So here we are. You have read all of my eloquent, informative, and groundbreaking (perhaps a minor exaggeration) blogs. You are ready. You have a pencil behind your ear and a calculator on your desk and you’re going to trust in the numbers! Slow down there cowboy, because the most important thing to remember is the numbers, formulas, and suggestions are just that – suggestions. They give you a reasonable starting point. They get you in the neighborhood but some good old CNC machinist trial and error might be in order. I know, if you bought an expensive GPS unit for your car (who buys those anymore?) and it GUARANTEED to get you within ten blocks of your destination and left the rest up to you then you might wonder why you bothered. Well my friend, if that building you were driving to was constantly moving and changing based on the simplest of variables then ten blocks isn’t too shabby. Besides, you got my Shop Math formulae for free. Stop complaining.
Control the Variables
The point is, no matter how sophisticated your CNC machine, software, tooling, or ego is you will always have to make adjustments. Sometimes minor, sometimes major. It all depends on what you are doing and how you are doing it. There are an endless number of variables involved with machining so I won’t even attempt to touch on all of them. The name of the game is eliminating or at the very least CONTROLLING those variables to get consistent results every time. Let’s say you set up a new job on Monday morning. This job uses eight tools and takes approximately two hours. The tools do well all day Monday and when you come in Tuesday morning the second shift guy tells you he had to look busy so he swapped out all the tools. When you ask if they were worn out he replies “I dunno.” You would think they would be smart enough to put a competent human being on second shift since there is far less supervision, but trust me I know how that goes. Anyway, you have many issues now. Judging by Gomer’s attitude and general work ethic you can assume that none of the new tools were properly measured before he put them in. So it’s time to get to work – measure all your tools, check your zero points, make sure your speeds and feeds are good. Should be all set, you say? Guess again. Same program, same machine, right tools, everything looks good. That doesn’t mean it will cut the same as it did yesterday. Or even two parts ago. Depending on the tolerance you are working with something as simple as the ambient temperature and humidity can affect the final result. This is where some CNC machinist trial and error comes in.
Warm Up the Spindle
Before you decide a program is ready for production and release it to Gomer, you need to make some determinations. First off, make sure no matter what that you always warm up the spindle. If you warm up the spindle properly before running your first job of the day then you will ensure that thermal expansion in the spindle will not become one of your variables. That way the fifth part will come off just like the first. Also, any time your machine is going to sit more than a couple hours it is a good idea to do a warm up, especially if you are working with tight tolerances.
Know Your Tools & Standardize Your Tool Library
Another consideration when preparing a job for production is tool life, so you can avoid the problem mentioned above. By testing and running through some “CNC machinist trial and error” you will learn a lot about tool life and be able to compile some simple information and expectations. You will find that the more you do this, the faster and easier it will become. You will reach a point (especially if you followed my advice from my previous blogs and set up a standard tool library) that the information will just be there and suddenly your reference material will be right off the top of your head. Once you know your tool life you can be much more proactive in your approach to shift change and work flow.
Trial and Error with Feeds and Speeds
CNC machinist trial and error also comes into play when efficiency and productivity are the goals (when are they not?). So my suggested speeds and feeds get the part done in twenty minutes, but if I take a 10% lighter cut and increase my feed 20% then the part is off the machine faster, and my tool will last for eight parts rather than five. I have cut time, increased tool life and made the boss happy. What about QC? Are they still happy? OK, so my surface finish suffered a little, but it’s still within specifications so we are good. Success!
Don’t be Afraid to Push the Envelope … or Break a Tool
If there is one thing for you to remember it’s that ERROR is half of trial and error. The best machinists I ever worked with broke tools on a regular basis, just because they wanted to see what they could do. A little bit of CNC machinist trial and error, pushing the envelope, will get you farther than you may think. You will discover very quickly that the envelope is far more expansive than you imagined.