BigAl's 4x8 CNC machine...

[mikeschn]

Hmmm... sorry about that, I posted to the wrong thread...

This is the machine I am thinking about...

http://www.cnczone.com/forums/attachmen ... cf1068_jpg[/img]

http://www.cnczone.com/forums/joes_cnc_ ... post738877

Mike...

[IASCOTT]

If you want to build a real work horse of a cnc router MechMate is the one to build. The X,Y, and Z axises are all rack and pinion driven. It can be built any size you want. The plans are free and the forum has lots of advice on the building of it.

http://www.mechmate.com/forums/index.php

I've just touch the surface of what is in the forum and my some of the builds shows real quality. You do have to join the forum to see the pictures and be able to download the free plans. They are in a pdf format and come in 5 separate downloads. They are located at the bottom of the forum in the downloads section.

Scott

[BigAl]

@Mike, Thanks for the 4x4 links, I will watch them over until I have noticed all the ingenuity.

@Scott, The MechMate is a thing of beauty. I'm going to download the plans to see what is going on underneath that exterior. Thanks for bringing it to my attention.

[Cliffmeister2000]

MechMate videos

http://www.youtube.com/watch?v=IGcQ-d2ymz8

http://www.youtube.com/watch?v=XwT_d--KCVg&NR=1

[mikeschn]

So the question is, what is the cheapest you can build a 4x8 CNC for?

Of course, it may not make sense for me to build a large CNC router, because I would only use it to route all the pieces for my next Winter Warrior. If the CNC machine costs $1000 to build, for example, that would buy me a lot of time on a real cnc machine!!!

Of course, I may decide to build one, just to build one, and then use it to cut parts for everyone else...

The last time I looked, the biggest expense was in the lead screws. What would a 9 foot lead screw cost now?

Mike...

[chorizon]

The last time I looked, the biggest expense was in the lead screws. What would a 9 foot lead screw cost now?

Mike...

Mike, There are a few on McMaster's website.

They are not ball-screws. But claimed accuracy is within .009"/ft. That doesn't seem unreasonable at all considering it will be used for routing wood.

Plus, they are all around $100-$200 for a 12 foot long piece (for 1" thru 1-1/2" diameters).

I can't ever seem to link to Mcmaster's site, so I can't post a link to a particular page, but a couple of clicks on the drill-down search brought me to several excellent candidates.

[mikeschn]

thanks Joshua... that seems affordable...

Meanwhile, does anyone have any more information on chain drives...?

How would one figure the IPM... ? Here I've include the standard calculations for a standard leadscrew...

source: http://www.probotix.com/stepper_motor_calculations/


Stepper Motor Calculations

IPM = (Frequency * 60) / (Turns Per Inch * Steps Per Revolution * Microsteps)

Configuring software steps per unit:

This variable stores how many steps to take to move the X axis 1 inch. You will need to set this as accurately as possible if you want your machine to be accurate. There are two ways to set it:

1. Move and Measure - slap a pen or marker on as a tool head and draw a 1000 step line. Measure it and divide 1000 by the length in inches.
2. Calculate Step Size - this one is the preferred way of doing things. Its rather easy to calculate step size based on your drive mechanism.

For threaded rod type drive systems:

Find your TPI (Turns Per Inch). for example, 1/4"-20 threaded rod means that there are 20 threads per inch single start (aka 20 turns = 1 inch.) Simply take that number and multiply it by the steps in a revolution. With a 400 step motor (200 steps per revolution at half step), it would be 8000 steps per inch.

For belt/pulley systems:

1. Find the circumference of your drive pulley. (remember circumference = 2*pi*r) (say: 2.75")
2. Calculate step size (ie: circumference / steps per revolution) (say: 2.75" / 400 = 0.00625")
3. Divide 1 inch by step size (1" / 0.00625" = 160 steps/inch)

Steps & Frequency
Step Angle: 1.8°
Physical Steps Per Revolution: 200 = 360° in a revolution ÷ 1.8° step angle
Micro-step Mode: 8th
Effective Steps Per Revolution: 1600 = 8 microsteps per step x 200 physical steps per revolution
RPM (Revolutions Per Minute): 1000
Steps Per Minute: 3200000 1600 effective steps per revolution x 2000 revolutions per minute
Hz (cycles per second): 26666.67 = 3200000 steps per minute ÷ 60 seconds in a minute
kHz: 26.7 Hz ÷ 1000

IPM
Lead Screw Threads Per Inch: 10
Number of Starts: 2
Turns Per Inch: 5 10 Threads Per Inch ÷ 2 start, so it takes 5 revolutions to move 1 inch
IPM (Inches Per Minute): 200 = 1000 RPM ÷ 5 Revs Per Inch

[BigAl]

Sounds like you are working up to build one of these Mike?

There is an online pulley ratio here.

[chorizon]

thanks Joshua... that seems affordable...

Meanwhile, does anyone have any more information on chain drives...?

How would one figure the IPM... ? Here I've include the standard calculations for a standard leadscrew...

Chain is sized with respect to it's "pitch".

The pitch is the nominal distance between the center of the rivets on the chain.

Therefore, if you have a 12T drive-sprocket driving a 1/4" pitch chain, one revolution would yield a linear movement of 3".

You can then use that figure in your normal IPM calculation.

For CNC application I'd consider supporting the chain with a guide where possible.

I'd also take into consideration the momentum of the moving assembly in the driven axis with respect to how fast it should run.

Upon accel/decel and direction-changeover the chain will stretch or compress.

Obviously with a 1000RPM stepper motor and even a small sprocket you could really move, but the machine might shake itself apart!

I've been kicking around the idea of building one of these, but I'd probably use my first build as a spring-board for learning and then build a 4 or 5 axis machine.

[mikeschn]

I've built a 24x36 cnc, which ended up not having any rigidity.

Then I built up a Fireball V90, which was considerably smaller, but it's very rigid, and it's also very quick.

4 ft x 8 ft stuff I've never thought about before. But I'd hate to build another machine without any rigidity. But I'm also not going to spend $3000 to buy the BlackToe kit... So I've got some studying to do!!!

If I can do a mostly wood machine, I figure that'll keep costs down...

Mike...

[BigAl]

I started yesterday morning, and by this evening I have the X-Axis complete and have started on the Y-Axis.



Usually I would have been further into a project such as this, but I am measuring and re-measuring everything, drilling pilot holes with a drill press, etc,. I figure every ten minutes spent rechecking saves one hour trying to correct mistakes later.

I am building a variant of the JGRO router table.

The X-Axis is quite accurate. It deviates about 1mm in 1000mm. The main bearing runs along steel square section. It is very rigid against pitch and roll, but I noticed a LOT of yaw, so I retrofitted a secondary "anti-yaw" bearing under the gantry base. Unfortunately it is mostly hidden in the photo.

[mikeschn]

That doesn't look like a 4x8 machine...

Mike...

[BigAl]

Yes, I'm trying out my joinery skills on something mid-sized that I will use to cut the parts for my larger table.

I read an interesting blog post by that guy at buildyourcnc.com who recommended building what he called an ugly machine to build the pretty one.

I should be able to reuse all the expensive bits.

[mikeschn]

Yes, I'm trying out my joinery skills

Okay, we can let you get away with that..

But you need to encourage us with some 4x8 ideas while you are playing.

So, what's your idea of the day?

Mike...

[BigAl]

I have finished the ugly medium sized CNC table. I am actually glad I built it, having learned a few more things along the way from this my second CNC build.

This one is actually much more accurate and powerful than my smaller table top. I am able to mill (in one pass) 2mm deep at 5mm/sec with a 3.2mm straight mill end in MDF.

The JGRO design is excellent!

A big improvement came from replacing my 700w rotozip type cutter with a cheapo 1020w router. The extra power sure does help it munch through the wood.

Another big improvement over my table top CNC mill is the software toolchain. I still do my CAD in Corel Draw 8 but now use CamBam for the CAM(on Windows XP) and EMC2 as the control software(on Linux Ubuntu). Two really great apps!

Now I need to restart the design of the larger CNC table capable of cutting a 4x8 sheet.

Will take a photo if I ever find the camera again.