[jseiler]

After spending too many hours reading the posts here, I've decided to take on prototyping a simple router based around 11/16" stainless hex rod, LV 20/7 ZZ yi-tong 120 degree bearings. (http://www.yi-tong.com/en/vee_bearings.htm).

I made a quick gantry (in the living room) out of 40mm 80/20 surplus extrusion. I took two sections 48" stainless hex and taped it down to rails, just for fun. I used shopbot 1 to cut a piece of 12mm baltic with a 8 mounting holes (4 on each side) to hold 7mm shoulder bolts. I ground the
heads on the shoulder bolts down to about 1/16 to the extrusion and used a few washers to lift the bearings off the board to clear the hex rod. I had to also grind the edges off the hex as yitong bearings have a 2mm (?) flat and hex rod doesn't have a flat on the points.

so using bearings to pinch the rod and adjust out the slop, it moved suprisingly easy and was quite slop free. So I secured the hex rods as securely as I could and rode the gantry like a shopping cart.

Since I only had 2 pieces of hex, I made another large frame for the prototype x axis, cut two more pieces for the ends of the gantry, 4 holes each to pinch the hex rod. It seemed to work so well, I climbed on it. Seemed pretty tight. I quickly weighed the prototype gantry. 40 lbs.

so far, so cheap.

bearings - 10ea (8 per axis) $80
hex rod - $60 (that's 6 5 foot sections of 416 from ebay)
80/20 40mm 6ft*50 ft (?) $300
baltic birch $ 5
misc corner braces/screws $100
thk style rails for the z $50
about 600 or so.

Now I want to mount the rails without drilling them. I thinking about 4905 3m vhb tape. I know that's not necessarily the most accurate way to do it, but it will allow one to "rerail" their system with a piece of piano wire, a quick grind, and $15 worth of vhb 3 times before recycling the rails.

on the x axis, I plan to place the hex down flat, and I'm trying to decide whether to hang the rails on the side for the y axis (makes the gantry slide a simple plate of aluminum in the nonproto), or do like joe did, top and bottom of the y axis.

there's nothing that precludes this design from being relatively cheap and big...

so what's the cheap accurate way to drive a 8 ft x axis? I figure that rack is cheap if you make your own gearboxes. vhb will hold that down the rack too (shopbot does it). the only hangup I have with rack is that you really need gearing to make it work right. can gearing be avoided with belt drive?
the third option is a spinning a delren nut around a fixed acme rod, but i've not seen many good examples here to look at.

the y axis, i'll probably use 1/2-10 2 start and a dumpster nut.

I guess that's a few questions. I'm hoping to bring in prototype 1 with 48" rails x and y for under 1200, motors and all.

J

[jseiler]

hmmm...after looking at

http://solutions.3m.com/wps/portal/3...ape-Selection/

it seems like static loading will move the tape. I'm going to need some sort of end fixity to make it work.

[txcowdog]

I get a 404 on this link http://www.yi-tong.com/en/vee_bearings.htm

Please verify.

[jseiler]

that's wierd. if I click the link, it doesn't work. if i type the link, no dice. if I surf to it through the front page, it works. I can copy the link and hit refresh and it works. wierd. anyway....

goto:
http://www.yi-tong.com/

click usa/uk
click products list
click LV…Track rollers with “V” groove profile
click specifications and dimensions

the ones i'm using are "LV 20/7 ZZ"

-----

I tried some vhb on the anodized aluminum and it creeps far too much. too bad. I'll probably just drill the hex and use tnuts to hold it down on washers. 11/16" was cheap, but bigger would completely cover the t slot and might work better.

J

[txcowdog]

This is what I am going to use to try to drive my rack & pinion http://www.mpja.com/prodinfo.asp?number=16237+MS This is already geared down at 18 to 1. That should remove the need for belts and pulleys to reduce the motor speed and at the same time increase accuracy.

[jseiler]

Do you have these inhand? its hard to believe they're low backlash, but since
their hooked to a stepper, you never know. is there a way to estimate the inoz torque from its voltage and phase current? I was trying to figure out what kind of omph one might expect...i'm guessing 80-100in oz before gear.

I ordered 4 on the offchance that someday I might want a plasma. I wonder
if they'll take the torque I'm going to unleash on them in a router.

thanks for the headsup. I bought a 15:1 low lash gearhead so i'm going forward on rack or belt, probably rack. is std steel still the best place? http://www.stdsteel.com/gr_stock.htm

how does one mount this stuff to 8020?

John

[txcowdog]

I have not felt any backlash but if there is any, by the time it gets to your pinion it will be 18 times smaller due to the gearing. With an 18:1 gearing, you will multiply the torque of the motor by 18 which essentially gives you a 1800 oz/in motor, but, you also slow down the rpm of the motor by 18. With a rack and pinion you want a slow but very precise rotation of the shaft driving the pinion. With the rack I have, one rotation of the pinion gives me about 1.5 inch travel. While I have not installed these yet they seemed like the perfect solution on paper.

SSS does have a lot of the rack and you might also try http://www.sdp-si.com/

[jseiler]

I bought some of those. while they are nice for the price, they do have
quite a bit of lash in the output shaft. I'd guesstimate it at a degree or so. on a pinion of 2", that would be 2 (pi)/360 inches of error or about 1/60".

I decided to start small and beat out a quick little 18x18 design with
dumpster nuts and 2 start acme. its 24x24 frame.

$200 80/20 extrusion and connectors. (could be a lot cheaper)
$25 in 416 11/16 stainless. (love to find hardened hex and bigger)
$50 misc hardware 5/16 bolts etc. (lowes prices)
$75 linistepper kit
$20 28V 8 amp supply (surplus)
$200 in dumpster nuts, couplers, collars
$60 in msc bearings (thrust bearings are pricey and I probably overbought)
$100 4140 amce mcmaster carr and stuff (didn't want to take a chance with emco)
$ 80 thk 4" z axis (steal on ebay).
$ 90 8 - 120 v bearings (cheaper in qntity).

so about 900. still need mach or freeware to move it around and a breakout board. most of it will carry to a bigger machine.

i'm using 24" rails (hex rod) with 120 V bearings pinching the sides. it works pretty well and it compensates for wear by making it easy to tighten the bearings up. when the hex is too torn up, throw it away. the outers on the bearings are hardened, so they're not going anywhere. one could use this machine to build the rail supports for hardened 10-12mm round. it would work just the same, if someone needed even more precision. Personally, I'll
probably never kill a set of rails even in relatively soft 416 stainless.

Once I get this one going, i'll start figuring out a reasonable way to do a rack drive. I've got a nema 34 100:1 and a nema 23 15:1 plantary's on the way for servo and stepper drives. I still need a good way to attach rack to aluminum.

John
cheap, fast, accurate, reliable...pick 2.

[txcowdog]

$100 for acme is a big gulp. I went with Enco Tuesday and they were having a sale. A 6' length of 1/2"-10 was only $4.50 but the old saying "you get what you pay for" probably applies. This will be my first build with 1/2" so I don't know what to expect from Enco. Guess I'll know in a few days.

[jseiler]

how's the enco look? the mcmaster is nice dark black. feels almost coated.

[txcowdog]

The Enco rods have a nice smooth feel and maybe .010"-.015" backlash. They are still on sale at $7 for a 6' length. They also have free shipping on a $50 purchase thru the end of Sept. with the use of a promo code.