Goal: increase the temperature until you extrude a Dia 0.782 mm extrudate.

If the temperature exceeds the plastics melt temperature, you will need to switch to a different material that has a higher flow rate,
and/or increase the orifice diameter. I suggest that you create a matrix of your empirical test results to help guide you.


Hob:
If you have access to a machine shop, and your goal is speed

I would not use a tap as the hob profile, as it has a helix, which will impart a lateral, and torsional force components to the filament.

The lateral force will tend to push the filament out of the hobbed profile,
and the torsional force will apply a shearing action to the groves in the filament, which will cause the filament to strip.

The lateral and torsional forces reduces your ability to push the filament through the die at maximum feed rate.
If speed is your game, then the helix is working against you.

Ideas for a redesigned hob:

1. Straight knurl: grind a knurling die to the desired width (use a steady rest).

3. If you don't like the profile generated from the straight knurl (rounded crest), make an OD broach using a wire EDM to form a straight helix.
Machine it from a tool steel and have it heat treated.
If the tooth profile is deep, machine two broaches.
Locate washers on either side to guide the filament.

HTH

A2

Edited 1 time(s). Last edit at 12/10/2013 06:51AM by A2.

The orifice length also plays a big role. If the 0.5 mm hole is too long, it will limit your speed. Too short will wear out quickly and also be prone to more ooze. Striking the right balance can play a big role in print quality.