Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: mckoz on November 23, 2008, 03:38:58 PM
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Hi all, I'm in the process of converting an Industrial Hobbies manual mill to a CNC (Mach3 of course), partially using their kit, and partially my own experience with Automation and Control systems.
Anyway, I'm going to use cnc4pc's step and direction board to talk to a vfd, and will probably use one of my old plc's as a modbus slave for certain aspects of control of the system. As I'm sure most folks don't have an unlimited budget and must balance performance against cost (as do I), I would like to get a recommendation on a spindle speed detection component to provide a closed loop speed control system. I'm currently planning on taking the speed reference off the top of the spindle shaft, since I've got about 5 inches exposed on the top of the gearbox head.
Thoughts and recommendations?
David
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OPB 917B is what I use for spindle index.
Hood
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Thanks for your reply! I looked it up, looks like a hall effect switch? I assume that is one pulse for every revolution, does that provide enough resolution for threading? Also, where did you purchase it?
David
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Dave,
Mine came out of the junk box. I use a single index and with the latest Mach update threading seems to work
well. Pic attached for one on a lathe for what it's worth.
RICH
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I am in the UK so I got it from RS Components http://uk.rs-online.com/web/search/searchBrowseAction.html?method=searchProducts&searchTerm=opb-917b&x=0&y=0
Yes one pulse per rev is all that is need for threading on the lathe but you said you have a mill so afraid if you are thinking of doing rigid tapping then Mach doesnt support that yet. The SmoothStepper may change that but only time will tell.
Some guys have done rigid tapping on a mill by using the swap axis feature and saying that their spindle is the A Axis and coding suitably for it however you will have to make sure your motor tuning is spot on as any bogging down will screw things up fast.
Also I would think this is only possible with stepper or servo spndle motors and dont think a VFD and induction motor would have the same control but could be worth a try.
Your other option of course is a floating holder for your tap and that should work if you code carefully.
Hood
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Thanks Hood and Rich. I've just mounted the 3-Phase AC drive, and I'm using the cnc4pc step/direction board to drive a tverter chinese knockoff VFD. I have a fairly large Moog bushless servo motor that is resolver based, and based on Hood's comments I've begun to look for a drive that will handle the Moog. I've found a resolver to encoder board out on the web, but I haven't found a drive yet, NOR have I decided whether to just ignore the resolver output and install an encoder. Any advice on either of those options will be appreciated!
I'm going to continue the build out with the 3-Phase VFD motor and try some of the threading while I'm looking for a servo drive, I'm very interested in the results that Rich is getting.
David
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What size is the moog motor? ie Continuous current, peak current and voltage?
Hood
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I'm waiting on a spec sheet request from Moog, once I get the info back I will post it - currently I only have the nameplate of the motor.
David
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What info is on the nameplate?
Hood
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It's out at the shop, I'll be out there later today and will get the nameplate info...
David
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ok, usually there is torque, current and voltage details on the plates.
Hood
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Ok got the spec sheets and the motor.
Moog D315-007B Brushless DC Servomotor (Resolver with IP65 housing)
Cont Stall Torque: 5.5 Nm, 48.675 in lbs, 4 ft lbs
Continuous Stall Current: 12.4 A
Peak Torque: 16 Nm
Nominal Speed: 4900 Rpm
Terminal Resistance: 0.88 ohms
Here is the Duty Cycle graph and graph legend as attachments. The current motor is a 3 phase 208 2hp 2500Rpm Chinojunk special, so this Moog will pull the teeth out of the AC motor at any RPM, and has a top end RPM that's almost double the speed. What's left is:
Deciding whether to mount an encoder (mounting will be difficult, and I'm worried about slop in the gear head, so I'm thinking I might need the encoder on the spindle itself), or go with some kind of Resolver to encoder converter board;
Sourcing a drive that will do 310VDC at 15 amps peak and 5.5 amps continous;
Finding a power supply.
Recommendations?
David
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I think you may struggle to find a suitable drive for that, I know brushless DC motors and AC Servos are very similar so possibly one of the drives I use would work, they are Allen Bradley DSD- HV drives. You would need 220-460v three phase but if you just had 220 then that would limit your voltage output from the drive to 220v which in turn would cut the max speed of the motor.
As to the encoder, can you not remove the resolver and fit the encoder in its place? Also a lot of the Yaskawa drives for AC Servos can accept resolver input but again not sure if they would be suitable for a DC Brushless. One other thing to note is a lot of industrial drives will either have models that accept Step/Dir or Analogue so be carefull if you find any on eBay and make sure you read the specs. The Allen Bradleys that I use can accept either.
Hood