Sunday, 28 September 2014

CNCing about Part 4

Hello again :o)

Sometimes you think you have figured something out and then it goes and does something else which leaves you baffled...

I had a few options on what I could have done today and I finally decided to carry on playing with the CNC mill, after all its still new and exciting for me plus the more I use it, the more confident I am getting with it. Todays fun with a CC milling machine was to try and generate some roof trusses for a GW engine shed I posted about a few posts ago. Maurice has been brilliant and sent me a laser cut shed he made but sadly it lacks any interior. As I need the interior (more than the exterior) I needed to get something together, so I made a start on the roof trusses.

These were modelled up in Inventor and the G Code was generated. I have decided to make them out of 0.5mm thick plasticard as it means I can glue them easily to each other and the wooden laser cut kit. Its also a bit easier to machine (or so I thought)..

I used one of the 0.3mm milling cutters I had and off it went, merrily profiling the trusses...

This is the result of today's efforts. The top one was the first attempt and as you can see I got the Z axis datum wrong and the cutter didn't cut deep enough. This was quickly corrected and the next four came out really well. Annoyingly though this is where it went downhill, the bottom truss in the photo is the result of what happened. Initially I thought it might be the cutter going blunt, picking up melted plastic, then braking but this is where is went weird. I replaced the cutter with a brand new one and it started doing the same, the plastic started to melt on the cutter and the cutter started to bend before eventually breaking. I then tried another (new) cutter and replaced the plastic with a different sheet, and guess what, it did it again! Strange as the feedrate and the spindle speed haven't changed.
OK, so I gave up on the trusses for today and tried to machine some more gears but once again it ended in the cutter breaking.

Maybe its the cheap Chinese cutters I am using? Maybe its the cheap plastic I got off eBay? Maybe its the ambient temp or something equally weird? Its time to have a rethink...

Julia.

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10 comments:

  1. Paul B.28 September 2014 at 08:25

    Have you used any coolant/lubricant Julia?

    ReplyDelete
  2. Julias Modelopolis28 September 2014 at 08:28

    Erm....no. Should I?

    ReplyDelete
  3. Paul B.28 September 2014 at 08:37

    Worth a try. I always used to flood the job with coolant when turning plastic. On a job that size squirting a little water at the cutter with a syringe might be benificial.

    ReplyDelete
  4. Julias Modelopolis28 September 2014 at 11:06

    Thanks Paul. As you say, its worth a try.

    I have convinced myself at the moment thats its something to do with the quality of the cutters. I am trying to find some higher spec ones but its hard finding any 0.3mm cutters that are longer than 0.6mm / 0.8mm, ot much use when some things you are machining are 1.5mm thick. The ones I am using right now are 5.5mm long and I fell these are causing more problems than they are solving right now.

    ReplyDelete
  5. MikeOxon29 September 2014 at 04:23

    'Coolant' can be a simple as blowing air from a compressor. There's a lot of useful information in this download: http://www.theplasticshop.co.uk/plastic_technical_data_sheets/engineering_plastics_machining_guide.pdf

    Mike

    ReplyDelete
    Replies
    1. Julias Modelopolis30 September 2014 at 05:16

      Thanks for the link Mike, that is very interesting.

      Julia.

      Delete
  6. richbrummitt1 October 2014 at 06:16

    I've been less successful than you trying to mill styrene on an MF70. I concluded that either the spindle speed was too high (min. 5krpm) or - less likely - I couldn't feed fast enough.

    Incidentally my first thought on reading a previous entry about gears was that 0.3 was a much more appropriate depth for a cutter of that diameter, but you know or found this out already.

    ReplyDelete
  7. Julias Modelopolis1 October 2014 at 11:17

    Thanks Rich.

    I think you are right about the feedrate when milling styrene, if its too slow the cutter will just rub, generate localised heat, then melt the plastic which is what I feel happened to me. I am a little constrained at the moment with the length of the 0.3mm cutter, at 5.5mm long it distorts easily and so I have to slow the feedrate right down. New milling cutters have been ordered now which will hopefully get past this problem.
    Yes, don't know quite why I tried cutting that deep to start with but you are right, 1D is about right.

    ReplyDelete
  8. Simon Dunkley3 November 2014 at 12:01

    Jim Guthrie (a.k.a. "Flubrush") has been milling coach sides from plastic. He has found that stated thicknesses of polystyrene sheets are nominal, so now begins by passing the cutter over the sheet to bring it down to a constant and consistent thickness. Might be worth following up.

    HtH

    Simon

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    Replies
    1. Julias Modelopolis4 November 2014 at 07:10

      Thanks for the tip Simon.

      I havent tried machining something like a coach side yet so I havent been overly concerned about the thickness but I will take that into consideration if the need ever arises.

      Julia.

      Delete

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