Hi guys,

I have all my linear rails etc. and need to decide whether to use rack and pinion or ACME screws. Is it harder to manage backlash on one or the other? It appears rack and pinion is easier on the wallet but ACME screws aren't terribly expensive. Ball screws are pretty spendy and I'm kinda avoiding that cost.

Here is my thread describing my current parts obatined. This will be a pretty high end router. All aluminum and steel with supported Thomson rails all the way around. 30" X 48" X 18"z. 450oz servo's etc...

http://www.cnczone.com/forums/showthread.php?t=12696


Any advice on pros and cons of either or other ideas would be great. Looks like I could use a 4:1 reduction. I'm dreading figuring out how to get the machine work done on ACME screws which is why rack and pinion caught my attention.

Actually both have good and bad points. I haven't yet tested my plasma that I am building with R&P. If you don't have a lathe to turn your screws, this may be the way to go.
You do need a spring loaded mechanism on them to try to keep pressure or contact between the rack and gear as it changes direction. I have seen these simple and complicated. My particular build has a jack shaft to drive the X and it took me awhile to get the design down. I think it should work great, but as yet untested. I think it is also easy to install and maintain. Can be installed upside down to alleviate trash on it. It will allow faster speeds on larger machines.

I initially bought Acme rod to build my first router with. I bought store bought brass nuts. Terrible fit on these things and they really weren't cheap. At that point, I decide to go with ball nuts and screws. I still have the acme rod and may use them with some Dumpster nuts. They look well made and will make acme acceptable. Not bad price wise either.

For a machine your size, I would likely go with acme and Dumpster nuts given the two choices. Either will work, though.
Good luck with it.

Well I did rack on my long axis. And like it. How ever I would love to have more gear reduction. But unless you pinion is REAL small I'd say 4:1 on servos is not enough. I'm running 10:1 planitary (thompson tranny) with a bit of a belt reduction after that to 1" diam pinions to my rack. I can fast jog up to 800ipm how ever i've got more RPM left on the top end of my servos even at that thus more gear reduction would alow me to up my resoloution a bit on the axis.

b/

Yeah, the 4:1 reduction was for the acme screws that were recommended to me. I have no idea what is needed for R&P.

Thanks for the info so far guys.

What's your price range???? I started out with McMaster Carr Acme Screws and Dumpster Anti-Backlash nuts and just recently discovered Roton..dirt cheap, if you don't buy the mounting flange. The dumpster nuts are like $18, the Roton ballnut is $22.50 for 5/8", and the ballscrew is about $1 an inch, that's cheaper than Mcmaster for the acme screw. So in my opinion go with ballscrews, I just ordered mine to convert.

What's your price range???? I started out with McMaster Carr Acme Screws and Dumpster Anti-Backlash nuts and just recently discovered Roton..dirt cheap, if you don't buy the mounting flange. The dumpster nuts are like $18, the Roton ballnut is $22.50 for 5/8", and the ballscrew is about $1 an inch, that's cheaper than Mcmaster for the acme screw. So in my opinion go with ballscrews, I just ordered mine to convert.

I don't have a set budget, just trying to remain reasonable. If I can find ball screws for that cheap then I'll go ballscrew! Now I just need to figure out which ones I need and look into that as well now.

I'm a little confused. The prices are all over the place. Is this what you are looking at? http://www.roton.com/Mating_Components.aspx?family=7059321

I'm having a terrible time determining what pitch etc... Every step I take it gets more complicated to get this right.

Yes, the 5/8" x .200 is the most resonable price there. It takes 5 turn per inch.

Sweet! That is an excellent price, looks like I'll be considering ballscrews. How will you turn them down to fit the bearings for mounting etc?

How long is your screw going to be? That's a key design point. If you are greater then 4 feet, then the rack in pinion is probably your best bet. Longer then 4 feet and your spinning rod will develop a significant wobble. I didn't look in detail at your other build thread. That's why I don't know how long your table is.

How fast do you want to jog and cut? Most guys are going with the 1 inch diameter. Enco.com sells a 1 inch 10 tpi single start acme for $10/6feet. I bought a 1 inch/10tpi, 2 start acme for $60/6 feet. You can jog a bit faster with the 2 start.

Dave

My longest axis is 48" but the travel will be a few inches short of course. I'm not sure I want to deal with the ballscrews right now so I'll probably begin with acme screws.

I'm not sure how fast I want or need. I just want to take advantage of my servo's and run them at what would be considered to be a respectable level of their overal capability. I'll be cutting aluminum and wood.

acme screws 1/2"-10 5 start, will work nicely for that size.

Joe

How long is your screw going to be? That's a key design point. If you are greater then 4 feet, then the rack in pinion is probably your best bet. Longer then 4 feet and your spinning rod will develop a significant wobble. I didn't look in detail at your other build thread. That's why I don't know how long your table is.

How fast do you want to jog and cut? Most guys are going with the 1 inch diameter. Enco.com sells a 1 inch 10 tpi single start acme for $10/6feet. I bought a 1 inch/8 2 start acme for $60/6 feet. You can jog a bit faster with the 2 start.

Dave
Can you tell me how well the Roton ball screws worked out. I'm looking to get some 5' ball screws for my machine. The price sounds great.:)

Sweet! That is an excellent price, looks like I'll be considering ballscrews. How will you turn them down to fit the bearings for mounting etc?
It looked to me like the roton screw had a shaft cut on the end for motor/spider gear mounting?:confused:

Joe, 5 Start? You mean 2 start right?

Sorry, I can't help you. I don't have that hardware.

Can you tell me how well the Roton ball screws worked out. I'm looking to get some 5' ball screws for my machine. The price sounds great.:)

:)What's your price range???? I started out with McMaster Carr Acme Screws and Dumpster Anti-Backlash nuts and just recently discovered Roton..dirt cheap, if you don't buy the mounting flange. The dumpster nuts are like $18, the Roton ballnut is $22.50 for 5/8", and the ballscrew is about $1 an inch, that's cheaper than Mcmaster for the acme screw. So in my opinion go with ballscrews, I just ordered mine to convert.
Off the top of your head, could you get the Roton's, say, 8' ft. long?

Joe, 5 Start? You mean 2 start right?


No 5 start, two turns per inch, trust me it is very smooth and dumpster nuts work great with it.

McMaster-Carr Part Number: 99030A704

Joe

Ok, so do I need any sort of reduction with my servo's or will 5 start give me what I need to keep the servo RPM up?

Personally I think it may also depend on the type of bearings you use, but at two turns per inch, that sucker should fly. You may need reduction to just gain torque though.
The screws I am using on my mill are 1 turn = 10 mm. That is a pretty high helix as well. I have chain drive gears on the Y and have tested that and quit at 120 IPM. For an 8 or 9" axis, that is plenty. No need to go higher, but will likely set it at about 80 IPM rapids. The gearing on it is 3 to 1. Probably way to much, but I will let the machine cut new parts when it is done, then change it to a belt drive about 1.5 to 1.
I got to work on my X this afternoon and just need to mount the motor now. May do that in the AM.

If you go back to your other thread, Jeff said you should run those servos at about 2500rpm. 5 start 1/2-10 screws are 2 turns per inch. You need to pick a target top speed, and gear to that. If you want to go 250ipm, that would be 500 rpm at the screw, and 5:1 gearing.

No 5 start, two turns per inch, trust me it is very smooth and dumpster nuts work great with it.

McMaster-Carr Part Number: 99030A704

Joe
Wow, I'll bet that really moves out. Yes, very smooth. What jog speeds do you get, 200ipm?

I have the 5 start 1/2-10 with Dumpster nuts on my machine. I'm using 3' screws on both axes. I get rapids of 300 IPM on my Y, and I've gotten 600 IPM (that's no typo) on my X. They're wicked fast, and help a lot with whip.

Ahren
www.cncrouterparts.com

Darren, I am building a machine using the 5/8-.200 ball screw set up. My long axis is 48". I am using 12mm ID 6201rs ball bearings (dirt cheap from VXB). This is just less than the root of the thread so it cleaned up nicely on the lathe. The OD will press fit into 1.25" with some forgiveness (1.2594" in theory). I combed CNCZone before I spent my dough and the majority say that ballscrews are the way to go. Lots of commercial machines use R+P, but the 5/8-.200 ballscrews are cheap! You must factor in the cost of a tap for the ballnuts, however, as the 15/16" - 16tpi tap is pure unobtanium except from Mcmaster ($58), unless you pay the premium for the prethreaded flanges. I think it is worth it to eliminate backlash and wear.