New 8020 build with a twist - looking for input

I'm building a new machine that attempts to take advantage of the strength of the larger series 15 extrusions and a test of their linear slide bearings. The gantry is built up of two 3030 3x3x36" extrusions stacked that are joined together with a number of joining plates and the 1/2" side plates. The side plates of the gantry each rest on a fully supported 20mm linear bearing, which is bolted to the lower support frame.

Motivation will be through 425inoz series 23 stepper motors powered at 36v, 2 steppers per power supply. The 1.5kw water cooled spindle will be ran 3 ph. 220v via a VFD.

I'd appreciate any suggestions.

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Please let us know how this works out, I also was going to use the linear slides, but when I read the specs, they are for compression loads only.

What are you looking to gain by sandwiching two 3030 beams together, over one 3060 beam?

With two 3030's sandwiched, most of the extra metal is concentrated at the center of the web, which adds no real strength in tension or compression to the beam, though you will get a slightly larger cross sectional area.

If you're looking for more mass, two 3030's with the extra mass of the plates and bolts would be the way to go though. Rigidity-wise though, the single 3060 is likely to be stiffer.

Mark

wendtmk - Thanks for the feedback. While there may be some difference in 3060 resisting twist (the primary issue we deal with after a certain cross sectional density is exceeded and surpasses what is need to carry the tooling load) I believe that the amount the one piece exceeds two 3030's is miniscule. So that left only one variable of any importance - the two 3030 pieces cost me $40 whereas a 36" section of 3060 costs $120.

In the end, as long as cheaper will do the job, it's all about economics. ;)

My existing machine is a Shapeoko. While it has allowed me to learn, I've never been happy with how flexible the gantry is and how I could see it twist with any tooling load. I believe one 3030 would have been good enough for this 36" span, I KNOW two will be. I talked with some of the staff at 8020 as I was planning this build and asked them what the real difference would be. Their response was that I'd never see any difference. I also checked their design specs for myself and while I no longer recall the exact numbers, the load carrying ability of 3030 x 2 was mere tens of pounds of difference out of tons of ability.

I was more curious about the effects of deflection on the two smaller beams tied together, rather than the twisting moment.

Your point about the economics is the deciding factor, however. :)

Attachment 277188Attachment 277190Things are coming along. A bit slower than I'd like, but every step forward is good.

You can see a bit of progress today as we now have the machine lifted up onto the work table where it will live. Not too surprising when we got it up there, I discovered that my overhead arm that will be hooked to the gantry slide is way too loo. It was perfect on the Shapeoko. :) Jacking it up won't be to hard.

More progress;


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Attachment 279468Sorry I've not updated for a while, but things have been very busy in my shop. Even with all the studio work, I've gotten things together a screw and a bolt at a time. Along the way I hit a wall with the 8020 linear slides and had to totally change my design for the gantry. I'll be adding some pics so that you can see what I did differently. First of all lets get to the problem I ran into, possible solutions I did not take and the final, working design.

One week ago I was right where I am today - the machine is assembled (except for the odd fiddley bit) power is to everything and I was running tests, calibrating the driver and linking my spindle control to the controller card. Then as my wife walked into the garage to see why I hadn't come to bed yet (3:30am), I press the "Cycle" button, the gantry begins a 45 degree traverse heading to the right rear corner (x+,y+) and for 10 seconds, everything looks and sounds great. On second 16 it suddenly started sounding like my 1961 VW Beetle did after I missed a shift as I was pulling into the high school parking lot in 1972. A sound I hadn't heard in a loooonnnng time. After a very brief inspection I discovered that the x axis lead screw had actually broken right where the diameter steps down from 1/2" to 8mm. Turn the lights our and go to bed.

The next day I inspected the screw and discovered a hidden flaw in the steel that looked like a casting sand inclusion (I know it couldn't be one, but that's what it looked like). the actual flaw was very close to the surface after it was turned down to 8mm. So now I can see why it broke there, but I can't see WHY it broke. Then I try moving the spindle by rotating the lead screw - it won't budge until I grab a pair of channlelocks and apply some force. OK, I'n not a body builder and the closest I get to a daily workout is rolling up to the supper table, but I'm not THAT weak! I start unbolting everything that linked to the drivetrain and removing to leadscrew - the spindle still will not move without applying great force in either direction. If you examine the pics I've posted of the CNC you may have noticed that in the back side of the gantry I linked the upper and lower linear slide "bearings" with a couple 8mm threaded rods and on the front with a plate of 1/2" aluminum. As soon as I removed the spindle and it's mount from the sliding carriage the carriage loosened considerably. After I removed the two threaded rods, the carriage became "free", able to freely slide from side to side with no load at all. I have a 4" 2x4 foot slab if steel that I use for a "plate" and I rolled the rods along it. Instantly I could see that they were bent - almost twisted. I have no idea "how" this happened since I always am very careful to assemble things loose and tighten assemblies from critical to less critical to avoid this very problem. I've yet to discover how things got so twisted, but that does not change what actually happened under load.

After looking at things for a couple days I decided that I would not try reinventing the wheel again, that I'd revert to building the machine with the more typical 20mm fully supported linear bearings like I used on the y axis. I also decided to forgo the 425 inoz stepper motors that were much more powerful than I really needed and replace them with 270 inoz steppers. In addition to not being powerful enough to twist off the end of my acme screws. I also decided to go with a different source for the acme screw I needed to replace on the x axis. This one is a full 1/2" down it's full length - no turned down zone at the bearings. (IMService we are THE source for low cost Cad-Cam and Desktop CNC Mills, lathes, and Routers)

Let me interject an observation here - the difference between the typical Ebay Chinese acme screws I bought first and these Acme screws is day and night. The steel is higher quality, the "threads" of the screw were smooth, it came with a pretty good zero backlash and GOOD bearings. I have listed the seller that my replacement came from and if anybody wants to know who supplied the substandard screws, feel free to PM or email. I see no benefit in listing a source for substandard parts. It is worth noting that the screw was not the cause of the "issue" I experienced, but it did not help at all. The replacement screw looked so good that I decided to take advantage of the availability or superior components and ordered replacements for the rest of the machine.

Another change of the gantry you will see is that I reconfigured the assembly by separating the two 3x3" extrusions that make up the gantry by 3" and run the amce screw centered in the space between the extrusions. The previous design placed the screw on the backside of the gantry. This change placed the screw 3" closer to the center line of the spindle.

All in all I've very happy with how things are turning out. The mechanical design is cleaner and it will run much smoother. Now it's time to get everything calibrating - again. :)

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Maybe it's just the way I'm looking at it, but your mount for the leadscrew nut in pictures 1 & 2 in post #9 looks a bit flimsy to me. From the pictures, it looks to be aluminum angle. With no forces against it, it may move your Y axis nice and smooth, but the minute cutting forces come up against it, it looks like it would flex. You might be better off using some kind of block to mount the leadscrew nut rather than a plate like that.

Mark