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This is probably the last thing that will hit this blog.

I made a pair of drums on bearings that were intended to allow me to roll off filament quickly, driven by an electric drill, while weighing it.

Both drums lift off the stands and have a quick remove side so that you can get a roll of ABS on or off.

In practice there were a bunch of problems:

When I put one of the 5kg rolls of batch 0 ABS on a drum and simply pulled the end, it jammed: the bit I was pulling went down between all the other coils and got stuck. I think drawing it out of the middle like this would have been better.

The timing belt from the drill drive was too loose and kept slipping.

The drill was hard to accelerate gently, and went too fast. Really I needed a gearbox.

When the drill stopped (quite suddenly) the rotational inertia of the drum and ABS was enough to either make the whole thing fall over, or undo one of the nuts on the stud I was using as a shaft (despite lock nuts and thread locker), or both.

I gave up on the drill and moved both drums by hand. After ~200m of rolling and about 10 weighings I hand the 2 kg roll I wanted. This is just too time consuming to do regularly, and at the moment I have neither time to make nor space to keep a machine to do it well. Sorry guys, but you’ll have to buy feedstock in the size rolls that it arrives in. Only 5kg rolls are in stock at the moment.

I’ll do a write up of the results when I’ve tested the prototype, but here’s the gist of the design:

The bearings are omitted from the images below. There will be a 624ZZ at each dimple around the edge, and 608ZZs around the drive shaft. I’ll machine the prototypes from nylon and 6082T6 Aluminium alloy, but this design should be printable.

100:1 Hypocycloidal reducer (exploded, view 2)

Here’s one that someone else made (using some printed and some machined parts as best I can see):

Here’s how it works, in short: the cranked drive shaft causes the lobed rotor to move eccentrically, such that the lobes on only one side at a time engage with the rollers around the edge of the casing. There is one more roller than there are lobes, so when the crank has rotated by one revolution the roller has counter-rotated by one roller. The second set of rollers on the output flange have a similar interaction, so for each full revolution of the rotor, the output flange rotates by one roller.

If you’d like more information on these gearboxes, I recommend the page I started with as a jumping off point: http://www.zincland.com/hypocycloid/

Hypocycloidal gearboxes are interesting here because they offer high torque and high ratio in a small package with large features, and so may be printable.

Why we want a better gearbox

Our extruders, even the geared ones, are torque limited: they could easily extrude faster except for the fact that the drive doesn’t have enough torque. Typically the extruder drive stepper motor will run below 10rpm. We are a long way from reaching the peak mechanical power in our drive motors, let alone the maximum step rate. The obvious improvement is to use a higher ratio gearbox.

Stepper motors have an S shaped torque/rpm curve: high torque at low rpm, rapidly falls off at medium rpm, and tails away gradually to nothing a very high rpm. Mechanical power = torque * speed. For any stepper there is an rpm that gives the most possible power, where torque is still high, but speed (rpm) is high too. If we can operate the motor at around that speed, we get the most out of it.

Consider a typical NEMA17 stepper: the Zapp Automation SY42STH47-1684B. PPM on stepper data sheets mean “pulses per minute” and that’s the same as “steps per minute”. From the data sheet we see that it has a torque of 0.32 Nm @ 1000 ppm (= 5 rpm) and 0.20 Nm @ 7k ppm (= 35 rpm).

0.32*5/60 = ~27mW mechanical power at 5 rpm

0.2*20/60 = ~70mW mechanical power at 35 rpm

ie: at the fastest speed that the datasheet provides a spec for, the mechanical power is still increasing. For the sake of argument, assume that our target motor speed is 80 rpm.

Suppose that we wanted the extruder to extrude at 32mm/s, and we decide on a 30mm diameter feed wheel, and we have a 0.3mm tip orifice.

The feedstock has a cross sectional area of 3*3 = 9mm^2
The extrusion has a cross sectional area (ignoring die swell, etc) of 0.3.*0.3 = 0.09mm^2

This means that for each mm we feed in, we get 9/0.09 = 100mm out.

So for an output rate of 32mm/s we input feedstock at 0.32mm/s

Which implies that we turn the feed wheel at:
60*0.32/(pi*30) = ~0.2 rpm

Since we want to get 0.2 rpm at the gearbox output with 80 rpm at the input, we need an 80/0.2 = 400:1 reducing gearbox.

Under no load, the stepper can probably do something like 1000 rpm, so with that ratio we will still have a good speed headroom for ooze prevention and other similar uses.

Why do we want a larger feed wheel? Two reasons:

Larger diameter should improve the grip on the filament (more contact area).

Larger diameter should increase the sytematic accuracy in the feed rate. As nophead commented, it’s hard to know what the effective diameter of a pinch wheel is, because it’s hard to know how far the filament is pressed into the ridges on the wheel. Since you don’t know the diameter, you can’t be sure exactly how much filament you’re feeding per rev of the wheel. As the wheel becomes larger this effect becomes less severe, since a given absolute diameter variation would be a smaller proportional variation, and so a smaller variation in the amount of filament feed per rev.

Ideally, I’d like a 400:1 reduction, but I’m starting with 100:1, and we can take it from there.

100:1 Hypocycloidal reducer (exploded, view 1)

This is a work in progress. I’ll post design files at some point. Comment if you have an urgent desire to work on them.

If you want to participate, you post a comment here (or email if you prefer not to post). Say what type of extruder you have. Anything is welcome.

I send you two 1.2m lengths of filament at my expense, and give you 20% discount from your next filament order.

You try the sample, and post your impressions here.

Everybody wins (which is always my favourite outcome. ).

I didn’t have any tool that would cut big rods of any material.

I considered getting a circular metal cutting chop saw, since they seem to work fast and simply on thin rods and all sorts of fairly thin walled profiles (Angles, pipes, etc). However, after substantial searching I couldn’t find any evidence that such a saw will cut big solid bars (eg 30mm diameter round let alone 100mm round). Also, most engineering plastics, such as Nylon, can’t be cut cleanly with a circular saw because the fast moving blade and widely spaced teeth tend to cause chipping.

I settled on getting a bandsaw instead. The bandsaw is far more versatile, has much cheaper blades, and was cheaper to buy too (I love ebay!).

Bandsaws come in two main types: Horizontal and Vertical. Vertical bandsaws are mostly used for cutting plate or sheets, and are more common in woodwork than metalwork. Horizontal bandsaws are mostly used for cutting long thin things (rod, bar, and profiles such as pipes and angles), and are mostly used in metalwork. After a bit of research and some hunting on ebay, I have a 12″x7″ Axminster horizontal bandsaw. The 12″x7″ means that it can cut (almost) anything that is no more than 12″ wide and 7″ tall. So I can use it to cut metal (even stainless steel) or plastic rods, bars, or bundles up to ~7″ (~180mm) diameter (which should be plenty!).

This is the most heavy duty tool I’ve ever owned, and it’s big too: 1.6m long. I’m excited about using it, and it’s a major step towards being able to do much bigger metal fabrication than I ever have before.

The new (to me) Axminster MB712 in all its glory

The saw was (as far as I can tell) in mostly good condition. I cleaned it, serviced it, fixed the cutting coolant pipe, and I’ve ordered a new No-Volt-release switch for the coolant pump, because the old one is broken – it doesn’t latch on. The total cost, including parts, was just GBP 165.

Bandsaw blades are available in two main materials and various tooth pitches and shapes.

Bigger stock and softer materials need more widely spaced teeth.

Carbon steel blades are heat tempered to a fairly high hardness along the edges to reduce wear, and left at a low temper in the middle. They are usually used on mild steel and softer materials, and are available in the widest range of sizes and tooth configurations. I got a pair of carbon steel 3 tpi (tooth per inch) wide set blades for cutting plastics. Wide set means that the teeth are bent out from the plane of the blade in alternate directions, so that the cut is wider than the thickness of the blade. This reduces contact between the blade and the stock, so it reduces frictional heating, which should reduce the tendency that plastic has to melt when you cut it. I may also use these blades when cutting large Aluminium bars.

Bi-metal blades have a wide strip of spring steel along the back and a narrow strip of HSS (high speed steel) along the front. At room temperature HSS and high carbon steel are similarly strong, but HSS keeps its hardness and strength much better as the temperature rises. This makes it a far superior tool material for cutting hard or tough stock such as stainless steel. It will also last much longer and may cut faster in softer stock (eg: mild steel or Al). I got two each of 4-6 tpi and 10-16 tpi M42 bi-metal blades.

The 4-6 tpi means that the tooth pitch varies continuously between 4 tpi and 6 tpi. This variation reduces the tendency of the blade to chatter or squeal because chatter and squeal are both resonant phenomena, and the wide variation in tooth pitch means that not all the teeth will excite all the resonances, whatever their frequencies. I think it also makes the blade able to cut a wider range of stock materials and thicknesses adequately, but I’m not sure about that.

The 4-6 tpi blade is for small stock, especially aluminium and plastic stock, and the 10-16 tpi is for 8mm steel and stainless steel rod.

I bought new blades from Just Bandsaw Blades. They were able to offer expert advice, and custom weld my Bi-metal blades in time for next day despatch. All six blades cost little more than a single aluminum cutting blade for a circular cut off saw (such as the Evolution Raptor) – Just under GBP 100 all in.

Changing the NVR switch looks like it will be pretty easy. After a bit of googling found a likely candidate on ebay, and it should arrive later today.

The broken NVR switch for my bandsaw

The cleaning and service only took a few hours, too.

Truing the vice to the blade with an engineers square

Making sure that the blade is perpendicular to the table

When I tested the coolant pump the coolant splashed onto the floor.

Coolant splashed widely when I tested the pump

I’m not sure how to stop this happening, or even if I can. I’ll try reducing the coolant flow and lengthening the delivery hose so that the coolant doesn’t have so far to fall.

Free, £1.50 postage and packing.

I’ll send you a couple of meters, wherever you are in the world. Note that the material has slightly variable dimensions along it’s length. This means that if the sample works for you then the full roll will almost certianly work too (but I can’t guarantee it).

The cross section is not quite circular. It’s more egg shaped, so it’s wider in one direction and thinner in the other. Here are some diameter measurements taken from a 5kg roll picked at random:

Near the cut end, in the thickest direction:

Measuring the diameter of a random sample from ABS feedstock filament batch 0, 3.27mm in this case.

… and in the thinnest:

Measuring the diameter of a random sample from ABS feedstock filament batch 0, 2.68mm in this case.

Somewhere in the middle, in the thickest direction:

Measuring the diameter of a random sample from ABS feedstock filament batch 0, 3.31mm in this case.

… and the thinnest.

Measuring the diameter of a random sample from ABS feedstock filament batch 0, 2.70mm in this case.

I’m told that it does vary a bit more than this on some rolls, but these measurements appear to be typical.

I’ve been making enquiries with a bunch of factories in China and India, to get top quality ABS feedstock for a low prices (so that you can have the best filament for the least money ).

Most factories want to deal in tons and container loads (~22 tons for 3mm ABS plastic rod). I’ve found three Chinese and one Indian factory that would like to work with me, and requested samples.

Today the first sample arrived. They didn’t have rolls of rod in stock, so they sent me 0.5kg of 1.2m rods.

The first ABS sample shown with its packaging

I’ve measured a random sample and they’re holding excellent tolerances, 3.04 +/- 0.07mm as you can see below. The rods are quite flexible, much more so than the last batch and more like the ABS that I bought from MakerBot just over a year ago.

Measuring the diameter of ABS feedstock with a digital caliper

Measuring the diameter of ABS feedstock with a digital caliper

Measuring the diameter of ABS feedstock with a digital caliper

Measuring the diameter of ABS feedstock with a digital caliper

Measuring the diameter of ABS feedstock with a digital caliper

Measuring the diameter of ABS feedstock with a digital caliper

Measuring the diameter of ABS feedstock with a digital caliper

Measuring the diameter of ABS feedstock with a digital caliper

I’ll test all the samples in my extruder (once that’s ready), and complete the an as soon as possible.

If you’re thinking of buying ABS soon, I advise you not to wait since I may be out of stock for a little while between these batches (Sea shipping takes a long time).

I’m proud to offer them for less than half the next best UK price I could find.

Please buy using the cart (and let me know of any problems with a comment here or an email).

Now I can get on with something more interesting.

For anyone else trying to do this:

• Here are a good tutorial, the google tutorial for international shipping, the ISO country codes, google checkout sandbox docs, google shipping restrictions and address filters docs, and google geographic areas docs. It isn’t actually hard. Just fiddly, especially if you want to specify shipping for ‘Europe’.
• Use the sandbox until everything seems to be working. The sandbox gives better error messages.
• Use a binary chop (remove half of the code at a time) to identify the sources of errors. Start removing at the bottom so that your fields are always numbered contiguously from 1.
• Get your numbering exactly right. Many of my problems were a field number missed out or duplicated.
• Don’t try to include or exclude Andorra. Don’t mention Andorra.
• The Checkout may confuse Alabama and Albania, as regards shipping restrictions: Residents of Alabama will be able to choose shipping options for Albania. Don’t sweat it too much.
• Feel free to use my page source as an example.
• Test it with the live system. The live system is more stringent.

Send me an email when you want to order. leo@fabricationsofthemind.com