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- Jun 12, 2014
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The proximity sensor has a repeatability of better than 0.001" if the speed/feed parameters stay the same, in most cases you are looking at 0.0004". This is even with fairly high feed rates and chuck speeds. It requires a VFD with electronic braking on the order of a second or less (requires a braking resistor with VFD external braking circuitry) , so stops happen quickly and predictably. This would not be possible to these tolerances with a mechanical system, or mechanical electrical system such as a mechanical stop switch, also you do not have the braking. The advantage of the mechanical stop,feed stop is that it stops the carriage at the same position in the Y axis, and is not affected by feed or speed, they disengage the feed system. An electrical stop in this type of setting, needs to be readjusted for any major change in the feed or speed, as the momentum in the system will change the stop point to some degree. Think of it as the travel distance feed system relative to the rotations of the chuck over 1 second. The maximum variation I could achieve in my system was about 0.050" more travel between a snails rate feed/speed and stupidly high feed/RPM rates.
The proximity sensor is an interesting concept, but like anything electronic, they can fail (usually due to damage) or you can have operator error. I do find it an additional safety factor when turning or threading to a fixed position, and makes it much less stressful of disengaging the feed at just the right time. The addition of the secondary "Kill Switch" which kicks in after the proximity stop is a real plus, I thank one of the forum members for suggesting the idea. Anyone with a VFD lathe system, should consider as a minimum adding a snap/end of travel switch which acts as an E stop if a particular travel limit is reached (same as feed systems). Wire it in just after your E-Stop, just beware that without a reset relay the system will activate when you release the switch.
Another point to consider with either system, is that the stop position is based on the carriage, not the tip of your cutter in the Y axis. So if your tool post position is changed, and it is not perpendicular to the cutting surface, the stop position of the tip changes with the tool post position. When cutting the diameter using a stop system, I use the cross slide, so there is no change in the cutting tip Y position. When cutting a thread, the cross slide is locked, I "0 or record the distance" my DRO Y axis at the position the cutting tip is to end with the tool post advanced to the depth of the final thread cut in the relief. Remember if you cut at a 30 degree angle the stop position changes as you advance your cutting tip. I then back out the tool post slide and set the electronic/mechanical stop position based on the DRO reading. I verify this with an extra pass. Then do my scratch pass for threading, and "0" the dial on the tool post slide, then get to cutting the thread in sequential passes only using the too post slide. You have the option to either disengage the half-nut and manually back up the carriage on each pass or electronically reversing the carriage by pressing the electronic bypass (proximity) switch and reversing the spindle. That latter is required for metric threads.
I will probably build another batch of the P sensor stop systems (two will go with complete systems), the new design works well but is a PTA to make
. I appreciate forum members comments on some of the systems I have built, as they have evolved with newer versions. I hope to post some more build schematics, with detailed parts lists for the VFD low voltage systems for others in a couple of weeks, time permitting. The revised design should work with any lathe and any VFD that has at least 5 inputs, and has circuity for an external braking resistors (which excludes most Chinese eBay VFD sellers). The last issue has been using the stock work light 24V 50W halogen bulb that comes with some machines, was not happy with anything less than a 100W power supply. I have switched mine out to an LED bulb, otherwise for any of these systems using the stock bulb, probably go with the recommended 100W DC power supply in my schematics (only one that fits).
The proximity sensor is an interesting concept, but like anything electronic, they can fail (usually due to damage) or you can have operator error. I do find it an additional safety factor when turning or threading to a fixed position, and makes it much less stressful of disengaging the feed at just the right time. The addition of the secondary "Kill Switch" which kicks in after the proximity stop is a real plus, I thank one of the forum members for suggesting the idea. Anyone with a VFD lathe system, should consider as a minimum adding a snap/end of travel switch which acts as an E stop if a particular travel limit is reached (same as feed systems). Wire it in just after your E-Stop, just beware that without a reset relay the system will activate when you release the switch.
Another point to consider with either system, is that the stop position is based on the carriage, not the tip of your cutter in the Y axis. So if your tool post position is changed, and it is not perpendicular to the cutting surface, the stop position of the tip changes with the tool post position. When cutting the diameter using a stop system, I use the cross slide, so there is no change in the cutting tip Y position. When cutting a thread, the cross slide is locked, I "0 or record the distance" my DRO Y axis at the position the cutting tip is to end with the tool post advanced to the depth of the final thread cut in the relief. Remember if you cut at a 30 degree angle the stop position changes as you advance your cutting tip. I then back out the tool post slide and set the electronic/mechanical stop position based on the DRO reading. I verify this with an extra pass. Then do my scratch pass for threading, and "0" the dial on the tool post slide, then get to cutting the thread in sequential passes only using the too post slide. You have the option to either disengage the half-nut and manually back up the carriage on each pass or electronically reversing the carriage by pressing the electronic bypass (proximity) switch and reversing the spindle. That latter is required for metric threads.
I will probably build another batch of the P sensor stop systems (two will go with complete systems), the new design works well but is a PTA to make
. I appreciate forum members comments on some of the systems I have built, as they have evolved with newer versions. I hope to post some more build schematics, with detailed parts lists for the VFD low voltage systems for others in a couple of weeks, time permitting. The revised design should work with any lathe and any VFD that has at least 5 inputs, and has circuity for an external braking resistors (which excludes most Chinese eBay VFD sellers). The last issue has been using the stock work light 24V 50W halogen bulb that comes with some machines, was not happy with anything less than a 100W power supply. I have switched mine out to an LED bulb, otherwise for any of these systems using the stock bulb, probably go with the recommended 100W DC power supply in my schematics (only one that fits).
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