Designing a not too heavy CNC for home use, some questions.

[EthanCNC]

I have one of those cheap belt driven small cnc machines. While I like the quiet nature (when cutting very soft materials like foam), I don't like the excessive flex etc.

I wanted to design then build my own machine.

First my constraints:
1. I don't have access to power cutting tools. I can cut aluminum extrusions but only by hand tools which will not be very accurate.
2. Because of the first constraint I would like build the frame with mostly aluminum extrusions that I can buy online.
3. I will be ok 3d printing some of the parts. I don't think 3d printed parts has the rigidity or durability of metal ones so I am planning to go for a hybrid design, that is reinforcing the 3d printed parts with aliuminum flat bars etc.
4. The size I am aiming for is close to or larger than 800x800mm. While I am not planning to cut any metals, I would like to build a machine rigid enough to be able to cut aluminum at slow speeds.
5 I would like to have silent motion (not much can be done about cutting but still.)


I will have many questions but I want to start with Z axis for my first post.

The cheap cnc's z axis is a v-wheel setup on an X axis which is 40x20 extrusion backed. Wheels are attached to acrylic plates. They seem to switch all aluminum for the new version of the same machine with a 6020 extrusion for the x axis which seems to be much more stiff. Needless to say I would like to have a significantly better Z axis.

I am planning to go for at least an 8040 C beam for the X axis and I am hoping that would help with eh Z axis flex. For the Z axis though I am trying to decide if I should go for a traditional rod setup, linear rails/box bearings or a c-beam setup like the openbuilds.

I can see that linear rails with sliding bearings will have a thinner profile and they are said to be more precise and rigid but I was wondering if they really make a significant difference for the Z axis. Moreover can we say that even the cheap linear rails/block bearings are better than the other options? I can but linear rails 12/15 size and attach it on a 3d printed plate but do you think the cheaper options are any good?

Attaching several pictures from machines being sold as examples and I will appreciate any comments.

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[EthanCNC]

Maybe I should ask another clearer question.

I was searching for the available linear rails. It seems 12mm rails used on 3d printers are widely available for shorter sizes.

For longer sizes, 1000mm+ it is interesting to see that HGR20 and similar rails, guides with ball screws are available for very good prices while 15mm and less rails are less common and more expensive.

So:
1. Does it make much difference to build Z axis from 12mm vs 20mm. Obviously 20mm will be much more stiff etc but it is going to be heavy and I will need to enlargen the plate to house the rails. I was thinking of using an 8020 extrusion as base and 12mm rails wih 8mm leadscrew on top as shown in my drawing here. But for $50 I can get 20 mm rails with hgr20 guides of length 250-300mm which is a hell of a deal.

2. I can move the rails on the sides, I wonder if it would have any drawbacks?

3. I am amazed that I can go ball screws with linear rails for such good prices, my concern is ball screws may be a bit too heavy and noisy. I was wondering if going for a belt setup, say 6-10mm while still using 20mm rails with hgr20 sliders makes sense?

[peteeng]

Hello Ethan,
Your aspiration to cut aluminium is the Holy Grail of many people wanting a small machine. The type of machine you describe will not cut aluminium like a mill and its not about going slow. I suggest you do not use aluminium construction extrusions they are not actually very stiff and they have limits in how you can connect them. I suggest you make a plywood machine. There's lots of examples around and if you really get to know the material you will be able to cut aluminium. Look up the Frankenrouter thread or plywood cncs in this forum. Don't be fooled, good plywood is a very good material to build a machine from. Stiff, damp, easy to change and easy to fabricate. Glue and screw your connections and it will be a very good machine. Peter

re your direct questions:
1) plastic is about 3MPa stiffness so is not suitable for structural parts on such a router eg aluminium is 70GPa and plywood is 17mpa so go plywood
2) 20mm rail is considerably stiffer than its smaller cousins. Size does matter in a CNC so always pick the largest component that does the job (thats in the budget)
3) I have built routers with 10mm and 16mm belts and would not build with less then 25mm these days. You will find that 12 or 16mm ballscrews will be cheaper and 10X stiffer then a belt system. Ballscrews are not noisy. I like belts but on the size machine you are talking about ballscrews will be hard to beat on all points
4) see plywood machine attached as an example and Frankenrouter the machine I'm building at the moment is mostly ply. You can do all of this with a jigsaw and hand tools and some patience. Better then construction extrusions. Can be aligned and set with epoxy very accurately, cant do that with al exts....
5) I would not use V wheels they require near constant adjustment. If budget is an issue use round rail. Your first machine has to be simple and with square rail its easy to make something that jams. Round rail is more forgiving for a first timer...
6) do not buy anything until you have resolved your design to its conclusion. All machine parts are connected and once you buy something you have fixed that variable. Keep the design on paper (CAD) make the best design you can then go shopping for the best bits that are what you designed... Good Luck your on a step slippery curve

[peteeng]

Heres Frankie - my current build. The base is formply and the gantry will be ply. With timber it depends on mass. Plywood is 700kg/m3 and aluminium is 2700 so you can use 4x the thickness of aluminium before you get to the same weight. At 4x the thickness its locally stiffer then the aluminium...and damper. Solid parts are damper and locally stiffer then tubular parts especially compared to AL construction extrusions,. You can use timber screws or normal screws. You can use PVA or epoxy to set the screws and they will never come out. Lots of possibilities with plywood and a hand router that can't be solved with extrusions, you will find these things out as your design progresses. Peter

[EthanCNC]

Pete, thank you, very nice advice.

Maybe I should clarify, my main purpose is not to cut aluminum but to be able to cut simpler shapes if I need to.

You are right plywood is much stiffer per weight. However, per the same volume I think 3d printed parts from pla and petg can be just as stiff.

I will focus on building all the rails from 20mm ones as you suggested.

While extrusions are weaker than solid aluiminum bars, I see that the thickness of aluminum used for lighter machines are in the 1/4-1/8" range, don't you think 20x xtrusions will be stronger than those?

I also see quite a few , simple extrusion+printed parts cnc that "can" cut aluminum such as this one , what you think would be the problems with such a build?

Thank you

[peteeng]

Hi Ethan - cutting aluminium is cutting aluminium period. Lightweight routers cut AL by using very small depth of cuts and going around many times. If your happy with this then all's good. But you will wear out cutters fast, your 400W spindle will have lots of run out so the edge quality is poor and its small shaft will break. Been there done that.

Sorry your 3D printed plastic parts will never be as stiff as plywood, volume or weight irrespective. Its not about strength machine design is all about stiffness and rigidity every component has to be maximised in stiffness to make a good machine. It also is about your expectations. If you expect your flimsy machine to machine aluminium like a 1 tonne mill then you will be disappointed. If your happy to take small cuts and lots of them then that's fine. You will learn as you go.

You as the designer have to commit to a certain medium. eg welded steel construction, bolted construction extrusions, plywood, aluminium plate, epoxy concrete, UHPC etc etc. each medium has pros and cons that you either learn about by doing or reading threads such as here. Each medium has advantages and disadvantages. No one medium is perfect. Beware taking the convenient path as this will always have negative consequences sometimes unknown till you cut something then your stuck with it.... Figure out the BEST path and best solution for the machine (not you)

eg 3d printing plastic parts - very convenient but as stiff as old chewing gum... great for electronic boxes and dust boots... keep at it Peter

[EthanCNC]

Hi Ethan - cutting aluminium is cutting aluminium period. Lightweight routers cut AL by using very small depth of cuts and going around many times. If your happy with this then all's good. But you will wear out cutters fast, your 400W spindle will have lots of run out so the edge quality is poor and its small shaft will break. Been there done that.

Sorry your 3D printed plastic parts will never be as stiff as plywood, volume or weight irrespective. Its not about strength machine design is all about stiffness and rigidity every component has to be maximized in stiffness to make a good machine. It also is about your expectations. If you expect your flimsy machine to machine aluminium like a 1 tonne mill then you will be disappointed. If your happy to take small cuts and lots of them then that's fine. You will learn as you go.

You as the designer have to commit to a certain medium. eg welded steel construction, bolted construction extrusions, plywood, aluminium plate, epoxy concrete, UHPC etc etc. each medium has pros and cons that you either learn about by doing or reading threads such as here. Each medium has advantages and disadvantages. No one medium is perfect. Beware taking the convenient path as this will always have negative consequences sometimes unknown till you cut something then your stuck with it.... Figure out the BEST path and best solution for the machine (not you)

eg 3d printing plastic parts - very convenient but as stiff as old chewing gum... great for electronic boxes and dust boots... keep at it Peter

Hi Peter,

I agree with all of your points and learning from you. Yeah they are as stiff as a chewing gum at times .

I should be aiming for shallow passes at reasonably slow speeds for aluminum. I am planning to use a Makita router.

For me unfortunately working with plywood is a bit harder atm compared to 3d printing and cutting/drilling aluminum extrusions since the latter is easier to do with a reasonable precision. I tried a couple of pieces printed with petg. Glued a 1/16" aluminum flat bar vertically in once piece and tried to bend them with pliers. The one without the bar easily deformed the other one always bounced back to the original shape and flexed much less. I think I will give this method a try to make router holder plates, side arms etc. I guess as you said it is about committing a medium and trying to design while keeping the strengths/weaknesses in mind. I may also try to replace the printed parts with machined aluminum(or plywood) parts after building the machine.


That being said I wonder what you would consider as a good cutting speed for aluminum? The machine in this video doesn't seem as good as many derlin wheel/aluminum extrusion machines. While it is all aluminum, the side plates seems to be maybe 1/4" at best and the derlin wheels which are not far apart doesn't inspire confidence in me. However, the guy claims that the can cut aluminum(even steel) at 0.8mm depth per pass around 800mm/minute which I would consider fine for my needs. What would you consider a good feedrate, plunge rate, doc etc while having a good finish for aluminum?

[peteeng]

Hi Ethan - Feed rates depend on tooling and machine rigidity. If you look at tool suppliers' sites, they will have cutting parameters specified for the tools. The issue with routers like a Makita is that it will be too fast for the feed speed of your machine. You will need to use single flute tools and fast feeds to get the correct chip load. There's lots of discussion in threads here that cover this. Your on several learning curves and you'll have to digest lots of info fast... Sure people publish videos of cutting steel and alum with their routers but can they do it all day every day and can they afford the wear rate of the tools? These things you will find out in good time. Peter

to explain the Makita if you have it say at 12000rpm then you have to have a feed rate of around 6m/min to acheive a clean chip load with a 6mm tool. Going slow means you will be rubbing the tool vs making chips so the tool is spinning, skidding and every so often cutting. Research chip loading and chip thickness the math is pretty easy...

[EthanCNC]

Hi Ethan - Feed rates depend on tooling and machine rigidity. If you look at tool suppliers' sites, they will have cutting parameters specified for the tools. The issue with routers like a Makita is that it will be too fast for the feed speed of your machine. You will need to use single flute tools and fast feeds to get the correct chip load. There's lots of discussion in threads here that cover this. Your on several learning curves and you'll have to digest lots of info fast... Sure people publish videos of cutting steel and alum with their routers but can they do it all day every day and can they afford the wear rate of the tools? These things you will find out in good time. Peter

to explain the Makita if you have it say at 12000rpm then you have to have a feed rate of around 6m/min to acheive a clean chip load with a 6mm tool. Going slow means you will be rubbing the tool vs making chips so the tool is spinning, skidding and every so often cutting. Research chip loading and chip thickness the math is pretty easy...

Hi Pete,
Again good points. Yes I don't see those machines repeating clean cuts for a long time.

I do know about chip loading. I actually adjusted it pretty nicely for foam on my current machine (The Cheap spindle is not really capable of cutting anything but foam at a decent speed). I use
1/8", 2 flute downcut bit, around 8000-9000rpm at a speed of 1500mm/minute giving a chipload around 0.003-0.004 which is very good for foam. My depth of cut was 6mm for eps 3mm for xps foam, clean edges, no dust just large chips.
Unfortunately I can't even cut thin plywood at a decent chipload. I went 10000rpm, 0.5mm depth and 800mm/min which is a dust fest, noisy and most likely wearing out the bit. So I decided not to until I build a better machine.

For makita I was hoping to stay around 10000rpm (the lowest setting) and go for around 1m/min with a single flute cutter which would be around 0.004" chipload so this is too small in your experience? I guess I may need to significantly adjust my expectations.

Maybe you can share the tool diameter, flute count, doc, rpm and feed rates.

[peteeng]

Hi Ethan- download the Gurhring catalogue and look at their tools. All the feeds and speeds, chip loads are noted. You will see how fast the feed has to be to get the chip- load right. This is proportional so you slow down the feed or speed up the spindle to get where you need to be for the tool. Often with a router you can't get to the ideal point. 0.004" is 0.10mm which is too thick. Looking at the charts its more like 0.01mm for most aluminium tools.

https://www.guhring.com.au/downloads/downloads.html

to achieve these figures (they are intended for mills) you may not have enough spindle torque to cut the chip, you may have too much runout so the tooth leading edge is wobbling around so can't start the cut and skids, your machine may not be stiff enough and the tool tip is being pushed away from the billet surface, with aluminium it picks up and blunts the tip with plaque then really picks up and gums up entirely. The issue list goes on and you will have to solve every one of them... I don't cut aluminium enough on my router to comment. If I need a good part done I email a file to a machinist and have them do it.... But with my new spindle I want to delve into AL more... Peter