@andrew Just be careful with 6040, not to mention the 6090, they mostly use the same round rods as guides and at that sizes you tend to get a lot of "droop" (sagging), more than 1 mm across the long axis, which is ok if you're using them to carve wood or whatever but is very bad for pcb engraving.

About the endstops, I don't have them and never felt the need for them, a cheap switch has a huge 0.1mm of error which can lead to holes milled between pads, cut traces, etc. What I do is place a hole in the sacrificial layer or the pcb near the edge, later if I need to reset the machine move the gantry manually so that the bit can enter the hole (spindle stopped of course), raise the bit and set zero.

For all of you wondering, just by connecting an Arduino to the parallel port of the control box you can turn any kind of cnc in a grbl machine, you just need the normal Mach 3 pinout: and grbl pinout
and connect the step&dir pins and Z probe

My toolchain is Altium Designer (Schematics->PCB->Gerber/Excellon) -> FlatCAM -> chilipeppr.com/grbl (autolevel->mill)

LE. Don't ask me about stupid taxes in a 3rd world country (Romania), in a small city where the nearest p-channel mosfet is 120miles away Here dhl morons ask $22 only for passing papers to the customs, plus 19% VAT for the whole amount, including the shipping(why since it's already arrived in my country?), plus additional taxes if they can find one to match the content, so I prefer to buy from ebay.co.uk from within EU, even though the shipping is insanely expensive compared to direct Chinese free shipping, that's how I got the CO2 laser, the CNC, the spindle and vacuum pump for it and a few other heavy items. For small items I have no problem to get them through normal post no matter what the cost is.

I tend to stock up on things. I prefer to buy 3 Arduino mega 2560 for $20 from ali and have a bad one than fry a $20 original. By the way I have more than 50 megas now, all good. A lot of 10 arrived malfunctioning, I made a video, got my money back, later I've found they all had a bad bootloader, reflashed and happy ever sice
Ebay is more cumbersome for me, the very long delays in postage can't extend the paypal/ebay protection, on ali you can. Ebay from Europe is an option for large, heavy, expensive items, I got my cnc, vacuum pump, soldering equipment and my laser from a German reseller, but shipping for smaller items from UK/DE to a country like Romania is many times more expensive than the item itself.
Tme&Farnell are both good options, low shipping charges, blazing fast, but they sell only original components, so an Atmega2560 is like $12, add a few dollars for a few resistor, oscillator, shipping...
To buy form ali I usually arrange the listings by order number, then filter by rating, then if the item is more than a dime I read the reviews to make sure they are real. (I left 5* after getting my money back a few times, afterall I haven't lost a dime but the waiting time). On ebay you sometimes get the "sold a bazzillion times" red flag, but you can't sort by nuber of orders, so you can't see the "sheep flock oppinion".

LE. Oh and I keep a spreadsheet of the orders I made so I can track the parcels untill they arrive. I buy in small quantities so I get the free shipping and free customs tax (under 10 Euro). For eg. Buying 30 Arduino megas can be more expensive than 5x 6pcs orders since many sellers on ali apply shipping taxes after a number of components, 1-4pcs free shipping, 5 pcs $1.71 shipping, 6 pcs $2.31 shipping !?! chinese algos involved Also chances of getting a bad lot are smaller. Divide and conquer!

Milled some PCA9615 differential I2C converters for the sensors ouside: magnetometer for the gas meter, temp/hum, baro.
Until now I've used a 7 meter long cable, but whenever the gas water heater fired up the Arduino would just freeze losing the count of gas pulses, I've tried shielded cable but it hasn't solved the issue.
Since Sparkfun's breakout boards are on the wrong side of the pond I decided to make my own.
Really hope the Arduino doesn't lock up anymore.

LE. That TSSOP10 was a b*tch to solder

Put an esp wifi node in the box with a beefier battery, or another box next to the first one if it isn't large enough. There is an "esp bridge" app that can easily do serial monitoring over wifi. That way you should have console output from the node when ot craps out. I've used it to debug some app that controls some contactors via a 5v relay board, turning contactors off sent the w5500 to Valhalla, the debug output via the esp saved the day since the device is located in a cielling a few meters up.

@neverdie Congrats for your result, I use a "copper pour" all over the board, that makes it pass at least 2 times, once for the track and once for the pour isolation. I never mess with flatcam's multiple passes, but I believe you can get the same results. What you see in the middle are copper "silvers" that is copper left behind between traces.
From the last picture of the width test is seems your bit is engraving 0.1mm wide.
Try to use OpenCNCPilot instead of Chillipeppr and set the lines to be split at 1-2mm lengths and probing each 2-3mm. That should make the engraving depth more uniform.
I see a couple copper flakes, maybe go even slower than 100? And give it a light sandpaper with 1000 grit or a scotchbrite sponge and some abrasive detergent for dishes?

@neverdie I believe the boards with a removable Arduino Nano are better, if the uC fries you can replace it without replacing the whole board.

@rfm69 the honest answear: "It depends!"
I've had much better luck at higher rpm. I've even posted a gcode to determine the best feed/rpm in this thread.
To mathematically know for sure you would need a few hundred thousand dollars worth of equipment, engraving pcbs is not milling, so chipload calculators won't work. Lower rpm rips the copper leaving rough edges, too high of a rpm increases runout. As I've said IMHO 24-30k rpm is enough for our machines, you need a far better, stiffer frame, servos instead of steppers and a very good controller board to be able to go over 1000mm/s (400in/s).

PLA smells like hot popcorn/cooking oil, it's corn based afterall
ABS smells bad and it's a little toxic
PETG is almost odourless, very resistent, easy to print, doesn't warp or shrink when cooling

Best slicer: Simplify3D

@crankycoder https://github.com/jeelabs/esp-link

I've used my CNC for milling PCBs for more than a year now.
Here are a few tips:
1st: don't skimp on the machine itself, the 3020 and 3040 are ages ahead of 2018 in terms of quality and reliability (the numbers show the machine size in cm) as the the 3020 and 3040 have no moving bed, but a moving gantry instead, search for the ballscrew ones (normally Z-DQ but also some T-DQ, initially T meant Trapezoidal screws and Z ballscrewZ, but now a lot of T-DQ come with ballscrews). Get the parallel port ones, the usb ones are flaky, you can run an old parallel port (centronix) computer with old WinXP or LinuxCNC or attach an arduino nano/uno (328p) and run GRBL, just attach that to the parallel cable and you're set.
https://www.ebay.co.uk/itm/USB-CNC-ROUTER-ENGRAVER-ENGRAVING-CUTTER-3-AXIS-3040T-DQ-WOODWORKING-3-USB-PORT/282593206065?hash=item41cbde1f31:g:ibsAAOSwC9VZfuV0

2nd: if you buy it stock it comes with a 300-400W aircooled 12,000rpm ER11 spindle, good but not good enough, I've upgraded for a watercooled 300W 60,000rpm spindle (costs almost as much as the machine itself) but I can run it at 1400mm/min and being watercooled it doesn't spread the "deadly" fiberglass dust everywhere.
LE. you can also buy just the frame and buy separately the spindle, 3 nema 23 steppers, 3 tb6560 or 6600 drivers and a 24v or 36v PSU, we are tinkerers after all, aren't we?
https://www.ebay.co.uk/itm/300W-ER8-Spindle-Motor-Water-Cooled-60000rpm-1-5KW-VFD-Inverter-Bracket-Pump-Kit/192035995150?hash=item2cb63cbe0e:g:kz8AAOSw-0xYNoey

3rd: it is said that milling fiberglass boards (FR4) can create very very very small dust ("charf") under 3 microns which when inhaled is never released from the lungs, gets it's way to the pleural membrane where it creates small cysts that can degenerate in lung cancer, a very slow an painful death. So a good vacuum cleaner which exhausts OUTSIDE is a must, unless you buy very high end and expensive professional HEPA filters. I use a Philips HEPA vacuum cleaner, being a hepa means all exhaust is from the back port and no additional airflow is created (my shopvac has dual airflow, one for debris and one to cool the motor, guess what they are not at all isolated between them) and after the exhaust filter I 3d printed a 50mm hose adapter and exhaust it outside. I've wrapped the contraption in a thick garbage bag and sealed it with tape so no air can escape inside.
Another way of doing it would be wet milling using a lubricant as WD40, oil or even liquid soap but might interfere with number 4 below (swell the mdf sacrificial layer).

4th: you have 3 options to fix the blank pcb to the sacrificial layer:
-clamps/screws: easiest, cheapest way, buy you will almost always get a bow in the board, it can be as bad as 0.5mm in a 100x150mm board, especially if you don't want to waste real estate and mill the whole board from side to side, I know you could use a 200x150mm board and leave an inch or two as a border and clamp there but that doubles the costs, I use 148x98mm out of the 150x100mm blank
-double sided tape: use 3M Scotch 665, it handles well, sticks well but is not very hard to remove the board from the mdf sacrificial layer in the end, some other double sided tapes hold so well that you bend the board trying to remove it, or jut let loose and you lose alignment
-vacuum table: I've just bought a vacuum pump but haven't got the time to play with it yet.

5th: ALWAYS use registration holes, put a small known size hole at a known position, if you reset or stop any GRBL controller you lose position, specially when milling small traces 0.1mm out of alignment can make a huge difference

6th: don't be afraid to test feeds and speeds, as other people said buy mills in bulk and sacrifice one of each to make tests, you can learn a bit GCODE and write a simple program to mill a zig-zag pattern at different feedrates (G1 X100 Y10 F300; G1 X0 Y20 F300; G1 X100 Y30 F350; G1 X0 Y 40 F350; etc) then inspect using a magnifier/microscope and settle for the best quality speed. I use titanium coated engraving bits from 0.1mm 10deg for very very small smd traces up to 0.4-0.5mm 30 deg for normal th boards. A 0.4mm 30 deg titanium coated bit can last for up to 10 heavily packed 100x150mm boards as you can see below. At the price it comes ( under $1/piece) it's cheaper to use a new bit then to destroy a blank board, which you will anyway at least a few dozen times
https://www.aliexpress.com/item/10x-Titanium-Coated-Carbide-PCB-Engraving-CNC-Bit-Router-Tool-10-Degree-0-1mm-Tip/1535712782.html?spm=2114.search0104.3.60.32Or0K&ws_ab_test=searchweb0_0,searchweb201602_5_10152_10065_10151_10344_10068_5000016_10345_10342_10343_51102_10340_5060016_10341_5130016_10609_10541_10084_10083_10304_10307_10301_10539_5080015_10312_10059_10313_10314_10534_100031_10604_10603_10103_10605_10594_10596_10142_10107,searchweb201603_14,ppcSwitch_3&algo_expid=e598dd4d-29ef-4a85-af37-7bfefdd92c46-7&algo_pvid=e598dd4d-29ef-4a85-af37-7bfefdd92c46&rmStoreLevelAB=0

7th: after milling sandpaper or use a scothbrite sponge on the board to remove copper edges

Here are some pics of a few boards, I mainly do through hole since I find easier (cheaper) to buy modules from ali than to order the original circuits, none has failed until now
LE. before you ask, the boards are HA light switches, 6 way, rotary encoded, led ring lights, led halo around them (as in car switches), MQTT enabled, w5500 eth connected, 100% designed and home made, 3d printed frame and buttons, laser cut acrylic and faceplates, cnc milled pcbs.

@NeverDie This is the one I've got: https://www.aliexpress.com/item/4000321437538.html?spm=a2g0o.productlist.0.0.3fb4457dTnpLL8&algo_pvid=d91fe6a2-335b-49cf-99fb-e01ae6fc9467&algo_expid=d91fe6a2-335b-49cf-99fb-e01ae6fc9467-3&btsid=0b0a556216043099802122070e932b&ws_ab_test=searchweb0_0,searchweb201602_,searchweb201603_

I've bought it via ebay from an European reseller, faster shipping and no taxes di*king around.

By "tuning" the endmill I mean: take out the collet/nut, blow the dust out, insert the endmill untill the chamfered edge passes through the collet. Insert collet/nut/endmill as it is in the spindle and tighten LIGHTLY!!! Do NOT overtighten! Start spindle, turn to max rpm, if it's noisy stop! take out whole collet/nut/endmill from spindle and rotate the endmill in the collet like 30-45 degrees, reinstall and run test again. If patient enough you can get to 60k rpm without much vibration.
Lately I'm not going over 35-40k rpm, the VFD shows the speed from 0 to 1000 and I stick around 700-750. For drills I run even lower, like 550-600 and don't bother tuning at all.
You can find the sweet spot as I said earlier, turn up the rpm, then slowly decrease until silent. You will find there are rpm ranges where vibration is high and ranges where is low, just use the lowest vibration one while still having a good rpm (find the vibration function poles).

Imagine the in the picture rightmost is 60k rpm, the blue line is the noise/vibration level, as you go to the left (lower rpm).

My 60k rpm is ER8, I guess the lighter collet/nut combo is less prone to vibrate, never seen a decent priced 60k rpm ER11.
ER 11 good is for heavy endmills, big collets, more than 3.175mm (1/8") shaft diameter, maybe 1/4" for table flattening.
The 60k rpm spindles are usually 300W, not a lot of power at lower rpm to do tough milling, so no use for larger endmills, ER8 is a good choice for fine milling with 1/8" shafted endmills.
My VFD is a Sunfar one, pretty decent I might say, I ran it for a few dozen hours now without issues.

PS. The best way to check for runout is the sound test, run the spindle empty, then with collet+nut, then with the endmill inserted. If I "tune" the position of a new endmill I can reach 60k rpm without any vibration at all, so I get almost no runout.

Regarding feedrate vs rpm the last engravings I ran at about 40k rpm, was to lazy to balance the endmill (turn spindle on, if it hums loudly then stop, undo the clamp, rotate 20-30 deg the endmill in the clamp, retighten clamp, rinse, repeat). I just tend to find the poles of harmonics: start spindle at full speed, hums badly, lower rpm slightly less hum, lower even more starts humming again, lower again, even more quiet... so find the sweetspot with least humm (vibration). If you go down enough at about 35-45k rpm you will get the quietest.
For best feedrate I've manually written a simple zig-zag gcode with different speeds: F1000 G1 X20Y5 G1 X0Y10 F1100 G1 X20Y15 G1 X0Y20 F1200... and inspect with the microscope for the best, burr free, engraving speed.
I normally run at 1400mm/min.

@NeverDie How about a dial gauge in a magnetic mount attached to the spindle and measuring against x/y/z frame points, and a simple gcode like g0x100 g0x0 a hundred times? At least that's how I did it to check for lost steps.

@NeverDie my UV lamp is made from a UV led strip glued to a cardboard box, I just place the UV over the board not the board in the lamp.

The so called 1080p cheap projectors are actually 640x480 real resolution, or even 320x240, they are able to accept a 1080p signal and scale it down so are falsely advertised as FullHD. That was the state of things a year ago when I last checked. Much better to get a sh brand projector instead.
About resolution: FullHD 1920x1080 over a 150x100mm board gets you 10pixels/mm roughly, that is 0.1mm resolution, I don't know if that is good enough, also focusing at such a close range would require some lens hacking, from the factory they focus at 50cm at least and get you a 60-100cm diagonal size.

It highly depends on the lead/ball screws, if the tolerances are in range of 0.01mm and backlash at about 0.1mm a 0.X deg of resolution at the steppers/servos doesn't matter since a normal lead screw is 4-8mm/turn.
Even 200 full steps/rev (1.8 deg stepper and full steps) with 4mm/rev screw means 50 full steps/mm or 20 micron/step (0.02mm/step) linear resolution which is much better compared to the rest tolerances of the machine.
On high precision rails, very tightly built machine it might make a difference, but for these cheap routers it doesn't.

LE. @NeverDie 0.27 deg error translates to 0.003mm linear error (3 microns) assuming 4mm/rev screws are being used. Do you think you can notice/measure that? (4mm/360)*0.27=0.003 Even if using HUGE 16mm/rev ball screws translates to 0.012mm of error, unnoticeable.

@NeverDie The UV laminator just gave me the craziest idea: Why don't we take a normal laminator, remove the heating coils and add a strip of UV leds to it??? The expected result: perfect pressing of the artwork to the board and curing at the same time, speed it travels through sets the hardening level. I can get rid of all those glass panels and stuff.
I'm going out hunting for a cheap laminator

@NeverDie for $99.99 I would roll the s#it out of it with my hand roll, heck I would even step on it with my pressure controlled 200 lbs weight in pink slippers.

@NeverDie My printer has a problem with transparencies (jams) and also the high cost for transparencies made me go the paper route, if the toner is facing down the distance is the same as when using transparency printed artwork, also the paper "softens" the uv led light and makes it more uniform I guess.

@NeverDie or just try some canola oil spilled on normal laser printer paper