[Dozer56]

I had been lurking around the benchtop millling machines forum for a few weeks looking for general info that would help me with my G0704, and stumbled into the CNC- DIY routers category, that was a big mistake .
Now couple weeks and about $700 later, after reading, I think, every build post and opinion on every component I could find, which I'm sure was not near enough, I got the frame built, with all 3 axis rails and everything installed. ( I think )
I looked at several builds posted here, first deciding that a wood frame was not for me and after seeing what was available in my price range I came up with this type of machine, like several designs and the ones gio666 shows. I thought it would work best for me, it's ended up with about a 30" X travel and 32" Y travel, and I think I have about 6" clearance at the gantry and alot more z travel than that or I nedded.
I don't have any desire to cut anything specific, I would like to be able to do a variety if materials and sizes and I'm thinking possibly after a while different spindles, right now it's probably going to be something inexpensive.
I have the screws and ball nuts ordered, and am thinking that the cncrouterparts 3 axis electronic kit, since I know nothing about the electronics, would be best for me being basically a plug and play solution, my question is if the 380 oz would be large enough for this machine.
I've measured what I can to see how it is tracking and dial gauges show the rails and ganty are way closer tolerance than I imagined but I need a table on it and legs to get more accurate readings, there coming soon also.

Please criticize or comment as I probably am missing some major componant or issue with getting this machine running which is my main goal at this point...

thanks ..

[stevespo]

Congrats. It looks like you did a great job on the frame. Doesn't look like you missed anything. What are you going to do for screws? Ballscrews at what TPI? 5 TPI?

As far as electronics goes, a good solution for this size machine seems to be the Gecko 540. I don't imagine you'd have any problems driving each axis with the 380 oz-in motors. I ran Gecko 203Vs with 425 oz-in motors (at 72V) on my (similar) machine for several years with good results. The G540 with 380 oz-in motors probably have a flatter torque curve and better overall performance.

I eventually wanted to close the loop and go servo, with BLDC motors and Granite drives. All around a great upgrade, but a more costly path than steppers. I do have to say that I love the feedback loop and my performance and accuracy has definitely improved.

Steve

[ger21]

Please criticize or comment

Your gantry sides are twice as high as they need to be. If you lowered the gantry beam to the bottom of the Z axis plate, you'd still have the same clearance. Another change would be to increase the height of the gantry beam, so the bearings on the back of the Z plate could be at the top and the bottom, rather than hanging down unsupported like that.

[stevespo]

Yup, Gerry is right on (as always). If you need that full (12"?) of clearance, you can leave the gantry where it is and modify your Z. If you really only need 6+" of clearance, you can lower the gantry and remount the Z so the bottoms line up. Great idea.

Steve

[Dozer56]

Thanks for the comments, all good points. I ordered 5 TPI Screw assembly for each axis thinking from what I've read it would work for general use for me.
I played with the height of the Y rails a bit and probably will drop the whole assembly or at least I can raise the z plate once I decide on the spindle. For now I like the clearance probably mostly because I can always make it shorter but hard to lengthen or stretch. I am curious if it will deflect when cutting as it is. Hopefully we'll see.

[stevespo]

It's great to have options, so wait and see when you start cutting. The 3.5" full sized routers are a popular option and reasonably powerful and accurate. The Chinese spindles may be something to consider as well. Start with a laminate trimmer if you really want to keep the costs down.

I'm also using .625" diameter/5 TPI ballscrews and they're a reasonable choice. Depending on the length, your top speeds will probably be limited either by motor RPM/torque or the critical speed of the ballscrews. I get some funky resonances around ~300 IPM, so I'm running 250 IPM rapids with great results. It's run to see 600 IPM rapids, but not necessary for my work.

Steve

[johnmac]

I would consider lowering the gantry now, before any further assembly. After its running, it will be a pain to disassemble. BTW, its looking good!

John

[Dozer56]

Still learning, that the more I get into this project the less I know about alot of things. Aligning and mounting the ball screws and were a bit more difficult that I had thought but I think I got it beat now, one more bracket to make and the Y ball nut mount and they will be complete and mounted , oh yea and wait for the bearings to come to replace the ones I beat all to pieces, trying to get them back off the screws after I dry fit them....
I did change Z plate so that it is more even and not all hanging below the gantry beam, I realized what Ger21 was talking about after taking a long look at it..
thanks for the input...
The one thing that I am curious about is the stepper motors and sizing them properly, I see machines that seem larger or with, well what I think is probably more resistance in the travel using acme screws etc,... but is there something I can do to measure or check the resistance in the axis' to be sure to correctly size the motors? and is it a bad thing to have steppers too large?
I have been able to move all the axis' with a 18v cordless drill easily, and can set the clutch to the lowest setting so that even grabbing it makes it slip, but it will move the gantry smoothly, without slipping or binding.
Can I measure the weight or resistance some how to get a better idea or am I just being anal?

[stevespo]

Dozer, nice progress on the machine. Not to give you a hard time, but it looks like your Z axis is still hanging a good 6" below the gantry. That combines the long moment arm of your tall gantry with the reduced clearance of the Z axis. No practical benefit from either one.

If you look at most designs (particularly the commercial routers), the bottom of the Z axis typically lines up with the bottom edge of the gantry. The mounting plate and router, then can move downward to perform cuts on the table, or get moved up and out of the way for clearance.

Here's a picture of my setup. Not implying that this is the only way to do it, but it's a common approach.

To answer your motor questions - there are formulas you can work through that take into consideration the weight of the gantry, friction losses, pitch of the screws, the diameter of the screws, etc. They will get you into the ballpark.

The difficult thing is knowing exactly what the steppers will provide (in terms of torque) at a particular RPM, given your specific driver and power supply. So, it is probably easy enough to calculate that you'll need (approximately) 50 to 100 oz-in of continuous torque to move your gantry at a given speed, but the real question is can your motors provide that at 1200 RPM? 1500 RPM? One approach is to look at (roughly) equivalent machines and see what they're using to achieve the type of performance you're hoping for. Combine the theoretical with real world application.

There is a real danger in oversizing the motors, because (often) as they get larger (for a given form factor) they become less efficient and the torque curve is less attractive, especially at higher RPMs. YMMV.

Steve

[Dozer56]

Very nice setup Stevespo, & I see exactly what your talking about with how much extra height I have. My inexperience had me thinking that the height of the gantry beam is where the clearance came from, but now I know that the bottom Z assembly is where the clearance comes from. I would really have to re-design and rebuild my Z axis assembly to lower it more than I have it now. Centering the backplate helped with the leverage I'm sure. I'm believing that it is rigid enough to do some cutting as it is, and after I get it running and if I see that it must be lowered it to do what I expect it won't be much more trouble than it would be to do it now, just some more money. I'm just not ready to spend the extra money to do it now. I may regret that decision, but I'm hardheaded..
I'll have to take a look around for the formula to get me an idea on stepper size.
I may be way off but I'm having fun and learning.....

[stevespo]

You're going to have a very productive and good performing machine, regardless of the clearance issues. It's going to be nearly impossible to get everything nailed the first time through. So, don't sweat it too much and see how it performs. You may choose to rework a few things down the road. I think we have all been through that a few times.

If you work though the thought process completely, the clearance is really dictated by the lowest hanging part of the router and the types of tools you're going to be using. So it could be the gantry, the Z axis, or even the router collet. They're all taken into consideration.

If you're moving your machine around with the cordless drill on it's lowest torque setting, then your alignment and motion must be pretty darn good. That's what I ended up with as well, and the machine glides with finger tip pressure.

This is one of the motor sizing tutorials I was thinking of.

Steve

[cyclestart]

Here's a picture of my setup.

Is that z a commercial unit ? The sticker on the side (I can't make out the words) suggests so. That pleated/accordian cover is a nice feature and something I'd like to attempt copying.

[stevespo]

Yes, that's a commercial actuator/positioner from Parker. Bought off eBay for around $300. Basically 6" wide with about 7.5" of total travel. I believe it's 15mm (maybe 12mm?) THK linear rails and a high grade ground ballscrew (.625" @ 5TPI). Everything is mounted to a machined aluminum base with steel endblocks holding the bearings and motor mount. Very solid and smooth.

Steve