Basic mechanical engineering questions for new build

[schoeftinger]

Hi guys,

I've been lurking around in this fine forum for a couple of months.
When I'm not reading this forum, I usually spend a lot of time building guitars.

And that's what I'd like to build a DIY CNC router for. - The goal is to be capable of routing wood (anything form low to high density) and certain metals such as brass and aluminium with precise enough for guitar necks, bodies and hardware.
Work space should be around 140x80x20cm...

I'm good with electronics, software, cad, and planning BUT I have only very little experience in mechanical construction using metals.
Also I'm not sure if/how I can provide the necessary precision for working with metal.

What tools do you deem necessary for building such a CNC?
How do manage to work accurate enough (e.g. when drilling an endless series of holes for screwing rails)?
Does it make sense financially, to "outsource" the mechanical part to more skilled/better equipped people?

Thanks for any hints!
-Tom

[mactec54]

schoeftinger
Does it make sense financially, to "outsource" the mechanical part to more skilled/better equipped people?

Yes it does make sence sometimes
If you want to build a precise & accurate well built machine, & you don't have the machines to do it, then it would be better to outsource that part of the build

If you draw up the parts you need made, get prices, from your local shops,when the parts are made, inspect the work/parts they have made for you, to make sure it is all to you drawing/spec

Take a look at my build this may give you some idea's as well http://www.cnczone.com/forums/cnc_wo...ld_friend.html

[jsheerin]

Instead of outsourcing custom designed parts, most people buy components that are already designed and available such as bearings and blocks from cncrouterparts.com. Or if you want higher grade stuff, you can buy bearing blocks, ball screws and linear bearings on ebay from Chinese vendors such as linearmotionbearings2008 and others. This lowers your cost as the setup costs are spread over a much larger number of people.

[schoeftinger]

Well, thanks for the infos!

I got a decent drill press (a heavy one) and a lot of hand tools.
Most of my electric tools were made for wood-working.
However, I don't think I have the capability of precisely cutting metal.

The thing, I have most respect for is to build a gantry from solid 2cm Aluminium and drilling 20 holes over a distance of 100cm dead straight in line for screwing the Y-axis rails to it...

I've been asking around at local hardware and machine builders/companies.
Their prices are exorbitant...
My estimation is around 2000 EUR just for cutting, sizing, and drilling the parts...

-Tom

[ger21]

By clamping a fence to your drill press table, and clamping the work to the fence, it's relatively easy to get all your holes in a straight line.

Aluminum can be cut with most woodworking tools. Miter saws, and table saws with sliding cut off tables have no problems cutting extrusions and smaller plates.

Larger pieces can be rough cut with a jigsaw, and the edges cleaned up with a router and straightedge.

[dkirtley]

You can achieve some pretty good precision just with a fine scribe, files, and punches. It has worked for hundreds of years. With experience building guitars, you have more than sufficient skills to work with metal. Just a few minor differences in the tools and the cutting speed.

There are good reasons for screws not to be in perfect alignment anyway. Holes in a straight line is a method used to *weaken* materials. That is how perforations make things break off at a pre-determined place. Staggered holes are pretty standard for structural drilling for most things.

Some of the drilling matters, much doesn't. It only matters that the holes align for the fastener. Just clamp things together and drill them at the same time. Many things are slotted or drilled oversize instead of just a single straight hole to allow movement for alignment.

Generally, the basic tools for cold working metal are: a square, a scribe, some dividers, calipers and a good rule for measuring, a drill with an assortment of drill bits, a set of files, a hammer, and saws for sawing straight and for curves. It's not that complicated.

[schoeftinger]

Thanks again, for encouraging me!
Yes - building a guitar requires some accuracy, but I guess precision is even more crucial when building a CNC router.

I guess I'll never find out unless I try building one!
This cries for a build thread...

Do you have any recommendations on where to get materials (heavy Alu profiles, Alu plates) at decent prices in Europe?

Thanks,
-Tom

[dkirtley]

No idea about stuff in Europe. I am down in the southernmost part of Texas in the US.

If your goal is to cut metal as well, you might as well think about doing steel structure instead of aluminum. A good place to start would be to find places that do repairs of car exhausts and other welding. Many of those places might have people that are available to do some small tasks in their spare time between jobs. Take plenty of beer with you. That usually helps

[schoeftinger]

Originally Posted by dkirtley No idea about stuff in Europe. I am down in the southernmost part of Texas in the US. If your goal is to cut metal as well, you might as well think about doing steel structure instead of aluminum. A good place to start would be to find places that do repairs of car exhausts and other welding. Many of those places might have people that are available to do some small tasks in their spare time between jobs. Take plenty of beer with you. That usually helps

Material - good topic!

I'd build the bottom frame/table from alu profiles - preferably heavy ones.
I'm open to any input on the gantry material.
The only limitation is that I can find rails and wagons (e.g. SBC Profilschienen) at a decent price which are able to carry the gantry.

I figured aluminium would be a good choice because it's light.
Using something heaver, such as steel, would require stronger steppers or a smaller thread pitch, IMO.

However, I assume that heavier means less vibrations and - ultimately - more precision, right?


Regarding the booze:
If I take lots of beer with me, there's usually little left for the others.

cheers,
-Tom

[dkirtley]

Mass on the stuff that doesn't move is your friend.
Mass on the gantry is your enemy.

If you are going to be cutting metal, you want a solid and distributed support. Each time the cutter comes in contact with metal, it acts much like a hammer blow. If it is too malleable (soft and bending when hit with a hammer) it will deform if the force is not distributed over an area so that the force is below the strength that will move the metal. Aluminum is usable for some stuff but for strength and wear resistance, steel is much better. Just a bit different to cut. You can build strong and light things from heavy materials. You can build heavy crap with light materials. They all have their good points and weaknesses. As an example, look at bicycle frames. You can have a strong light frame from steel, aluminum, titanium, or carbon fiber. Even wood.

It doesn't only have to be just thick rectangular tubings and angle iron. There are many structural shapes available. Take an afternoon and go talk to a welder. They will know what is available in your area. They might even have left over pieces that meet your needs. A lot of this stuff comes in large sections and they end up having to buy a 7m piece when they only need 4m as an example. They would much rather get their money back selling it than having to store it and watch it rust. I again really advise going to a place that does auto exhausts and mufflers. That is where I have seen people with some of the best welding skills. They constantly deal with intricate stuff. Places that work on trailers and garage doors are also good sources. Also, don't overlook the places that make things with sheet metal. Many shapes that would otherwise cost a lot of money can be bent quickly with sheet metal tools.

I realize that this is awfully vague but from half a world away it is hard to get specific.

[schoeftinger]

Originally Posted by dkirtley Mass on the stuff that doesn't move is your friend. Mass on the gantry is your enemy.

I get that idea, but wouldn't it be good if the gantry had a certain minimum mass in order to avoid vibrations?

Originally Posted by dkirtley If you are going to be cutting metal, you want a solid and distributed support. Each time the cutter comes in contact with metal, it acts much like a hammer blow. If it is too malleable (soft and bending when hit with a hammer) it will deform if the force is not distributed over an area so that the force is below the strength that will move the metal. Aluminum is usable for some stuff but for strength and wear resistance, steel is much better. Just a bit different to cut. You can build strong and light things from heavy materials. You can build heavy crap with light materials. They all have their good points and weaknesses. As an example, look at bicycle frames. You can have a strong light frame from steel, aluminum, titanium, or carbon fiber. Even wood.

I'd use alu profiles (e.g. http://62.75.219.46/mod_show_image.p....jpg&width=270) for the cutting table because they come with flutes and matching nuts.
This should simplify srewing the y-axis rails to them.
So far, I couldn't find any similar profile systems from steel!?

I'm still not sure on the gantry material.
Some commercial CNCs I've seen are using 12mm aluminimum (cut from sheets) for the gantry.
If I went steel, how thick would the gantry material have to be?

Originally Posted by dkirtley It doesn't only have to be just thick rectangular tubings and angle iron. There are many structural shapes available. Take an afternoon and go talk to a welder. They will know what is available in your area. They might even have left over pieces that meet your needs. A lot of this stuff comes in large sections and they end up having to buy a 7m piece when they only need 4m as an example. They would much rather get their money back selling it than having to store it and watch it rust. I again really advise going to a place that does auto exhausts and mufflers. That is where I have seen people with some of the best welding skills. They constantly deal with intricate stuff. Places that work on trailers and garage doors are also good sources. Also, don't overlook the places that make things with sheet metal. Many shapes that would otherwise cost a lot of money can be bent quickly with sheet metal tools.

I'll start asking around as soon as I have a concrete plan which materials to use.
Right now, you got me thinking about steel vs. aluminium...

Originally Posted by dkirtley I realize that this is awfully vague but from half a world away it is hard to get specific.

Ha - good one!

Thanks!
-Tom

[dkirtley]

There will be plenty of mass just with the required structure. For the most part, adding mass only changes the frequency of the vibrations. You stop vibrations by not moving.

My own personal opinion is that that those types of extrusions are not rigid enough for supporting the workpiece for cutting metal by themselves. I wouldn't be happy with it for cutting wood. Some people build with them and are happy. We all work to our own tolerances.

I think you are too worried about the difficulty of drilling holes.

Much of the gantry could easily be built rigidly with sheet in the range of 1mm - 2mm if supported properly in steel or aluminum without getting fancy with the design. Car bodies are made with thinner material. You just need some structure to bolt the rails onto.