1.7*0.74*0.4m Mill / Router building...

[Jonathan]

Ummm like this???

Yes, exactly. I'm sticking with one bearing for now though as I think it will be ok.

[routercnc]

Mulled it over a bit more and managing to visualise this now. Yes, it does work. Nice idea, and worth a look compared to the cost of ballscrews.

[Jonathan]

Mulled it over a bit more and managing to visualise this now. Yes, it does work. Nice idea, and worth a look compared to the cost of ballscrews.

Certainly worth a try. £8 for a meter of M16 threaded rod, plus a few quid for the bearing compares very nicely to the price of a ballscrew! Even a *standard* ballnut has some backlash whereas I can't really see how this could have any? Granted the pitch of the thread being slightly uneven with threaded rod isn't ideal ' but that could be solved by using ACME.

[routercnc]

I think there will be a torque limit (from the stepper motor) which can be applied to the leadscrew, otherwise the bearing will want to jump over the teeth. Depends on the spring preload holding it against the teeth - higher is better for this, but then more bending load on the leadscrew which is less good. This effect will probably be the determining factor in the success of this idea, but good thinking!

Better tooth engagement from a brass insert, as you suggest, might help although there will still be a limit - you'll have to see how it goes. You might be able to arrange a locking up nut so that when the spring tension is developed you can lock it off at that position. Of course this won't compensate for small variations along the thread, but might be a fallback position if it keeps jumping.

[Jonathan]

I can see bearing jumping could be a big issue with a long screw since for the bearing to jump the screw would need to bend a bit...

Locking nut on the spring is a good plan. Just thought instead of using a standard spring I could use a couple of belleville washers since the travel required is tiny. Can also adjust the spring constant by stacking the washers different ways etc...

Back to the main build.

I went into school today. Their laser cutter had arrived over the summer...it's huge! Only 80W but still useful I guess.

I milled the 600x160x20mm Z axis plate square and drilled and tapped a couple of holes to bolt the rotary table to it for when I want a 4th/5th axis, not that that's very likely since I can't afford the software.

I also cut and milled the steel gantry sides square which took much longer than anticipated. It's not a nice grade of steel to mill.... Started milling the surface flat on either end to make a better contact between the gantry cross piece and this. I'm putting a piece of 20mm aluminium plate in both ends of the 80x80 box section to make a good strong joint.

Photos coming soon!

[routercnc]

Hi Jonathan,

I'd need to draw it out and look at the angles and forces etc. , but I suspect that the bearing jumping would occur on any length of leadscrew. If the bearing is mounted in a sort of gimble as you have shown and held against the screw thread via a spring, then if you push the leadscrew it will try to rotate (not spin) the bearing about the gimble pivot axis against the spring force, and ratchet the bearing over the screw threads. A bit like a handbrake ratchet on a car where the pawl runs over the teeth against the pressure of a spring.
If you zoom in close to the exact point where the bearing contacts the thread you'd see an inclined plane sliding under the angled face of the bearing. When the inclined plane of the thread moves forward (either you pushing it or the action of the stepper rotating the leadscrew) it will push the bearing upward and away. If it pushes the bearing far enough away it will jump a thread.

Anyway, I was thinking of a way around this and was thinking about a setscrew with a small rubber buffer on the end, but your belleville washer is similar and neater. Hope it all works out because it's a nice idea and will save some cash. I went with 3 ballscrews for the fit-and-forget method, but it was not cheap.

Look forward to your photos . . .

[irving2008]

There's a big thread regarding this idea on CNCZone... seems some have used it successfully and theres a commercial offering based on the idea.

[Jonathan]

There's a big thread regarding this idea on CNCZone... seems some have used it successfully and theres a commercial offering based on the idea.

Yes I spotted that. There's two threads, one quite short one about the method I'm using and one huge thread about roller screws which is another story entirely. I may well try making a roller screw if this method doesn't turn out so well.

These photos wore taken yesterday:
Pretty self explanatory - Z-rails are bolted down and really smooth.


I milled the alu plate square at school, shiny! Never seen such a good finish on aluminium - must be the grade.


Today I've milled the gantry sides flat at either end and drilled them. Also had to nip a few mm off the end of one of the Z rails. I was a bit worried with it being hardened, but it milled OK taking small cuts. Plenty of smoke...

Also started machining the M16 rod for the Y axis.

I'll post some more photos tonight.

routercnc:
I see what you mean with the bearing jumping now - that is a bit of a worry.

[Jonathan]

Some more photos ...

This is how the gantry fits together.


Detail of gantry sides. I've milled them flat etc. Unfortunately I drilled those 5 holes (for M8 bolts) in exactly the same place on both sides, forgetting that the other one is flipped over! Looks like I'm going to have to mill those 5 holes on one side to move them across by 6.5mm...dow!


Just after milling block to fix box section to gantry sides:


It fits!


Pretty accurate too I sawed the plate to roughly 80x80mm then used a 20mm 4 flute endmill to cut it to size. I used 2mm cut depth and 1000mm/min - no problem at all! I'm sure I could have gone faster or cut deeper since it didn't get hot. I then did a single finishing pass taking off 0.2mm at full 20mm depth and 300mm/min.

I'm very pleased with the accuracy seeing as this is with trapezoidal screws and using backlash compensation. I guess the brand new cutter and having set the backlash compensation values to the nearest 0.002mm a few days ago helped.


Shiny :)


Just a shame I didn't measure the box section beforehand and relied on the sellers dimensions. Turns out it's more like 74.6mm, not 74. I think it'll be ok though...

I'll post a vid of that latest bit of milling when I've compiled it.

[Jonathan]

http://www.youtube.com/watch?v=iC7lXNTPYrg

Some progress today. I made another of the above part and assembled the 50kg(!) gantry, then assembled it: