New build-Why aren't there many tables out there usings Thompson rod and acme screws?

[mpascino]

Hi guys, I am starting a joint new build and have some table specfic questions. I have spent a long while researching the forums and have not found many builds using acme screws or Thompson rod. I understand the downside of keeping everything clean and the acme screws getting gummed up. The long story short is I have access to 2 5 foot lengths of 1 1/4 thompson rod and 4 bearings to match. Acme rod is cheap from Mcmaster, enco, or surplus center. The table will have about 4'x4' of usable cutting area. My partner on the project has purchased a complete blade runner kit from CancCNC. I would be extremely happy with a tolerance of .010-.020". The table will be used to cut 14 gage and up, more likely .25"-.5" steel plate. I was thinking of using 3/4-6 acme rods for each side of the x axis. The y and z axis will likely be some type of linear rail setup with acme screws as well. I figure the 6 TPI will still give the speed needed assuming a max loaded RPM of 800 at the stepper motors. I know 5:1 is the recommend, but I don't see the harm slowing things down a little especially if the machine is going to spend most of it's time cutting plate. For starters it will be mated to a Thermal Dynamics Pak 50 Xl, but eventually will likely find a home mated to a 1" capable plasma.

My thought was to build the table with the cutting head mounted BELOW all of the X axis mechinisms. This would allow the gantry to be built very short, reducing weight and hopefully reduce the amount of dust that would be availble to harm the bearings and Thompson rods. The holding table would then be built below the rest of the setup. I would likely go a step further and encase the rods, bearings, and acme screws in square tube with a split. rubber strip in the top(much like a DRO on a mill or lathe) to reduce contamination.

Am I thinking out in left field? I would think the acme screws on the x axis would reduce moving weight since the stepper motors will be stationary. If backlash becomes a problem I could build a zero backlash acme nut. The gantry itself will be made mostly from 2x3" aluminum extruded rail.

Everyone's feed back is appreicated. As everyone else here I need to keep the table costs to a minimum, hench me using what I've got. Oh and by the way, a water table will come later on since I don't see this machine getting used everyday(at first).
Thanks.

[ger21]

Most router builders building that size are using 1/2-8 2 start or 1/2-10 5 starts acme. You can get a lot of speed out of them.

[rchacich]

Based on your table specifications, you are on the right track for a very inexpensive table build. Most people are looking for more speed, thus the use of gear rack or multiple start acme rod. There is nothing wrong with your approach. Torchmate just released a 2 x 2 multipurpose machine using a screw. Search the web for Torchmate and 2 x 2 and you will find some blogs about it as you may get some ideas for your build. Maybe you would want to use your shafting on the gantry and CRS with bearings on the side rails to keep costs down. You could put the shafting in a formed enclosure for protection from dust. If I had to guess, they may be using 5/8"-6 acme rod from Enco. I would suggest investing in antibacklash nuts to maintain a constant velocity while cutting rather than relying on software for the antibacklash compensation. Dumpster CNC is a low cost source. I have 2 more things for you to consider. Rod has whip at higher rpms and acme threaded rod is never straight.
Best Regards,
Ron Chacich

[WSS]

Check this machine out, round rails and ballscrews

http://www.cncsteelcutting.com/index.html

HAPPY NEW YEAR!!
WSS

[multiplex]

thats a nice table there!

i've kinda been battling the same thoughts. I have some left over supported thompson rail from a previous build that i want to use for the Y axis. I like what they have there, but I will still probably go with rack for the movement.

[cNcCharlie]

I would suggest investing in antibacklash nuts to maintain a constant velocity while cutting rather than relying on software for the antibacklash compensation. Dumpster CNC is a low cost source.

Any idea where to find antibacklash nuts for 3/4-6 acme? From what I've seen, 1/2" is the common big one... with an occaisonal 5/8". Who has bigger ones?

[WSS]

Any idea where to find antibacklash nuts for 3/4-6 acme? From what I've seen, 1/2" is the common big one... with an occaisonal 5/8". Who has bigger ones?

Try here:

http://www.nookindustries.com/acme/AcmeTabNut.cfm

Nook says they are going through some engineering changes with their anti-backlash line. I believe they have or had 3/4-6 acme nuts like you are looking for.

Good luck!

WSS