Beginner Troubleshooting and Building Considerations

[coherent]

Having been a member of the CNCzone forums for some time now, one of the most common home built machine issues I have run across is home builders who’s machines either miss steps or just don’t cut the part or design like you expected it to. Having made every mistake one can imagine when building a small home built machine, I’ll try and offer a little insight. This may be a bit lengthy but here is a little very general guidance for beginners who are building or have built a machine who are new to CNC. Most “first machine” builders use low cost stepper motors, surplus power supplies. Everything from drawer slides to Ebay found mismatched linear slides rails and bearings. We use parallel port driven chopper 3 axis boards and low cost acme screws or threaded rods and home made nuts… once we’re hooked, we may move up from MDF to aluminum, from threaded rod to ballscrews, from steppers to servos, but provided here is a little insight to possible problem areas for beginner types of machines. This is by no means meant as in-depth or all-inclusive. I’m not going to address specific driver boards etc. One important thing to keep in mind that even the very best linear slides or the best ground and polished ballscrews won’t make up for a poorly designed or misaligned machine. By nature, CNC is precise and not forgiving when things aren’t square and true. After all, if you didn’t want things cut precisely with repeatability, you’d just use a hand saw and hand drill right? Geared for beginners, most of what follows is just a little of what those of us who have been there, done that, have learned along the way.

1. First, if you have a problem with your home build machine here's what I would suggest... Use some process of elimination. Start by determining if the problem is software, electrical (including the computer) or mechanical. First produce some basic Gcode (use a simple text editor like notepad if you wish) and make up a simple back and forth pattern that uses all of your axis's to their extents or close to it. Or, even better test one axis at a time. Repeat/cycle it 10-15 times or even more. Zero your axis(s) and mark it’s/their exact location. Now take Windows out of the loop and boot a computer in DOS (not a windows DOS screen, you must boot in DOS), and run the simple gcode in a DOS based CNC program like TurboCad. Return all to zero... is everything back to exactly where you started?

2. If the above eliminated your problem, software may have been the issue due to windows software. You may need to either upgrade your computer if it’s dated or very slow or use a DOS based program to drive your machine. Newer windows based programs like Mach2 need faster processors to work efficiently and eliminate timing issues. Or, in the process you used to create your previous gcode from your design, your previous program produced gcode that caused problems. This could be something as simple as a gcode command that your driver software doesn’t recognize or interprets differently than intended. The gcode produced must be compatable with the machine and driver software you are using. Stick with basic files and learn at least basic gcode commands. It’s a necessity when troubleshooting or correcting gcode. Take the time to research your software's gcode capabilities and limitations.

3. If you still had missed steps or interference, your computers parallel port, parallel power or other interference may be the problem. Other drivers (like for printers or scanners) can interfere with CNC driver/software. Although most windows based CNC software like Mach 2 makes it’s use of the port as a computer priority, it can still be an issue with DOS or Windos based software.

4. Change the computer, try running the gcode with spindle motor off and route wires as to not have power cables laying against motor cables etc., re-run the gcode after each change. If one or more of these help try to isolate cables or spindle power supply and/or upgrade computer as necessary to eliminate problem.

5. If you’ve tried all the above with no results determining the problem, it’s still possible that you have a software or computer problem, but things are pointing to a mechanical issue. Try setting your feed & rapid rate to very low. Try half the rates you experienced the problem at, then half again and so on. This will increase stepper torque because generally speaking, torque decreases on steppers as rpms increase past a certain point. Run the gcode again. If the problem was eliminated, your motors do not have sufficient power/torque to move your axis(s) at higher feed rates. This may be due to: 1)motor size too small, 2)insufficient power to motors, 3) binding or excessive friction on the linear rails, bearing or ways 4) excessive friction on the screws, ballscrews, nuts or mounting bearings, 5) driver, motor mismatch or incorrect power/resistor settings or 6) a combination of one or more of the above.

6. If you have tested all the above within your machines capabilities and are still unable to successfully eliminate the problem, some considerations at this point are:

- Is your driver, power supply and motor package capable of moving the axis easily? Have you tried half, quarter or other microstep settings? . Follow driver instructions regarding power supply and motor recommendations and the settings for your driver card. If applicable double check the type/size of power resistors if used or the boards axis motor power adjustable “pot” settings. Are others using the same driver board, motors and power supply setup as you with good results? If only one axis is in error or dropping steps, try swapping motors and/or connect another driver board axis to that axis on your machine and see if this eliminates the problem. You may have a bad motor, cable or board.

- Check your motor tuning/settings if available in the software like Mach 1 or 2. They can affect torque greatly. Start slow and work your way up. Are your software steps per unit set correctly to correspond to your motors steps per revolution, full, or microstep driver board settings and screw/starts TPI?

- On smaller systems with the power off you should be able to turn that axis’s screw by hand without too much effort. With no screw attached to the axis, can you slide the axis, gantry or table easily? Is there binding? Stuttering? If any of these problems are noted then you may have a general design flaw or an alignment or “squareness” issue. You may just not be using “precision” enough rails or bearings for your purpose. With nothing attached (motor, screw etc) to the axis you should be able to tilt or lift one end and it slide on it’s own. If it doesn’t you may have an excessive friction issue.

- Is your screw/nut assembly clean, and turn smoothly and easily? Some types of screws and nuts are more forgiving of misalignment, but if your nut and axis mount assembly is not aligned or the nut or mount is not tapped/threaded perfectly perpendicular to the screw or ballnut, friction will occur. Once things are bolted down, does your screw still turn easily within the nut? It should.
- With smaller motors the screw diameter and pitch (TPI) can be an issue, as these both effect torque required to drive the axis. Generally the more threads or turns required to move the nut a specific distance, the less torque required to turn the screw/nut assembly. As an example a 20 tpi threaded rod will require less torque/force to turn than 5 tpi threaded rod of the same type and diameter. Other, variables include the screw/nut diameter, type, and precision. Belt drives, gear drives and rack and pinion systems will all have similar issues depending on the reduction, gearing and quality.

- Solid motor to screw couplers are very unforgiving and also very hard on the motor’s bearing if things aren’t aligned perfectly. Couplers as simple as stiff tubing and small hose clamps to high dollar bellows type couplers offer much more leeway regarding misalignment. This doesn’t mean a solid coupler won’t work well, you just need you use what is appropriate for your motor and machine configuration based on how precisely things are built and aligned.

- Running gcode from a DOS platform will eliminate windows timing issues, but other things to consider are: Laptops may have problems with the power supplied to the parallel port. Try a desktop system. The best way to ensure the port is not part of the problem is to use a breakout board, which isolates the parallel port on the computer from the driver board and provides stable voltage. Also, your computer itself can cause interference to the stepper motors. Try placing the computer away from the driver card, steppers and stepper cables.

- Spindle (router, dremel, rotozip) motors can cause interference to steppers. This problem is more common than you think especially on hobby/home built systems. Isolate power/line noise or try using a different power/plug source/circuit for the spindle power. Shielded cables may have little or no effect, so don’t take it for granted that interference isn’t an issue just because you use shielded cables.

- Missed steps or interference can occur from everything from grounding issues to the Florissant lighting noise in your shop. Take a systematic approach to troubleshooting the problem and try simple things first like turning off the overhead garage lights and re-testing.

- Binding or missed steps may not occur until a force in put on the spindle. This is especially true with moving gantry designs when the spindle is moved to one or more extremes of its an axis. Keep this in mind when trying to isolate the error.

There are so many variables with home built machines it would be impossible to list specifics for each type here, but hopefully this will at least give you a starting point if you’re new to CNC and learning. Some problems can be very frustrating when starting out and forums like the CNCzone are a fantastic source of information. Read, read and read some more!

Happy CNCing
-Marc

[kriskanhai]

For a 3 axis machine, can you use screws of different TPI. I am totally new to these machines so bear with me.

[Peter Schuldt]

kriskanhai-
Yes you can use screws of differing TPI on the same machine but NOT on the same axis. The motor settings can be tuned to the screw they will turn. Some people chose to use two motors on a given axis. Usually the X axis. It would be very hard to tune motors with different screws on the same axis.

[Daveb572]

Hi Folks! Ok I'm new to the CNC community and would like a little direction. First off, I already bought the plans for HobbyCNC's smallest cutter and I'm shopping for a woodworker who knows what he's doing. I may have found one. Talking to him as soon as the plans arrive. Let me emphasize that I'm a hobbyist. I intend to use this cutter to make wing ribs, bulkheads and other curved parts for various RC Aircraft. Several questions arise in my mind.
1. Anyone have any experience with HobbyCNC's products? Opinions of them?

2. Best affordable software to run the cutter?

Any help would be greatly appreciated. Thanks! Dave Bryer