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[g4yvm]

Right, I got the thick acrylic sheet for the end caps and the pistons but have discovered, as with all things, that it aint as easy as it looks to machine the discs with me lathe.  Rather than spend the cash and the time on trying to reinvent the wheel and cut the disks, has anyone developed a little production line for piston tank bits?  Im after something like 100mm WTC and pistons for the Darnell Type VIIc wot I am making.

I want two PTs really, a fore and an aft one so I can propo trim them.

David

[Davy1]

Well I'm making a couple of tanks for my own use but you will probably be better in the long run making them yourself, since you have a lathe. Once you can make disks the world is your oyster for WTCs , piston tanks all sorts of things.

What sort of problems are you encountering? BTW  I prefer polycarbonate rather than acrylic for machining and I turn the disks held against a faceplate with  a rotating centre on my small Peatol (Taig) lathe.

Let us know what went wrong and we can maybe  help.

David

[g4yvm]

Basically the disk edges melted and became ragged.

I also think they werent held securely enough because they went eccentric.  How are you securing yours into your chuck?

D

[treeboa]

sounds like your running too fast or too deep cuts and melting it

[andyn]

sounds like your running too fast or too deep cuts and melting it

What he said

Slow things down a little, and take shallower cuts. I found this out once while making a master up out of acrylic bar...

Andy

[Davy1]

I basically stick a piece of polycarbonate onto the faceplate with double sided tape and further hold the work against the faceplate with a rotating centre.

(I actually also have a piece of wood screwed onto the faceplate  to prevent my gouging the faceplate. I stick the polycarbonate sheet to this. I occasionally take a skim cut of the wood to remove the gouges from time to time. It is  a bit crude but works for me!)

I have had problems with Perspex cracking in the past so I favour polycarbonate and I also have a big sheet of it already! I make thicker pieces by sticking layers together with "Solvent Weld" I think it is.


As the previous posts say, you maybe need to experiment a bit with cutting speed and depth.

Have fun!

David

[Circlip]

It would also pay to make a "Slug" of metal with a centre for the rotating tailstock centre to run in to use as a clamping piece. Saves a hole or marking the centre of the job. Masking tape on a sacrificial (Mylar faced chipboard) faceplate also saves the cost of using double sided adhesive tape.

  Regards  Ian.

[Subculture]

I have found polycarbonate is funny stuff to work with, it's totally different to PMMA (perspex). You need to ensure your cutting tools are razor sharp, because this plastic is softer than acrylic, and thus tends to drag on the tool a little. Also worth mentioning that I find clear plastics can vary a lot in quality. If you want consistency, then try and stick with good brand names- e.g. GE's Lexan, Makrolon

A suitable alternative is PVC- easier to machine, cheaper, and perfectly suitable for piston tanks. You can also use aluminium (although be mindful of anodic corrosion), and I've even seen G10 epoxy sheet used.

[andyn]

Or just do what I do and use resin......

[g4yvm]

Thanks guys, much to work with.  My acrylic (I think it is acrylic) is a thick clear screen from an old shop display: heaven knows what it actually is!

Andy, MMB, yes as I was driving to work today it suddenly popped into my head to use a tube offcut and cast the pistons!  D'oh!!!!


I'll keep you posted.

D

[Subculture]

Do bear in mind that the internal bore accuracy can be very inconsistent on most plastic tubing, so you will still need to machine the piston, and also machine a groove for the seal.

I can see the virtue in casting parts if you're going into small batch production, but for one or two off I would sooner work from sheet stock. If you have resin to hand, then by all means use it, but if not, an off-cut of PVC or Lexan may be cheaper and simpler to work with.

[Davy1]

If it is an old shop display you may be lucky and have polycarbonate because it is commonly used for security "glass". (It is a sheet of tinted polycarb  I was given some years ago) You will know it is polycarbonate because you will just not be able to break it or crack it. It turns very nicely actually - I use carbide cutting tools, which may help.

Nice suggestions Circlip. In fact I think I use a bit of polycarb as the equivalent of your slug.

So go on g4yvm back to the lathe and give it a go!

[Circlip]

You can also use aluminium (although be mindful of anodic corrosion),

  If you are going down the Alumininininium route, the grade you need is 6082 - T6 or HE30 - HT30 which are basically aerospace grades These grades machine smoothly with good turnings. Avoid HE9 or HT9 as these are soft and gummy. "30" grades tend to ring when struck, "9" grades tend to clunk.

  As an addition, if you want to heat form any of the acrylics, they can be heated in the domestic oven and formed on a male mold but have to be held in shape until cooled. Polycarbonate forms air bubbles its thickness when heated in an uncontrolled manner.

   Regards  Ian.