hextatic

Hextatic mini hexapod  

The Hextatic will be a high speed 5 axis benchtop milling machine/router. It is intended as an experiment to try out various ideas before embarking on a physically larger project, the hexatonchires. At this point the rigidity of the structure is unknown, but my goal is to be able to rapidly mill aluminum and do light milling on steel. It will be lightweight, modular, extremely portable, and inexpensive. (well, inexpensive as far as servo CNC machines go at least!)

Click here if you don't know what a hexapod is!

new  

A grant proposal I submitted to the Buckminster Fuller Institute:
http://fennetic.net/pub/emc/bfi/

Major components of the hextatic include:

• six iskewb integrated motor/drive modules (estimated cost: $40 each) which include:
  • japan servo 127K9720 servo motor,
  • irfz44n-based h-bridge motor power stage
• netier xl1000 thin client computer running linux with realtime extensions and emc2
• the motherchip, a high speed parallel port interface/breakout board ($20) (DIY/open source)
  • handles communications between a standard PC and up to 8 iskewb modules
• A 24V 5A unregulated power supply ($30-$50 bought)
• multiple spindle attachments
  • a cheap harbor freight laminate trimmer with 1/4" collet ($20 p/n 44914-2VGA)
    
  • an air powered 1/8" collet ($10 p/n 47869-2VGA)
    
  • custom built planetary geared R8 or ER collet spindle for heavy milling and drilling, with encoder position feedback and automatic tool changer($??)

What follows is an idea and part number dump. Pardon the mess. This page is for mechanics and geometry only, no electrical stuff here please.

timing pulleys  

What I want: 90 or 120-groove MXL double flanged 1/4" belt width 8mm shaft size, with some sort of hub or way to lock the pulley to the shaft.
MSC is being a pain and hardly has any timing pulleys in stock. All the other outlets have even less (mcmaster) or are ridiculously expensive (sdp-si at $12 each for plastic unflanged) At least you can check to see how many are in stock on the MSC item's page. (17 as of 4/29/06)

I ended up ordering:
04894838 100 groove MXL plastic single flange no hub $1.48 qty 6

Well actually i ordered a bunch of different types to try them out and see which worked best, but that's all they sent me. Nuts.

I considered hobbing my own out of plastic or aluminum stock but since I'm running out of time and have too many irons in the fire, I was forced to buy them. Too bad, because otherwise I'd have an unlimited supply of custom timing pulleys. Maybe I'll do it some time in the future anyway.

Section view of the pulley assembly I hacked together. The pulley comes with a stub on one side that possibly could be drilled/tapped for a setscrew? But it didnt look very sturdy and had to be trimmed off with the lathe because it was in the way of the three 8-32 mounting screws for my custom aluminum setscrew hub and flange. A job made easier by gripping the pulley inside the rim with the 3-jaw chuck.

I'm using a largish flange with a notch cut out, in conjunction with an optointerrupter as a primitive 1 line per rev optical encoder. This will enable precision homing without any modification to the current incremental-only encoder assembly.

Pulley housing  

The motor mounts are coming together. The blue styrofoam will be cast into aluminum using the lostfoam method.

Timing belt

McMaster-carr p/n 7887K85 1/4" wide 165 groove neoprene/fiberglass MXL timing belt, $3.57 ea min qty 1 - Should work..

608RS ABEC7 skate bearings, qty 2

to support radial and thrust loads on the screw. Ordered from VXB bearings on ebay.com - $0.50 each or so in qty 100 --these might be a bit wussy in the axial direction. I'd prefer angular contact bearings but they cost a heck of a lot more and you cant reliably find them in the dumpster either

Anti-backlash nuts  

Will be cutting these out of acetate or some nylatron that I have laying around. Acme threaded rod can be made into a tap by turning a taper and grinding flutes into it. Basic design is a split nut that is squeezed closed by a rubber O-ring. Someone suggested the o-ring might stretch over time and lose its elasticity, so I may go with a 2-piece spring and tapered collar design.

__        __ O___
  --------  |_|  |
                 |
/\/\/\/\/\/\/\/\/\        O-ring anti-backlash nut 

\/\/\/\/\/\/\/\/\/
             _   |
__--------__|O|__|

____ spring ___
    |/\/\/\| / ____
    |______|//    |
                  |
/\/\/\/\/\/\/\/\/\/        sleeve/collar a-b nut 

\/\/\/\/\/\/\/\/\/\
                  |
     ________     |
    |      |\\____|
____|/\/\/\|_\

need to get springs to make this work - msc part numbers of interest while i'm ordering from MSC:

02312395 0.688" OD 89lb/in stainless spring - $0.71 ea in qty 6

jmk suggests using wave washers instead of springs

02566628 0.588 ID 0.734 OD rusty steel wave washer $0.34 min qty 25

struts  

stainless 316 tubing from scrapyard - qty 2 dimensions 10ft OD 1" ID 0.835"

aluminum tubing crusty 9ft OD 1.25" ID 1.125" - has holes 18" from either end

1/2"-10 threaded rod, cut into 18" pieces - qty 8 (2 6 foot sticks) -- 18" was chosen because it divides nicely into 3 foot and 6 foot sections, is reasonably large for a tabletop sized machine, and isnt too long. (I was worried that 24" screws might have excessive bending/buckling when cutting tougher materials.)

Bearings and the drive pulley will take up approximately 3 inches, leaving us 15" of screw thread. The nut will be another 1" or so, and if I size all the tubes and slide bearings correctly I should end up with 14" of usable travel.

Will I ever even approach critical speed?  

read critical speed section (critical speed is rpm at resonant frequency which can lead to severe whipping of the leadscrew)
http://www.nookindustries.com/acme/AcmeGlossary.cfm#ScrewCritical

critical speed calculation for 1/2-10 acme rod 15 inches long:
1.47*4.76*10^6*0.33/15^2=10262 rpm this value double-checks with the chart provided
10262 rpm * 10 tpi = 1026 ipm max (434mm/sec), not too shabby at all.

However I will only be going at a maximum motor speed of 2500 rpm with a 30/100 gear ratio, so my real leadscrew speed will never be over 750rpm == 75ipm (30mm/sec) linear elongation speed. So, the answer is no.

What is the maximum compressive force the struts can withstand?  

Formula from http://www.nookindustries.com/acme/AcmeGlossary.cfm#ScrewColumn

Calculation for 1/2-10 acme rod 15 inches long with a 1 inch nut, fully extended:
14.03*10^6*2*0.33^4/14^2 = 1431 pounds of force max before the rod exceeds its elastic limit and bends permanently. This corresponds with the chart but seems to suggest that I'm approaching the maxmimum practical length of the screws. Hey that means I'm doing something right! The struts can carry much more load when only partially extended (as will be the case for most work) so we shouldn't worry about these numbers so much. Also the chart tells us the maximum load for 1/2-10 screws at any length is 4000 lbs, but I expect something else will break before that.