Build Thread- 8020 based CNC router with Ahrens motion system

[ilya]

Ok. I've decided to make my own CNC since it would be much fun. And cheaper of course
I've chosen the 8020 profiles and Ahrens linear motion system and it looks like this would be as close to the bolt-together kit as it can get.
To be a little more specific about my router, I'm planning to machine aluminum plates and panels (mostly cut-outs and engraving). Working area - not less then 21"x12".
I've come up with this design and this machine has actually bigger working area (around 22"x16") and a footprint of 24"x24". It's a fixed gantry design with moving table - I feel it would be more rigid this way.
I REALLY-really need your comments on this 'cause I'm building a BOM now and most components will come from US (which is quite far from Russia) so I'd like to catch as many quirks/mistakes at the design stage as possible, so I don't have to ship something again .

I haven't decided on the router yet. I'd really like to know what routers are suggested for cutting light metals effectively.
I haven't done stepper calculations as well, but I'm going to do it asap. Any recommendations are appreciated! I think my Z-axis would weight around 10 kg, so I need powerfull motors to move this baby...
Here go pictures.

[jsheerin]

Hi Ilya,
If you change your z-axis design, you can get less overhang for a smaller moment arm and also not have to raise and lower your z-axis motor. There are other examples of how to do this like Glacial Wanderer's design, but basically mount your steel Z-axis rail to the piece of extrusion that connects your Y-axis bearings and make that piece of extrusion longer. Then mount your Z-axis bearings to a plate of aluminum (like the one Ahren sells) and mount your router to that.

[ilya]

Hi jsheerin,
Thanks for comments. Actually this design you were describing was my starting point - I tried to design it as Glacial Wanderer did. Gantry height was 13", however the clearance between the Z-axis extrusion end and the table is only 3.3" (that means maximum part height is 3.3"), and Z-plate stopped at 4.47" over the table and couldn't go any lower. That is too high (I may be wrong though). I can't increase clearance because Z-plate will stop even heigher which is unusable.
With movable motor design I get the 6.1" clearance between the Z-axis extrusion and the table, the distance between the edge of Z-plate and the table is 1.39" all the way down and 7" all the way up. I admit that gantry is 3" taller (16" vs 13") however even if I make it 13" I'll still gain than in the lowest position of the plate.
Now, I can see how the gantry height affects the arm length, but I can't see how it differs with different design approach.
Sorry if the explanation is "cloudy", I can't post a picture right now but will do it if needed.
Let me know what you think.

[jsheerin]

The arm length I mentioned is horizontal from the gantry to the cutting bit. Consider plunging the bit down into material. The more the bit is offset from the gantry, the larger the torque generated on the gantry, bearings, etc.

Instead of using the full size bearing block at the bottom of the z-axis, try using the reduced size one that Ahren sells. The Z-axis plate will be able to clear this bearing block and go lower. With the short bearing blocks, this should be able to let the bottom of the Z-plate go about 1.25" below the bottom of the Z-axis extrusion. If you use a PC892 router, that would put the bottom surface of the collet about 4.9" below the bottom surface of the Z-axis extrusion.

[ilya]

Ok, got the arm thing you were referring to.
I've measured the distance from the gantry leg to the Z-plate. In static motor design it is indeed less, however only 0.375" less then in moveable motor design. I don't know if this is significant or can be ignored.
Thanks for pointing at the low profile block. Unfortunately it has bushing insteat of bearing. And PC routers are impossible to get locally, I'm thinking of Bosh, Hitachi or Cress, so the lowest travel point of Z-plate is important to me 'cause it may be the start of the collet with some routers.
I'll thinks some more and "play" with both models but I'm leaning towards moveable motor design.

[ahren]

jsheerin, thanks for giving good advice while I'm away from the forum!

Ilya, first of all, let me say that I think your approach will work well, and it looks like you have a good handle on it. However, I also think you'll find the low profile blocks work really well if you decide to go that route. There were a few issues with the first round of them (the press fit of the bushing was too heavy, causing some people to have to re-drill the bushings). However, this has been resolved now, and every block is checked for fit with a piece of precision rod before entering inventory. The motion is very smooth, despite the lack of a rolling element.

Also, if you're considering a Hitachi router, K2CNC sells a mount that bolts right on to my z-plate. It's set up to be mounted as low as possible on the z-plate to maximize travel. You can get about 5" of part height under it this way. Just FYI.

Best regards,

Ahren
www.cncrouterparts.com

[ilya]

Ahren,
Thanks for chiming in.
I think I'll put the low profile bearing block in my BOM and if this design won't satisfy me it would be easy to convert it to the more common one.
I'd really love to use the K2CNC holders, but I need to decide on a router. Since I don't have any experience with this I'll rely on some knowledgeable persons oppinion. I know that Hitachi is a variable speed router and I think it's a benefit. However I'm curious about other options available.
Now, there's a question. I'm not going to use a drill rod. Do I still need to sandwitch the bearing blocks with washers between shaft collars or would it be sufficient to clamp the outer edges of the screw (as pictured at dumpstercnc site)? This is really important 'cause all those washers and collars are quite pricey and I don't want to stockpile them at home if they are not needed.

EDIT: Ahren, I've just remembered of 2 more questions. 1st - can I use M8 screws with the bearing block? I'd like to keep it metric where possible. 2nd - what is the distance between holes in pre-drilled steel rails? Is it something like 3"? And what's an offset of the first hole from the edge? 1.5"? I guess the holes are for 5/16 screws, right?

[ahren]

Ilya,
The washers, needle bearings and shaft collars are there to provide resistance to thrust -- they have nothing to do with the drill rod. If you don't have this sandwich, your motor will be taking up all the thrust loads, which its bearings are not designed to handle. You can still accomplish the sandwich without using drill rod, however -- the same concept applies.

You can use M8 screws in lieu of 5/16 -- they're almost the same size. For the pre-drilled rails, it depends on which rail you are talking about, but most ar 4" apart. Contact Nate at www.finelineautomation.com -- those are his products, and he's happy to send you a drawing of them.

Best regards,

Ahren
www.cncrouterparts.com

[ilya]

Ahren,
Thanks for clearing this for me, now I get the concept. Is there any difference if the acme screw is locked at one bearing block like this:

acme screw - collar - thrust bearing - bearing block 1 - thrust bearing - collar ===(acme)=== bearing block 2

Or between two:

acme screw - collar - thrust bearing - bearing block 1 ===(acme)=== bearing block 2 - thrust bearing - collar

Pardon my crude description, I can't upload pictures from my phone.

[ahren]

Ilya,
Either way should work. For super-precision setups, the bearings are typically set up around one bearing block to avoid stress if the screw grows due to thermal changes. For wood routers, however, this is rarely a concern. Additionally, by spreading the thrust bearings between two blocks, you can tension the screw between them, which can help to prevent whip at higher speeds.

Ahren
www.cncrouterparts.com

[nikbrewer]

on your z-design, i have a 12" tall z with a 9" screw and get 7" of travel from it. the gantry clears the stock by 4" , and i can get my collect to with in a 1/2" of the deck. you can probally shorten your z a lot if you just think kinda creatively. i also used the cnc router parts.com parts

[ilya]

Ok. Here's the first update on the build.
It took a while to gather the funds and order stuff. Since I have to order most of the parts from America it took about three weeks for each parcel to get here. And don't even get me started about customs (I have luckily avoided any problems so far though). Strangely, the first package arrived with no problems, but to get the other two I had to go to customs first to settle everything. The contents of the packages was identical, so go figure...
Anyway, here are 3 parcels with 8020 extrusions - we had to split the order to make the packages fit the international post restrictions on size and weight. Finally I've gathered all the extrusions at my house.
Didn't realized how bulky the 8020 15xx series is. Untill I actually took it in my hands. You can't really judge from the pictures. Will see how it works out