Frequently Asked Questions

Printed Parts

Minimum printer size required to print the MPCNC parts- X&Y – (XYZ turned) 135mmx135mm on a Cartesian, 160mm Diameter on a delta, min. Z– (toolmount) 120mm min. You can add these parts to your slicer and verify if it works with your own printer.

M=Mirrored parts- Some printers can’t do low overhang angles well or don’t print well in general, so there is a mirrored version of the rollers.The mirrored version is to prevent the gantry from twisting with cruddy prints. If the angle is on the same side of the rail there is no twist, if it is on opposite sides there is 2x’s the twist (print error). The mirrored version of other parts is for functionality, corner top and bottom. Other parts are mirrored for symmetry, feet, locks.

 

Rails

-Steel or stainless steel only, aluminum will not work for long (high point loads from the bearings will wear in a flat spot making it a loose fit) and it is about 3 times less rigid. The LowRider CNC requires Stainless rails.

-We use either steel EMT conduit (sold by ID), or stainless steel tubes (sold by OD. 0.049″ wall thickness seems to be the sweet spot Q&D test) on the MPCNC.

-0.065″ stainless is fine, or thicker, if the nut traps will need to be modified to fit in your tubes or you can print custom ones.

-Before you start see what rails size is available to you. Physically go measure, it will save a lot of headaches in the end.

-Anything over 36″ should use mid span supports on the outer rails on the MPCNC.

-Maximum length for the z axis on the MPCNC- about 12″ using a bowden style setup with the extruder mounted to the xyz piece of the gantry, 8″ max for an mk style extruder, 3″ for an aluminum router.

-How big to cut your rails? MPCNC Calculators… LowRider, Zen.

Please do not attach anything to the Z axis assembly above the tool mount.  You will see lots of images of people attaching cable chains, wires, and vacuum hoses to the Z motor mount. This is the absolute worst place on the entire machine to attach anything! It will act as a torque multiplier and completely ruin your accuracy/precision. Anything necessary should be attached to the main gantry assembly lots of room on the XYZ piece. With 100% certainly anything attached to your gantry that is taller than the upper xyz part will have a noticeable and detrimental effect. This also includes things that bolt to the XYZ and then have a higher attachment point (Extension Bracket), same effect.

 

Z-Axis –

-Stock Z axis max speed is 8.5mm/s (30mm/s for the T8, firmware limited), I usually use 3-5mm/s for routing jobs with an 8mm/s rapid.

 

Getting it as square as possible –

-Before you start your cut on the MPCNC, measure each roller to its closest corner block, take the same measurement on its counterpart. You should be able to tweak these by hand to get all axis equal to its counterpart at the same time before you start your cut. Once you start your cut they are locked in place. Dual endstops can do this for you with the cost being more CAM planning. If you can’t get them set well enough see the next FAQ.

Gantry not Square enough -This is starting to come up more not that people are starting to want greater tolerances. If you find you just can’t get the center assembly square enough for your needs try this. Assemble the machine and use it for a while, plot some things, do some test cuts for feeds and speeds for materials you plan on using. This seems to break it in, stretch the printed parts, whatever it is doing.  After a few hours use, take the gantry out or at least take off the belts. Make sure the z axis is still parallel, and rebuild the center by taking out the 7 longest bolts then put it back together. Most find the second time things work much better. Check it at each step for square. Checking the tension on the middle bearings by rolling it with your finger can reveal a lot about unequal tension.

 

 

Power Supply

-5A is way more than enough for the CNC, it is even enough to add an extruder, 30A is only needed if you are running a heated bed. There is no added benefit to running the CNC with a larger power supply.

Wiring

 

Tolerances to expect –

-Everything affects the accuracy/precision/resolution. Theoretical resolution is 0.005mm @16th stepping, do not expect that in real life.

  • Physical size of the machine and each axis, smaller is better, Z axis is most critical.
  • How well you built it.
  • The rigidity of your table.
  • The rigidity of your rails used.
  • The accuracy/run out of your tool/spindle.
  • The accuracy/run out of your cutting bit and are you using the right kind.
  • How well you clamped your material.
  • What material you are using.
  • Knowledge of your Cam software and how you approach your cuts.
  • And the big ones, feeds and speeds.

– Here are a few links so you have an understanding of some of the numbers that get thrown around, Protomold is using I believe large Haas mills, top tier software, and trained machinists. They say “In general, tolerances of ±0.005 in. (0.13mm) are expected” from here. The tiny Hass is bare bones at $65,000 to try and put things in perspective. I chose protomold because you can see the actual cost, example.

– Here is an idea of what each added zero cost in your measurements. Rough is .030″ (.762mm). link

– Wood can change size by up to 4% seasonally due to humidity (1m = up to  40mm change), so be mindful of what you expect out of wood. A 500mm piece of Aluminum grows or shrinks by about 0.1mm per 5 degrees change in Celsius.

– Last one to drive it home. The calipers you use are only accurate to 1 decimal place less than they display. So if your calipers have 2 decimal places you are only really certain of the first decimal place, the second decimal place has been rounded up or down. If you measure something at 10.53mm it is 10.53mm+/-.01mm so .02mm accuracy for digital calipers (for really good ones, not the $10 HF ones).

 

More Power-

-Nema 23’s are no longer supported. They are not needed. The MPCNC uses 2 steppers in the X and Y axis and ran great with small 42OZ/IN steppers wired in parallel. The kits, currently, come with 72OZ/IN steppers wired in series.

-Nema 23 is a size designation, 2.3inches. It has nothing to do with strength. They typically are stronger but not always. saying “I want 23’s for strength”, means nothing.

 

Spindles-

-A more powerful spindle isn’t always better. The MPCNC has been designed for something roughly the physical size and weight of the Dewalt dw660. Physically larger spindles move the cutter further from the gantry decreasing the rigidity. Heavier means slower accelerations in all axis, on all machines.

-A Dremel style tool usually is not a good option. They tend to have high run out, low power, and high price tag. Most people already have one and they do work, I understand this, but don’t expect stellar results, it is a good option to get your feet wet with a minimal initial expense if you already own one.

-Good international options are kress, and the import 400-800W spindles.

-The DeWalt is by far the best bang for your buck where available, but I am not positive it is the best choice if money were not an option. The dewalt is 600W and about $65 comes with 1/8 and 1/4″ collets, it does not have a speed control and it is a good option at about $20, so $85 total. Unfortunately I have not had the luxury of trying one but I think the import spindles might be a really nice option they have an er11 collet system so any size tools you want, most come with a huge assortment of collet sizes. You can get the 300, 500, or 800 W spindles with power supply and speed control they are a bit smaller making the sit closer to the gantry meaning slight rigidity increase, but they do weigh more. They are more than double the price, I kind of doubt they are twice as good. Diminishing returns kicks in really fast in the CNC world.

-Gcode control of your AC powered spindle is possible. The two easiest ways right now are an off the shelf plug, or the control module in the shop (that can be used with or without PID control).

 

CAD Files

-I do not have any CAD files available other than a few mounts that makes it easy for anyone to edit. I do this intentionally. I hope no one takes this the wrong way but the parts are easy to replicate. If you can replicate the parts from scratch you can probably make an informed edit and understand how the pieces interact and how to keep them printable as well. This means people who are making new parts should be making pretty high quality parts, no one wants to have to try several versions to figure out what actually works, right?

-If you need a dimension, please ask I am not hiding anything, but I am not going to supply you with technical drawings. I am trying to keep poorly made things from popping up everywhere. It happened in the first few months of the release. I have seen nothing but well designed parts coming out lately and I think this is best for everyone. I love user designed things, and I support them as much as I can.

-Some parts could be better or stronger with added hardware, or the use of support when printing, this is not my design intent. I am trying to keep the cost down and the world wide usability at a maximum. Specialty hardware is a pet peeve.

 

Shipping –

– I know shipping prices are a sore spot.  I have checked everywhere to make shipping less expensive, but USPS flat rate is by far the best deal and easiest to use, that I could find. I do not mark up shipping (some international rates are a few dollars more or less for some reason).

-Using your business account is a possibility but really it is a hassle for me. I use flat rate boxes and to ship any other carrier I need to buy a different box and repackage and drop it off, so please only ask if you are saving a substantial amount of money because it cost me a lot of time and effort.

 

End Stops – 

-Most people do not use endstops, I would say much higher than 90% do not.

-In almost every page you will notice no talk of endstops and the forum is full of why not and when to use them. If you are coming from a 3D printer background, CNC maching is different. 3D printers are all basically a repeat job, meaning your bed is always in the same place. In a CNC router your bed is now your work piece, and your work piece is located where your fasten it down. This means your home is in a different place every single time you use it, this would mean you would have to adjust your endstops every single time you use it for no reason. This is why we do not use end stops unless we do repeat fixtured jobs, or some tool change jobs.

-The new dual endstop firmware is an advanced piece of software, please learn the basics before you try this.

 

Belts vs. Lead or Ball Screws –

-Belts and screws can be sized for any ratio, or resolution. The current designs theoretical resolution is 0.005mm, or 0.00025 depending on your drivers, you do not need higher resolution. Belts are much easy to size and adjust the ratio.

-Whip. Horizontal (or even long vertical) screws have “whip” . To counteract this you have to support them at both and and have a diameter large enough it does not sag and whip, expensive, very very expensive.

-Lubricant. All screws need lubricant, very bad in a dirty environment.

-Backlash. Screws and belts have backlash, belts have very very extremely small amount most of us can’t measure it. Screws have backlash, a lot of it, and every single time it gets used the backlash gets larger. To counter act this you need ball-screws, which are stupidly expensive and need to be adjusted. You can use a spring driven anti backlash but that adds a large amount of friction. In either case you will need to up the power for that axis to overcome all this added mass and friction.

 

LowRider

 – Just wheels on a table? – There is a much smaller load on the machine that you think. This machine uses less “table touching” that the previous version, because we simply did not need it. You start your machine square, simply by starting it on the hard stop belt blocks. From there, why would it every turn? The wheels are pointed in the exact same direction and the steppers are moving at the exact same time. You don’t need anything for corrections because by that point your cut is already ruined.

I keep using this number that came out of a calc, but have never verified it. Aluminum with an 1/8″ bit requires a 1.5kg load, so worse case on a directly X axis cut you would have a force of 1.5kg pushing sideways….nothing. Wood is drastically less.

Now, extrapolate that a little and you can see the flaw, with a tall Z axis you can get a wonky cut (under sized) as the leverage comes into play, but is still should not wander. But as the z axis progresses through the cut, the machine actually gets more rigid and is correcting the previously undersized cut. Yup, it works, the shorter the better.

 

DXF’s

-You can print 1 to 1, measure, view DXF’s with this free SolidWorks tool. https://www.solidworks.com/sw/support/edrawings/e2_downloadcheck.htm In Windows you can even Print to PDF, for easy conversion. Use the measure tool to make sure you printed it correctly sized.