[Dylwad]

I would have to see pictures of the ends of the ballscrews. Keeping the fixed ends like they are and doing an AC on the simple end and drilling and tapping the end of the ballscrews comes to mind, using a short SHCS and washer to tighten up and tension the rod. Dont know how easy it is to drill and tap your ballscrews, Might need to be annealed if the ends are more than surface hardened.

Its tricky with your bearing block setup as far as I can tell, tension might bend the angle brackets used to bolt them to the 8020, might have to make the bearing blocks wider for through bolting to the 8020, or support both sides of them with angle brackets, or cap them from the sides.

[stevespo]

Dylan, I understand what you're describing. It's going to be difficult/impossible with my current end block. I'll monkey around a bit more with alignment and then just run it below the critical speed. That's certainly decent performance.

Perhaps on a future build, I'll use a fixed AC bearing on both ends, as that clearly provides the best support and tensioning options.

Steve

[stevespo]

A little more monkeying around. I added (2) bolts to the bracket where the X axis ball nut attaches. This does seem to stabilize the screw a bit better and there is less resonance at higher speeds.

I also rearranged my Z axis. I originally attached it so that the table was fixed to the gantry and the entire actuator moved up and down (25lbs + spindle + motor). I thought this would be the stiffest possible arrangement. The by-product of this design was that my 7+" of travel was turned into 4.5" because of the position where the router was mounted. Most of the range of motion was on the downward side, which is not really where I need it.

So, I flipped it around and modified the mounting plates. Pretty easy. Now I have a good 6" of clearance over the table, and the full range of motion on the Z. I can bring a tool way up and over a workpiece without any interference. Another advantage is now the motor is just moving the spindle up and down and not the entire actuator.

Did I lose any stiffness? Hard to say just yet because I haven't done any heavy duty carving since the mod. I could definitely imagine a heavier/stiffer mounting plate which could help (if necessary).

Where it's positioned in the picture is for the smallest/shortest bit I'd ever use, so the spindle would be under very little stress. With larger bits, I'd slide the spindle up and that would help with the "lever" effect on the mounting bracket. I think it will be ok, and I really need the extra travel.

Any comments or thoughts on this mod?

Steve

[Jason3]

Nice change. I think it's better around this way - it really doesn't look like you're compromising very much, if at all, for the extra travel. It would be interesting to see whether you can detect any difference at all in the cut quality.

Less mass moving on the Z can't be bad

Best regards,

Jason

[Dylwad]

I think it looks great, If anything you could add a gusset between the top of the spindle mount and the Z plate on both sides to tie things together, should be easy to do. Spreading the spindle mount plates apart to the extremes could only help too. Looks easy to slide the spindle up on the mount to stiffen things up for heavy cuts.

[mhasting2004]

Nice build Steve

I'm at that stepper/servo crossroad myselfand am happy to see it working so well for you. My build is a bit bigger ...http://www.cnczone.com/forums/showthread.php?t=53824&page=7 ... and made with junk thrown from work so no design has gone into the choice of drives or motors its just what I happen to have. This means I've saved alot in build costs and I feel justified to spend wisely on servos to enhance the machines capabilities.

One question I have is are you using a smooth stepper? I see no mention in the thread and wonder if it wouldn't help with your pc clock issues?

I have no vested interest but I have one and am driving my machine from a laptop at the moment.

Cheers

Mark

[stevespo]

Mark,

Great looking machine you've got there. The 300W Zeal servos should be a great fit for your needs. Not quite as large as a NEMA 34, but bigger than a NEMA 23. I'm sure you could adapt the 70mm motors to your existing mounts and they should have loads of power for your application.

I'm still using the parallel port, so no smooth stepper. My PC issues seem to have gone away with my most recent machine, a Dell Dimension 4400 running at 1.7GHz. I have my steps/inch turned down to 2000 (previously 10,000 with the steppers), so the load on the machine is fairly low. Even a 25K Mach kernel is fine. No issues with the pulse train, even at speeds of 600 IPM (3000 RPM).

No doubt people are doing great things with steppers, but if you want a closed loop system, servos are the way to go. The Granite drives provide a lot of flexibility with tuning and setup, but I really haven't done much other than use the default settings for the motors and verify that the PID settings are reasonable. It was easy to setup and configure and I'm thrilled with the performance.

Steve

[Dylwad]

Steve,

I'm using the same Dimension 4400, I Highly recommend a CPU upgrade, the 1.7's were better than the 1.6's, more from the 1.6 having a maze for a cache, but for $10-$15 for a 2.4Ghz + CPU its worth the upgrade, and its as simple as plugging it in.

http://www.geeks.com/details.asp?inv...78-533&cat=CPU will de-rate to 2.08Ghz after installation due to the 400Mhz front side bus on the 4400 motherboard.

My 4400 tripled its speed and then some, I had the CPU laying around from another build.

Dont know if you know this, but some 4400's had problems with service pack 2, the fix is a newer hard drive believe it or not. I used a 120GB WD out of a "dead" DVR. SP3 runs great on it now, wouldnt load windows at all with the stock 40GB HD with SP2.

Dylan

[stevespo]

Dylan,

I can never remember if it's 1.6 or 1.7, so I double checked - and it's a 1.6Ghz Dimension 4400. So, the CPU has never really been a "problem" for me, but it wouldn't hurt to do the upgrade either.

I actually did replace the hard drive when I did my clean XP install. Pretty certain it auto-updated itself to SP3, but I'll check when I'm back in the shop. Thanks for the tips. This Dell has been super reliable for me, so why not give it a little boost.

Steve

[stevespo]

Installed the 2.8GHz P4 (actually clocking at 2.10 Ghz) and the benchmarks are telling me it's running 25-40% faster for compute intensive applications. Pretty good for $15 + s/h.

Steve

[RicknBeachcrest]

Hi Steve,

It's been a while since this thread has had any postings. I'm in the process of building an 8020 CNC router. I have read your build thread several times and each time see something I hadn't seen before. Your thread is one of two that I am using for guidance in my build. I'm getting real close to making some decisions on the drive screw for my machine. I am planning on using ball screws for my X and Y axis.

I have a few of questions, if you don't mind.

1) You used 30mm 7200B angular contact bearings on your ball screws. From my understanding these are not sealed. How have these worked out for you? The sealed bearings are a little over 3 mm wider (5200-2RS Double-row Angular Contact Ball Bearing).
5200-2RS Bearing Angular Contact Sealed 10x30x14.3 Ball Bearings

2) What was the thickness of the aluminum that you used for each half of the bearing blocks?

3) My biggest problem (that I can see now) is finding a source for machining the ends of the ball screws. Did your have to adjust the length of yours?

4) I hope this isn't too dumb of a question. My understanding is that there are two AC bearings in each bearing block and the nut on the end of the ball screw squeezes these to AC bearing together. Is this right?

Thanks for your help.
Rick

[stevespo]

Hi Rick, this thread has been quiet, but not the machine! I'm still using it regularly to make parts for myself and some long term customers.

1) You used 30mm 7200B angular contact bearings on your ball screws. From my understanding these are not sealed. How have these worked out for you? The sealed bearings are a little over 3 mm wider (5200-2RS Double-row Angular Contact Ball Bearing).
5200-2RS Bearing Angular Contact Sealed 10x30x14.3 Ball Bearings

The 7200B bearings have worked out very well. They are not sealed, but in my case, one set is under the table and the other is in back of the upper gantry and well protected from dust. It would not be hard to add some type of dust cover to the end blocks, or you could certainly go with sealed bearings and play it safe.

It looks like the 5200B are a double row AC bearing and (probably?) don't require a matched set with a manual preload. If that is the case, then the 5200Bs have two advantages over the 7200B and seem like a great choice.

2) What was the thickness of the aluminum that you used for each half of the bearing blocks?

It was .500" 6061 aluminum and I milled the parts with the CNC.

3) My biggest problem (that I can see now) is finding a source for machining the ends of the ball screws. Did your have to adjust the length of yours?

I had a local machine shop shorten (and turn the end of) one of my screws. The screws that I had were hardened steel and required some finesse to turn. I don't think it was anything exotic or difficult, but the machinist was a little surprised at how tough they were (which is a good thing!)

4) I hope this isn't too dumb of a question. My understanding is that there are two AC bearings in each bearing block and the nut on the end of the ball screw squeezes these to AC bearing together. Is this right?

Not dumb at all. With the 7200B, they are used as a pair within the bearing blocks. I'm trying to remember how they went together. I believe the inner raceways are clamped tightly together on the screw shaft (with the lock nut) and the preload was managed with some metal shim stock to allow them to turn freely (as a unit), but resist the axial forces that the screw might encounter. I may have some photos or a description somewhere to refresh my memory.

In theory, that wouldn't be necessary with something like the 5200B - but then you also can't control the preload because it is going to be a function of the dual row unit. I might actually have some of those around for another project, so I will look at my supplies and see if I can tell you anything more about that tonight or tomorrow.

Just found your build thread - looking forward to checking it out.

Steve

[RicknBeachcrest]

Steve,
Thanks for the quick reply. As far as my CNC build, I truly am standing on the shoulders of those who have come before me. I have absolutely no machining experience, but do a lot of wood working. This is my driving motivation, also the personal satisfaction of building such a machine. I am very grateful to all on this forum who have taken the time to describe (in copious detail) their builds.

My present task has been building a bench for the CNC router. I need to get the machine together so as to determine how exactly things will fit together. Currently I just have an assortment of sub-assemblies sitting on the floor. It will be exciting to see it in a more 3 dimensional form.

Rick

[stevespo]

Hi Rick, just pulled out my "box of extra CNC stuff" and I do have some extra 7200Bs and a pair of 5200Bs as well. I'm pretty certain I read about them after I used my 7200s and I thought it might be a reasonable replacement as the OD is the same (30mm). I remember there was some debate as to whether they were "true" AC bearings or not. I can't say. They do appear to be quite solid and I don't detect any side play while handling them.

The 7200s have a lot of play (individually) and the key is using them in a matched pair. If you put the logo sides together and clamp the inner raceway, the slop disappears and you can tune the preload with shims. Precision ground AC bearings are all ready to go, but the low cost version requires some trial and error. With the bearings clamped on the screw, you can measure the axial play and adjust the load.

One set felt just fine "as-is" and the other one used a very small shim to get the same type of feel/resistance. I'm definitely NOT an expert, but relied on several other threads here that went through the process.

Steve

[stevespo]

Here are the files from post #34 converted to IGS format. Just the mounting plates and other misc small parts.

Steve

[RicknBeachcrest]

Keeping busy.

The rails are attached with long strips of steel, that were pre-drilled and tapped for 4mm socket head screws on 60mm centers. Very handy. I knew I wouldn't be able to accurately drill/tap those holes, so this helped me out tremendously. Thank you 8020 Surplus. All my 8020 supplies were purchased through the outlet and very easy dealing with them.

Steve

Hi Steve,
I think I need to use some of these long strips of steel you used in your buld. I have been using 4mm T-nuts and it has been giving me some problems. The T-nuts have a corrugated seat, so when they are mounted they bit into the extrusion. Now this would be good (I guess), if I didn't need to fine tune the placement of the rails.

So any idea where I could find some of these long strips of steel you used?

Thanks.
Rick

PS. I checked 8020 surplus, but didn't find any there. Maybe I didn't look in the right place.

[stevespo]

It's in the PDF catalog. I think the part number is 8900-36 for the 36" lengths of undrilled/untapped 15 series nut stock. I sometimes see it on eBay in the 8020 surplus store, which is where I found mine. Just search for "8020 8900" and you should find it there. They will drill/tap it for an additional charge.

Steve

[RicknBeachcrest]

Thanks Steve,
I emailed them to see what they would charge to drill and tap the holes for four 25" bars.
Rick

[RicknBeachcrest]

Hi Steve,

I am in the process of deciding on ball screws. Are you satisfied with the lead(.2") on your screws?

I was thinking that lead is a good compromise between speed and accuracy. But then again, I know next to nothing about it. My machine will only have a cutting space of about 18" square, so there really isn't a whole bunch of real estate to cover.

Also what kind of couplers are you using?

Thanks.
Rick

[stevespo]

Rick, the .2"/turn lead has been good, but I (personally) would go with something coarser next time around. For my machine, with a 25x32" work envelope, a .5"/turn lead (or probably 10mm, 12mm, 15mm, metric) would be a very good fit.

You're correct that is is a bit of a trade off in terms of speed/accuracy. With steppers you may lose some positional accuracy because you're dividing each screw revolution into 200 steps. Fewer revolutions per inch means less granularity with motion.

My servos have 1000 CPR (quad) encoders, which I am only using in 200 step/turn mode, so I could definitely use a coarser screw and adjust the step count to keep the same accuracy I'm used to. The accuracy is a function of the encoder and not the motor.

The downside of my current screws is top speed. Theoretically, I could drive them at 3000 RPM (600 IPM), but in reality I start to see resonance (whipping) at more like 300 IPM. Do I need to move that fast? No, not really - but it's kind of sad not to be taking full advantage of the great motors and electronics I have!

For a smaller machine, with 18x18" of travel, screw whip is not going to be an issue even at very high speeds. You can calculate the max speed with a few variables.

So, for your machine - I would probably go with the .2"/turn screws, which are a good balance. If you're using brushed DC servos, it's a good choice because you get increased mechanical advantage with the finer pitch and you can run the servos at very high RPM.

If you are using steppers, the max speed is dependent on the motors, electronics and screws. You may hit your max RPM with the steppers and .2"/lead, but it's (probably) going to be over 250 IPM. Pretty fast. If you really need more speed than that and want to keep the RPMs down into the higher torque range of a stepper, then a coarser screw would also work well, but you trade off some resolution.

Oh yes - I was using the aluminum helical couplers and then switched to the Lovejoy style with the urethane spiders when I went to the DC servos. I couldn't go from the 16mm motor shaft to the .25" screw end with anything else.

Steve