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CNC Build #1: DIY CNC - phase 1 : planning

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Blog entry by loupitou06 posted 08-19-2020 01:28 AM 1076 reads 0 times favorited 3 comments Add to Favorites Watch
no previous part Part 1 of CNC Build series Part 2: DIY CNC - phase 1: planning, continued »

Hi fellow Jocks,

Let me preface this blog series by saying I am by no mean an expert in CNC, I own and operate a Tormach and build a CNC router from scratch (see this post : https://www.lumberjocks.com/projects/413158 ).

But I picked up some useful tips along my journey that I think could benefit the community so take this blog for what it is – just a hobbyist CNCer’s opinion.

So phase 1 of any CNC build is the planning:
1. What do you plan to do with your CNC ?
2. How big of a machine do you need ?
3. What is your budget ?

If budget is no object, then you can skip 1. and 2., buy a pro level CNC all assembled and go to town.
As a matter of fact I’ll try to give you some insight of what a DIY CNC build will cost you and it may be worst buying a CNC kit or even a smaller commercial machine.

But if you have budget constrains and if you are curious and passionate then it make sense to go the DIY route, you will learn a lot about CNC along the way and be able to troubleshoot your machine easily.

The size of your CNC is very important for 2 reasons:
1. it will dictate your work
2. the price of your CNC will dramatically increase with the machinable area or work envelope of your machine.

Let’s break down the components of a CNC machine:
1. The frame/base
2. The linear rails and motion transmission in X, Y and Z
3. The gantry
4. The motors and drivers
5. A spindle
6. The controller and software

Let’s discuss several options for each components, starting with

1. The Frame

The Frame must be sturdy, the cutting forces of a CNC machine are substantial (more on that later) and therefore the frame must absorb the reactive forces of the cutting process.
Large, industrial machines rely on cast iron frames and base:

pro-entry level machines are often based on 8020 extrusions

and hobbyist machines are usually made of what is available on hand.
In my case I already had an outfeed table that looked sturdy (torsion box construction showed here https://www.lumberjocks.com/projects/348778)

The frame is often overlooked in a CNC construction, and mine was no different, I enjoy the flat surface with precision holes for workholding but I regret building it too light and not being able to access the back of the work surface.

One more thing to consider when building your frame is your work holding strategy: most hobbyist will rely an a waste board and screws/clams and superglue even. Pro-level machines usually come with a vacuum system and that is something you want to be build in your frame/base from the beginning if you are going that route.

2. Linear motion.

So the good news about building a DIY CNC machine is that fundamentally it is a machine composed of 3 independent axis (X, Y and Z) and you can build each axis one at a time.
The precision of your machine will depend in first order on the choices you make for the linear motion components. In a nutshell:

Linear rails (MGN) > linear guides (SBR)> wheels on extrusions > wheels on angles > nothing

The other good news is that linear rails are more and more affordable if you have the patience to order import (aliexpress, ebay,...). The bad new is that of course the price will increase faster than the size.

For example, I paid 835$ back in 2018 for a HGH20 rails and 1605 ballscrews for my cnc and my work area is about 5 by 3 ft and a Z height of over 12in.

You can spend less (or more of course) if you choose less rigid rails but keep in mind the larger your work envelope the more critical rigidity become. For instance I choose to limit by X axis to 1600mm (~5ft) because linear screws longer than 4ft start to oscillate with movements.

3. The Gantry

In most CNC router designs, the work is fixed on the table/frame and the gantry moved the spindle above the work. This is done to allow large and heavy objects on the table (slabs anyone ?). Interestingly enough, in most metal CNC machines, the stock is held on the table that moves in X & Y (and A & B,...) and the spindle only moves in Z, this is done primarily to increase the rigidity to be able to cut hard materials.

Keep that in mind when you ask yourself “what will I be able to do with my machine”: in short you can cut pretty much any piece of wood, aluminum and brass – the rest will be a challenge.

The design of the gantry is pretty simple but you must keep in mind a few principles:
1. keep the height of the gantry as small as possible
2. keep the spindle as close as possible to the Y axis to minimize the moment of the cutting forces.
3. Keep the gantry rigid is your priority while trying to keep the weight down

Most designs use some form of aluminum extrusion for the best compromise of weight/rigidity and straightness. My CNC is no different, I used both a 40-8016 and 40-8080 because I wanted to keep the 12” Z range. In retrospect this was a mistake, my gantry is very heavy because of all that (and the sides) and as a consequence my cutting speeds are low as well. Keep the gantry light !

side note: I am blessed to have a CNC in order to design all the brackets and other components used to join the gantry together – this is something you will have to account for in your budget if you go the DIY route.

I paid ~400$ with shipping for these extrusions – but you really only need one so ~250$ with shipment but expect to spend ~200$ for the rest of the gantry parts (less if you machine them of course).

Next post we will discuss motors, controller, spindle, wiring and other ancillary costs.

-- 100 fois sur le metier remettez votre ouvrage



3 comments so far

View JimYoung's profile

JimYoung

426 posts in 2830 days


#1 posted 08-19-2020 12:37 PM

Following along…

-- -Jim, "Nothing says poor craftsmanship more than wrinkles in your duck tape"

View sras's profile

sras

6346 posts in 4372 days


#2 posted 08-20-2020 12:52 AM

This should be interesting!

Your first project link has a few extra characters after the “8” – you can edit that and fix it if you wish.

-- Steve - Impatience is Expensive

View LittleBlackDuck's profile

LittleBlackDuck

8097 posts in 2064 days


#3 posted 08-20-2020 07:01 AM

I’m ready for the haul!

-- If your first cut is too short... Take the second cut from the longer end... LBD

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