Overwhelmed, have many parts... need guidance

[Darren]

Yes, this is my first machine and I do tend to want the best of everything when setting out on a project like this but I offset that with finding good deals. My priorities have changed a bit and money isn't as big an issue at this point. I don't need the machine, I'm building it solely for fun and to learn. I'll definitely get some use out of it for hobbies as well.

Spending $1,000 more to finish would be desireable. My intention is to be able to cut aluminum reliably with a decent amount of precision although I'll be cutting mdf etc... as well. I've probably only spent about $800 on materials so far. I've been waiting for good deals but at this point I have most of the parts so getting the right materials to finish is more important than cost.

I'm ok with acme screws unless there is some reason I shouldn't be. I do tend to succomb to upgradeitis. I'm a little hesitant to start putting things together without the lead screws in hand. I'd like to have them for mocking up etc... am I being too freaky?

Actually the one thing holding me back from starting to assemble is getting a proper base to build from. I'm not sure I have the appropriate materials to build a solid platform for the Y axis to mount on.

[ahren]

Darren,

8020 extrusion has been mentioned by others as an effective and easy way to build your frame, as well as your gantry. In a bit of shameless self promotion, I might suggest using the setup described on my website (www.cncrouterparts.com) for acme screws and end bearings, which just happens to work well with 8020. Cheap, easy to assemble, and adjustable. With the motors you have, high pitch ACME screws (which are far more efficient than standard 1/2-10), and anti-backlash nuts from dumpstercnc, you'll have a pretty solid machine without breaking the bank. You can upgrade to ball screws later if you feel the need.

Ahren
www.cncrouterparts.com

[Darren]

Thanks for the info Ahren, I had thought about 8020 for the base or table but haven't ever worked with it. I'd think I would need rather large extrusions to prevent trosional issues and racking etc... Am I overthinking the table?

Your site is a great resource.

[ahren]

For the main part of your table, a surplus steel table or even a bit of pallet rack might be a more economical way to go. However, for all of the parts you want to keep relatively straight (and adjustable!) 8020 is pretty great. It's easy to work with, and the t-slots give you lots of options for your design. I've made some progress on my router this week -- it's time for me to re-open my build log. I'm using some of the parts I sell on my website now. I'll post some more pictures this weekend, and will let you know when I do.

[Darren]

Thanks, I'll be anxious to see the pics.

[sansbury]

$800 for all those parts isn't half bad. I wouldn't hesitate to start out with 1/2-10 acme though Ahren is right to say it's not the best. I suspect however that it is unlikely to be a limiting factor in your first iteration, and it's comparatively inexpensive to work with and easy to find. Good luck with your build!

[Darren]

Ok guys, I'm having a hard time judging what size acme rod I'll need. I don't know much about this part.

Earlier in the thread 5 tpi with a 4:1 gear reduction was suggested and appears to be a good recommendation. I'm looking at http://www.use-enco.com/CGI/INPDFF?P...TEM=AB408-0220 and am trying to figure out which rod to buy. It doesn't go by tpi from what I can tell. Should I have a specific diameter for a 48" length? I want to get all axis purchased today if possible. How do I tell which rod is 5tpi??? Yup, I'm pretty dumb about this part but anxious to learn.


1. do I use different diameters for different axis?
2. should all axis have the same tpi?
3. help, I'm getting confused with all the info

[sansbury]

Don't spin your wheels!

To a large extent the only thing that matters is the total drivetrain ratio between the motor shaft and the moving table. If you have a 4:1 drive on a 5:1 screw, you have effectively got the same thing as a motor direct-driving a 20:1 screw. Rack drives usually require a reduction gear since the drive mechanism itself does not provide reduction like a lead screw does.

On my machine a 125oz motor driving a 1/2-10 acme screw at 50ipm will generate enough force to push the machine over if you hold an axis steady. I would start with 1/2-10 screws with direct drive since this will be easier to get right. If it works well and you still need more, you can swap in 2-start 1/2-5 screws and/or bigger motors. 1/2-10 is dirt cheap and steppers go for very good prices on eBay so the upgrade path is very simple.

In terms of axes, all controllers (well, I know EMC and Mach, at least) will allow you to specify something like a "SCALE" for each axis. This is usually specified in the number of steps to move the axis one inch or one mm, depending on which unit you use. A lot of people will use steeper (fewer turns) on the XY where jog speed is important and steeper on the Z where you're moving a lot more weight. However, my impression is that most people over-specify their requirements by at least 200%--since the price gap between a 125oz and 200oz motor is sometimes just a few dollars, it does allow you to get away with more misalignment, etc.

[Darren]

Ok, I'm a little confused and a little better off than I was a few minutes ago after reading your post

1. So, 1/2 - 10 would be 1/2" diameter 10 tpi???
2. 2 start... I assume that means two threads start off? Do I need to consdier this when buying nuts?


I'm using servo motors as was trying to avoid direct connect coupling for ease of mounting etc. It was recommended that I use a 4:1 reduction so I figured I could kill two birds with one stone. Good idea? Bad?

[sansbury]

Screws are typically specified by diameter, thread pitch, thread type, and number of "starts" or threads. A normal threaded rod has one continuous groove while a 2-start has two, and so on. You definitely need to keep this in mind when buying nuts! All other things being equal, a 2-start thread will move you twice as far per turn as a single start, and so on.

It is easy to get lost in the weeds here. Axis travel per "step" is still what governs your operating limits so a 2:1 reduction drive with a 2-start screw gets you to the same place as a direct-drive on a single-start. A 2-start screw is not "better" for all applications. Keep in mind that the screws that McMaster sells are used for a million applications other than CNC so there are a lot of other reasons to specify these.

I do not have a lot of experience with servos so you will have to ask someone else. Steppers generate more torque at lower speeds so reduction drives can actually work against you. I am not sure how a belt drive simplifies mounting (alignment is still going to matter, and you'll have more things to align) but if that's the only reason then you can do a 1:1 drive. However I think servos like higher speeds more than steppers so you want want or need the reduction. Look around elsewhere on here for more on that.

[ahren]

Your real limitation is the critical speed of the screw -- if you rotate faster than this, you will get whip. Nook industries has a critical speed calculator here. For reference, you will probably have "B" style end-fixity:

http://www.nookindustries.com/acme/AcmeCharts.cfm

I'll save you the trouble of reading it and tell you that for a 48" axis, you can go about 200 IPM with a 1/2-10 5 start, vs. 40 IPM for regular 1/2-10 1 start. Even going up to 1-5 (1" diameter, which quadruples your inertia, meaning your motors will have to work 4x harder) still only gets you to about 160 IPM. It's all about how fast you're spinning, and 1/2-10 5 start is tough to beat for price/performance, since you only have to spin it 1/5 as fast to get the same linear speed.

I highly recommend using the 1/2-10 5 start from mcmaster carr. A 6 foot piece is part number 98940A020. I'm using the 3 foot length pieces on my router, and they are awesome. Also, while it doesn't say the manufacturer on McMaster's site, you will most likely get the screw from Nook. This has advantages in that it is probably much straighter than what you'd get from Enco or some other company.

Other than that, might I suggest end bearings and motor mounts from www.cncrouterparts.com ? Oh, and dumpster anti-backlash nuts: www.dumpstercnc.com.

Good luck,

Ahren
www.cncrouterparts.com

[ahren]

The reduction between the motor and the screw won't matter when calculating critical speed -- it's the speed of the screw that matters. It's probably a good idea to use reduction for your servos, since servos operate more efficiently at higher RPM's.

[Darren]

Ok, good info ahren. A couple of questions for ya...

1. If I went with a 5 start how does that affect my reduction? I'm understanding what you said but trying to get my ratio correct is becoming more complicated from what I can tell. So, my motor will be spinning faster but the screw will be counter acting that correct?

2. If I need a 4:1 reduction for my motor, how does this look at the screw?

[ahren]

1) Just the opposite. The multiple starts is in fact an "inverse" reduction -- your motor will be turning slower for the same linear speed, which requires more torque, not less (but this is fundamentally necessary to get around the whip problem). With the 5 start, you'll be moving 1/2" linearly for every rotation with a straight connection to your motor.

2) The 4:1 ratio for your motor isn't a hard and fast rule -- it's just to bump the RPM's to get your servo into a more efficient range, so what you really want to look at is RPM's. To get the same linear speed as you would have with 1/2-10 single start and a 4:1 reducer, you'd have to have a 20:1 reducer with the 5 start. However, one of the benefits of the 5 start IS higher speeds, so I don't know if you'd want to lose all of that.

If it were me, I'd go with a 5:1 ratio on the servo and the 5 start screw. This will mean that at 200 IPM, the screw will be rotating at 400 RPM (200"/min * 2 rotation/1"), and your motor will be spinning 2000 RPM, which is a respectable speed for a servo.

[Darren]

Ok, so in another thread it was pointed out that Roton has ball screws for a really good price $10.00 a foot for the following:

http://www.roton.com/Mating_Componen...family=7059321

How would I determine what reduction etc... I'd need for something like this?

[ahren]

Ball screws have a pitch just like lead screws, which is given in inches / rotation: as an example, a pitch of 0.2' means 0.2" of linear travel per rotation, or 5 turns per inch. Use the nook table above to find a screw with the same diameter and pitch as your roton screw to get the critical speed. This will be in IPM -- divide this by the screw pitch to get critical RPM. Then, gear up until your servo is running at an acceptable speed. I suggest 2000 RPM for your servo, but some can go much faster -- check your motor specs. Basically, multiply your motor gearing ratio by the critical RPM to see how fast your motor will be running at max linear speed -- you want this to be towards the upper range of your servo speed.

Keep in mind that ball screws are usually hardened, and turning the ends down to fit in a bearing can be a bear -- you can't just slide them in the bearing like you can get away with in ACME. Plus, you'll need a preloaded nut or a double nut to remove backlash from the system. It can be done, and when it's done right, the results are great. However, it isn't as simple as the ACME system I proposed, and if it isn't done right, you'll have paid extra for a system that's kind of sloppy.

My $0.02,

Ahren
www.cncrouterparts.com

[Darren]

Good food for thought. ACME might just be the ticket. I have a friend that can turn the screws down for me but I'm not sure he can handle hardened steel.

I'll browse your website a bit and do some other research. One way or another I'll get something ordered.

[ahren]

Darren,
Another option if you have access to a lathe (or better yet, a friend with a lathe) is to make adapters for the ball screws. A clamping adapter might be easier to make than actually turning the screw, and this could let you adapt to my bearings (or someone else's). You would still have to deal with preload on the nut, but it might be another option for you.

Ahren
www.cncrouterparts.com

[Darren]

Darren,
Another option if you have access to a lathe (or better yet, a friend with a lathe) is to make adapters for the ball screws. A clamping adapter might be easier to make than actually turning the screw, and this could let you adapt to my bearings (or someone else's). You would still have to deal with preload on the nut, but it might be another option for you.

Ahren
www.cncrouterparts.com

I do indeed have a friend with a lathe. He said he could probably even turn down a hardened ballscrew. He said that hardening is usually easy to grind through then machine the rest of the way. I personally don't know much about it but he seems confident.

By clamping mechanism are you thinking something that would clamp around the threads that has the correct diameter end on it for the bearings or coupling?

[ahren]

If your friend can grind through the hardening, you probably don't need an adapter, but yes, I was thinking of something that slipped over the ballscrew threads, and then had a smaller diameter cut to fit in a bearing.

If your friend is cutting them down for you, buy your bearings first -- FYI angular contact bearings are usually metric. Once you have those, you'll want to cut a shoulder long enough to fit through two of them mounted back to back, and put a thread after that that you can use to generate a preload on the bearings. Good luck with either way you go.