View Full Version : Need Help! RMU Baja Team plasma table build

11-22-2009, 05:04 PM
Well, this is the RMU SAEĀ® baja team speaking (well 1/5th of it) And this year we(me) are going to build a roughly 4x4' cnc plasma table (its actually 42"x44") to cut out out templates and brackets instead of milling them or cutting them by hand like last year.


Our budget for the table is around $300 total(its low because our budget is whats left of my last paycheck)
And I have thanksgiving week to get all the parts so lets get started!

For the frame rails our machinst Gary donated a 40x80mm extrusion section from a old robot with two nice linear rails and a box of eight good 20mm linear bearings and a dozen or so 12mm bearings.

The gantry will be two upright 40x80mm extrusions attach to that will be two 60"inch long 20mm rods/bearings with pillow blocks, on the one side opposite the rails will be another set of pillow block and another 60"inch 20mm rod/bearings.
Now which type of 20mm rod? hardened or plain? Is there any sites that sell cheap 20mm rod? In the box with the bearing where alot of 12mm bearing if that rod is cheap I could use three 12mm rods instead of the two 20mm ones

For the the steppers Gary had a small nema17(99oz.in)and acme screw that will work for the Z
For the X and Y I've found two baldor DSM34F-175 Nema 34's with a built in drivers which should be good for 380oz.in each for $49
Does anybody have any experiance with these motors? or their drivers?
(found a manual .pdf) http://www.baldor.com/downloads/manuals/_downloads/1940-105.pdf

With my limited budget I was thinking Acme thread the problem is which?
Normal acme rod or B7 grade? Is B7 that much better? of would normal acme rod work?
I was leaning towards 1/2-10TPI(since it seems a bunch of people use 1/2 acme rods)
I can get normal 1/2-10 for $13 or 1/2-10 (B7 grade) for $37 (both from mcmaster)
I can get two nuts for the 1/2-10 for $30 on ebay.

40x80mm 55" lite extrusion $50
Acme screws? $13 or $37
Nuts $30
Steppers $98($49x2)
break out board $26
20mm rod? $?? (need three 60" sections)

Any suggestions?

-Robert Morris University Baja team

Here is a picture of the existing frame and rails.

11-23-2009, 02:00 PM
I would recommend NOT using Acme for a plasma table (at least the X and Y - the Z is probably OK). My actual CNC plasma experience is limited to about 20 mins of actual cutting time (mainly have been using my recently home built table in router configuration)... but speed is important for plasma. Greater than 100 IPM will probably be necessary for anything thinner than 1/8" (others will probably chime in with better data, or you can easily search). My table uses belt drive ("L" series timing belt), rack and pinion is probably the most popular, some use a chain drive (like bicycle chain - this may be the cheapest route for you).

One thing I have found in my brief experience is that plasma is MESSY (metal grit that blows through, as well as gets in the air and covers EVERYTHING around. I would recommend using it outside or with a water table... the water table is on my project list before I do any more plasma cutting! Your linear rails look nice (mine uses similar ones, 20mm THK on Y and 15mm Hiwin on X) - be careful to keep them clean. Knowing what I know now about the mess, if I was planning to do mainly plasma (vs router) I would use V-rails. But, the linear rails were easier to work with (using similar extrusion for X/gantry axis). And, you have them available.

On the motors, I would recommend 3 - one on the gantry, and one on each end. Mach3 can "slave" the two motors on the ends of the gantry and based on your limit switch setup can actually "square" the gantry - really cool! Or, at least couple across the gantry (belt, etc) and drive both sides.

I wish you the best of luck on your project... I am currently a college student too (U of Cinci). And I actually have a class starting in 3 mins! So I need to get going.

I think you will really enjoy Baja - I had a lot of friends who did it at Letourneau Univ (I was there a few yrs ago), and did quite well. It is a lot of work but a lot of fun. I used to hang out down there a lot, I was actually on the Formula SAE team. We used to give the Baja guys a hard time about how they needed our engine! I did take my dirt bike out a few times when they drove the car and hung out. We all wished we would have had ANY CNC equipment! :-) But I did enjoy the bridgeports and nice big lathes... just stuck with smaller mill and lathe in a home shop now.

Let me know if you need anything...


11-24-2009, 05:10 PM
I've just got two sections of 1/2-10 from enco $9.36 and am going to get two sections of 1/2-8 acme 8 start from mcmaster $48.56 each.
The 1/2-10 will give me 1/10" per turn for 60ipm and the 1/2-8 would give me 1/2" a turn for 300ipm.

Now its just to find a supplier that has 60" sections of 12 or 20mm rod CHEAP.

11-24-2009, 06:32 PM
I would highly recommend not using acme rod, possibly for the z is fine but thats it.... You will run into alot of problems I think with acme, go belt drive or rack and pinion. Your budget is very low, so I would probably start out with belt drive ( since belts and pulleys are cheap). You can also use roller skate wheels to drive your axis. Compress the neoprene wheels to a frame surface with alot of force so their is no slipping, (kinda like a tire on a car and the road is the frame of your machine). This would eliminate rack and pinion and belt pulley expenses and help keep your budget down. $300 will be extremely difficult.... I wish you the best.

11-25-2009, 11:48 AM
I know acme thread would get dirty with continuous cutting but this table will probably in total cut 4'x6' of material for the team before ending up in my garage and becoming a router/plasma (60/40%). With wipers on the screws and a water box, I think it will be OK.
As for price acme and belts are similar enough $49 acme and $69 but that's because I already have a bunch of 1/2" bearing and couplers, and the fact the timing belt pulleys for a 14mm shaft are a bit expensive.

But for skate bearings that might works since I'm having a hard time finding 20mm rod in the length I need at the price I can afford.
I could make the gantry out of extruded aluminum and mill out the T slots to fit the bearings in on all the sides

12-06-2009, 01:27 PM
Well Its a update,

First things first God love McMaster-Carr I ordered two sections of 1/2"-8tpi 8start rod on weds got them on thurs.
I ordered some spider couplers but they have to much slop in them(probably would give me 1/8" of backlash) so I'm going to have to bore out some rod and make some.

Instead of 20mm rod for the linear bearings I'm going to 1" bearings and using some 1"x.125" 4130 chromoly tube left over from our first car I thought it would have to much dimensional inconstancy but its only at max +/- a thousandth I think the bearings can take that much inconsistency if not I'll chuck the tube up in the lathe and hit it with a piece of 1200 grit sand paper and polish it up.
With the one inch linear bearings I be in $4.50 a piece so another $36 bucks for the 8 bearings

Now its for the hard part because the 1/2-8 8start cost more than I budgeted for $48 vs $37 I'll need to make my own nuts, problem is the tap is crazy expensive so I'll have to make one.

This is my first try at making a tap with some scrap 1/2-10 single start I used a 3/16" end mill to slot it, I think I should have used longer taper.

The second try I used the lathe to add a longer taper but I think its still not enough, I used a 3/16 ball end to give it a more hooked 'sharp' cutting edge it worked a bit better I'm still thinking a deeper cut next time and a longer taper.

side by side

But the problem is how to do the 1/2-8 8start tap? if you look at a side by side between the 1/2-10 and 1/2-8 the thread is much tighter. So a single flute? double? eight flutes?


12-07-2009, 07:33 PM
Another update, I got working on the base of the table today, got it all welded up and the pillow blocks mounted, and started experimenting with making 1/2-8 8 start taps

the second 1/2-10 tap vs the 1/2-8 tap, they both remove a hell of a lot of material I need to make the 1/2-8 tap longer its only about 3-4" long

The first holes with the 1/2-8 tap into some scrap PTFE, after running the tap through it a couple dozen times it slid like butter on the full length rod a inch per turn wow

The legs before I welded them up, I now just have to put some jacking screws together, I welded 1/2-10 acme nuts inside the legs to put the screws in.

Here is the extruded alumnium mocked up to the frame, it will fold up at the pillow blocks to save space. Still waiting for the last piece of extrusion to finish the frame.

Here is the Z axis got a nice linear rail and a 5/16 acme with 99oz-in stepper

Edit: just went to get my mail, and found this out side waiting for me!

A 51" section of extruded aluminum