Acrylic, Polycarbonate, Wall Thickness And Commissioning Work From A Machine Shop

[public alias]

Hello, I'm in the very early stages of developing a pen design which employs a somewhat unusual piston filling mechanism, and I'm hoping to glean some insight from the craftspeople of FPN.

 

I'm at a stage where I'm designing the parts on CAD, but before I get too carried away, I figure I should seek some understanding of what is actually possible in terms of turning acrylic on a computer controlled lathe. Would 1mm wall thicknesses be reasonable for a cylindrical feature which is about ~15mm in length? Should I go thicker if this part has internal threading?

 

Are there alternative plastics to be considered, which will hold up to machining stresses on very fine features? For example would poly-carbonate be any more advantageous than acrylic?

 

Lastly, does anyone have any experience with sending their designs to machine-shops? I found one, emachineshop.com which seems to be geared toward providing unusual one-off parts for hobbyists. I'm currently working on Inventor, but emachineshop offers a free cad package with built in price quotes for machined parts, so I'll be experimenting with that later today. Another shop I'm looking into is protolabs.com

Edited June 15, 2015 by public alias

[richardandtracy]

After threading, 0.5mm wall thickness is just about OK on cast acrylic, but will be weak. Better to aim for 0.75mm or above at the root of the thread, so depending on the thread you want, 1mm wall may be OK if the pitch is fine enough. If using acrylic with veins or colour changes in it, the boundary between the colours is a danger point, as the colours sometimes don't adhere well together in the blank, and 'snick' is the worst sound a pen maker/machinist will hear - it's the sound of the colours detaching from one another.

 

If the threads are machined, I fear polycarbonate or extruded acrylic will crack at the root of the thread. If moulded, PC may be OK, but it is finicky stuff and occasionally cracks regardless of good design & injection practice (as happened with a number of the TWSBI 5xx versions). As for machining fine features in PC - I have never had it work out for me due to cracks developing at the root of sharp features, and always use Delrin (Acetal) for such bits. Delrin does not take a polish though, and you are limited to slightly translucent white or black. Polyester is not really a great material to use, it does have an advantage in that it can be cast to size, but is weak, moderately brittle and subject to shattering at low temperatures.

 

As for using a jobbing machine shop - no, I've never used one. Always machine my own.

 

Regards,

 

Richard.

Edited June 16, 2015 by richardandtracy
Added detail

[public alias]

Thank you for your reply RichardAndTracy,

 

It has been a interesting challenge, trying to make something that's sleek and slim, yet also possible to manufacture. At the same time, there are plenty of people who enjoy big pens, and after seeing your reply I went ahead and thickened the effected areas.

 

I already heard from two machine shops, both of which say they don't have the capabilities to machine the part. The issue seems to be that one part has a relatively shallow section of internal 9/16-32 threading which needs to actually be milled out - I'm told that taps don't work well for very shallow sections.

[richardandtracy]

Absolutely, taps don't work well on thin walls. There is a tendency for the material to burst unless it is tapped before turning the outside.

 

If someone you know has access to CNC, and is willing to try thread milling with a single point cutter, then I have written some PC software to generate G Code toolpath codes for threads, multi start/single start etc. This is available here: http://www.chestnutpens.co.uk/misc/downloads.html and scroll down the left hand pane to 'CNC Thread Milling'. If you have access to this facility, any thread is available to you with only the investment in a single point cutter - which is a great deal cheaper than a special tap/die pair. This thread milling works on any 3 axis CNC mill, or even a dead simple, lightweight engraver like one of these: http://www.ebay.com/itm/NEW-3-AXIS-3040-CNC-ROUTER-ENGRAVER-ENGRAVING-DRILLING-MILLING-MACHINE-CUTTER-/180615224369?pt=LH_DefaultDomain_3&hash=item2a0d819c31 Well, it will do threads in plastics.

 

I am sorry, but I do metric only programs. I have had a gent ask for a conversion to imperial, but it really wasn't worth the faff, using a calculator to divide by 25.4 the numbers is not hard.

 

Regards,

 

Richard

[public alias]

Sadly I don't know anyone with access to CNC. Also, most of the machine shops I have communicated with aren't interested in milling internal threads into a relatively thin piece of plastic. The one shop that is willing charges around $15,000 to produce a pen prototype, understandably too, they work with major pen manufacturers. I've been trying to keep my prototype production cost under $500.

 

I've decided to give additive manufacturing some serious thought here. I know printed parts won't have the same tensile strength of a machined part, but one company I spoke to might be able to polish some of the internal surfaces, allowing my piston to function. At least if a printed version works and holds together for some time, I'll have a proof of concept.