The Mistakes I've Made

Here's a list of things I've done wrong while getting started as an amateur machinist. If you're a professional, then enjoy the laugh. If you've already done these, then feel free to commiserate. If you've wound up at this page while doing research before starting, read carefully.

I bought myself a 3 in 1 combination machine: mill, drill, and lathe. It has some fairly impressive specs, the 16" swing being one of them. I figured "hey, with a swing like that, I might even be able to turn my own brake rotors." It didn't occur to me that the cutting speed near the outside of a 16" diameter disk would be scorching fast, even at the slowest speed I can turn the work. Another thing I learned is that having the cutter that far away from the lathe bed is a great way to introduce chatter. Chatter is the worst enemy of an amateur machinist using cheap Chinese tools.

So, lesson 1: read the specs carefully and understand what they mean. Bigger numbers are not necessarily better. I spend almost all of my lathe time an inch or so from the chuck. Sure, it's nice to have the capacity if I need it but most of the time it is a serious liability. A small machine with big specs equals flex. Flex equals chatter. Now, chatter is manageable so don't think my tools are useless; it's just that I didn't quite get what I was expecting.

Lesson 2 is about carbide. When I first started, I thought, "all those cutting angles are just confusing; I'll buy carbide cutters and be done with it." You guessed it, that was another mistake. First, carbide may be really hard but that makes it brittle. If you make a mistake, say by turning the work backwards by hand while the cutter is rubbing against it, or feeding the cutter into the work the wrong way, you will chip or flake the carbide. This ruins the cutting edge. Most times, this ruins the whole cutter. Carbide cutters are unforgiving; this is not a good thing for beginners. The second thing I learned about carbide is that while it holds an edge better than HHS, it isn't as sharp. Also, many carbide cutters are designed with negative rakes. These two things are a real problem with light machines where you need to worry a lot about chatter. It takes a powerful and rigid machine to exploit the benefits of carbide; even a poorly ground and sharpened HSS cutter works much better on my machine. Better yet, when I screw up, I don't waste the cutter; worst case, I have to resharpen. I still do that a lot.

Lesson 3 is that while my machine can hold 1/2" cutters, this does not mean I should buy 1/2" HSS blanks. This is another capacity verses common sense thing that I seemed to have missed when I started out. First, with my machine the flex of the actual cutter is insignificant; I don't have the power or rigidity. Properly supported, a 1/4" or 3/8" cutter is just fine. Please realise this is not about cost; the cost of a larger cutter is only marginally higher than the smaller ones. If you want to know why smaller is better, go try to grind the relief angles into 1/2" of HSS. Remember, don't grind too fast or it will overheat and loose it's HSS properties. Grind, grind, grind, grind... HSS is HARD... grind, grind, grind, grind. Now, repeat for the different shape cutters you will need. Grinding various shapes into smaller cutters is much easier, and they work just as well.

Lesson 4 is about milling - and specs. My machine is "capable" of cutting with a 3/4" endmill, or so the documentation says. So, being the kind of guy that I am, I went out and bought 3/4" endmills. You guessed it, chatter was a significant problem and I couldn't find any speed/feed combination that would settle it down. One day, I had to mill a 1/2" slot. With the 1/2" endmill in place, my little machine cut like a dream. What little chatter there was cleared right up by increasing the feed rate. I can cut a given amount of material much faster with a 1/2" endmill than I can with a 3/4" one. Bigger is not always better, at least on a small machine.

Lesson 5 is that "amateur" means "not making money." In the world of professional machining, time is money. The best machinist is the one that spends the least amount of time removing the most amount of metal, while leaving precisely the right amount behind with the right finish. Because of this, all the speed and feed charts are maximum safe starting speeds, which professionals then exceed to whatever their particular task will bear. For us hobby people, there really isn't that kind of rush so it's okay to go slower. With some materials, cutting faster will give a better finish but most times going faster just leads to more mistakes. Over the last while, I've tended to slow down the speeds as this gives me more control over the feed rates. After all, it's the machining that I enjoy more than the final product. People ask my what I make with all my tools, I tell them: swarf.

Lesson 6 is about reading. All the tools in the world will not make your metal into the right shape. You have to know how to hold it down, how to make the cut, and how to put it all together. Machining is partly about the tools, but being a machinist, amateur or otherwise, is a learned skill. But, you're here reading this so you probably already know that.

Lesson 7 is about precision. Cabinet makers and house framers both make things out of wood but they work to different levels of precision. Working with metal is like that too. Most of the stuff you read on metalworking will be from people telling you how to do things right: precisely and correctly. However, if you're making a cart axle and just want to turn down a steel rod so the wheels you have will fit over, then if you can safely get the pointy end of a cutter near the spinning rod, it will probably be good enough. Sometimes, I like to put a lot of effort into making a part more precisely than is actually necessary. I know it's a waste of time but I'm using a non-critical part as a learning exercise. If I'm off, it's no big deal as the part will still work. I'm doing it as a learning exercise because I enjoy it; I'm still not very good at it, but I enjoy it. However, when you're making something to a rough fit, you don't have to worry about it unless you want to.

Lesson 8: The more power tools I have, the more I appreciate a good hacksaw and file. It amazes me how much I can get done with basic hand tools; it also amazes me how long it takes to get a machine set up to make even a simple cut. I've learned, over the years, that if I need to get something done, and there's not too much metal to remove, it is often faster to hack it to shape and then file it smooth. I mean, I like my power tools but there are times when getting the work held down properly for a cut, at the correct angle and in the correct place, takes way longer than clamping the thing in a vice and having at it. The more you use a hacksaw or file, the better you will get at it, and the faster you will be able to shape metal. Sometimes, the simple things are the right tools for the job.

So, if you are looking to get into amateur machining, here are my recommendations:
  • Get the right machine for what you will spend the most time doing.
  • Remember that bigger specs on smaller machines means, in all likelihood, more chatter.
  • Start with HSS cutters; try carbide after you know what you're doing.
  • Avoid running your machines at maximum capacity. Their "maximum" is probably a sales pitch.
  • You don't need to run at the maximum speed and feed.
  • Learn as much as you can about techniques.
  • Be as precise as you want to be and not necessarily as precise as you've read about.
  • Don't get belligerent about using a machine when hand tools can do a better job.

Oh, and one more thing: don't forget to have fun!

No comments: