Tag Archives: g code

Bridgeport CNC Conversion – Is It Worth It?

Converting your manual mill (Bridgeport) to CNC is a cheaper way to get into running production runs. While manual mills can still be useful in shops today for certain operations on one-off part, they are virtually obsolete for production runs if you want to make money. However, if you are starting up a small CNC shop of your own or are a hobbyist on a budget, a CNC conversion kit may be your answer. However, lets take a look at what it will take to convert, how much it will cost, and how it compares to a a VMC (vertical milling center).

Conversion?

You can buy a CNC conversion kit and piece it together yourself, or you can buy a mill that has already been converted. Of course buying one that’s all set up and ready to go would be ideal, but you may not have that much cash to spend right away. This is why many machinists end up buying the parts as funds allow.

If you want to convert your Bridgeport (or similar) manual mill to CNC, I suggest doing a full 3-axis conversion. It will be more expensive, but if you are going to do the swap, you might as well go all the way. Being able to program for Z-axis moves in addition to the X and Y-axis will allow for shorter machining times.

Bridgeport CNC Conversion
Bridgeport CNC Conversion

So how much will it cost? A knee-mill (bridgeport) converted to CNC will cost anywhere from 10K-25K. The newer and nicer set-up the more expensive it will be. Shopping around and waiting for deals may help lower that cost, but you should still expect to end up in this margin for a ready-to-go mill.

Buying a VMC

While buying a vertical milling center will be more expensive in most cases, they are much more capable machines. Faster rapid moves, a lot sturdier, more horsepower, coolant, automatic tool changes, and the list can go on. It really depends on how much you want to spend and how big of a machine you want. $20k can get you a used CNC mill, but it will probably be 15+ years old and will need a lot of maintenance sooner rather than later. For another 10-20 grand you can get a newer and nicer machine that will actually last a while depending on how you use it and what kind of deal you get.

What Do I Recommend?

Without a doubt, a VMC is the better choice IF, and that’s a big if, you have enough dough. Of course, many of us that are middle or lower-class citizens cannot just throw $30,000+ at a machine at any given time. This would be a long-term goal, but the capabilities are are vastly greater than a converted knee-mill.

I Should Buy/Build A Converted Manual Mill If I:

  • Am on a budget
  • Have time to convert it as funds/time allow
  • Am just a home hobbyist
  • Want to DIY to save money

I Should Buy A VMC If I:

  • Want a faster, sturdier, more powerful and capable machine
  • Want to make a business out of it
  • Have a bigger budget
  • Have patience to save up for one (If funds don’t currently allow it)

There’s advantages to buying each kind of machine. While I haven’t said which one is better for YOU specifically, I have tried to lay out reasons why you would or wouldn’t want to go a certain route. If you have any questions, feel free to post a comment.

2 vs. 3 Axis Machining – CNC Profiling

Traditional 2-axis milling on a CNC machine is still very common, you adding another axis to the equation greatly expands your possibilities. You can make parts that you might not have been able to before when just using 2-axis programming, and it can possibly reduce cycle times.

First, we must establish the difference between two and three axis machining. Most CNC mills these days should be able to accept and perform programs with 3-axis machining. Two and three axis machines both have an X, Y, and Z axis, but using that third axis for milling profiles can allow you to profile the surface of a part.

In 2-axis milling, you can move in the X and Y-axis at the same time if you’re milling the outside or inside profile of a part. If you’re using the third axis, you can make X and Z-axis moves while milling a profile, such as a waving contour. You can also move in the Y and Z-axis if you simply change the plane that you program in.2 vs. 3 Axis Machining - CNC Profiling

If you’re hand-writing the program, G17, G18, and G19 are the CNC commands for selecting which plane you want to machine on.

G17 is the XY plane.

G18 is the XZ plane.

G19 is the YZ plane.

Other than that, programming is virtually the same as any other G-code program. If you want to make a positive Z and negative Y move, an example would be:

G90 G20 G19 (To set the YZ plane and absolute)

 

G1 Y-.5 Z2.23

If you want to go back to the traditional XY plane, a line with G17 will be needed.

If you have rendering software or a program that simulates your program, I strongly recommend using it on a new program, especially if this full 3-axis machining is new to you. Good luck, and go experiment! The best is experience is with machine time and trying new things.

Absolute Vs. Incremental – CNC Programming

To be able to write and edit programs, you must know when and how to use Absolute and Incremental modes effectively. More programs are in absolute, but there are times when it’s easier to use incremental.

There are major differences between to the two, so if you don’t know how each one works, don’t start programming until you read this. Mixing the two up can and will cause a disaster

What Is Absolute?

When programming in absolute, all of your coordinates and movement values will come from the origin (0,0) point. If you want to be in Absolute, the G-code that defines this is G90, which is a modal code.

Absolute Programming
Absolute Dimensioning

Most CNC programs are written in absolute because it is easier to understand. Why is it easier, you ask? Because if you have a lot of coordinates to move, you always know where the center of the tool is in relation to the work offset.

What Is Incremental?

How is Incremental different from Absolute? Well, instead of all of your coordinates/numbers coming from one location (0,0 offset), each move is the distance from your current location. That means if you want 2 holes that are 4.000″ apart and start 4.000″ from your start location, you would use X4.0 twice, as opposed to using X4.0 and then X8.0 for the second hole if you were to use Absolute. G91 is the G-code that puts you in Incremental mode, and it is modal as well.

Incremental Programming
Incremental Dimensioning

Which one is better? That depends on what you’re doing, but 99% of the time Absolute programming will be easier. If you’re hand-programming, it may require a little more math, depending on how the blueprint is laid out, but it will be much easier to go back and read or edit the program if there is a mistake.

In G90 (absolute), no matter where your tool is, you can always go move to a certain location by inputting those coordinates, such as X1.625Y-.875. However, if you’re in G91, you can’t just punch those numbers in if your tool is somewhere other than the origin. If you put in those coordinates, your tool will move a positive 1.625 in the X direction, and a negative .875 in the Y direction from where it currently is.

So, how do you get to that location in G91? You have to know where you tool is, then add or subtract the distance of the location from where it is relative to the origin. You see now why incremental can be very confusing? If you’re in G91 and have dozens, or even hundreds of moves, one mistake in the middle of the program and all of the following numbers will be skewed because they all come from the previous location.

On the flip side, you can alternate between G90 and G91. If it’s easier to use incremental for a few moves, use G91, then when you want to go back to absolute, just put a G90 on the line of the next move.