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After a long time lurking, I've (like many others) finally taken the plunge and acquired most of the parts for my first router build. My plan is to be able to cut full sheets of MDF with a little wiggle room on each side. Because of the cost of materials and shipping (to Canada) I sadly decided against 8020 and have opted instead for a torsion box out of 3/4" MDF. Here's a shot of what I'm planning for the main base.
I've already received a fairly complete package of parts from Ahren, including 5 extended carriages, 3 standard carriages, 3 NEMA 23 rack and pinion drives, a G540, parts for my Z-axis, etc, etc.
I'm planning to run my X-axis rails vertically, similar to the arrangement of angles that Joe uses on his Hybrid. This is going to mean some changes from the way the carriages are usually used, but I think it will work. The CRS that I bought for the X and Y are 6" tall (to fit the 5" thick torsion box). This should help stiffen the box over its 9' length as well as the 5' wide gantry. I'm running dual R&P drives on the X, and a 5 start 1/2" ACME screw for the Z. Nothing groundbreaking here, but I really wanted to make the machine big enough from day 1.
Here are some pics of the design. I just started 3 months of parental leave from my job today, so my access to SolidWorks has come to an end (thus some differences between the pictures that I saved at different times over the last month or so). I might try to pick up the design process in Sketchup, but I'm getting the wood for the torsion box in a few days, so I'll likely skip more modeling and start posting build pics.
Let the criticism begin!
pulse equivalency，working speed, spindle power, accuracy is very important. if you choose pinion and rack and want to do 3D carving, surface working piece is not very smooth. it's better for you to choose ball screw in x axis because x is main axis when engraving. Usually 3/4'' MDF can be cut off by 3-5kw spindle
you can use Artcam or other CAM softwares to convert BMP to relife and generate nc file. BIts is very important. There are many bits, The best bits is ball tip bits. overlap is also important. When it is small, surface is smooth but taking a lot of time.
Not true. There are plenty of people using rack and pinion On similar machines with very good results. Just look at ShopBot. All their machines are rack and pinion.Quote:
Originally Posted by DHSYCNC
What ever your gantry sides are made of, they look like a weak point. Are you using hardwood? Whatever it is, it need to be thicker than the 3/4" it appears to be. Especially where the top carriage is bolted to it.Quote:
Let the criticism begin!
Thanks for the feedback. Yeah, that's the part that I'm struggling with. The way the pinion has to line up with the rack means I have a particular thickness I can use to join the upper and lower carriages if the R&P drive is going to mount to the outside as shown in the pictures. In this case it's 5/8", so I can either do hardwood, plywood or MDF. I want to thicken the gantry sides above the top carriage as much as possible (like I've shown, but maybe all the way to the inside edge of the top carriage) and bolt the thickened gantry sides to the top of the carriage in addition to the side. I could do 5/8" aluminum for the sides if I have to. I wonder though if that would be stiffer than having the sides be 2-1/2" thick from the top of the carriage to the bottom of the main gantry beam. I still haven't thought much about the connection between the gantry sides and beam.Quote:
What ever your gantry sides are made of, they look like a weak point. Are you using hardwood? Whatever it is, it need to be thicker than the 3/4" it appears to be. Especially where the top carriage is bolted to it.
I was originally thinking of doing something like this older screenshot I made, but I read a comment that said it's better to have the gantry overhang the sides so that the carriage bearings aren't pulled out of line with the 1/4" rails...
This picture is a better shot of what I was planning for the gantry sides, although the gantry was just slapped together at this stage.
Notice how the sides have been thickened above the top carriage and extend to the front of the carriage. I still have concerns about the 5/8" thick piece that connects the upper and lower carriages especially with the cutout that I have to make to accommodate the R&P belt drive. If there's any merit at all to this design, maybe I need to consider extending the part between the two carriages further forward so that the hole for the belt drive is in the middle of the part (i.e. the wood is closed at the front instead of the open "birds mouth" that I've shown here)
May I suggest, that you over build the gantry support beam and laminate it from 5/8 th's AAA baltic birch plywood, with 3/4 in maple substrate. And the side members that connect the beam to the linear unit be wider on those members as, this, is the weakest point of the whole unit. I believe this will help eliminate all the annoying flex and slop of this design.
Not sure I follow. Are you suggesting I make the gantry support beam out of laminations of wood all going in the same direction as opposed to building it like a long, thin box?Quote:
Originally Posted by SPEEDRE
The machine I'm currently designing will use torsion box type sides, with the rack and pinion mounting plate mounted inside. Haven't worked all the details out yet. Another option is to reinforce with steel tubing. I'd opt to bolt as well as bond with epoxy, for maximum stiffness.
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Originally Posted by ger21
if module of gear is1.25, rate of reduce 1:2, pulse equivalency is 0.0216mm/step. but we use ball screw, it's 0.0025mm/step. surface is quite different. we had compared two kind of results. if only one result engraving sample, you can't recognize it. On other hand, gear has gap when direction is oppisite, the gap will cause result worse. we had made test by engraving stone, wood, soft and hard wood. more pulse equivalency is small, result is fine.
I am kind of confused why you are persistent not using rack and pinions and must use ballscrews? I don't like it when people come in and just use blanket statements like these and assume that something is not worth using when they have no clue. :confused:
Sean
Quote:
Originally Posted by DHSYCNC
I agree with Sean. Sure ballscrews are good, but that's not what I'm using. I said in my second post that I have my R&P parts. That part isn't up for discussion. I'm looking for help building the best machine I can with the parts I have. 5 years ago I thought about using all thread and doing it cheap. I'm glad I convinced myself not to bother, and I'm pretty sure I can make a good machine with R&P drives. (wrong) Lots of other people have!Quote:
Originally Posted by freecncplans
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Originally Posted by freecncplans
when building small size machine, ball screw is good because ball screw is short and machine bed is not bend. But when building big size machine,like more than 2800mm of lengh. machine will be bend. So we will choose rack and pinion. As we know, rack and pinion has gap and accuracy is lower than ball screw.
To keep high accuracy, we must choose ball screw. To keep high speed in big machine, we must choose rack and pinion. On the hand, if using stepper motor, it max speed is 800RPM, can we choose it to build big machine with ball screw and speed is up to 20mm/min? if don't know pulse equivalency, how to choose drive and reducer? it's clue.
If you're really concerned about backlash in a high-speed r&p, why not a helicak rack and pinion, with either a split-or dual-pinion (to eliminate backlash)?Quote:
Originally Posted by DHSYCNC
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Originally Posted by louieatienza
thanks a lot. we're going to choose helicak rack and pinion.
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Originally Posted by louieatienza
we want to choose control system to compensate it. It's simple and easy. If not get good result, we have to choose above rack.
Torsion box started...
Finally got around to picking up some 5' x 12' sheets of 3/4" MDF (they were out of 5' x 10'). Cut all of the internal and external strips for the torsion box and dry fit the internal framework. I chose to do half lap joints on the parts as shown in one of my first posts. The fit up is very good. Next on the list is getting the bottom torsion box skin set up nice and level so that I can start the final assembly of the box.
That's looking great! The 5x 12 sheets must've weighed a ton!Quote:
Originally Posted by ivojnob
BTW I take you're a bon jovi fan? (I'm a big Jumble player...)