Cross posted from SMW3D Forum:
I have completed (99%) of the mechanical assembly for my new R7 build. I am currently laying out my control box and front panel layout and will begin wiring soon. I hope to be making chips before the new year. I have a few questions and comments for discussion with the vast unpaid R7 R&D Department.
I spent some time reading through both the two Google+ R7 Communities and this forum, to get up to speed with Best Practices and other common issues that could pop up during the build. It was very informative and worth the time but sometimes a bit confusing. Due to the nature of R7 build improvements and changes over time, some of the older issues have been corrected or improved on. Here is a list of comments and questions for review:
Shield vs unshielded cabling
Several threads discussed wiring for the Spindle /inverter run. The 4 Cond - 16 AWG wire supplied with the kit is not shielded but some people have moved to use shielded cables for this run. The large toroid included with the kit is to help reduce interference and may work well, but just wondering if a shielded run, tied back to a common Ground in the control box would be best. Beldon makes a specific 4-16AWG Shielded cable for VFC drives that is very flexible and tough. Hard to locate for purchase by-the-foot, but I did fine a source at $2.61/Ft at: http://www.wireandcableyourway.com/BELDEN-CABLE/VFD-Cable/belden-29500-16-awg-4-conductor-shielded-vfd-1000v-ul-flexible-motor-supply-cable.html
Beldon 29500-16awg-4cond/shielded
The same goes for using a shielded 4 wire - 22AWG wire for the Stepper runs, with the shield tied only at the common ground in the control box. The kit supplied wire was 4 cond - 18AWG, unshielded. Same source but cost is $0.98/Ft (https://www.wireandcableyourway.com/belden-9418-18-awg-4-conductor-foil-shield-300v.html)
Home/Limits: I plan to add limit/home switches to my system at some point. From the threads, many builders do not use them. As the Limit lines are direct inputs to the control IC on the TinyG, with little protection from static, etc and run at the 3.3V logic level, I could see where they would be less immune to noise. Those not using Limit switches would not see this as an issue. I plan to wire them as normally=closed (NC), and may add some .01uF-10V caps across the switches to reduce noise. I may use some surplus Cat6E cable for these, as they will be twisted and shielded. A Best Practice here may be to make some optic-isolator circuits to buffer these signals between the TinyG and the outside world, maybe even running at a 5V or 24V logic level. Would add to cost, which is alway a factor, but would better protect the TinyG and add noise immunity.
GFIC usage:
One thread discussed adding a Ground Fault Interrupt Circuit on the AC Mains, since this unit was mostly metal, uses higher voltages on the Spindle Drive, and has water present for cooling. On a good day, the water and HV will never met, but GFIC’s are for bad days, so I would like to include this. In the thread, it was suggested to remove the AC ground from the input to the inverter to prevent the GFIC from having false trips. I would think you would need to make sure the Z, X, and Y (both) C-beams were grounded to Mains to make sure the GFIC would still function correctly. They are each isolated due to the Extreme wheels/Delrin ACME block being the only contact points in common. Comments?
Tensioning the Y ACME Rods
The Kit came with a plastic ACME nut to allow you to apply tension to the rods. The videos show an ACME rods with enough excess length to allow this to work. In my kit, I got the nut, but the rods are pretty much and exact length and will not allow this method to work. I tried a light tightening of the collar and using it to apply tension to the rod but cannot get all the slack out of them. I tried to do the “M5 drill/tap” method shown in a couple of threads but I cannot seem to get the hole started in this hard rod. Any other ideas? Will not being able to do this be an issue?
General Comment - Encoder Feedback?
With the cost of good encoders coming down these days (CUI, etc), is there a controller that allows encoder feedback for Steppers? I have seen some individual Stepper drivers and cost seems to be coming down on those, as well. Maybe using these only on the long axis X and Y runs. Seems accuracy would be improved. Again, since I am knew to the CNC world, it may just not be needed for R7 sized systems who run smooth and have correct Feeds/speeds for materials under the cutter.
#1. I have not had problems with interference on any of the machines I have built. I also have never used shielded wire. Some people seem to have issues with it though. I bought 500’ rolls of 18-4 and 22-4 unshielded, because the cost is much much lower than just buying custom cuts from stores. I don’t use a vfd, so I’m unsure if something needs to be shielded for that. For limit switches, it may be a good idea to use shielded.
I’m one of the guys who never runs limit switches. Sure, it would be nice to have them sometimes. If I were doing more production runs of things they would save time, it would be nice being able to click once and your machine knows it’s location and home. I change things with my machine so much I would always be fiddling with them if I had them.
Wouldn’t hurt to run a Gfci. All my outlets have one somewhere in the circuit. Don’t even notice then there. About grounding the machine to the main circuit…i Don’t think it’s necessary. I don’t know if you have had much experience with gfci outlets, but just about any thing can set them off. Sometimes they trip here in the summer from excess humidity/condensation, and winter from a frost cord end.
Tension on lead screws. I think what your referring to is the anti backlash nut? This is just to reduce backlash of power transmission from lead screw to machine axis movement. As far as not having extra sticking out of the bearing, do you have your plates backwards-corner to corner? I can’t remember if that is possible our not. On one machine I have no tensioner on the end of axis’, and another I just fiddled around with the lock collars and coupling until I got no play. That is the main reason for putting tension on screws.
I am a big proponent of shielded cables… Then again it’s all we would think of using at work for industrial automation since the cable cost isuch less than a site visit for troubleshooting. So I used end cuts from work left over from jobs and is free to me. It is cheap insurance.
There have been some reported issues (not many) with VFDs from china with loss wiring/grnd, which could lead to shock or EMI transmissions (nosie). (poor Quality Control usually)
Plus Inverters and VFDs are known for EMI, could be more too old technology or assembly methods. Either way, most people with the desire to build/use kits like the R7, are going want to make sure it works without issues. This at times can lead to over engineering or extra caution in some aspect.
As far as the components Brandon has pulled together for this CNC, I have not seen or heard of any issues without all the extra precautions. Guess, comes down to your comfort level and will you use/need all the bells and whistles (limit/home switches).
@Scott_Sanders yes, the engineer in me is coming out but I like to do what I can to head off issues that could cause me headaches later. The assembly of the R7 Kit has gone together quickly and now I am packaging the electronic bits into a control box. Just making sure I do not have to back up. Coming from industry, we always used twisted pair shielded for signals with very clear grounding rules to prevent intermittent issues in high noise equipment. Those type issues can drive a guy crazy working them out and can eat up a lot of time.
@Scott_Sanders well said. Also, whether your the kind of person who likes spending time working on and building machines, or the kind who wants to spend (more of) their time making projects.
@Mark_Leino as for #4, not the backlash nuts. Getting the tension on the ACME rods is the issue. As tight as I can get, they still have some movement. I am setup per the build sheets on the Y C-beams with the bearings on the outsides of the end plates. The ACME is setting almost flush with the end stop collar. Nothing to grab onto on the non-motor ends past the stop collars.
@Mark_Leino I am in the process of laying it out. I was able to pick up a very nice surplus HyperLink box and have it ready to mount. Need to fit the TinyG, Rpi3, Inverter, 24 and 5volt supplies, AC screw strip and a small 4-relay board for AC power control. On the front panel, I am adding a little LCD Temp meter to monitor box heat, as Texas gets a bit warm in the summer. I am also adding a voltage and current display for the 24v supply, remote mounting the Inverter front panel, main powers and an E-Stop switches. Maybe others. I will add side-panel USB jacks for WiFi and XHC links as I think they need to be outside the box for best signal strength. plus a USB port for direct TinyG access, incase I want to run it off a laptop. I have already added a larger fan output and a filtered input port to vent the box. The 24v supply, inverter, and TinyG will stir up the inside well plus the added fan to vent heat should be more than enough. Will post a pic or two of my mockup, so far, but it is subject to change as I work through it.
@Donald_Loocke may want to consider a DC to DC converter vs an extra pwr sply. Less heat and space taken up. (ex: 24VDC pwr sply and 2 DC converters, 5VDC and 12VDC (LED strip, etc…).
Thought about the Dc/DC for the 5V. But would need one that output 3-4 amps for Rpi, usb port loads and some LED lighting. I will take another run at locating one that will handle the load and not cost too much ( and can be sourced quickly).
Spent today gathering Mech drawings of parts and working out layouts. Have some parts and AL plate coming in this week and I will need them to get final sizing right and start drilling holes.