CNC design for consideration

[blackburn mark]

Ok so Chai has replied with a quote. For the 4 rails, 8 blocks, and three machined ball screw assemblies 529usd shipped?!? That seems awfully cheap to me. Does that sound right? I'm about to rip his arm off.

told you it would be silly not giving him a shout :)

only three screws ?? if you are not using two screws on your x axis id have a good think about the implications before you press buy
that extra screw will more than earn its keep!

your design shows four screws? have you already got one for your Z axis?

[John S]

That sounds OK to me, as I said before he's 110% genuine.

[John S]

Sorry, didn't realise it was illegal for a company to try to make a bit of money.
You could always collect from Chi

[mocha]

You could always collect from Chi

Anyone got an address for Chai? PM please. (EDIT: Thanks, got it now)

[jonbabbz]

Hi guys, been really busy lately so updates will be few and far between. Im collecting my parts together at the moment but just wanted to show you a revised y and z axis on my machine and see what you think. Ive moved the rails above and below and gone for supported on z. Ive tried to make z as light and thin as possible so I can get more movement on y. As for the previous design LotusPack, it is driven from both sides you just cant see it because of the angle of the drawing that's all. I considered going for profile rails but I don't want to spend that amount on a first build. Maybe the next one ;)

[LotusPack]

Hi guys, been really busy lately so updates will be few and far between. Im collecting my parts together at the moment but just wanted to show you a revised y and z axis on my machine and see what you think. Ive moved the rails above and below and gone for supported on z. Ive tried to make z as light and thin as possible so I can get more movement on y. As for the previous design LotusPack, it is driven from both sides you just cant see it because of the angle of the drawing that's all. I considered going for profile rails but I don't want to spend that amount on a first build. Maybe the next one ;)

Hi CNC design firend, this looks really good, have a few points to consider on your design:
1) The Y Axis is very close to the X axis (A), going on the diagram there seems to be little space under the X axis for your work piece, (Work Area my sketch) if the Y axis bearing is level with the work table.
2) The Z axis feed drive plate is working outside the bearing range (C), 4 bearings below the Z leadscrew
3) Can't quite see how the Y axix bearings will work (D) assume there are two bars 1 top and 1 bottom
4) It would be better to increase the X axis bearing distance (B) at each end, the wider the stronger, although you will lose some travel distance.
Attachment 5993

The tool stress area is best kept inside the bearing array, if possible.

hope it goes well...

[Jonathan]

1) The Y Axis is very close to the X axis (A), going on the diagram there seems to be little space under the X axis for your work piece, (Work Area my sketch) if the Y axis bearing is level with the work table.
2) The Z axis feed drive plate is working outside the bearing range (C), 4 bearings below the Z leadscrew
3) Can't quite see how the Y axix bearings will work (D) assume there are two bars 1 top and 1 bottom
4) It would be better to increase the X axis bearing distance (B) at each end, the wider the stronger, although you will lose some travel distance.

1) The bed is well below the X-rails and presumably its height will be adjustable. Having that distance as small as possible is excellent to maintain rigidity.
2) That shouldn't make a difference.
4) Yes it would be better to space the X-bearings out more. What is their current spacing?

[jonbabbz]

So its all ordered then! Very nice guy. He sent me the standard drawings for the machined ends and I had a peruse to see if they would fit my machine and wouldn't. So I simply emailed him my requirements and he has not charged me anything extra! I will definitely do business with him again. The screws I ordered are just for x and y. i need to redesign the z axis because I'm not sure I want to go with round rail. I think there will be too much deflection at the cutter so ill just buy another smaller ballscrew when I'm ready for it. I want to change the bed too. Here's what I ordered screw wise. I'll create a build log when I get all my parts together. Can't wait :)

[LotusPack]

Hi all,

I just wanted maybe a more experienced builder, to have a look at my design and see if there is anything obvious that might hold my design back before I commit. I posted on cnczone and took all comments on a previous design on board and have now come up with this one. Figured it would be more beneficial to be a member of a UK forum. I am already a CNC owner too so not a complete noob, although it would be naive of me to say I'm not still learning.

The goal was to create it as cheap as possible but try and keep rigidity. I've used 12mm multi-start trapezoidal lead screws on x, y and 10mm on z. Fully supported 16mm rails on x and y and 16mm round rail on z. My main concern is the lead screws on y and the 16mm rail. I used that size because it is easier to implement with the size of aluminium profile I'm using. The machine is 1500mm x 1000mm at the rails, ignore the length of z as im going to shorten it anyway. I designed it with a view to implement a fifth axis at a later date. I will be cutting MDF and PLY only.

Any advise would be greatly appreciated. Thanks

Hi The Y axis looks like it is set on one side; if it is, this will jam as the axis drive needs to be central or with two stepper drives simultaneously operated one on each side of the machine. I would also consider widening the Y axis bearings so the machine has less Y wobble. As you are only cutting MDF and Plywood accuracy is not an isse here that design is good enough. If you can squeese a bit more out of your budget, go for the linier bearings as in the photo.
These are far more accurate and rigid and will increase rigidity and production speed. Keeping this design square in the X & Y plane is going to be dificult.

[LotusPack]

1. Ah yes, I see, that sounds good.
2. Ok cool but the further away the Z plate drive point is from the bearings the more compound the tolerance will be on the drive forces. You are right it should not affect the travel accuracy.

The Z axis travel looks quite short compared to the X&Y if you are designing for large surfaces and need to jump up and down the job work plane will this have enough for your need? I originally had 250mm Z distance and have since increased to 400mm for those extra long tools and thicker jobs. My Z axis runs from the collet face at 20mm above the table to 420mm +Z my machine has a fixed 200mm throat under the gantry. This now allows for quite a long tool for example drilling through 200mm you obviously need at least 410mm travel. As the -Z movement starts to put the pressure on when drilling the whole gantry structure starts to go under stress, which is why I advocate centering the spindle in middle of the Y axis bearings, if it is off centre the gantry start to tilt back against all those bearing tolerances and pulls the drill out of square and jams up the drill.

I have a distance of around 400mm between the bearings on a 1.3m Y axis total length so I get an affective length of approx 900mm which is perfect for the standard linear scale I have fitted.

This is a good design as most fixed table machines have the "Throat" restriction under the gantry. How are you going to adjust the table up and down and keep things square? I would think a secondary Z knee adjustment for the work piece height, suggest a manual mechanism that can be locked after part fitting otherwise although flexible just like a knee milling machine, will bring in further rigidity issues.