There’s a reason tight tolerance parts cost more time. Yes, I use the word “Cost” because time is money, and it can take a considerable more time to set-up and run a part if the tolerances are closed up. Tool run-out and tool deflection can both be an issue when trying to hold a close tolerance. However, they are not the same thing.
Tool run-out is how far off of the rotating axis the tool is. While in the machine, you want to check just above the bottom of the tool, as that will give you the most accurate reading and is almost always worse than the top of the tool where it goes in the holder. To check it, put an indicator on a vice and touch the tool off of it. Zero it out and then rotate the tool (usually in a counter-clockwise rotation so it doesn’t catch the cutting edge). If there’s run-out, one side of the tool will give you a different reading on the indicator.
Before we go any further, let me explain what run-out actually does when machining a part with a tool that has it. If an end mill or a drill has excessive run-out, the side (or flute) that is bigger will do more cutting. If you’re milling out a hole with an end mill, it will cause the hole to go over-size if all of your program and offset numbers are right. A drill can also go oversize, as well as drill an oblong hole.
Now lets take a look at what causes a tool to have run-out. A brand new and unused tool can have run-out. Why? Not all tools are made the same, and if you buy cheap tooling, there’s a better chance that it was made with the same precision as a higher quality tool.
Not only can the tool be at fault, but a defective tool holder can cause run-out as well. On the other hand, you may check the tool while in the spindle and see that their is run-out, but certain tools (such as a reamer or drill) will allow you to slightly move it without removing it. This can often get rid of many run-out problems with longer tools.
Tool deflection should not be confused with run-out. It is a common term used when side milling with an end mill, and it causes a taper in the part feature that is being milled.
Take this as an example; you’re milling the outside profile of a part (2x2x2.25″) that has blueprint dimensions of 1.950″ wide, long, and 2.000″ tall. Using a 3/4″ end mill with greater than 2 inches of flute length, you mill around the part once. The top of the part is 1.951 all the way around, but the bottom is upwards of 1.954″. This is because the end mill is too long and was ‘deflecting’ because it couldn’t handle the pressure of removing all of that material.
That’s the most simplistic scenario of tool deflection. So, how do I combat this? Great question, and there’s quite a few ways to make sure your part is square, perpendicular, and/or parallel.
First of all, how deep of a cut are you taking? If you’re drilling a 1/4″ hole that’s .375″ thru, you don’t need a jobber drill with 4″ of flute length. When milling, a bigger diameter tool will be stronger and resist deflection better. If you’re milling a feature that is .400″ deep, using an end mill with 1/2″ flute length will achieve the best results. One last thing on tool length is that you should have the tool as short as possible in the holder. Do not clamp on the flutes, but if you choke up on the tool, this will also help prevent deflection.
Feeds and speeds. I’ve said it before, and I’ll continue to say it. Having the right RPM and feedrate for your tools is one of the major keys to success in the Machining industry. Even if the surface foot is close, having a high feedrate will naturally produce more tool pressure and in turn cause deflection.
Slowing down the feedrate can help, but in the end, you may have to take multiple passes to make a feature square, especially if you’re profiling out a part with an end mill.