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Old 12-06-2013, 09:31 PM   #51
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Just a general questions for you folks who have some experience with CNC... How important are LIMIT switches? Anybody running without them?

Chuck
Never installed limit switches. If you run the machine to its outer limit it will skip steps and make a horrible noise but wont hurt anything. If you break your machine from maxing out a motor then you need to re-examine the way you built the machine.


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Old 12-06-2013, 10:03 PM   #52
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Hi Chuck

Well here is my .02$ worth; I ran my knee mill for years without working limit switches. They worked originally when I did the conversion but over the years a couple of the wires broke and I just jumped them out.


My old round Pacific Scientific steppers didn’t have enough power or speed to do any damage if you hit the end of travel. Although it was a pain to be cutting a large part and run out of travel, stall the motor and have to shift the setup and start over.

I converted my machine over to Mach3 about 3 years ago and replaced all the switches and repaired the wiring. At this point I wouldn’t be without them. Within Mach you can enter your axis travels and also a safe distance to start slowing down. The first thing you do is home your machine then Mach knows the exact travel limits of your machine and shows you on the monitor; it will let you know if your job is going to fit within these limits before you ever start machining.

Also because of the safe limit distance you define you can do a rapid jog in to a limit and the software will start slowing down the axis when it reaches the distance away from the switch that you set. I think I have mine set at 1”. The last inch of travel the axis ramps to a crawl as it comes into the switch.

I used decent quality switches, Omron snap action switch with a roller actuator; surprisingly these easily repeat within .001” or less. So if I have fixture offset stored (G54, G55, etc.) I can shut my machine off and when I restart it and home it; I can go right back to that offset without having to pick it up again. I have checked this with an edge finder many times and it’s usually dead on.

In my opinion it is really worth the time now while you are in the build phase to fab up some mounts and get them working; you will be glad you did.


Dave


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Old 12-06-2013, 10:50 PM   #53
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I dont have them on any of my machines. I do have Home switches on the lathe, but unless you use some kind of permanent vise or fixture on the mill its hardly worth the effort to Home on a mill.

For a stepper driven machine like you are doing I dont think limit switches are worth the trouble. The motors simply dont have enough power to break anything. If you run to the limit you will just stall the motor. You have of course lost your position with or without switches.

A suggestion for setup so you dont run out of room. With the CAD system print out a full size drawing of the part you are about to make. Move that paper around the table top as a guide to where to mount your stock. This has saved me running off the end of the stock, or hitting a clamp many times.

Last edited by RonGinger; 12-06-2013 at 10:55 PM.
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Old 12-07-2013, 11:10 PM   #54
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Thanks for the input, guys. I may add limit switches at some point, but for now I think I will forego them.

After some more work today, I am 2 timing belts and a DB25 cable away from having this thing operational.



That's not to say that I'm finished... still have a bunch of tidying up to do including keying the pulleys, making guards and covers, and doing some cable routing, not to mention a bunch of testing and, most likely, some debugging. But with the two belts installed and a DB25 cable to hook it up to the computer, it would be capable of making parts. I looked all over town yesterday for a DB25 cable, but none was to be found. So, I wound up buying one on Ebay and will have to wait several days for it to arrive.

Chuck
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Old 12-08-2013, 12:17 AM   #55
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Hello Chuck!

That stepper behind the table worries me a little. When the table moves all the way back that stepper won't contact the column will it?
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Old 12-08-2013, 01:35 AM   #56
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Hello Chuck!

That stepper behind the table worries me a little. When the table moves all the way back that stepper won't contact the column will it?
It could hit the column if the if the table was all the way to the right. Most of the time it won't be a problem and if it ever is, the motor mount can be swiveled up so the motor is over the table.

Chuck
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Old 12-08-2013, 08:39 PM   #57
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Waiting on belts and cables, so I started making the keyways in the pulleys today. I plan to use a Woodruff key on the shaft.



I figured it was easiest to use my keyway broach to make the slot in the pulley. Naturally, I didn't have a 17mm bushing so I had to make one. You can see that I milled two slots in the bushing, one shallower than the other. I first made a pass using the deeper slot, the a second pass with the shallower slot to make the keyway in the pulley deeper. Still need to do the pulley on the Y-axis. It's kind of nerve-racking pushing a broach through the pulley using my hydraulic press. Gotta make sure everything is completely lined up to keep from snapping the broach.

Chuck
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Old 12-09-2013, 12:52 AM   #58
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Just a general questions for you folks who have some experience with CNC... How important are LIMIT switches? Anybody running without them? Chuck
As far as I'm concerned they are very important. I've seen lathes snap off substantial castiron mounting lugs when a servo ran away. You probably aren't working with system hardware that powerful but damage can happen or sneaky things like loosening a coupling can happen.
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Old 12-10-2013, 01:17 AM   #59
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Still waiting for the belts and the DB25 cable, so today I finished up the keyways.



I cut a standard keyway instead of woodruff. That gave me a lot longer key. I used a 3/32" carbide end mill to cut the keyway in the shaft. Easier than I thought it would be. I made the key out of some .093" sheet steel I had on hand.

Hope the cable and belts get her tomorrow. I'm anxious to get this thing working!

Chuck
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Old 12-14-2013, 01:44 AM   #60
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Just a quick update. The DB25 cable was delivered a couple of days ago. I got the motors, the Gecko controller, cables and the power supply all hooked up. Loaded up Mach3 and got everything configured. The steppers all respond and move correctly to the Mach3 commands. Even loaded up the G-Code file to print my name and ran it. Unfortunately, I still haven't received my X & Y timing belts (Aarrgghh!). So, even though the motors spin, the table doesn't move... Wish I'd ordered the belts sooner. Now that the conversion is virtually done, the waiting is killing me!
Chuck


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