I guarantee that an M5 or M6 hole will not strip in 5mm of aluminium for the loads that are imposed upon it on your gantry.
When calculating the length of thread engagement required the standard method is to calculate the required thread for it to require a higher force to strip the thread than for the bolt to snap when placed under tension.
For M5 the calculation (reference here) results in minimum 4.0mm engagement and 5.0mm engagement for M6. However this assumes the thread is steel, so you have to compensate by multiplying by the ratio of the material's tensile strength. This clearly results in slightly greater values.
This is still not a problem since the force on each bolt is small in your application, particularly if there are a lot of them. You are not going to get anywhere near the force required to snap a bolt. The concern is more ensuring they do not work loose, so I would thread-lock them to be safe. Since you are using SBR rails I reccomend thread-locking the bolts that fix the rail to the aluminium mount since they can also work loose.
Last edited by Jonathan; 07-08-2012 at 07:43 PM.
Following the change in gantry design it made sense to rationalise some material used, as the minimum profile length from Aluminium Warehouse seems to be 2500.
Have also tweaked and simplified my overall design... and am sure there is still more tweaking to come... Gawd I can go off on a tangent and over-complicate things unnecessarily!
Have corrected the collision-ing at the extremities of the X-axis travel in the virtual world... Can now easily slide the gantry along it's entire length of travel within SketchUp with a few mm to spare!
From pics I've seen I'm guessing that the drive shaft of the Nema23 motor is too short to accommodate a pulley if the motor is mounted with the shaft going through 10mm plate... Please shout if this isn't the case... So still have to design a mount to solve this.
I think that so far virtually everything is within my limited capabilities and my limited workshop/tools facilities, particularly if I can get Aluminium Warehouse to cut the aluminium profile to the required lengths. The one exception is that I am looking at ballscrew housing mounts (purple) on the X-axis that also serve to brace the gantry-end angle profile.
Have just started on the Y and Z carriages... Will probably use Zapp's Hiwin HGR20R profile-rails (800mm and 400mm) and HGH20CAZA medium-preload four-row bearing blocks. Pretty sure it will require machining to keep
the Z-Axis really close to the Y-Axis, so when I get there I'm hoping that someone on the forum offers machining as a service and would have the time to build these for me?
Drive shaft length of a Nema23 is 20mm of the main face, 19mm off the reference lip and you need a 38.1mm hole to accomodate the reference lip on the face of the motor (or bigger if you are not using the reference). So if your pulley is <38mm dia and <10mm wide you can fit the pulley before mounting the motor. Otherwise you'll have to recess the motor by milling a 56mm wide recess, 5mm deep, in the 10mm plate... or find another way
The Following User Says Thank You to irving2008 For This Useful Post:
Apologies for chopping and changing like the weather... In mitigation I have never built anything out of alu, so reckon it's best to get the design correct-ish up front in the virtual world. My fault is being far too hasty in posting newer versions!
I have further rationalised the frame material used... Have got my head around bolting plates at 90deg so am rid of the unequal angle profiles. Main materials are now KJN 45x90 alu-extrusion and Cut-To-Order alu-plate from Aluminium Warehouse. And the frame is now far easier for me to build and assemble.
Have also decided to go with profile-rail on the X-axis... Am more than a tad worried about the rising costs, but suppose in for a penny... and if a job's worth doing...
Although some relatively minor tweaking will be needed once I get into the Y and Z carriages, I am finally well pleased with the frame and won't bore you with any more iterations of it.
Quick question... my supports for the BK and BF blocks... made from 20mm plate... do these look strong enough... particularly the BF side that is not braced?
Last edited by WandrinAndy; 14-08-2012 at 03:11 PM.
I'd replicate the bracing in reverse as per the near end, it'll look better too, not sure thats rigid enough and thinner pieces can resonate unexpectedly.
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