Bridgeport Clone CNC Conversion

[Cartierusm]

So I'm thinking about converting my First brand Mill to CNC. I've build CNC's from scratch before, so this shouldn't be too complicated. I will have to work somthing out for the screws as I don't want to spend a fortune. Also the bed needs some work as it's looser in the center than at the ends, of course, but if the motors I use can drive the tighter spots then I'll leave it.

I've already got a CNC router I build from scratch using Keling 8078 drives, which I hooked up with quick disconnects so luckily I'll use one computer, drives and PS for both, one at a time of course.

I want to use belt drive system as I think it will be better than direct drive as my bed is probably a little stiffer than it should be, maybe, I haven't really testing anyone else's. I'm going to use a 72VDC PS and the 8078 Keling Drives with 1810 oz/in. Steppers.

So the questions are:
Are belt drives fine to use, will they stretch or anything else I need to consider. Will it be just as accurate with the belt and pulley?

Using a 2 to 1 ratio with the bigger pulley on the screw will that increase the power from the motor as there is more leverage?

I'll be driving the quill, I figure I'll find a way to drive it from where the quill handle is, is there any reason not to do this?

If I were to drive the Z from the Knee obviously it's way heavier how does Mach 3 know that it needs more inertia to drive it, or is it all based on the signal the motor recieves. Meaning when it's tol to go 100 steps, it also give a time it should get there so the motor speeds up to meet the speed it's trying to achieve? I figure if it can't meet the speed by driving the mass of the knee it's missteps?

Well that's it for now, tomorrow i'll be removing the bed and measuring the screws and taking lots of picture.

[HimyKabibble]

Originally Posted by Cartierusm So I'm thinking about converting my First brand Mill to CNC. I've build CNC's from scratch before, so this shouldn't be too complicated. I will have to work somthing out for the screws as I don't want to spend a fortune. Also the bed needs some work as it's looser in the center than at the ends, of course, but if the motors I use can drive the tighter spots then I'll leave it. I've already got a CNC router I build from scratch using Keling 8078 drives, which I hooked up with quick disconnects so luckily I'll use one computer, drives and PS for both, one at a time of course. I want to use belt drive system as I think it will be better than direct drive as my bed is probably a little stiffer than it should be, maybe, I haven't really testing anyone else's. I'm going to use a 72VDC PS and the 8078 Keling Drives with 1810 oz/in. Steppers. So the questions are: Are belt drives fine to use, will they stretch or anything else I need to consider. Will it be just as accurate with the belt and pulley? Using a 2 to 1 ratio with the bigger pulley on the screw will that increase the power from the motor as there is more leverage? I'll be driving the quill, I figure I'll find a way to drive it from where the quill handle is, is there any reason not to do this? If I were to drive the Z from the Knee obviously it's way heavier how does Mach 3 know that it needs more inertia to drive it, or is it all based on the signal the motor recieves. Meaning when it's tol to go 100 steps, it also give a time it should get there so the motor speeds up to meet the speed it's trying to achieve? I figure if it can't meet the speed by driving the mass of the knee it's missteps? Well that's it for now, tomorrow i'll be removing the bed and measuring the screws and taking lots of picture.

Timing belts work just fine, if used properly. Pick a low-backlash design (I like the GT2s), and be sure you tension them properly - timing belts require considerable tension to work their best. Stretch is not a major problem.

Using a belt reducer will multiply torque, but reduce top speed. I don't think you want to go beyond about 2:1 with steppers, as your rapid speeds will suffer, depending on the RPM capability of your motors. Personally, I'd seriously consider using servos on a BP clone. The 850 oz-in motors from www.homeshopcnc.com work very well. I'd use a 4:1 reducer with them, to keep the rapids from being TOO fast.

If you plan to do precision work, do NOT drive through the quill handle - you'll have LOTS of backlash. The knee also has problems, because of the weight, though it can be done. I'd recommend CNC'ing the quill directly.

You might want to consider moving this thread to the Bridgeport forum, as most people here have smaller table-top machines, so are facing some very different problems and trade-offs.

Regards,
Ray L.

[Regnar]

I cannot speak as far as cnc'ing the mill but I have seen the Knee get cnc and the quill left alone. To me it seemed like a great ideal because you can quill the tool down to the project to zero off. Lock in place and hit start. Here is a video of what I am talking about.

YouTube - Milling fork clamps and tool changes with power drawbar

YouTube - Milling fork clamps and tool changes with power drawbar

[metlcutr55]

i have a knee driven cnc mill, google "cnc automation m3x" to see it. i really like the driven knee, it gives 14" of z travel, leaves the spindle free for hand feel drilling, and allows me to chuck a tap and tap around a part using the table to quickly position. also can release & retract the quill to prove a program with the tool above the part. i have been thru a z (and a y) axis motor, but i dont attribute the failure to knee weight, i found at disassy the motor(s) had oil and/or coolant internal. placing a "condom" made of a soda bottle over the motor(s) has made them last. i would guess i have in excess of 7000 hours on this machine.

[Cartierusm]

Thanks for the help. How do I move the thread? I looked under everything?

I would like to stay with stepper so I can use my current drives, BOB and Computer. I don't care too much about super speed rapids, I know everyone says that but I already have some cnc machines so I know what it'll be like.

I'll search around but does anyone have a pic of how it looks attached to the quill directly, I MEAN a pic without a housing cover on, all I find are ones that show complete I want to see how it's acutally hooked up.

Also if someone can answer the last question in my first post about speed and timing.

Himy, I forgot to ask in the PM how did you machine the ends of the ballscrews? Did you do it or have it done? Thanks.

As far as tension should I be using somekind measuring device to make sure I have enough tension?

[HimyKabibble]

Originally Posted by Cartierusm Thanks for the help. How do I move the thread? I looked under everything? I would like to stay with stepper so I can use my current drives, BOB and Computer. I don't care too much about super speed rapids, I know everyone says that but I already have some cnc machines so I know what it'll be like. I'll search around but does anyone have a pic of how it looks attached to the quill directly, I MEAN a pic without a housing cover on, all I find are ones that show complete I want to see how it's acutally hooked up. Also if someone can answer the last question in my first post about speed and timing. Himy, I forgot to ask in the PM how did you machine the ends of the ballscrews? Did you do it or have it done? Thanks. As far as tension should I be using somekind measuring device to make sure I have enough tension?

Go to www.google.com/patents, and search on "Elrod quill", and you can see the patents for the Elrod conversion which spells out exactly how they do it. Their conversion is very highly regarded.

*You* determine acceleration and top speed when you setup Mach3, so it is entirely under your control.

I had the screws machined by a shop in the mid-west. I tried doing them myself, but those Nook XPR screws are *really* hard, and I couldn't get a decent finish. I think it cost me about $300 to do all three, including round-trip shipping.

Belt tension you can either use a scale, or just do it be "feel". I don't think it's real critical, as long as it's tight enough. Too loose, and you'll have some backlash in the belt. Too tight, and you'll shorten belt life, and maybe the bearing life in your motors.

Regards,
Ray L.

[Cartierusm]

Thanks again. As for setting speed and accel. I've done that a milllion times in mach, but my question may have not been phrased correctly. I mean if the motor is sent a signal to move 1" at 60 ipm on the X and it moves that because the mass of the bed is not that much, and if you move 1" at 60 ipm on the knee the mass is much greater? So how does it keep up, I guess by typing this I've answered my question the motor speeds up to meet that speed until it stalls if the mass is too great.