Design Help Pt2 Required for CNC design/Build

[MikeyC38]

Hi All,

Firstly many thanks to Jonathan and John S for their kind help and advice, for HiltonSteve for his inspired build log and drawings and all you others who have contributed build logs, posts and to routercnc for the fantastic spreadsheets!!. I have now bought my ballscrews, linear rails and bearings from Chai on Ebay. They turned up on time, and everything is straight and not bent. A really nice guy and fantastic value. Thanks John S for that one and the source for the stepper drivers.

Attached are various CAD diagrams for which I would appreciate your comments. You know the old adage, measure twice cut once! The ballscrew sizes are X - 1100mm, Y - 750mm and Z 300mm with 20mm supported round rails on the X and 16mm everywhere else. I plan on using Nema 23 steppers at 3.1Nm with 36V ps at 11A. The main purpose is to (mill or rout John S -:) )wood for guitar bodies , but also have the capability to do light 6060 aluminum milling. The extrusions are 40x80 (heavy 4kg/m) sections with 15mm toolplate for the gantry sizes and bed ends. The Y axis is 120x40 (heavy 8kg/m) extrusion. Z axis is 10mm toolplate. Router will be Kress 1050FME-1 with a planned upgrade to one of the 2.2Kw VFD Chineese spindles once I've got more funds. The drawings don't show the ballscrews, steppers, router mounting plate or the plate to join the gantry sides as yet - after all it is Christmas! but they will be in the ususal positions.

Comments please?

Kind Regards
Mike

[JAZZCNC]

I can make a few suggestions that you might consider having built a few machines very similiar for the same job and actually have one part built which I just don't seem to find time to finish.! I'll attach pics of it.

Looking at the drawings I presume you intended using twin screws.? If so then I would put a cross brace between the gantrys sides running under the bed, this will help make the gantry more ridged but it also then gives you the possibilty to run the ballscrews under the bed out off harmsway rather than exposed on the outside it also offers some protection from dust and rubbish.
On the machine I build the motors under there as well connected to the screws with belts. Other than the protection advantage the belts also allow geariing if needed and reduce the chance of resonance, but the main reason in my design was to keep the foot print as small as possible because the original machine was to live in a typical 8 x4 garden shed/workshop so space was precious. As you will see from the pics the Y axis motor/screw setup was inverted the same and the Z axis motor flipped to keep the height down so it could sit on a bench without hitting the roof, there are no components sticking out side the initial footprint.
I've built several versions both with single screws and twin screws, some with belts as mentioned, some with direct drive and motors on outside when space saving doesn't matter as it does ease the build cost slighlty slightly. Thats why some pics will look differant to others.

Regards the largish kink'd back gantry side, thou not a problem as such it's more material related.! . . Depending on the material width you intend to use you may want to check that you can actually get that much kink out the width's.? Unless you intend to use one large and expensive sheet then it could be tight using STD width's from the likes of Alu warehouse etc.? Most stop at 300mm width.!

Regards the Z axis I would run the rails on the Front plate and put the bearings on the backplate, this will help strengthen the front plate slightly. I would also recommend you at least beef up the Z axis with min 19mm plate. Remember it doesn't matter how well the rest of the machine is built if the Z axis flexs then accurecy goes out the window.!

Hope this helps and happy building.

[MikeyC38]

Thanks Jazzcnc for taking the time to review this. The photos are really useful. My drawings are incomplete at this stage as I will be using a single ballscrew on the long axis as per your photo with a plate underneath between the gantry sides . But the tip on adding the extra reinforcement on the long axis is taken and accepted. HiltonSteve's router/mill also had this. It also solves the problem of mounting the BF/BK blocks very elegantly and protects the ballscrew from muck! Many thanks!!

Again the comment on the material thickness on the Z axis is accepted and you given me some food for thought on the gantry side plate lay back. My original idea is to get the cg of the Z axis between the bearings on the side plate, but as you say well, cost of getting the plate for the gantry sides has to be taken into account!

I note your comment about reversing the position of the Z axis linear slides and bearings - I see in your photos you are using Hwin/THK slides which have better torque load characteristics than Supported round rail and linear slides. I'll have a think about that one.

Measure twice cut once is the adage!!!

Kind Regards
Mike

[John S]

You can buy plate cut to size in any size, not limited to a specific width only from certain suppliers.
If the design is though about you could nest the two sides like spoons and cut wastage to a minimum.

Worth giving these people a call.

http://www.ascmetalsgroup.com/northampton.html

[JAZZCNC]

I spent some time experimenting with cutting MDF templates for the best layback and what you see in the pics is the outcome. I found it gives the best balance and can still be got from 12" x 3/4" plate from Alu warehouse with the minimum waste. Thou not exactly on centre of bearings it's pritty close.

Regards the Z axis I really would consider putting the rails on the front.! It does help regardsless of rail type used.
Remember the Z axis when extended is basicly a lever and a weak lever will bend and flex.!!
Like I allways tell folks.!!. . The machine is only has good as it's weakist part or component.!!. . . Unfortunatly I see lots make the mistake of building weak or inaccurate Z axis's. Which makes the rest of the machine no matter how well built or quality components used pointless because at the end of the day it's the Z axis which is at the sharp end taking the strain.

[JAZZCNC]

You can buy plate cut to size in any size, not limited to a specific width only from certain suppliers.
If the design is though about you could nest the two sides like spoons and cut wastage to a minimum.

Worth giving these people a call.

http://www.ascmetalsgroup.com/northampton.html

Agree John but to be honest I've always found it works out cheaper for small Qty's buying from AW. It seems unless your buying from them on a regular basis as business in decent Qty they load up the price.?

Mike. . . Something I meant to say and partly touched upon before with my material comment and I'm sure others will agree or have found out them selfs while building.?
You have to be very carefull in the design stages that you don't plan on using or design something that hisn't easily available. It's often the simple things that can be a right pain in the arse to source or may not even be available in the intended size or length. IE Timing belts lengths often catch folks out.

[MikeyC38]

Regards the Z axis I really would consider putting the rails on the front.! It does help regardsless of rail type used.
Remember the Z axis when extended is basicly a lever and a weak lever will bend and flex.!!
Like I allways tell folks.!!. . The machine is only has good as it's weakist part or component.!!. . . Unfortunatly I see lots make the mistake of building weak or inaccurate Z axis's. Which makes the rest of the machine no matter how well built or quality components used pointless because at the end of the day it's the Z axis which is at the sharp end taking the strain.

Good reasoning there JazzCNC...the placing the supported round rails on the moving part of the Z axis will add stiffness to that axis...

[Jonathan]

About the Z-axis... It is foolish to do anything other than what Jazz has suggested, namely putting the rails on the front plate. With the rails on the front plate the distance from the tool to the support (i.e. the bearings) is some constant (overhang of spindle plus length of tool) plus how far down the Z-axis is. So the minimum is the constant, and the maximum is the travel plus that constant. Whereas with rails the other way round the overhang is always the same constant plus the travel - i.e. it's always at the worst case scenario for the other orientation!

I don't really see the point of going above 20mm on the Z-axis, the gain is absolutely minuscule compared to the added cost (especially when you're using just 16mm round rails) as I'm sure you'll have found from routercnc's spreadsheet. I used 20mm for this reason.

Aluminium warehouse sell plate cut to size which is a good choice, as unlike the flat bar, it should be flat! It's a little more expensive.

http://www.aluminiumwarehouse.co.uk/cutting_calc.php

However, I urge you to consider not having gantry sides and using the style, like mine and Jazz's bigger machine where the cutter operates below the level of the X-axis rails. That eliminates the flexing of the gantry sides, as there aren't any, and puts the X-axis ballnut(s) in a much more optimal location in terms of the forces they receive.

Y-axis of 750mm sounds a bit much for just one ballscrew on X, especially considering you want to cut aluminium.

[John S]

Agree John but to be honest I've always found it works out cheaper for small Qty's buying from AW. It seems unless your buying from them on a regular basis as business in decent Qty they load up the price.?

Jazz,
To be honest I don't buy large quantities but buy just what I need unless it's small section and then it doesn't pay to get a bar cut.
I have to rely on their delivery service as my nearest branch is Lincoln, delivery is a reasonable £7.50 no matter what but their rep tells me that if I'm ever in the area to call in and take advantage of offcuts they have lying around.

Not going to happen with me but others who are not so tied to a workshop all working hours could score on this.

[JAZZCNC]

However, I urge you to consider not having gantry sides and using the style, like mine and Jazz's bigger machine where the cutter operates below the level of the X-axis rails. That eliminates the flexing of the gantry sides, as there aren't any, and puts the X-axis ballnut(s) in a much more optimal location in terms of the forces they receive.

Y-axis of 750mm sounds a bit much for just one ballscrew on X, especially considering you want to cut aluminium.

Seen as Jonathan agree'd with me I'll agree with him. . Lol . . . . Thou Ive actually built the same type machine as shown, it was with the specific intension of cutting just Wood which it does very very well and not to disagree with jonathan it also cutts Ali ok.
That said If building an allround machine then I'd go with what Jonathan says and actually I will be building just such a machine over the next few months ounce I've got other commitments out the way.
I've attached the first ruff models thou it may change in dimensions before it's built with ballscrews covered and rear of gantry enclosed along with Cable chain etc. The bed will be adjustable because a forth axis will be needed at some point also allows for milling vises etc.

Edit: Oh by the way It's going to sit on a bench not floor mounted.!