Quite an Unusual one

[Boyan Silyavski]

I'm not sure I agree with your theory for putting the legs in the corners, but we won't worry about that now.

I'm interested in how to make it moveabe and still no twist. Obviously, making it very stiff is a good start. But what you really want is to know the reaction forces with the ground so you can install the machine elsewhere with same contact forces. The reactions don't need to be equal but equal would be sensible.

If you have screw adjusters on each foot, then the torque is an approximate measure of the contact force, so you would carefully go around the machine and even up the torque on each foot adjuster.

You want to do this before your epoxy pour.


Or maybe thick elastomeric pads, where you can measure the deflection as an indicator of reaction force. These might be good for vibration too. What is your plan for fixing to the floor? or does it just sit on the floor ?

Some numbers would be useful. Have you calculated the stiffness of the frame? eg if all the weight is on 2 opposite corners how much dip at the other 2 corners?

i am still not sure. When all is ready and un-leveled, we will see if i am right or wrong. If i had made a table and not all on the floor, the frame would be even stiffer, so that would not be an issue. But As you can see the garage is very small, lets say it will be the machine enclosure :-).

On paper if the table hangs in the air one side and somehow is supported only by one side/cantilever/ the bend will be around 0.03mm or less. In normal conditions with 500 kg gantry in the middle less than 0.03, so for the side twist.
So basically if i have not made any mistake all must be well under 0.05, which seems quite an achievement to me for 3000x1800frame, having in mind it has only 500kg steel in it.



Now my wife went a week before to Bulgaria on a business trip. It is our country of origin so i know where to make it cheaper than here, she managed to order the rest of the laser cut parts, the rotating nut parts, collect them , pack the and bring them to me. Meanwhile emptying on the way back all personal things of hers, cause this weight exactly 32 kg with the suitcase. I could only bow to her for the support.







I wonder how the 2 bearings go against each other? The thick inside ring side should be outside or facing each other? Somebody shed a light please!

I have to find my bloody caliper in the garage, cause the bearings don't enter without force at all. I called today and they said that they made all so that it must be press fit, something like 0.03mm more. I dont have an idea about that, will see how i fix all inside. First will have to bore the 30T pulley. Though actually i am still in a doubt to use the Chinese aluminum pulleys i bought from ali express or order new from belting online. cause some of the chinese pulleys the 20t ones were not straight, though i kept them for the sake of the price. 10x30t pulleys+10x20t pulleys=~86 euro. Having in mind i could not find cheaper than 13-15euro aluminum pulley 30t, one piece.
Unluckily, belting online had only steel ones and even they were getting expensive with the machining.

[jimbo_cnc]

On axial type bearings, the fatter ring faces are where the thrust is applied. Because there is a taper or similar in there.

[Jonathan]

Yes, you want the larger side of the inner rings facing outwards so that when you do up the nuts you apply a preload to the bearings. Don't put the bearings in the wrong way round, as they'll be pushed apart by the nut and probably be broken. Don't do the nuts up too tight else the bearings will fail prematurely and don't leave it too loose else the shaft can float in the housing.

From the machining marks in the picture it looks like the bearing bores were milled? If this is the case then it's a bit concerning - if their milling machine doesn't have zero backlash then the bore will be less round, which would distort the outer ring. It's also generally less accurate than boring, which is why bearing housings are usually bored.

When I make these I bore them on the lathe and make the bearings a light press fit, to get better rigidity and ensure the outer rings don't creep. Also, if you get more made I'd be inclined to increase the axial spacing of the angular contact bearings, as it's currently insufficient to get the best stiffness. You'll probably be OK, but the critical speed should be higher if the bearing spacing is increased a little.


[QUOTE=silyavski;64503]i am still not sure. When all is ready and un-leveled, we will see if i am right or wrong. If i had made a table and not all on the floor, the frame would be even stiffer, so that would not be an issue. But As you can see the garage is very small, lets say it will be the machine enclosure :-).

On paper if the table hangs in the air one side and somehow is supported only by one side/cantilever/ the bend will be around 0.03mm or less. In normal conditions with 500 kg gantry in the middle less than 0.03, so for the side twist.
So basically if i have not made any mistake all must be well under 0.05, which seems quite an achievement to me for 3000x1800frame, having in mind it has only 500kg steel in it.

Regarding the pulley selection, you know the dimensions of the rotating ballnut shaft and the rest can be estimated well enough, so work out the inertia with aluminium and steel pulleys and see if aluminium is worthwhile? Or post the following numbers here and I'll show you:

• Mass of gantry (i.e. mass that both rotating nuts are moving).
• Desired rapid feedrate.
• Ballnut size (RM2510?).
• Motor ratings.

I think you'll find aluminium pulleys aren't worth it, due to the relatively high inertia of the ballnut - but no point speculating...