Thread: Gantry design advice please
im down to my last chance to get the damn thing working perfect lol after that its the scrap yard lol
Just a little advice guys if you dont mind.... take a look at the JPG i have attached, are the rails too close together? i would rather not spend anything else on this machine (Unless one of you gurus want to design one for me :p) so im trying to use what i have.
Also is the X & Z axis plate too narrow? again hopfully this will be ok as im not impressed with the proice of this ali plate carry on haha
Assuming you are going to use a ballscrew to move your z-axis assembly along the gantry, where does that go and how is the bearing attached to the z-axis assembly it looks like a very tight fit?
I was thinking about powering the gantry and the rest of the machine via rack and pinion. The rack will be mounted on the top of the gantry. So from the verticle Ali plate will be a piece of Ali coming off at 90 degrees, to accommodate the motor and possible rack and pinion drive. I will attach a picture of the gantry as it is now.... Please note that the machine is still in raw state simply until I get her performing how I would like her to. Also the 25mm X axis is also temp so I can cut parts as I need them to upgrade
Thinking about traverse speed and acceleration, lets say you want 1.5m/s2 acceleration and your gantry weighs in at 25kg. F = ma = 25 * 1.5 = 37.5N. with a motor each side you need 18.75N from each. Lets assume you want to use a 50mm pd (25mm radius) gear then torque required = 18.75 * .025 = 0.46Nm. A 50mm pd gear gives 50 * pi = 157mm traverse per rev, a resolution of 157/200 = 0.785mm, So you'll probably want to 1/8 step it for 0.1mm resolution (guessing wood only). Now check the inertia of that 50mm gearwheel, its about 1.875e-5Nm2 together with the motor inertia of 8.4e-5 is 1.02e-4Nm2. Angular acceleration at 1.5m/s2 = 1.5/.157 * 2pi = 60rad/s2, so at the motor you'll need another 1.02e-4 * 60 = 0.01Nm of torque adding 20% or so for losses, etc. Microstepping 1/8 step means the motor only gives 20% of stall torque approx which means a motor torque of (.46+.01)/.2 = 2.35Nm at the required traverse speed
OK, so we are geared at 1:1 from stepper to drive gear. 1 rev of drive gear = 157mm = 1 rev of stepper, so for a traverse speed of 6m/min... stepper revs = 6000/157 = 38rpm. At 38rpm a 3Nm stepper should get close to that 2.35Nm but it could be marginal, so I'd advise a smaller drive gear, say 40mm and 1/8 stepping requires 2Nm @ 48rpm which is safer and gives a resolution of 0.078mm/step. You could even go down to a 30mm gear, but the smaller you go the less teeth in contact so backlash and other issues arise...
Other considerations: with a 50mm or smller gear, the physicality of mounting the driver stepper gets harder, there's not a lot of room to play and the stepper needs to be outboard of the rack, making it very bulky width wise. As an alternative consider direct belt drive. Stepper remains static at one end of the bed and drives a belt thats fixed to the gantry. With a larger double ended stepper you can drive belts on both sides with an additional coupler and axle off the back of the stepper, or drive a common intermediate axle which drives the belt on both sides.
Last edited by irving2008; 21-10-2012 at 09:49 PM.
Wow I've never been so overwhelmed my numbers. Irving I know you took your time to write that out for me buddy but a lot of that means nothing to me as I'm still learning. But I will read it over until I fully understand.
I would rather stay away from the belt drive, from what I have read there would be too much stretching it the belts.
I first need to get the gantry to the point where i know it's going to have a good outcome. Did you have a look at my diagram? Do you think they are too close that it would cause some slop or would not be sturdy enough? I will get this right first but also leave the option for both screw or rack and pinion to be used.
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Hey, no problem on the numbers. I'm trying to give various people who read these responses some insight into the thinking as some have indicated they'd like to understand more about how to go about the calculations. IMHO better to have them there and for people to get their heads around them as and when than not to have them.
Intuitively they are too close IMHO to give much torsional stiffness, tho its the plate rather than the box section that'll distort. For the sort of cutting forces you'll encounter the box section will twist a miniscule fraction of a degree but the ali plate will bend about .2mm so at the tool tip that'll be considerably more...
I'd want to consider double the spacing at least...
Not sure I understand whats wrong with your current setup?
Well managed to get some more done on the gantry today.
Sides are made from 25mm HDPE. This is not the finished product, it's simply a get me up and running kinda thing. Prototype if you like. All is well and it's fairly solid. But will defiantly be going with Ali.
What you think?
If you added some aluminium/steel box section or angle or extrusion (or anything really!) vertically down the sides then it would improve the stiffness greatly for forces parallel to the Y-axis.
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