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Old 11-05-2008, 04:34 PM   #1
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Default IH Mill CNC Conversion

After making a series of modifications to increase the rigidity of my IH mill and add a one shot oiling system, I'm now turning to the conversion of the mill to CNC. I'll be working with an Industrial Hobbies "mechanicals only" kit and using my own servo motors and electronics.

Let's start with the design and layout of the electronics. I purchased a NEMA-style enclosure from eBay which I mean to mount to the side of a rolling tool cabinet I got from Sears on sale for Labor Day:



and here is the NEMA enclosure:



Some kind of phone system had been inside, I think. I'll be clearing out all the guts. To get the door open I had to make a little key as well:



Some lathe work, cross drill on the mill, and a 1/8" roll pin and I was in like Flynn.

<continued>



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Old 11-05-2008, 04:44 PM   #2
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Default Re: IH Mill CNC Conversion

I'll be using the ubiquitous Geckodrives to control my servos. I've used them in the past and like them well. This will be my first time for servos instead of steppers though. Servos have a number of advantages over steppers, though they are slightly more expensive--perhaps 20-30% all told. Their chief advantages are in speed and accuracy. They maintain torque for a broader rpm range, and they are closed loop, meaning they have a sensor that measures how far they moved. A well-designed control system can compensate if the motion comes up short, and this is what the Geckodrive does for you.

I'm going to create what I call "axis modules" for each axis of the mill. I'll start with 3, but Mach 3 (the software on the PC that controls the servo motors) can manage up to 6 axes. I may actually use all 6 over time, so I want to make provision for expandibility in my design. Here is a CAD drawing of what a typical axis module would look like:



Here's what you get. The red box on top is a Geckdrive. It is mounted to an upside down heatsink. Remember, this is a very smart little motor controller, but it can manage up to 20 amps at 80V. That's about 2 HP, and it is a little tiny box, so I like to use a heatsink!

Underneath is the front panel. The 3 components directly under the heatsink are a socket for the servo motor power cord (I plan to use standard IES computer-style three prong cords), a connector for the encoder on the servo (that's the sensor that tells how far it has moved), and a 3AG fuse holder. That round purple cylinder is the back of a 15 amp ammeter. The ammeter will tell me the load on the servo. It's handy to know how hard the machine is working when you're doing things like tuning up the gibs. One of the commercial VMC (Vertical Machining Center) recommends tightening the gibs on their mills to 30% of the maximum load for the axis. Tighter gibs run more accurately right up until you get them too tight. On a CNC, there is no feel, so a meter is helpful. Seeing loads creep up would be an indication to try the one shot oiler first, and then re-adjust the gibs if there was still to much trouble moving the axis.

OK, now these axis modules will be mounted in my NEMA enclosure so there is room for up to 6 of them. That looks like this:



4 modules accessed from the front, and 2 more in the rear. Also inside the enclosure will be the DC power supply for the servos, a board to interface the PC to all of this, and some relays to help control the VFD for the spindle and optionally coolant or compressed air to blow the chips away while cutting.

I'm not going to try to squeeze a PC in here, though a little larger box and that would certainly be possible.



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Old 11-05-2008, 04:50 PM   #3
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Default Re: IH Mill CNC Conversion

While I'm planning and constructing the electronics, there are a myriad of other little things to do on the mill. One of them is to install the home/limit switches. These are precision switches that serve a couple of purposes. First, they stop the mill from running an axis off the edge and hurting itself, and potentially the operator too!

Second, they are precision so that the machine knows to a very close degree exactly where the axis is when the switch is tripped. When a CNC is first turned on, it will test those switches to calibrate its position.

Here is the X-axis limit switch mounted just where'd you'd mount the switch for a power feed:



The two pins are triggered by stops mounted on the table's side T-slot.

Here's what's inside one of these limits:



These are optical limit switches. What happens is a little shutter crosses a light beam to trigger the switch. There are a number of other kinds of limit switch available. One of the chief things to look for is a switch that can survive coolant and chips. This one is inside a sealed enclosure with a rubber gasket.



The other 2 axes work by those long metal rods. A color on the rod allows you to set where the switch is tripped. The rod runs through a hole in the moving block which is on the part of the axis that moves. The block will run up against the collar on the rod and trip the switch.

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Old 11-05-2008, 04:57 PM   #4
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Default Re: IH Mill CNC Conversion

Getting back to building the axis modules, I started with a big piece of surplus heat sink material from eBay. It was too big for my bandsaw or chopsaw, so I sliced it up on the mill:





Next step was to mill off the fins to create a mounting rail on 2 sides:



Making 6 axis modules involves a lot of repetitive tasks, so you'll see me using stops a lot so I just setup once and can do each part quickly.

Then I used my CAD program to make a 1:1 actual size template for drilling. I glued that to one a piece of steel plate to make a drilling jig (yes, this jig will guide the bit as well as locating as a fixture does):



Kant-Twist clamps made it fast to drill my holes in each of the 6 heat sinks.

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Old 11-05-2008, 05:01 PM   #5
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Default Re: IH Mill CNC Conversion

I had some 304 (yucky stuff) stainless rods laying around that I threw on the lathe to make 4 legs for each module. 304 is hard to cut, so I simply drilled the proper sized holes for tapping in the ends and faced the ends. I finished by parting to make sure the legs were all the same length.

I managed to break off one of my cheap 4-40 taps in a heat sink (hate the taps that come with the sets, but I got in a hurry, DOH!), so I had to stop until new taps arrived from Enco. A brand new spiral flute tap makes even the 304 stainless not that big a challenge!

Here is a heatsink on its legs with a Gecko mounted:



Next stop is to make the sheet metal panels for each axis module. I'm only going to do 4 panels for the front. The 2 rears will just be blanks until (and unless) I actually need the modules.

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Old 11-06-2008, 03:59 AM   #6
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Default Re: IH Mill CNC Conversion

Hey Bob!

That's looking great. Got a couple of questions for you as I start my CNC conversion this week.

How is the epoxy granite mix working for you? Do you think it is going to be worth it?

What are you going to use for a controller? I am getting a CNCbrain from SR robotics:



Click the pic for a link.


Eric

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Old 11-06-2008, 04:51 AM   #7
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Default Re: IH Mill CNC Conversion

RE the Epoxy Granite, it is too soon to tell. It was very easy to do, and the castings sound noticeably more dead when you rap them with a wrench. But, I have no real cutting experience on which to base an informed opinion. I am optimistic about it, however.

RE controller, I plan to go with a SmoothStepper. One piece of advice--don't go too far out on the bleeding edge. I did that on my last CNC project with a GRex and never got it finished. The controller wound up being orphaned and the Mach 3 support for it was never quite right. Ever since I resolved not to do anything but the #1 or #2 most popular controller for Mach 3. Right now that would be the parallel port or Smoothstepper.

Cheers,

BW

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Old 11-06-2008, 05:12 AM   #8
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Default Re: IH Mill CNC Conversion

Bob, many thanks for posting this. CNC is going to exist in my shop one day. It is a "down the road" plan, but every little bit of information that I can tuck away will be a great benefit when the time comes. Until then, I am enjoying the shared journey with yours and can't wait to see it come to life!

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Old 11-16-2008, 06:47 PM   #9
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Default Re: IH Mill CNC Conversion

A bit more progress to report after having most of yesterday afternoon available in the shop.

First, I soldered the cables on my servo motors. They come with short tails, so I just soldered on some IES power cords to extend their reach to 10 feet. These are the same power cords you'd use on a computer with the distinctive three pronged connector. For splicing, I like to protect each conductor with a piece of heat shrink tubing and then protect the overall splice with another piece. Unfortunately, I didn't have the latter 1/2" tubing on hand, so I did 2 of the cables with electrical tape wrap. When I ran out of even the smaller diameter I had to make a run to Radio Shack for it. Here are the three servo motors that replace my 3 handwheels on the mill:



You can see an additional white connector on each motor that looks like a serial port connector for a computer. That's the encoder connector. Encoders are one of the big things that separate servos from steppers. You can drive a CNC with either, but servos offer higher performance (at a higher cost, natch!) than steppers. I like using standard connectors so I can buy off the shelf cables already made up. Hence the IES power cords, and now these serial port connectors. The latter were installed by Homeshopcnc, which is where I got my servo motors. To give you an idea, these are 850 oz/in torque motors that cost $235 apiece with the encoder housings, so they're not cheap for this kind of performance. Steppers in that range would cost circa $130.

Next, I managed to finish a couple front panels for my axis modules. Here is one mocked up with the parts, but not wired:



So the encoder plugs in to the left of the fuse, and the servo power cord to the right. I can get at the fuse without opening up my enclosure with the panel mounted holder. The ammeter is a peculiarity of my setup. I wanted to measure the current draw of each axis as a way of understanding how "tight" the axis is in order to adjust the gibs and monitor wear. Remember, you lose feel for gibs with a CNC!

Here is the back of the panel:



As you can see from the photos I have an issue with meter clearance and the mounting bolts, so I made an oversized hole to try to create some "wiggle room". This happened due to an error in laying out the big square face during the CAD design. What I need to do is relocate the whole meter 1/4" down the panel and all would be well.

I've gotten it close enough, I think. I can't go much further or I'll lose the mounting holes for the meter as you can see in this behind shot. In the end, I'm planning to remake these panels anyway once the CNC is up and running. I'll make them out of 1/4" aluminum plate and put some engraving and other decorative touches on so they'll look a lot nicer.

Still an awful lot to do on this project!

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Old 12-22-2008, 04:23 AM   #10
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Default Re: IH Mill CNC Conversion

More updates.

Been cutting openings in the enclosure with an air shear (nice tool!):



Love my air tools!



And I'm mounting all sorts of little CNC4PC boards plus an Antek DC power supply:



Getting closer to being able to spin a servo or two...



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