New Machine Build- My first desktop CNC

[ma1]

Hello folks,

I have finally got enough parts and settled on a design to build my small desktop CNC machine. The build has recently started. I am utilizing my
desktop milling machine to drill/cut accurate holes/locations.
I am sorry that I can't post pictures yet until i can find a suitable and
free hosting site (hopefully very soon).


Here were my criteria for the router:
- Mainly for PCB prototyping, possible thin aluminum/plastic front panels
- Spend less than $75-100 on mechanicals
- Overall size of the CNC can be ~18"x12"x12", with ~8"x4"x3" cutting volume
- Not looking for a lot of power and speed, but rather accuracy on the order of 0.01" for small circuit components
- Use as much parts as possible from HomeDepot/Lowes
- Fixed gantry

1) I had the ?ingenius? idea of using ball-bearing heavy duty drawer slides for smooth linear motion. I have bought and tested the 100lb duty, ball bearing full-extension drawer slides, and they don't have any play in the vertical direction, but have some play out of the plane (which can easily be constrained).

Liberty 14-1/100 in. Full Extension Drawer Slides (2-Pack) at The Home Depot
$12+tax

This is for the x-axis (long side). For the y and z axes, I have broken into some old scanners and got some 8mm and 10mm linear shafts to go along
with homemade delrin sleeves.


2) The frame will be constructed of 3/4 MDF (4x8 sheet=$10 from HomeDepot), and the x-axis tray will be (3/4 In. x 7.5 In. x 48 In. RubberMaid White Melamine Shelf). Dados and steel construction L-brackets (from Homedepot again) will be utilized for best structural stability.


3) For the screws, my initial trial with a 1/4 rod and a coupling nut and hardwood ended in the realization that i need a thicker diameter rod, and a different base material to make sure warping is minimum. The first stage whipped slightly as it was moving due to the 10"-cut threaded rod being bent. (However, the screw/nut combination was very smooth without any lubrication). Next, I purchased:

GP acme 3/8-12, 36" single start, standard steel screw+ brass nut = $10+tax+ship at Mcmaster

The brass nut has very little to no backlash to start with, but I am constructing a hybrid brass+delrin nut to eliminate all backlash. The lead
screw had a very slight bend to it, which was surprising to me at this
thickness. Maybe I should have got a 1/2"-10 ?

As a side note, the deal with multiple starts some people may be ignoring:
5 start 1/2"-10 screw is like having a 1/2"-2 screw in terms of speed and
accuracy. The only difference is more contact area with the nut, giving more load bearing capabilities. It is good for a metal-milling machine, but in my case, I rather have the high precision then speed/load bearing.
With a 200 steps/turn motor and 1/8 microstepping, the 5-start screw will yield at best 0.0003" accuracy.
The single-start screw would give 0.00006" accuracy with the same
electronics.


4) The bearings were purchased from Mcmaster (open style 3/8 ID steel ball-bearings, made in USA, not rated, $4 each)

((Home depot also sells pure steel ball-bearings under the disguise of door rollers (but i didn't end up using these):
Prime-Line Products 1-1/4 in. Steel Ball Bearing Rollers (2-Pack) - D 1502 at The Home Depot))

I will not machine the ends of the ACME screws, but rather use electrical tape and fit them to the bearings (there is very small play without tape). This will save me a lot of headache - and that i don't have a lathe.

5) I am planning to use a Black/Decker $30, 2A dremel for the spindle. I have a 1/8 shaft carbide endmill for initial test tries.


I hope to update as much as possible, especially the pictures. Stay tuned

ma1

[DonFrambach]

Looking forward to seeing your build. You know you can upload pictures directly to this site. Click on the little paperclip icon on the top of the window used to enter text like this.

[judleroy]

As you mentioned the accuracy of your different screw options i will mention a few flaws with your reasoning. You can spend 100,000 dollars on a Vmc and at best get a true machine accuracy of around .0002. The .0003 accuracy of the multi start screws will yeild higher speeds.The accuracy you can actually ever expect of the type of machine your building based on your materials list is maybe between .001-.005 at best. As far as I know there have only been machines around that can hold a tolerance of 5 decimals for a few years. The price of those machines... Wow.
Judleroy

[ma1]

DonFrambach, thanks for the tip.

Here are some pictures showing the overall concept, the drawer slide that is used, and the initial stages of construction.

As you can tell, I am going to use tee-nuts for securing metal brackets/holders to mdf.

[ma1]

And here is a simple technique to make backlash free nuts:

First slide a rectangular acme nut onto acme screw, then slide a second one
and make it tight against the first (first picture). At this point,
the screw will be locked and will not rotate.

Now, rotate the second nut a small angle counterclockwise, until
the sides square up. Make sure the screw rotates freely through
both nuts (second picture).

Now, if you can machine a rectangular cutout from a
delrin/aluminum/mdf block for the nuts to fit very tight, this will
be a mostly backlash free setup.

[sirkossorg]

"I will not machine the ends of the ACME screws, but rather use electrical tape and fit them to the bearings (there is very small play without tape). This will save me a lot of headache - and that i don't have a lathe."



I'm a huge fan of the DIY, hardware-store build. That's how mine started, and now it's become more mainstream as I've found weaknesses that are easier to buy your way out of with conventional cnc assemblies than to keep messing with it.

From the been-there, tried-that school: You may want to try something other than tape to get rid of the play in the bearing. I did that, and the tape wears out more quickly than you might think. Once it does, you get wobble in your system and it tends to start jacking up your slides. I went to using a strip of aluminum foil, which worked a bit better. I've since gone to properly-fitting shafts and bearings because it's just not worth the trouble, in my opinion.

FWIW,

Chris

[ma1]

I have been unable to find much time to work on this unfortunately, here is the latest status:

x and y axis are finished, motorized and tested with Mach3. The drawer slides work !! There is very little whip in the leadscrews (3/8") due to non-straightness, not much I can do about it probably. The work table is also finished. only thing remaining is z axis, and dremel attachment. Please look at pictures below.

One thing that was not straightforward is how do I type some text in Lazycam and have it carve in Mach3? I tried a couple of times, but the G-code that was generated only had one arc in it instead of the full text. I might switch to EMC2, maybe it is easier to use. Any good FREE cam programs out there that can convert dwg, dxf and/or step files into G-code?

[lamesh99]

great build! i will sure be keeping an eye on this one, maybe steal some ideas
tell me one thing, the MDF you are using, you wrote that 4x8 plate would cost you $10 at home depot. is this 4x8 feet or something else? because where i live MDF of the same thickness, 20mm, around 130x200cm (cca 4x6.5ft) costs minimum around $70...

one more thing, the leadscrew you used, >>GP acme 3/8-12, 36" single start, standard steel screw+ brass nut<<, can you tell me the size in mm? i mean, outer diameter of the rod, etc...

[ma1]

Originally Posted by lamesh99 great build! i will sure be keeping an eye on this one, maybe steal some ideas tell me one thing, the MDF you are using, you wrote that 4x8 plate would cost you $10 at home depot. is this 4x8 feet or something else? because where i live MDF of the same thickness, 20mm, around 130x200cm (cca 4x6.5ft) costs minimum around $70... one more thing, the leadscrew you used, >>GP acme 3/8-12, 36" single start, standard steel screw+ brass nut<<, can you tell me the size in mm? i mean, outer diameter of the rod, etc...

I realized that I made a typo in the original post, the MDF size is smaller.The MDF sheet at homedepot is 2 feet x 4 feet x 0.75 feet for $11. It is probably cheap MDF, not the highest quality, but works for my purpose. The router is very solid with the tight fitting design, wood glue and the steel supports. You might want to check furniture builders in your area for left over MDF pieces that might be cheap. I have attached a photo of how i segmented the 2x4 mdf (all numbers are inches).

The screw diameter is 3/8" = 9.5 mm. I suggest that you use a bigger screw, since mine was slightly warped. Thicker screws should be more straight. it has 12 turns per 1" movement. If you want your machine to move faster, you need less turns per inch, or multiple start screw.

good luck, feel free to copy ideas

[ma1]

Hello again after a long break,

I wanted to give updates on the machine, and ask some questions in the meantime.

The x-axis is swapped to a supported-shaft design from the drawer slides. There was some wobble in the x-axis due to the leadscrew not being straight, or some small alignment problems. The drawer slides were strong enough to overcome this, but needed very good tensioning inside the plane. Instead, I saw a great sale on 20mm supported linear shafts on Glacern Machine Tools website, and pulled the plug. 2 480mm long shafts plus 4 open pillow blocks were less than $150 with shipping (Glacern Machine Tools - Linear Rails and Bearings). Highly satisfied with these.

I have finished the z-axis and the router mount, and did my first test cut today! I used a 1/8" solid carbide endmill in the BlackDecker rotary tool collet. The wood is scrap, most probably pine. The cut went okay (see pictures below), however I notice a small zigzag pattern in the y-axis straight line trials (last picture). I can visually see the y-axis wobbling as I move it without any load. I tried to pre-tension the gantry to one side around the y-rotational axis, but that didn't help much. The shaking corresponds with the lead-screw whipping a pretty good amount. I also noticed the whipping is so strong that it moves the inside of the ball bearing that is mounted close to the motor (or the motor if I loosen the motor screws). I replaced the screw with another one of the same batch, and still the same result. I checked the screw on my work table as I rotate it, and it doesn't look severely warped? Alignment looks pretty good when I look through the hole of the ball bearing. I machined the MDF and aluminum plates in an accurate milling machine to <0.002" precision.

I can't figure out what's going on. Only four things that I can think of (but unlikely to cause this much problem):

1- The y-rails are unsupported, 0.25" diameter and 12" long
2- The brass acme nut is on one side of the y-gantry (not centered)
3- Some miniscule alignment issues
4- Ball bearings busted somehow? They were brand new when I put it in.
They are rated to 350 lbs dynamic load (McMaster-Carr)


Does anyone have any suggestion on how to fix the y-axis wobble? I appreciate all the suggestions.


**Legend to images:
1)x,y, and z axis shown
2)y-axis motor side , homemade delrin coupling and ball bearing visible in black aluminum plate
3)y-axis other side, ball bearing visible in black aluminum plate
4)x-axis motor side , homemade delrin coupling and ball bearing visible in black aluminum plate
5)bunch of roadrunner booboos and cuts, and straight line trials
6)zoom into straight line trials showing y-axis wobble zigzag cuts (horizontal line)

[PaulRowntree]

1- The y-rails are unsupported, 0.25" diameter and 12" long

I would have thought that this was far too light a support, and would lead to flexing along all axes.

It looks like the T-nuts are not brought home into the MDF in photos1,2 & 3. I am not sure what they are securing, but they seem to be standing almost on their points in some cases, which can't be good for stability or strength.
Cheers!

[louieatienza]

Like PaulRowntree said, the 1/4" unsupported shafts might not be adequate enough. Even the MakerBot uses 3/8" shafting, and it's a 3D printer! Especially when cutting some wood, which has varying densities and hardnesses across the grain. Those Glacern shafts would work great for the Y as well, and should be an easy retrofit.

Also realize that with 12tpi leadscrew, you have to spin them relatively fast to get a speed good enough for cutting wood, which could magnify the screw whip. You could run slower, but then your spindle may not spin slow enough or have enough "torque" to cut well at low rpms.

You should be able to align your drive nut relatively easily, by bringing it all the way to one side and disconnecting it from the carriage; you'll see where it wants to sit. You could then slot the mounting holes or shim/shape teh nut block so that it lines up correctly.