Thread: A sufficiently strong machine
Well thanks to your work on the machining for the gantry its now just about alive with all axis running. Watch this space and I will try and get a build log up soon. Working on the control box now.
Thanks again for the great machining work you did for me. ..Clive
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Jonathan, this is awesome. A master class in how to do it properly!
Last edited by JoeHarris; 19-02-2014 at 07:07 PM.
I am tinkering on the design, as i find it great. I am wondering if i can build something similar, simplifying it a bit?
- 2 x 100x100 steel box for the gantry
-The bearing blocks and rails on the gantry to be at the upper side/or at the lower side/ so i can use epoxy and level them easy
-2x ballscrews at the gantry near the rails
What worries me most is the Z, making the z box and adjusting rails and fitting everything in place there. maybe using some ready box section and fit the spindle inside and fix the bearing blocks to the sides of it.
Last edited by Jonathan; 20-09-2013 at 05:41 PM.
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Would also add another ballscrew to Y axis rather than increase bearing plates has the action of 2 screws pushing/pulling together would be smoother than widening the plates.?
Like you say it's very strong already but feel these changes would make it the close to ULTIMATE strong machine.!
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Thanks for your reply Jonathan. To add a second rail, i was going to set it up and level it following your excellent instructions, then use the level surface prepared for the first rail, to mark the second. I was thinking of adding the second rail 90deg relative to the first ( on the side ) for the reasons stated in JAZZCNC above.
In fact, if i get the first rail level, and use it to mark the second, it would be quite hard to make it to far out ( as long as am carfull ). The main reasons i require stiffnes to the gantry is so i can add a B/C head or plate if needed. Am also adding a turret/lathe to the bed.
A couple of questions as i am designing my own build, using your ideas:
-so you believe 3kw spindle would be better than 2.2kw?
-how you did the oiling system? did you use the original nipples and modify them? why not grease? how much oil is used under operational condition? What oil?
-where did you get that spring shims that make the preload? what are the specifications? how did you calculate the distance between the 2 nuts , so that they would be mirroring each other exactly and the shims to fit in?
The linear guides do have some damping effect, due to the oil layer between the bearings and rail. Adding rail with two bearings, as you suggest, would therefore increase the damping factor, as it increases the surface area in contact. Instead of adding an additional rail and two bearings, you could increase the stiffness and damping by adding one bearing to each of the existing rails which is likely more cost effective. This also has the added slight bonus of evening out errors in rail straightness and generally aligning two rails is a easier than three. Either way, this damping effect isn't that large as the surface area in contact is small, so you're probably better off improving damping in other areas such as joints in the frame. You can also add non-load bearing sliding contact bearings to further improve the damping effect, which will make a bigger difference as sliding bearings will have a much greater surface area in contact.
If you take the load ratings from the Hiwin datasheet and plot them versus rail size, it seems that the load rating is proportional to the rail size raised to the power 1.7. So if we assume the stiffness of the rail is proportional to the load rating and that the magnitude of the force on each linear bearing is similar, to increase the stiffness by the same factor as adding one additional rail (i.e. 50% as you're spreading the load between 6 bearings instead of 4), you only have to increase the rail size by 27%. So for example going from a 15mm rail to 20mm, or 20mm to 25mm would gain slightly more stiffness than adding the additional rail. Similarly if you want to double the stiffness, then instead of going from two rails to four, you could increase the rail size by 50% (e.g use 30mm rails instead of 20mm).
Looking at it a different way, if you plot the price of the linear rails and linear bearings versus their size, it's a pretty convincing linear relationship. So by increasing the rail size you have an exponential gain (x^1.7) in stiffness for a linear increase in price. If you add more rails you have a linear gain in stiffness for a linear gain in price.
1) You need to get more power than the 2.2kW spindle can deliver at low speed, e.g. for cutting steel, but still require a high speed spindle.
2) The machine is rigid enough for the stiffness of the spindle to be the limiting factor.
3) The machine is rigid enough to make cuts which exceed the power rating of the spindle.
Number 3) is definately the case for the macine in this thread. I'm not yet certain about the rest so wont comment.
Lee Spring, spring manufacturer for a variety of uses. The preload force is set as a percentage of the screw's rating, so you need to find the rating for your screws and then find disc springs which can apply that force without being fully squashed. I didn't calculate the distance, instead a gap is left, measured, then a spacer made to the correct thickness to squash the springs by the calculated amount to obtain the required force.
Last edited by Jonathan; 26-10-2013 at 07:35 PM. Reason: Formatting
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