New Machine Build- Lead Screw Recommendations

[sst4270]

Hello everyone,

I'm Steve. I'm a mechanical engineer for Polk Audio and I'm in the process of acquiring parts to build a diy-cnc router. I have a lot of experience with lathes and milling machines. I have successfully programmed and run many g-code runs on the machine we have at work. And I intend to mill a lot of the parts myself such as the bearing carriages.
I've been collecting parts for years, but I'm finally going to put this whole thing together. I'm building in the vein of the 80/20 type builds I've seen here.
I'm modeling the whole thing in Pro/E and still in the design phase. Base will be approximately 42" x 16". Likely to have about 4~6" of Z travel.

I build custom tube guitar amps as side business and I intend to use this machine for several purposes towards that end, some of which are:
Engraving custom face plates in plastic, aluminum, and wood.
Cutting holes out of .080" 5000 series aluminum sheet
Cutting 3-d relief logos
Drilling holes in 0.125" thick phenolic boards
Possibly cutting copper clad pcbs as well.

My questions are:

1. I have acquired and intend to use 1/2" - 8tpi 2-start lead screws. acceptable or not for my application?

2. What are most of you using for lead screws, precision of general purpose acme? I think mine are general purpose. I can't remember because I acquired them about 10 years ago.

That's it for now, but I'm sure more questions will follow as I actually get to putting this thing together.

Thanks in advance for your reponse.

Steve

[ger21]

1) They'll work fine, unless you want to go really fast. I use them on my router, and have had the 40" Y axis screw moving at 220ipm. If you want to go much higher with a screw that long, you might see some whipping.

2. I'd recommend the precision, which should be what your 2 start screws are. They tend to be smoother and more accurate.

[ger21]

You should take a look at the ab nuts at www.dumpstercnc.com

[sst4270]

thanks for the reply, and I DO plan to use the dumpstercnc couplers and nuts. But that brings me to another question.

In reviewing some of the 8020 build pages on the web I notice that there is a bearing block located near the motors. Some choose to install a drill rod there (necessitating an additional coupler between the drill rod and the acme) and some choose to run the acme straight through ( so the only coupler needed is between the acme and the motor. (Thrust washers still used, just installed on the acme instead)
Is the drill rod necessary? Is its purpose to further reduce backlash? Or to decouple thrust forces on the motor?
I would think that in the long term the addition of the drill rod would cause error to accumulate in the machine and reduce accuracy and negate any anti-backlash savings it provided. Maybe I don't fully understand its purpose.

Could someone shed some light on the role of this part?

Thanks,

Steve

[judleroy]

You dont need the drill rod. I dont know what its for but on a 100,000 dollar vmc they mount the bearing to the screw so thats good enough for me. Judleroy

[mactec54]

I have not done used the drill rod but it will give you a better dia for the bearings & thrust bearing

You can not put the bearing straight on to the thread of the screw you would have to turn it down then you are getting to a small diameter for the bearing, This will work fine & many do it this way

But with using the drill rod or something like it you can start with a bigger diameter (like
.750"/1.0" ) & turn it down to fit your bearings you will then have a shoulder & bigger diameter for the bearings & thrust.

This only needs to be on the drive end, the other end you can just turn it down to suit a bearing to support the shaft

[ger21]

I've never seen anyone use drill rod? Here's what I did.
Turning Acme Screw Ends Without a Lathe

The other end that's not shown, is turned down to slip through a bearing (8mm), then steps down to 1/4" for the coupler to the motor.

[jsheerin]

You can put a bearing directly on the OD of the acme screw. That's how mine is. The purpose of the drill rod is to prevent the washers (races) in the thrust bearings from falling down into the threads of the acme screw. If you had low lead screws this could be a problem. I've got 1/2-10 5 start screws and at first glance it was not a problem - the thread spirals enough that the washers don't get stuck in the threads. The problem I did eventually have is that the washers for the thrust bearings slid down to where they were touching the screw and started grinding into the screw. My solution was to glue the washers to the bearing blocks so they wouldn't move radially. If you're making bearing blocks from scratch, I'd recommend a little recess to locate the washers so they don't do that.

[sst4270]

thanks for the replies. I am now more curious about the actual purpose of the drill rod. In the examples I've seen (fine line automation builds):

http://www.peakeff.com/beta/PostDetail.aspx?PostID=11 (uses drill rod)

http://cncrouterparts.com/product_in...products_id=60 (appears to be using a drill rod as well)

http://www.glacialwanderer.com/hobbyrobotics/?p=17 (no drill rod used) (link appears to be dead today)

None of these builds are tapering down. They're using 1/2" drill rod to 1/2" acme screw. The only step down is from a coupler used to join the drill rod to the 1/4" or 3/8" motor shaft.

So it still begs the question... Why the intermediate drill rod for the thrust bearing assembly?

Regards,

Steve

[ger21]

After posting my last post, I saw a machine with the drill rod.

The reason for using it is get an easy to assemble, bolt on screw with no machining required. It does add a few more parts, though.

[jsheerin]

No machining is required without the drill rod using acme screws. Screw carrying shaft collar, thrust bearing, radial bearing in bearing block, second thrust bearing, second shaft collar, motor coupler. That's it. The drill rod, as I said, is to prevent the thrust bearing races from falling into the screw threads on fine pitch screws. Personally I don't like it as now the screw is not support directly in radial bearings.

[Senna]

Another possibility is the drill rod is used as a low cost extender. Most ACME leadscrew is sold in 3 or 6 ft lengths... perhaps to save from buying a 6 ft leadscrew for use on say... a 42" axis where the actual travel may be say... 34 inches or so... a 36" leadscrew would suffice with the drill rod extending the mechanical length of a 36" screw.

Senna