Machining transparent acrylic - Cast or extruded?

[piscov]

Hello all,


Anyone has experience working with acrylic?

I want make a fountain pen with a ink window to see the ink level inside the pen. I will use transparent or translucent acrylic.
So far I have worked only with extruded acrylic. Its not easy to leave the interior surface "clean" without scratches because of the scratches the swarf makes when machining the inside.

I will describe the process m using and will appreciate your inputs to be able to have a clear ink window:

1. Reduce the acrylic stock to the external diameter needed;
2. drill the hole using a drill bit in the lathe
3. machine to the internal diameter using a Horn R108 internal insert
4. Sand internal and externally with 1500 and 2000 sand paper.
5. Polish inside and outside using a car polisher cream

I was told by a friend that cast acrylic is easier to machine and finish as it will produce less swarf and therefore scratch less the inside of the barrel, is this correct?

Cast acrylic is much more expensive and I don't want to trow the money out the window...
Will cast acrylic work better and will be easier to have a clear ink-window? Is the cost difference worth while??

This is a picture of of 3 clear inkwindows to show what I want to do.


Thanks in advance for your inputs.

Best regards

 

[PurpLev]

extruded acrylic will always have subpar transparency to cast as it has the extrusion lines embedded in the material (not only on the surface) whereas cast does not and is really clear.

 

[piscov]

the stock extruded acrylic rods I have are completely clean and tranparent before I start to work on them. Are the extruded ones suposed to have those lines before work them?

 

[refinery Mike]

If you drill in acrylic you must us a bit with 0 rake. A regular drill bit will shatter the acrylic for sure. You can use a wood spur bit (nothing with a screw tip) and then bore it to final diameter. Then to get the acrylic polished clear. you need to sand it wet or it will gall all up. Sand it to the smallest grit wet and dry paper you can get and then polish it with tin oxide with water to the consistency of whipped cream. even with a lathe bit make it plane flat on center no rake in any direction. Have you considered using Lexan in place of acrylic, It is much tougher and is almost shatter proof. It's only down side is that it is more likely to scratch in use. So you can choose.

 

[piscov]

Thanks guys this posts are really helpfull!! I did not knew I needed a zero rake drill bit to drill acrylic or brass.

How can I make my drill bits zero rake? Is only taking the angle of the flutes and put them perpendicular to the surface to cut? Is this easy to do?



Lexan is not an option... no one would buy a pen that scratches very easily.

 

[refinery Mike]

It is hard to explain but easy to make a standard bit 0 rake. You simply grind the cutting edge straight up and down. That can be done on a good bench grinder, with the end of a wide dremmel grinder, even by hand with a diamond hone. It really is easy if you practice doing it with the grinder turned off first, and visualise how you are going to grind the lip. then turn the machine on and grind the cutting edge straight up and down. Just a small flat, like about an 1/8 inch.

 

[stern]

Not sure if it would work for your application, but when I work with plexiglass I have found you can get hazy sections clear just by running a bit of "adhesive" over them. Just have to get the proper stuff for the material you are working with. With plexiglass I used Ethylene Dichloride (hard to get now, as they use Methylene Chloride instead which isnt quite as good) and pour it over the hazy area.

 

[dogcatcher]

I use cast acrylic for duck call barrels and inserts, the extruded doesn't machine worth a flip.

 

[Armor]

As a pen turner , you will have to wet sand as said.

They make a polish that goes first and they do sell pads that are used wet from 3000 to 36000 grit real fine.

If you take real light cuts, you should have no problem.

If drilling a blank you really do not want to drill all the way thru, cut the blank a bit over sized and then flush cut the end after you drill to depth.

If the blank is not big enough ,I suggest you use a set of brad point drill bit and use a lube, water would work.

But you will need a gouge to turn a pen and best to have a fingernail profile.:thumbsup:

Good luck , not that hard just takes time ,and slow speed on the Lathe.

Jeff

 

[dogcatcher]

I use a round nose scraper when turning on my wood lathe. I wet sand using automatic transmission fluid as my wetting agent, I start at what ever grit I think I need depending on how good the cuts are, sometimes 80 grit, sometimes 220. Sand through 1200 grit, at the end of each grit I stop the lathe and cross sand until I see no lines in the acrylic. After 1200 grit I clean off the ATF mess and use Maquiars PlastX, it is used for polishing scratched headlight covers, you can find it at auto supply houses. At medium speed I polish the acrylic using a small piece of cotton cloth using the PlastX. By now I should have no scratches showing, but I still buff it with Tripoli and white diamond compound.