Dave slapped this together for you quickly so you can see approximately what you may end up with, without needing to get down and dirty in CAD.
Nothing is to scale obviously as it is just a drawing, note the X Axis screw mount position and that it would probably need moving back (away from spindle) to allow for clearance of the nut and mount (at the spindle plate), side plates would need to be bigger and other plates also bigger, bigger side plates are not a bad thing outside of additional cost because you really donít want to go any less then 2-250mm centres for your rail bearings, both designs pretty much force you in that direction anyway so I didnít mention it before but I thought that was a little bit ignorant of me, so here it is.
You could close things up with datumís and pockets for the rails and bearings and so on, keep an eye on the proportions as well, if you look at the original image Iím working from, the gantry looks small next to the spindle, if something looks small, it probably is...resize to match the proportions if you can.
Also, others will probably say itís not as important as getting other things right, and they are right but try and keep everything lined up and parallel, so your rails, lead screw and nuts. On the Y Axis (bed) the nut could be moved back towards the spindle in my previous image, vertically aligning it with the rails and lead screw, the middle of that plate ďlooksĒ nice but would it be better for the loads, if they are all inline?, they all come together to work as one.
Just my thoughts...What do I know...hope it helps!
No need for that box design around gantry it add's very little but expense and more potential for binding. Also with this design the ballnut won't fit between the Z axis backplate and gantry. Those wide top n bottom plates with rails sat back just make long levers acting upon the bearings which will could cause binding.!! . . . . Not a design I'd go for.!
Your design in post #67 will be ok. Don't waste money or time on the box around gantry setup no point or bennifit IMO unless bracing sides.!
Last edited by JAZZCNC; 09-07-2014 at 11:36 PM.
The significant advantage from putting the rails one the top and front of the top and bottom beams respectively (as Jazz suggested earlier), is that you can make the rails a bit longer and thus increase the spacing of the bearing blocks a bit without widening the machine or loosing travel. I'd certainly recommend that since the stiffness due to the bearings is proportional to the bearing spacing squared (assuming what they're mounted to is rigid etc), so every little helps.
Not much progress but changed the "L" shape on the gantry as JAZZ suggested. I'm now using 2 pieces of 80x120 and 2 pieces of 40x40 (mainly for filling the gap). This used to be 60x30 but thought I better bulk it up. Also the height (total height 350mm) and width (now 175mm) of the gantry side plates has increased a little to accommodate. Still undecided with what to do at the back of the gantry... I need to leave enough room to mount the motor on the inside (using pulleys).
Last edited by biketrialsdave; 12-07-2014 at 10:29 AM.
Ahhhh! really not sure which direction to go with the gantry design. It just does't look right now. Maybe I should mount the motor on the outside of the plate and the fixed (FK12) ballscrew mount on the inside of the gantry plate? (I.e. direct connection, no pulley).
Who knew this would be so hard!
Lol, I know exactly what you mean. I must have gone through a dozen different designs before I settled on what I was going to build. The most disheartening bit is when you think you've got it done then notice there's a collision or something doesn't quite line up correctly.
I'd be interested to see a front view as your design you seems to have a full height side plate on the gantry. Do your x-axis screws stick out a long way?
"I'd be interested to see a front view as your design you seems to have a full height side plate on the gantry. Do your x-axis screws stick out a long way?"
Not quite sure what you mean by that?
Here are some options for the gantry..really don't know which route to go down now...Option A would allow an internally mounted motor (without increasing the side plate width). However I am also leaning towards option B. No matter which option I go with I still need to do something about the big gap where swarf could get to the ballscrew.
Option A isn't going to be very strong, it has almost no vertical bracing. Essentially all the stiffness would come from the bolts through the side plates into the profile.
Option B is ok but Option C is going to be a bit stronger. If you want the space internally to mount the stepper then you could go with option A but put a 10 or 15mm plate across the back (I assume the sloped side is the front). You should really have a look through some of Jazz's threads if you want to see a good design that keeps everything internal, the design I'm thinking of has a 20mm plate a the front of the gantry with a slot machined out of it for the screw connection. It served as inspiration for my design.
If you want to do a quick experiment get a couple of lengths of 2x4 timber and nail them to some ply in roughly the design of option A, it'll take 5 minutes to build and you'll immediately see why it won't work as a gantry as it'll wobble around like a jelly. Grab yourself a bit of hardboard and nail it across the back, you could probably stand on it them.
If you want a simple design that works is strong and neat then try this setup. This machine design with higher sides is just for cutting woods and soft materials but you can use the same "L" arrangement without the high sides. Before anyone says it the ballscrew at the rear makes no noticable difference to cutting and the advantages of keeping chips away from screw are clear to see has is the neatness.
If you want stronger than this then look at the All aluminium gantry but it's much more expensive, thou it is very strong and again neat with good protection to components.
Last edited by JAZZCNC; 13-07-2014 at 10:05 PM.
Thanks Jazz! I had thought of something similar but was afraid monting the ballscrew like that would cause problems. Is the general idea of the gantry sides to keep them as short as possible? Well not the sides, but the distance between the top and bottom profile carriages on the X axis?
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