Thread: Wobblycogs CNC Mk2
Hello again everyone,
My first CNC design was, in hindsight, perhaps a little bit ambitious for a first build. After building one of the sides I realized that welding was the only realistic option and that my current workshop just wasn't big enough to fit the machine (and it would weigh too much for the floor).
Rather than wait for the workshop to be built I thought I'd come up with a new design for a more modest sized CNC machine that will fit in the space I currently have. There's not much to see yet but I was hoping to get a bit of feedback on the pulley arrangement for the x-axis as this is an area that I'm not very familiar with. Firstly some pictures...
The frame is going to be all extruded aluminium if possible. The blue sheet forming the bed is 600x600mm to (hopefully) give a cutting area of 400x400mm. Z axis travel will be fairly limited, perhaps as little as 50mm, as this machine is being designed to mainly cut aluminium.
The x-axis rails are SBR16 although I could probably fit SBR20 if necessary. The rails are sitting on 60x90 heavy extrusion as sold by KJN. There's 180mm between bearing block centres.
The ball screws are currently spec'ed as 1605 with 16 tooth HTD5 pulleys on the end (with extra long end machining). The stepper will be a nema23 but I've not picked one out yet. The stepper is fitted with a 32 tooth HTD5 pulley to give a 2:1 ratio.
The stepper and the two idler pulleys are mounted on a 10mm aluminium plate. For build simplicity the idlers are used to take up the slack on the belt rather than the stepper. The stepper is mounted using stand off legs. The belt length (at central idler adjustment) calculates as 1736mm
The plan is to have this machine standing vertically, or perhaps wall mounted, so that the x-axis belt is at the top.
My questions so far...
- Does the belt make enough contact with stepper motor pulley? I'd guess about 12 to 15 teeth are in contact with the belt.
- How is the 32 tooth pulley attached to the stepper shaft? There's only 15mm of shaft on the stepper so I'm guessing it will have to be a taper lock (also Belting Online don't bore 32 tooth pulleys at 6.35mm).
- Where can I get idler wheels from? Neither Belting Online nor Bearing Station seem to do them. I'm tempted to follow Jazz and use bearings and penny washers...
Ok so first don't go with 1605 and 2:1 ratio it's not need and you'll want more speed than that.
Direct 1:1 will be enough for cutting aluminium. If you want to go 1:2 then buy 1610 then you'll get same resolution and 1:1 speed has 1605 but double the torque of motors so lots of torque while cutting at low speeds. You'll rarely cut above 1500mm/min in aluminium.
With 1:2 ratio on 1610 then it's a simple switch of pulley if you want speed for routing, plastics etc.
Next the frame is no where near strong enough for cutting aluminium. Beef up the lower portion, I'd suggest an outer frame with inner cross braces. Really those corner angle brackets are rubbish so I'd think about some good strong plates with plenty of bolts.
If your going vertical then you'll need to think about the gantry, Z axis has depending on which way you orientate it will depend on how chips collect on surfaces etc. You'll certainly be wanting to put covers on Z axis and belts to help stop chips collecting if your putting belts at top.
It's like this Aluminium chips fly every where and they go up before coming down so just think about what they will land on and or were could collect.
In general it's not a problem more of an annoyance because on my machine the Z axis front plate faces down and if doing again I'd face up and put cover on because the chips collect on rear of front plate. Basicly Z axis is on wrong side of gantry facing down, I'd put on top side facing up.
Hope this helps.
Last edited by JAZZCNC; 15-08-2013 at 07:39 PM.
Thanks, as always Jazz your reasoning makes perfect sense on the ballscrews so I'll change then to 1610's. Hopefully this new design will be stiff enough. As for the bely I was thinking I could save a bit of money if I just used a bit of string pulled tight ;-)
Everywhere you see a hole that looks like it should have a bolt there will be one.
This shot is of the base with the new base plate removed showing the new end pieces on the bed rails - these will be bolted on. The corner brackets are still there but they are more to affix the x-axis to the bed rather than hold the whole thing rigid.
The back plate has grown "ears" and bolt holes. Long bolts will run down into the x-axis and the rails at the end of the bed.
There's a new front plate (10mm) which again has bolt holes for the x-axis and the bed.
Finally the base has been covered in a 5mm sheet of aluminium and bolted all over the place for bracing.
I forgot to mention in my first post but the blue bed piece will also be bolted to the bed for bracing.
The only downside I can see of this is chips will accumulate on the front plate, a small price to pay if it's cutting aluminium accurately!
Last edited by Wobblycogs; 15-08-2013 at 05:29 PM.
sorry if I'm getting this completely wrong, but the op wrote a gearing ratio of 2:1, but also has written and drawn a 32 tooth pulley on the motor and 16 tooth pulley on the screw. Doesn't this setup actually give him a 1:2 ratio, so 1 turn motor gives him 2 turns screw. So theres plenty of speed, which isn't needed as you say 1:1 is ample for aluminium, the op's main working material.
I think I've got the ratio the wrong way around, it should probably read 1:2. To clarify there is a 32 tooth pulley on the stepper and 16 tooth pulleys on the ballscrews.
The design has progressed a bit so I thought it was time to get a bit of feedback...
I've decided to try to increase the maximum material thickness to more like 100mm (up from 50mm before). To achieve this though I seem to have ended up with an unusual bearing block placement on the z-axis, see image 2. The idea is to be able to lift the z-axis high enough so that the bottom of the gantry is the limiting factor for the material depth. As drawn I have 180mm top to bottom between the z-axis bearing blocks but I could easily increase this - I was thinking 250mm would compensate for the additional over hang of the z-axis.
The gantry has changed design several times . It started out as two pieces of 45x45 heavy (a bit like this) then got changed to two pieces of 60x90 heavy , then to a single piece of 90x90 heavy.
I finally settled on two pieces of 45x60 heavy with a backing plate of 10mm aluminium, the unsupported span is 610mm. The deflection calculator on the Metallin website (using the closest match) seems to think the z-axis deflection should be ok. Using 60x90 heavy would reduce deflection, I'm sure, but it was expensive and reduced the x-axis cutting. I think this is probably a fair compromise.
The belt arrangement of the x-axis has been improved. The ratio is now 1:1 and there's better belt contact with the stepper pulley.
I've not added the y or z-axis steppers into the diagram yet (should be fun). Also still to be drawn is a chip guard for the y-axis screw. There will be a thin plate with a long slot (and maybe brushes) for the y-axis connection to the z-axis assembly.
Ok it's getting there now.!! . . . Few more things I'll comment on.
I can see what you trying to achieve with Z axis but doing this your compromising it's strength where it will probably spend most of it's time cutting and that is Extended further out. Because the bearings are high up the extension distance is long when cutting thinner material and I'd estimate you'll rarely cut thicker than 50mm material.
Much better way would be to put bearings lower down but not flush with bottom, leave clearance for bolt access to Y axis bearing plate. Then make the spindle mount adjustable on front plate if you ever need to cut tall material.
Looks like your planning on using WC spindle so that alone can slide in the mount easy enough so you'll find a intermediate location for spindle mount that will allow you to adjust for all materials.
Next the material thickness of the Front and Rear Z plates are not thick enough and they could resonate and flex. Wouldn't go less than 20mm here.
The plate on the rear of moving Y axis carriage isn't needed or adding much strength so drop that It's only adding weight and cost. Good way to help gain strength in this area is to make a cover for the Z axis from folded 3mm ali plate and bolt into sides of Z rear plate and Y axis top bearing plate. This way it braces this area and protects the Z axis ballscrew belts etc see pic of cad 3d model and unfolded sheet, Next week I'll show real thing cause I've got to make one.
Also Got a way for you to increase cutting area and increase Y axis strength but will be hard to explain so may end up drawing it to best show.? I'll try explaining first.!
Drop the profile spacers on bearing plates this will allow you to use longer rails that extend right up to gantry sides. This will allow the Y axis bearings to travel over the X axis bearings. Now because you can still have the same Y axis bearing spacing(or more) you can drop the width of Z axis front plate and therefore gain some cutting width with no loss strength.
You'll need to brace the X axis bearing/gantry plate area but that's easy enough done with triangle plates.!
Hope this makes sense.??
I know what you mean if re-design but to accomplish this he'd have to turn rails/bearings 90Deg and that causes other weakness and issues which IMO would be worse than spindle extending out.? . . . If you have another way on the same theme he's taking now that I'm not seeing then get doodling.!!
Last edited by JAZZCNC; 17-08-2013 at 01:05 PM.
BUILD LOG: Wobblycogs CNC Mk1By Wobblycogs in forum DIY Router Build LogsReplies: 91Last Post: 02-01-2013, 06:38 PM