CNC Router: Mantis Build 1

I have started building my little Mantis CNC Router / PCB Engraver. If you’re not aware what the Mantis is, you can checkout David Carr‘s (the designer of the Mantis) blog/wiki at http://makeyourbot.org/mantis9-1 . It’s a little CNC router (footprint is 10″ x 10″) purpose built with a custom, low cost yet highly accurate and tightly built spindle specifically for engraving PC boards but it can be used for most any kind of light engraving or carving – the designer actually carved his face in chocolate in 3D relief! I am hoping to use the finished Mantis to carve PCBs for electronics hacks and maybe even for electronics kits available for sale here.  I will continue to post my progress to the blog and let anyone interested in building this neat little machine some of the issues and gotcha’s I come across. So lets get everyone up to date here:

David Carr was nice enough to provide the DXF file of the machine’s wood parts and a bill of materials (BOM) in .xls format FREE for anyone interested in building the machine themselves which is great and I thank him for it! However, one of the first gotcha’s I ran into was that while he designed the machine to be simple and to allow anyone to make it with basic hand/shop tools – he doesn’t provide any dimensioned drawings at all. In fact, the DXF is designed to allow him to cut the machine out using a CNC router like a Shop-Bot which I think can be seen in the background of one of his instruction videos. His intent, I think, in providing the files and the instructional videos, was that they were to go along with a kit given by him or purchased from him – probably to FabLabs – so they wouldn’t need to cut the wood themselves.  If one starts to dimension the DXF drawings, you find that he also included some graphical representations of parts from other sub-assemblies to show fit, etc… which makes it confusing to look at but the most glaring issue of all is the fact that many of the dimensions, particularly of holes are oddball sizes which makes it a little more difficult to work with at home. David relies a lot on very slightly undersized holes so things can be force fit snug into place whereas someone building this in a home shop without the benefit of a CNC router, like me, has to use slightly larger drills potentially leading to some added slop. This is very evident when I had to drill 3/8″ holes for the steel rods. When you watch his instructional video, when he inserts his rods, he has to bang them in with a mallet whereas mine went right through nice and smooth making it harder to keep them in place while arranging them for gluing.

I have actually taken the time to dimension his DXF drawings so if anyone is interested in getting these, please let me know.

This is a picture of nearly all the parts and hardware required to build a mantis.

Another gotcha I ran into is that his bill of materials is incomplete. He mentions most of the major components but some of his part #’s are now obsolete/unavailable (not a big deal since they all had readily available substitutes) but things like his stepper motor driver boards and power supply are completely absent from the BOM. That’s probably due to the fact that he milled his own so, again  not a huge deal. He did miss the little metal pins that hold up the spindle motor altogether. He doesn’t have the Loctite listed on the BOM either – it tends to be quite expensive so that is bothersome. He mentions ABEC3 R4ZZ bearings but doesn’t list a source. Nit picky things to be sure but the reason I mention it is that the $84 he lists for the cost of the machine doesn’t give someone trying to build this from scratch, in their home shop a good idea of what it will take and most importantly COST to make a Mantis. To that end, I have created my own BOM that includes all of the parts I had to purchase, their sources, their proper current part #’s, and their current prices.

You may download my Mantis 9.1 BOM (.xls) here: Download

You can download the Track Hacker Mantis DXF file here: Download

This DXF is fully dimensioned, although, since it was based on David Carr’s Mantis 9.1 design which was intended for fabrication on a ShopBot CNC router, you will have round some of the dimensions up or down to suite, which I had to do. The other major improvement on this DXF file over the original is that the original contained profile views of all the mechanical parts as well (i.e. the spindle, motors, rods, etc…) which made the original DXF file quite confusing. Hopefully you find this file useful.

Here is a picture of my Mantis with the base, back, and front and the Y axis rods and bearings installed

That should give you a good idea what this machine costs to build assuming you have nothing on hand but your tools – mine has come in at $359.82 for all parts and hardware including the wood, glue, and Loctite.

One last little thing I found so far and this one is on me – I got my NEMA17 stepper motors from a seller on eBay. They had the same part #’s as the ones spec’d by David, they looked like the same motors and they are, for the most part. However these are probably slightly older models that are now in surplus because I found out the hard way that the 4-40 screws David uses to attach his steppers to his machine don’t fit my motors. I then pulled the Lin Engineering‘s (manufacturer of the motors) datasheet on this part # and found that they are spec’d as M3 0.5 pitch screws well that turned out to be wrong t0o – that’s too big. I finally just decided to take out one of the screws holding the stepper together hoping it was the same size as the mounting screws – it was. I then measured this with a caliper under a magnifying glass and finally determined the correct size mounting screws are M2.5 0.45 pitch screws. Whew!

I also decided to deviate a bit from David’s instructions regarding the leadscrew drive nuts. David doesn’t spec anything for his drive nuts since he demonstrates that he makes these from scratch using a 3/8″-12 Acme tap. The average home builder is not going to have this type of tap so I thought it worthwhile to mention that you can take a little extra threaded rod and using a Dremel or similar rotary tool, carve a somewhat deep groove at one end leaving two sharp cutting edges and use this as an improvised tap. That will work with some effort. Well I decided that was to big a pain in the butt so I decided to by some premade flanged leadscrew drive nuts. As it turns out, the flanges on the nuts I got are a little big but overall the nuts are workable. However, that being said, the flanges do make it impossible to follow Davids “glue it to the bottom of the table” instructions so I had to come up with another idea. I decided to attach another small piece of 1/2″ plywood below the table and then bore a hole right down the part line between that piece of ply and the table dead center on the leading edge of the table. This allows me to mount the nut on the side using either screws, glue, or both. Check out my pictures to get an idea what I am talking about. This, however opens up another problem where I now have to relocate the stepper motor to account for the change in position. If you do move the nut like I did, make sure you don’t predrill the NEMA17 motor mount holes without first relocating them.

This is the Y axis table with the additional piece of 1/2″ ply I added to capture the drive nut – this is before boring

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This is the hole bored in the Y axis table to capture the leadscrew drive nut

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This is the Y axis table with the drive nut in place

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This is me gluing the rods and bearings onto the Y axis table

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I also decided not to glue my Y axis rods into place – I used a screw and a washer to hold them in place, at least on one side. I may change my mind yet but this was quickest and easiset for me to do. We’ll see.

Here is a closeup of the Y axis rod holding screw setup

Well, that’s it for now – I’ll tell you all about what happened when I drilled the “relocated” stepper mounting hole for my Y axis table and how my Z axis assembly is coming out in the next installment. I am thinking about selling the pre-cut, pre-drilled wood parts for building the Mantis as a kit – let me know if you’re interested.

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