I assume you will be using servos. So the encoder count + ballscrew pitch will dictate your gearing, as it will determine resolution, max speed of the machine.
I am planning to use 2x 750w servos on the X, 1x 750w servo on the Y and a 400w servo on Z.
The shaft on the 750w servo is 19mm dia and 31mm long so in fact that is what determines my smallest pulley.
I am planning to use 25mm HTD belts as my gantry will be close to 200 kg plus the belts are rated for -25 degrees Celsius.
In the Case of a 5 pitch pulley for 25 mm belts F= 30.5 which means that that is my whole 31mm shaft. So if I would want to use the whole L which is 38 I would have 7mm of the pulley hanging of the servo shaft. If I want to use grub screw I would be limited by Dm and an even larger Dm if I want to use key. So I have decided to cut of F from L and use just F and key cutting therefore I can use a smaller pulley and I am limited only by (De) and have nothing hanging of the shaft.
So basically 24 t pulley on the servo where Dp=38.20 and 72 t pulley on ballscrew where Dp = 114.59 giving me
a ballscrew rpm of 1000.09 on the ballscrew with 3000 rpm on the servo side so essentially 3:1.
With a 2010 ballscrew my critical speed is 1128 so would be able to get 10m/m rapids or I can go with 2020 ballscrew and get 20m/m.
With the resin my worry is that here in the uk we are getting around 21-25 degrees Celsius during day time and it goes down to 15 during the night. But I did not look into it so deeply yet and maybe that isn't a problem with the 105+209. If By any chance you already know it please let me know.
Vass you cannot just rely on the Key you will need Grub screws as well to stop the pulleys coming off the shaft. You can buy pulleys with larger Boss so can get more screws in but also not uncommon to put the screws thru the teeth and provided they are below the teeth and deburred correctly it's not a problem.
Regards the servo's the rated Continous speed will probably be 3000rpm which is the Max speed they will provide continous rated torque but in practice they often will reach higher speeds at lower torque 4 or 5K is not uncommon.
Really thou you don't want to go past the rated speed/torque esp with a heavy gantry so your calcs are correct but with 3:1 ratio and 750W servos's you'll have much more torque than you need so won't be a problem within reason.
To be honest you could go lower on the ratio and increase the speed and lower inertia slightly at same time. Like I said in emails the critical speed isn't critical and you can run above it without any issues so 2010 run at 2:1 would give 1500rpm and 15mtr/min rapids which is more than enough. You won't be running above critical speed all the time and infact rarely will be for more than 2-3s at a time so it never becomes an issue. My own machine as been running 2005 screws with 1:2 ratio so double speed and well above the critical speed for 6-7yrs without any issues.
I would only use 2020 screws if the screw length was long and needed to lower screw speed to resist whip. In this case I'd go with 3:1 ratio to increase torque and make up for lower mechanical advantage from larger pitch.
Last edited by JAZZCNC; 21-09-2014 at 03:08 PM.
I was thinking of the grub screw through the teeth option but wasn't sure thanks for confirming that.
What do you mean by the larger Boss?
Hello Sir! Amazing work!
How did you adjust Y axis bearing blocks against the Y rails? Did you weld the hole assembly of 10mm lasercut steel?
Yes, i welded the whole assembly, in fact all on the machine is fully welded except the Z assembly, which is spot welded almost fully-spot by spot each next to the other.
You mean how i squared the gantry? Y is my 2 long rails. After the epoxy they were on 1 plane. Then using straight edge and precision square i squared them. Then mounted the bearing blocks. Then mounted the gantry on top of the bearing blocks with bolts loose. Then using again straight edge touching the bearing blocks i squared the bearing blocks both sides. Then tightened the bolts that hold the gantry over the bearing blocks. So the gantry was now square.
Now have in mind all was was welded <1mm at least. Also the rails on the gantry were not drilled yet. Later i squared the rails on the gantry, fixed them with clamps and then drilled on place. Look at post #93
The Following User Says Thank You to Boyan Silyavski For This Useful Post:
Actually I meant X-axis then.
I understand that you ensure with epoxy that the rails are parallel. But how did you adjust HIWIN bearing blocks to match the rails?
Surely you did not weld the laser-cut parts so precisely?
The machine when long rails epoxy is poured must be glued to the floor so you will not bump it later. Then long rails mounted. Then first i poured epoxy on the gantry top , the gantry in normal position as near as it would be in final setup. Then flipped the gantry 180 degrees and the leveled top epoxy now down resting on the already leveled long Y rails. Then i poured epoxy on the low gantry rail and on the legs. So now we had everuthing parallel, gantry legs, low rail and upper rail.
Then i squared the gantry top rail as described bellow. In short i squared a straight edge on the long rails. Then using 2 equal sized aluminum bars and checking them square against the straight edge i squared the top rail. Then additionally using only one piece at both sides i carefully finished the job feeling the scratching . Drilled and mounted the same way. See first picture
Now i had 1 top rail ready and square.
The bottom gantry rail.
I mounted the plates on both rails. The plates that screw to the bearing blocks. The Z was still only pieces, not soldered. Then via 2 clamps i mounted the lower rail more or less. But clamps not tightened very much. Then using the front Z plate that will be later soldered to the both plates that screw to the bearing blocks i checked and aligned perpendicularity to the squared straight edge resting on the long rails , picture below.
Note should be taken here that when i bolted the 2 plates from the Z i pushed them from behind till they touched the screws, so i know they are alligned to the bearing blockes, as the holes are laser cut. I mean because of this i am sure that their edges are parallel with the corresponding rails. Later i will have some additional opportunity to align them on one plane/the z face plate/ as i can move one of them a bit back
So i mounted the lower rail. Z was carefully spot weld fixed while all was bolted on its place. No paint anywhere on the machine where bearing blocks contact with frame, metal only.
I swear I read this section many times before! But could not understand...
I guess my brain refused to believe that it is possible to fix the bearing blocks and then weld the parts together and end up with perfect alignment...
Even hammering some 10mm steel plate to perfect plane seems unbelievable. Guess it depends who is holding the hammer or welding torch :)
Sorry you had to copy hole page from your thread!
All the best,
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