Wednesday, 24 September 2014

CNCing About Part 2


Things dont always go according to plan. Below is a good example of my imagination overwhelming my abilities (or should that be the abilities of machines)...

 One thing I wanted to try out was to see if it was possible to machine gears using the CNC mill. In my head it made sense, I had the ability to draw one, generate the G Code, and machine it, so why not? I duly generated a 3D model of a 0.3mod 22t gear which is one I need for a future engine build. This was then put through the machine G Code generator which promptly created 2700 lines of G Code. The smallest diameter was just over 0.3mm so a 0.3mm diameter end mill was purchased, along with some 1.5mm thick PTFE sheet. Everything was then loaded into the machine and the start button was pressed....

Everything seemed to go well. The cutter machined the PTFE without a hitch where I was honestly expecting it to break under the loads. The whole program ran in just over 6 mins and then it was time to remove the gear...

As you hopefully can see from the above picture the gear hasn't come out that well. Certainly not as good as I expected, especially considering how well it apparently machined. The gear profile isnt anywhere near as well defined as the model, even after an attempt to clean it up a little with a steel wire brush.

This leaves me with a little dilemma. I was really hoping to have the ability to produce my own gears, especially 0.3mod as its the most common size I use. Now, I am thinking the problem could be one of two things. Firstly am I bordering on the backlash of the machine which is affecting how much the machine is actually traveling? The mill doesnt have any positional feedback so its reliant upon the step motor revolutions to position the bed correctly and there is definitely no anti-backlash devices fitted. The second could be the end mill, I mean its 0.3mm diameter which is small! There is a chance it could have been distorting and flexing or similar during machining but I feel this is unlikely. Mainly because the feedrate was low, the spindle speed was high, and the material is relatively soft.
So, what could the problem be? At the moment I dont know and therefore dont really have an answer. Has anyone else got any thoughts?



  1. Its a shame that it didn't work, but full marks for trying. I've never heard of gears bing CNC milled like that, usually they're generated using specialised cutters with the blank held in a dividing head, or better still a dedicated machine (I used to set and run Fellowes spur gear and Gleason bevel gear cutters). I doubt that a slot drill would give the required surface finish. But please prove me wrong!
    Just out of curiousity, is the slot drill as far in the collet chuck as it will go? There seems to be a lot of the shank sticking out, which could lead to excess vibration.

  2. Thanks for the comment Paul. I was starting to think I had posted something in a different language or something!
    I know gears are normally cut with a gear hob and dividing head but as I had a machine which should be capable of doing them, then why not try? If I can figure out how to cut them then it will help me and my engine building out loads. It wont be the end of the world if it doesn't work though.
    You are right about how far the cutter is sticking out of the chuck so thanks but I feel this isn't the problem. I have spent a large chunk of today looking into a way of adding some anti-backlash nuts to the bed.

  3. Why not try indeed. It might be usefull to further your method by cutting larger gears, so you can see exactly whats going on not only whilst the machine is running but also when examining the finished product, especially regarding surface finish.

    Another possible avenue might be to make up a little mandrel to mount a disc on so that the gear can be cut in two passes, roughing and finishing, to hopefully achieve the required finish.

    And it probably is a different language to most, cutting gears at home is usually the preserve of model engineers, a body of people who like to make everything as complicated and time-consuming as possible...

  4. I can't help you with the problems you're having but I recently found this webpage that might be of interest. It explains how to create tiny gears with a CNC mill and casting resin.

    1. Hi Anonymous.

      Thank you very much for the link, there is some really useful stuff there.


    2. Hi Julia

      You're welcome. I couldn't stop thinking of the possibilities when I first saw that page.
      Good luck with the gears.

      P.S. Nice blog and beautiful modelling.


    3. Thank you Simon.

      I agree, there is plenty of good ideas there. As a result I spent some time last night looking for some modelling board as it seems to be ideal for CNC machining.