Here we go again . . . MK4

[routercnc]

Good to know thank you. I'd ordered a solid carbide 3mm drill a few day ago, but these look better still.

[JAZZCNC]

Please Don't get me wrong on what I'm about to say has I'm not pulling down what you have designed, it looks and sure will work great. But This design seems an awful lot of work and complexity and extra cost for no major benifit over a fixed gantry design.!

By this I mean the wide Gantry and wasted space at each end mean it will have about the same foot print and give about the same cutting area of Fixed Gantry.
If used Fixed gantry design with lifting Y axis allowing none lifting Z axis meaning minimal Z axis extension, essentialy just tool length would have been much stronger and far simpler design IMO.

Like I say just an observation which for the sake of others thought I'd mention and not pulling down what you are building or doing. It's looking great sure will work great and i'm looking forward to seeing it come along.

[routercnc]

Hi Dean,

Thank you for the comments. I understand where you are coming from and I agonised and developed this design over the last 6 months or so, including looking at fixed gantry ( I think you are referring to that blue framed fixed gantry on youtube ?). I looked at different parameters trading this and that and trying to work through all the options. I think this is version 14 !

In the end I'm pretty happy overall with this concept and think it will be pretty stiff. I also thought long and hard about the arrangement of bits to make sure slip planes were in the right direction so that bearings were not trapped and pre-loaded, and that it could actually be built in a particular order without leaving impossible joints to make.

I also wanted it to look as neat as possible so a bit of the design is for aesthetic reasons as much as anything (e.g. curved cover plates on Y axis, metal junction boxes where the energy chains end)

I'll keep you all posted, but this is probably a long build so you'll all have to wait to see if it is any good at the end. I intend to do a few simple cuts on the current machine at different feed, speed, DOC,etc and film them before it is decommissioned. Then do the same simple cuts when this one is complete to compare the finish, sound (when does resonance, chatter start kick in) plus measure the static stiffness.

[routercnc]

These arrived recently as an early Christmas present. Another ballscrew for the Y axis, and another pair of 15mm rails and carriages for the Z axis. All the other mechanical and electrical bits will come from the MK3 machine.

I ordered from Fred at BST automation. I should mention how the ordering works for custom orders. Obviously they cannot list every part in every size to I asked for a quote on the lengths I needed. This was a bit less than the website price, which was nice. I was then asked to add various items to the basket which were 'similar' to the parts I wanted. During the checking out I had to add a note to say the actual lengths I wanted, and then wait at the payment stage for Fred to amend the price. I could then confirm the order.

It's a but of a concern that the order shows stock listed parts and lengths, and you have to take it on faith (and Fred's confirmation emails) that the parts you will receive are the ones you asked for.

But I didn't need to worry - everything was exactly the length I'd asked for. They arrived in a cardboard box, not wooden, but it was well packed out.



I immediately checked the ballscrew and it ran dead true to the eye (unlike the previous ones from other sellers which arrived bent). Bearings were a nice fit on the machined ends, and the end machining was nicely done.






Last job was not forgetting to confirm goods received on AliExpress, which then releases the payment to Fred. Note how this is different to ebay. Overall very pleased with my first order from Fred. Would recommend.

[Jonathan]

Are the ballscrew end bearings any good? The cheap ones I've seen, admittedly >2 years ago, all seemed to be quite poorly made as they exhibited end float, unless you fiddle about adding shims or better bearings. Have they improved?

For you ballnut mounts, I hope your spindle is accurately trammed, else the the surface the ballnut flange mounts on may not be perpendicular to the base within sufficient tolerance.

Looking at your design, I'm fairly sure that the weakest point will be the Y-axis. You've compensated for the small bearing spacing causing racking by using two ballscrews, which is good, however the carriage can still twist about the X-axis when a force is applied parallel to Y, due to the vectical compliance of the 20mm rail bearings. Assuming you got medium pre-load bearings (I hope not zero), their vertical stiffness will be about 274N/um. The cutter is central, so if we assume the force is distributed equally upon the Y bearigs, but in opposite directions, you'll get twice that stiffness as the bearings act in pairs (I think), so 548N/um. Bearing spacing looks like 78mm and about 200mm to the tool tip from the bearings in the Z direction. This means that the deflection at the bearings is amplified at the tool tip by a factor of 200/78 = 2.56. The stiffness of the bearings is therefore effectively reduced by the same factor, so 548/2.56=206N/um. Hmm, too be fair that's a fair bit higher that the ballscrew stiffness (I estimate ~50N/um for the two in parallel) so maybe it wont be dominant.

EDIT: Just noticed that's you've put the ballscrews on the ends of the X and Y carriages, not at the center (of stiffness), so the stiffness will be a bit worse.

[routercnc]

Hi Jonathan,

Bearings:
I've not opened the bearing packets yet, but I'm expecting to have to make up a shim like I did last time. I've been able to make them work on the current machine so am not too concerned.

Ballnut mounts:
The spindle is as true as I can get it - note I'm at the mercy of the flatness of the supported rail on X, and on the flatness of the extrusion for the Y rail. Based on things I've made from the MK3 machine I think it will be OK. If not there are ways and means . . .

Stiffness:
Double Y ballscrew design went in at the outset because of the reasons you mention - it cannot rack so you can push the bearings closer together. But you are right, and it bothered me for ages, that for loads in the Y direction, where it rotates around X, there is a moment between the cutting tool (reaction) and the ballscrew (input) which is resolved in the bearings. As they are close together this moment is higher, meaning more deflection.
Ways around this are to lower the ballscrew and reduce the moment - I tried between the box sections but this makes the gantry even wider fore/aft. I tried underneath, which is very good for forces, but it would get swarf on it and if it hit a clamp it would be nasty. So it went on top. The mori-seki is like that so that sealed it.


p.s. What did you mean here?
". . .Just noticed that's you've put the ballscrews on the ends of the X and Y carriages, not at the center (of stiffness), so the stiffness will be a bit worse. . . "

[Jonathan]

Bearings: Ok, thanks for the info - I'll continue to give those a miss.
Mounts: Sounds reasonable.
Stiffness: Understood - I wouldn't say Mori-Seki doing something one way is any reason to copy, as the relevance of their reasoning is unknown. Also I'm not sure my calculation in the previous post is any good, as the bearings are close they probably can't reasonably be considered as points, plus I ignored the position of the ballnut.
P.S: See here, from page 53.

[JAZZCNC]

Are the ballscrew end bearings any good? The cheap ones I've seen, admittedly >2 years ago, all seemed to be quite poorly made as they exhibited end float, unless you fiddle about adding shims or better bearings. Have they improved?

Jonathan the ones I've been getting from Fred are excellent far far superior to those things from Chai in every way. Finish quality and operation, they don't need a thing doing to them. They come sealed in Bags and lubed up inside a box and are light years better.!

[gojones]

The major benefit of this design is the ability to machine a workpiece that is way larger than you could fit in a conventional fixed gantry mill. You could place one end of a 20' beam on the bed and machine one end, or even machine the entire length in stages.

[routercnc]

Since I have an air compressor . . . I decided it was about time !