[N.Constans]

Hi everybody,

I am working on a CNC milling machine since a few months now and I would like some advice from you.
Some technical details :

- Aluminium structure,
- X axis : 220mm movement, 20mm linear rails and blocks, NEMA24 4Nm steppermotor coupled to 1605 ball screw
- Y axis : 400mm movement, 15mm linear rails and blocks, NEMA24 4Nm steppermotor coupled to 1605 ball screw
- Z axis : 140mm movement, 20mm linear rails and blocks, NEMA24 4Nm steppermotor coupled to 1605 ball screw, with counterweight into vertical column to relieve steppermotor.
- HF motor (Teknomotor C3140) 0.75Kw

Actually, I have some doubt regarding sensor to use for each homing : inductive? Mechanical? is 4Nm enough for steppermotor?
Please don't hesitate to give me your feedback ! Thanks

Some screens of it (designed with Solidworks) :

[louieatienza]

I like this design... fully supported bed, just like the big VMCs. 4N-m steppers are fine, if you are using a PSU and drive that can take advantage of them!

I would suggest for extrusions Misumi GFS series, probably the thickest wall t-slot extrusion (6mm) that I've found. Nice design, good luck with the build!

[N.Constans]

Thanks for your comment.

I searched on Misumi site GFS serie but I don't find anything with 6mm t-slot? Do you have more details about this?
Thanks.

[louieatienza]

Thanks for your comment.

I searched on Misumi site GFS serie but I don't find anything with 6mm t-slot? Do you have more details about this?
Thanks.

Sorry if not clear. The WALL thickness is about 6mm, not slot width.

[pippin88]

Consider buying double nut ball screws. Extra cost is not that much. Less backlash.

[N.Constans]

"Consider buying double nut ball screws. Extra cost is not that much. Less backlash."
What do you mean exactly? having nut ball screws for each axis?

[N.Constans]

Consider buying double nut ball screws. Extra cost is not that much. Less backlash.

So if I understand, it's to have 2 nut balls on the same screw?

[louieatienza]

So if I understand, it's to have 2 nut balls on the same screw?

Yes, preloaded against each other

[N.Constans]

Yes, preloaded against each other

the problem is that I will used round flange housing like this :



Is it also possible?

[louieatienza]

the problem is that I will used round flange housing like this :



Is it also possible?

Depending on design, no modification may be needed.

And contrary to what is otherwise said, backlash does matter significantly. Of course a lot depends on machine use. Truing casting surfaces? Maybe not. Jewelry work, PCBs? Maybe. But by far, the biggest problem I see here in this entire forum regarding positioning? The circles are not round. I have minus tolerances on my inside cuts and plus tolerances on my outside cuts. I saw one thread where the guy couldn't grasp this concept and it drove him nuts, to the point of selling his mill.

And no, in a perfect world we'd make all our machines of cast iron and epoxy granite. But we make with what's available to us and what we can work with. And there's absolutely no way to say what is ideal without asking the ultimate use of the machine. For cast iron, stainless, Inconel... maybe not. For aluminium and brass, plastics, PCBs, it's well adequate for the job.

[G59]

Holy. This is poor and unfounded advice, Don't do it. Buy good quality screw and ballnut. Save yourself having to compensate for that extra ballnut. Not worth it one bit. A myth perpetuated by ignorance. You won't end up with a 'Mori' even if you put in the double nuts. I wish people would think a little before posting just anything. Think of the two or three inches you will lose on each axis.

[N.Constans]

Especially that the empty weight of the table is less than 10kg and the effort points are well distributed

[pippin88]

Holy. This is poor and unfounded advice, Don't do it. Buy good quality screw and ballnut. Save yourself having to compensate for that extra ballnut. Not worth it one bit. A myth perpetuated by ignorance. You won't end up with a 'Mori' even if you put in the double nuts. I wish people would think a little before posting just anything. Think of the two or three inches you will lose on each axis.

Look at the spacing of his bearing blocks and the length of his screws. There is plenty of room room for a double ballnut without losing travel.

Which good quality screws would you recommend he buy for this aluminium based mill? Should he buy ground screws?

[N.Constans]

I will order ball nut without backlash - DIN 69051 precision C7.

[N.Constans]

sorry for my failure but what is exactly a precision ground ballscrews?

[pippin88]

I will order ball nut without backlash - DIN 69051 precision C7.

Double ball nuts have two nuts pushed apart (preloaded) to remove backlash.

These are the ones (random seller):
Online Shop Ballscrew 1605 -L 820mm DFU1605 ball screw + one Double ballnut|Aliexpress Mobile

If you are buying Chinese 1605 C7 rolled screws, they will have backlash.

[G59]

Travel is determined by overall rail length minus bearing spacing (or ballscrew travel, if that is shorter).

You forgot, and length of ballnut assembly.

[G59]

My advice, forget the counter balance. Your head isn't very heavy. In this design, unnecessary.
You're worried about backfeed(drop) when power is off? Get a motor with integral brake. So much simpler. Maybe an extra fifty bucks.

I agree with the cheap ball screws having less than desirable accuraccy, but look at the big picture and use of aluminum extrusions, which is far from the material of choice if it really mattered.

There is so much info available on this forum. So many people want advice and if they only bothered to type in the "New Machine Build/Design" into the search bar, they would find answers to all their questions. It's been rehashed over a thousand times. Nothing has changed in mechanical engineering for over 300years. Just look at previous designs/builds.

[G59]

Should he buy ground screws?

Absolutely. Nothing wrong with that. The extra room maybe was intentional???

Maybe the OP wants the extra travel and has designed the machine with those numbers in mind???

Too many projects here go dead by people overlooking a design and right of way dismissing it.

My advice to the OP, build it. Tweak it if your not happy with it. Otherwise, you will give up half way through.

[pippin88]

Absolutely. Nothing wrong with that. The extra room maybe was intentional???

Maybe the OP wants the extra travel and has designed the machine with those numbers in mind???

Too many projects here go dead by people overlooking a design and right of way dismissing it.

My advice to the OP, build it. Tweak it if your not happy with it. Otherwise, you will give up half way through.

Where are new ground screws available for a price in the same league as Chinese 1605 screws?

I said "consider" buying. That means think about it. I didn't say he must, or that the machine won't work without them.

The point of posting design threads is so people get feedback and suggestions. My suggestion was double ball nuts which have lower backlash. Many people spend ages trying to track down and correct backlash. Repacking with larger balls etc. I wish I had bought double nuts for my mill. Maybe he can get the benefit of others experience.

What extra travel? Travel is determined by overall rail length minus bearing spacing (or ballscrew travel, if that is shorter). On his design, the ballscrew travel appears to be longer than the rail travel. 4cm or so longer double nut looks like it won't impact travel.