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Discussion in 'CNC Mills/Routers' started by Giuliano M, Oct 30, 2015.
C-Beam Router using 1500mm lengths, 48x48" build area, and rack and pinion movement.
Giuliano M published a new build:
Read more about this build...
Quick shot of using the larger solid V-wheels, inside of the C-beam structure.
Mounted to the top portion of the rail is 20x20mm Mod 2 rack - the pictures show straight tooth rack, but I'm scoping out some helical/angled tooth rack from Aliexpress. I may make it 15mm wide rack instead.
The Bearings will be mounted to a bar to give a gantry plate enough clearance to go over the 20mm rack.
Bearing block - based on a 5/8" thick piece of 6061 aluminum bar stock.
The gear rack is 20mm tall, 15mm wide - so the surface of the bearing block sticks out about 3mm for clearance.
A gantry plate will be mounted on top of the block.
Since the eccentrics on the top bearings only go into the block by about 3-4mm, I will probably have to turn a piece of round stock with a matching eccentric to fill in the wider hole in the block for support.
Inside view of the gantry plates with the wheels that go inside the C-beam.
I managed to find a Modulus 1.0 gear rack set in 1500mm lengths with pinions for about $200.. so I'm going to go with those.
I would suggest inverting the rack and pointing the teeth downward to keep debris out of them. That would require eliminating the top center wheel and using that area for the pinion but this should not be a problem as the center wheel is really not doing much for you. A simple hollowed out round in the bearing block will allow sufficient space for the pinion and keep it protected at the same time.
The wheels on the top are the ones with the eccentrics, and the ones on the bottom are the fixed load bearing wheels.
Do you think two eccentrics is enough, or should I add two more on the sides, and leave the center open for the pinion?
I am planning on using some kind of belt/gear reduction on the outside of the plate, between the nema23 stepper and the pinion.
Possibly on a separate assembly that can be spring loaded for tensioning.
Here's what I've come up with so far, inverting the rack (teeth down) and cutting out a hollow for the pinion.
I've also replaced the low-profile bolts with standard M5 sizes - 35mm for the wheels, and 30mm for the bolts to hold the plates together.
Now I have to figure out the gearing - I'm going to try to stick with 1/4" / 6.35mm shaft diameters, since the pinion is 1/4" and so are the Nema 23 steppers.
I figure a 3:1 or 4:1 ratio, with 20T / 60T or 15T / 60T gears... if I can find them with 1/4" bores.
Here is a concept I dabbled in previously. It is for belted drives but you may be able to adapt something similar and convert it over to hinge based. CNCRouterparts offers a better solution but it might be a bit more difficult to implement. You will need to slide the pinion opening to one side or the other in order to have enough back span to put a hinge pin. This will leave you with enough space for another top wheel if you are concerned about only having two currently.
As far as limiting yourself to 1/4" bore gears, good luck trying to get to 3:1 or better without custom manufacturing. Best I have found would be 2.5 to 1. B&B (et. al.) will do custom gears for a price but I have no idea what the premium is. You might think about JB Welding a snug fit piece of aluminum tubing inside a larger bore gear and then drilling the inside back to 1/4". This would mess with the set screw but that could easily be drilled up a size and re-threaded.
Not sure what the issue is, link works fine on my system (XP, chrome browser). Try http://www.cncrouterparts.com/pro-rack-and-pinion-drive-nema-23-p-227.html
Here's the racks, if anyone's interested:
CNC Precision Rack and Pinion M1.0 - 1500mm (59") - 3pcs Racks and Pinions Set
The pinions are 1/4" bore, so I probably won't use those...
Instead, I managed to find some 60T and 20T Mod 1.0 spur gears with 8mm bores, for a reasonable cost:
1M-60T, 8mm Bore
2 X 1M-60T 8mm Bore Hole 60 Teeth 60T Module 1 Motor Metal Gear Wheel Top Screw
1M-20T, 8mm Bore
1M-20T 8mm Bore Hole 20 Teeth 20T Module 1 Motor Metal Gear Wheel Top Screw
The pitch diameter of those is 30mm and 10mm, so the center distance would be 40mm.
I was able to come up with a reasonably accurate model of the gears, and visually check the mesh..
Thanks for the link - I was thinking of encasing the gears in between two aluminum plates.
The gearbox assembly would be mounted vertically to the outside of the gantry plate, with some slots to provide a little bit of vertical travel - pre-loaded with some spring tension on top to pull the spur gear up towards the rack.
Better view of the pinion and rack mesh, from the inside..
The clearance between the upper part of the bearing block and the rack teeth is a little close.. I will probably have to trim it down a few mm.
Revised the wheel layout a bit, spaced them out in 20mm increments.
Decided I may try to make my own eccentric cams out of 10mm hex bar stock, turned on centers to an 8mm diameter shaft and a 5mm bore, offset by 0.8 mm.
I don't know if I can turn stainless 303 steel on a mini-lathe, but I could probably do mild steel, or aluminum if necessary.
The dark gray bolts are M5 SHCS, 50mm long, and carry the wheels. The ones on the eccentrics are 60mm long.
The light gray bolts are M5 SHCS 25mm long, and they'll be threaded directly into the backing plate - backing plate is 15mm thick, gantry plate is 9mm thick.
I figure I will take some 5/8" plate and fly cut it to 15mm thick, and do the same with 3/8" plate for 9mm thick.
Side profiles of the eccentric cam, the eccentric wheel assembly, and the static wheel assembly.
Reworked the drive assembly by quite a bit..
Gear Rack is now mounted below the gantry plates, eliminating having to bulk out the plate to go "around" the gear rack.
Designed a geared Nema 23 hinged pinion system, similar to the CNCRP type. The stepper motor has a 20T Mod 1.0 gear, driving a 60T Mod 1.0 gear, which is co-axialed with the pinion 20T Mod 1.0 gear that drives against the rack.
The distance between the pinion gear center and hinge point is 80mm. The plate is 9.5mm thick (3/8"), and is angled down 50 degrees from horizontal. The drive assembly will be mounted to the gantry plate with a 8mm shoulder bolt, 40mm long, with an aluminum spacer in between.
Tension will be provided with a tension spring, attached to another shoulder bolt and one of the stepper motor bolts.
The big 60T gear and pinion gear shaft (probably a M8 bolt) will be supported on the swing arm end by two flanged F608-ZZ bearings, in set flush with the plate, one on each side.
Added the drive bolt, M8 socket head cap screw, 60mm long.
Also added the pivot bar, a 20mm OD x 8mm ID x 28mm high cylinder.
The assembly is secured to the gantry plate with a 8mm x 40mm shoulder bolt - the bolt head is recessed into the plate by 3mm to get that 40mm depth.
The inner round edge of the big gear is about 3mm away from the outside edge of the rack gear.
I also fixed the mirrored stepper.
Moving on to the gantry X axis... I decided to use two C-beams back to back - an I-Beam, if you will. They will be bolted together.
Adapted the wheel system I set up for the Y axis and laid out a carriage that is supported on both sides of the I-beam.
The drive assembly hangs off the back side of the beam, and the Z-axis will be bolted onto the front side (the longer side) with some riser blocks.
Lastly, the front and back plates are tied together on top with another 3/8" plate - it will be bolted through to both plates, though I may make the top plate fit in between the side plates instead of on top.. Either way I will have to drill and tap some holes into the edge of some plates.
If you can find stock thick enough, you might consider using two pieces of heavy aluminum angle and lapping them almost full width at the top. Having solid metal extending through the corners would be far more rigid than could ever be achieved with a screwed joint. Lapping at the top would also offer a slight bit of adjustability toward getting the faces at perfect spacing.
Something along these lines, then?
1/4" x 1.5" x 1.5" 6063 aluminum angle, square edged.
I could even go with 2" angle, though I'd have to make the top taller to clear the eccentric bolts below..
Pivot point, plus a spring tension point down towards the pinion.
I haven't modeled that part yet.
For the pivot, I started with an 8mm diameter shoulder bolt - but I can just as easily go up to a 10mm, 12mm, 16mm, or even 20mm shoulder bolt instead.
Actually, I was thinking more along these lines. (wheels and bolts not shown as that is way more effort than I have time for)
Bolting at the top near both edges would have both pieces acting as a single rigid block.
I think I may have a winner here - 6063 angle 2" x 3" x 1/4", butted together on top with a plate bolted on top of that.
The inside dimensions make it so that it's almost exactly 89mm, and I can work with that.
The 3" long portion of the aluminum angle is long enough to support the wheel axles - even a bit longer if I move the frame down a bit..
I'm leaving 10mm clearance between the "I-beam" top and the material so I can add strips of plate along the length of the two C-beams so they can be bolted together.
I would images it would like kinda like the one on cncrouterparts.com site
which is kinda the route I going with when I get the new space ready for the project build
Yeah, pretty much the same concept as the CNCRP rack and pinion system, except I'm going to use gears instead of a belt and pulleys.
Rounded the inner dimension up to 89mm, and replaced the 6mm spacers and 1mm shims with 9mm spacers.
There's about 0.1mm of wiggle room, you can barely see the gap in between the angle pieces.
Also replaced the top plate with 1/4" instead of 3/8"...
Worked out a mechanism for providing the spring tension on the drive assembly.
I replaced the pivot point with a 12mm diameter shoulder bolt (up from 8mm), 40mm long and recessed into the motor plate by 3mm.
The screw portion of the shoulder bolt is threaded M10, and is long enough to go through the 1/4" plate and secured by a M10 nut behind the plate.
I also redesigned the motor mount plate so that it can be cut from 3/8" plate, from one side - without having to use locator pins or flipping the part. The plate thickness is 11mm, plus 2mm standoffs for the stepper motor.
The tensioner consists of 2 x 20mm cylinders with countersunk holes for M5 SHCS, with 12mm flats and cross-drilled for a M6 bolt.
They're designed to mount against the end of the motor assembly and against the gantry plate, with a heavy duty spring providing for tension.
The M6 bolt is 90-100mm long, and the spring is a "medium load flat coil spring", commonly used in injection molding processes.
12mm OD, 6mm ID, and 20mm length.
Red Medium Load Press Flat Coil Compression Die Spring 12mm x 6mm x 20mm
Revised the drive assembly some more..
Two 6mm / 1/4" plates for support instead of one 11mm plate.
Drive gear supported on both sides by the flanged 608 bearings, with the flanges on the inside of the plates.
50mm shoulder bolt instead of a 40mm bolt, with a 25mm long spacer between the drive plates, and a 13mm spacer between the drive plates and the gantry plate.
Tensioner bolt end now supported on either side of the plate - a pair of M5 bolts will secure it to the plate, one on either side.
Also the gantry plate is thicker - back up to 9.5mm / 3/8".