another 4th axis...... and a 5th

[ninefinger]

Plastic, metal rubber - as long it works. I'm using large plastic pulleys (80T) on my cnc mill conversion (have a small aluminum hub), been fine for years. The belt is rubber. Don't get caught up in flash and bling for something that doesn't need it.
Another way of thinking - if the rubber tooth can transmit the load why not a plastic tooth? There are better things to spend money on (like tooling!)
Mike

 

[blighty]

im going for ali pulleys 'cos i like shiny things

all the bearings turned up today and i have been going through blogwitch's website he posted. need to give them a buzz tomorrow as they don't stock the 120t pulley you have to phone and ask for them. i have decided to go for 6:1 gearing as it seems a lot of other people use the ratio on there home build. even manufactured 4th axis (ebay ones, not HAAS) use 6:1.

 

[Blogwitch]

I should have added 'while you are prototyping'.

 

[ninefinger]

One of the considerations you haven't mentioned is what you will be machining. This will drive the consideration of holding torque and moving torque.
You don't get a torque multiplication effect of 6:1 though with this setup unless you are machining at the very center of the table. Inside the diameter of the large pulley you'll have a torque multiplication effect to the stepper, the closer to center the closer to the 6:1 ratio. Outside it you'll be reducing the effective torque. At your above mentioned 4" radius/8" diam (~200mm diameter), the torque reduction will be very pronounced. 200mm working radius:77mm pulley = 2.6:1 effective torque reduction.
Those Aliexpress tables tend to advertise 'engraving', as machining hard materials or taking significant cuts would probably cause the steppers to loose steps.
A workaround is to use very small diameter cutter to keep the forces down - i.e. no hogging or taking big cuts with a facemill. You'll probably need to keep the cuts small anyways due to loss of rigidity with all the joints and suspended working platform.

 

[Wizard69]

call me a snob, but not to keen on plastic pulleys, but they do have selection of ali ones

I have mixed feelings about plastics myself but the newer resins can produce pulleys and gears as good as aluminum.

 

[rodw]

Blighty, if you are in the UK, maybe also look at RS Components I've bought a couple of pulleys from them and they were reasonably priced. T5 is a metric size (5mm pitch). My stepper reduction drives are using a 5M belt which is similar to T5 except the grooves are rounded and are meant to have longer life and less torque loss. I'd never heard of them before until IO bought these reduction drives. I think it is a pretty new standard. I'm sure the belts will be hard to find when the time comes to replace them.

 

[blighty]

One of the considerations you haven't mentioned is what you will be machining. This will drive the consideration of holding torque and moving torque.
You don't get a torque multiplication effect of 6:1 though with this setup unless you are machining at the very center of the table. Inside the diameter of the large pulley you'll have a torque multiplication effect to the stepper, the closer to center the closer to the 6:1 ratio. Outside it you'll be reducing the effective torque. At your above mentioned 4" radius/8" diam (~200mm diameter), the torque reduction will be very pronounced. 200mm working radius:77mm pulley = 2.6:1 effective torque reduction.
Those Aliexpress tables tend to advertise 'engraving', as machining hard materials or taking significant cuts would probably cause the steppers to loose steps.
A workaround is to use very small diameter cutter to keep the forces down - i.e. no hogging or taking big cuts with a facemill. You'll probably need to keep the cuts small anyways due to loss of rigidity with all the joints and suspended working platform.

good points to bring up.

i tend to cut Ali and brass with the odd mild steel thrown in there from time to time. more so with the steel as i have just started a 2" Burrell build.

for the time being, cuts will be cut on the light side. any holes that need drilling away from the center will be done at 33mm/min and not the normal 140. at a later date I will be putting some form of disk brake on the axis's. controlled by a pneumatic piston.

 

[blighty]

a quick up date.

after a week or so of trying to sort some pulleys out...... was trying to get 20 /120 tooth pulley. no one seemed to have the 120 in stock, so had to be made. one of the quotes i got was £195 each. so i finally went for 15/90tooth. price cam down to 70 odd quid all in. should turn up middle of next week.

 

[blighty]

few weeks later................bits have turned up. not really what i was expecting. thought they would be Ali, but their made of steel and a bit on the heavy side.
the 90mm pulleys have cam locks, one of the 15 tooth has a 6mm hole in it and the other just has a pilot hole, but both need an 8mm.

i'll have a play over the next few days see what i can come up with. thinking about making the 4th axis first. with a means of fitting it to the 5th axis when needed.

 

[blighty]

stupid ruddy cnc!!

started to machine up the top plate for the 4th axis. needed to cut 177mm x 223.35mm (stupid size i know) Y axis travel is 182mm minus the 12 mm cutter
just had enough to make the cut, but a bit on the risky side. so i changed to a 10mm cutter to give me a bit more room. still a bit risky. then thought i would change the size from 177mm to 170mm that should give me loads of room.....hit the button and off she goes. it does all the inner work fine, takes the first pass around the out side all fine, takes another pass, still looks ok. looks like a got away with it. meanwhile the swarf is piling up on the Y way cover so on the 3rd pass the Y jams up, stepper jumps about a million steps into the work........... C**K IT!!

so started with 177mm end up with 168mm :wall:

need a bigger mill.

 

[blighty]

well that took longer than i thought it would, next thing is the shaft.

random pics.

 

[blighty]

not been in the shed for a bit, but just had to weeks off and its not 32deg in the shed, plus I got a massive fan

few weeks back (before it got hot) i made the shaft for the 4th axis. used the Myford MG12 for the first time in anger. well happy with the results.

have made all the side to the box that holds all the stuff in. turned out a bit bigger than i thought it would, might not as compact job as i first intended. for the 5th axis will have to go for the traditional trunnion type. also had a bit of a problem fitting the stepper. if i had of made the box an inch longer it would of been fine, but had to machine out a lot of one of the sides so the stepper could fit in there. then had the trouble of mounting it.

as i said the box is all made, screwed, doweled in place then squared up. sorted the wiring in the control box and set up Mach. turned it all on, hit the button..................and nothing happened:wall::wall::wall:
wont bore you with the details, but 2 hours latter it works:thumbup:

 

[blighty]

started on the plate last night, faced off the back and board the hole for the shaft to fit. plate is 160mm dia. 15mm thick.

to night i will put it on the mill, pick up the 8mm hole and drill the clearance holes so the plate can be mounted to the shaft. once on the shaft, put the assembly in a 4 jaw, clock it up and face of the face and turn down to 160mm dia. then figure out how to hold a chuck on it.

not to sure if i'm going to put T slots in the plate and threaded holes all over the place. might do that at a latter date.

 

[blighty]

it's all up and running
had a few problems mounting the thing to the bed of the mill. in all the excitement of making a 4th axis i some how forgot to build in some way of clamping the thing to the bed:wall: as luck would have it there is enough room inside to slide two 12mm threaded rod through the body with a nut on top to clamp it down to the table...... that was lucky. a few things turned out to be lucky with this build. if you lay it down in lets say 4th axis mode the center of the axis is in the middle of the Y axis travel. when used in 5th axis mode its about 5mm of center, so still lots of Y travel, but also when the 5th axis is 90deg the chuck will be just out side the Y axis travel so i should still get all my Y travel.

other problem i thought i would have and ended up with....... the plate that holds the chuck is 160mm dia. the bits that holds on to it (the shaft) is about 50mm. when something is in the chuck you can bend it from side to side and you can see the 160mm plate moving. found a big needle roller thrust bearing. I will fit this between the body and the plate, leaving a few thou on. so when you tighten the plate up it should squeeze the thrust bearing and support the plate on the outside edge.

 

[blighty]

done some more work on the old 4th the other week, machined up the plate to fit the needle roller thrust bearing. thing the thickness of the bearing was about 5.something mil. so i machined to a depth a bit less than that. first to allow for the 0.2mm gap between the plate and the body of the 4th. and a bit more so when you tightened up the plate to the shaft it would pull the plate down onto the bearing.this seems to of taken out all the play that i was getting. so now all you get is a bit of flex in the material it's self. will have to look for a tail stock for long peaces.

as for setting up mach.......
did all the calc's for the step per unit. came out at 33.33 steps per deg. tried this and it never got back to zero it was all ways off. so if you pressed the "zero all" button, the 4th axis would go from 359.999deg to 0.009. zero again and it would go to 359.992 and so on. would never go back to 0. so i started playing with the step count..... this went on for a bit (kinda like this post) ended up with 33.3333333333333333333333333333333. problem solved

just have to think of something to make with it now.

 

[jeep534]

This is all very cool
pictures of it on the mill please

Happy Hunting
archie

 

[blighty]

best i can do for the time being.