Re: [CAD_CAM_EDM_DRO] Size of leadscrew clarification

John,
All of the posters so far are on target here..... You will want to watch that you CAN run a bit faster to avoid burning bits. Right now, you may FEEL that rapids over 50ipm are not important. Wait
until you are stuck with that on a router ! You'll change your mind real quick, especially if you are going with a trim router instead of a gimpy dremel tool.

I used to have .200 "lead" on a flatbed router with 450oz motors running 1:1. At the time, I thought it was a better choice than the .5" lead I was going to use, thinking I would have "such great
resolution". Now just to be clear, I only had 1/2 diameter screws on it and by the time you get 48" of travel, you may have near 60" of lead screw. In order to get a comfortable travel speed, you
will send those screws into "wobble land" and beyond ! I think I could run to about 125ipm without being too concerned I would end up with a lead screw in the shape of a coat hanger. But, thats
about it. Didn't take long for me to realize a router is not a mill, neither does a router need that kind of resolution.

So, either up your "lead" or get bigger diameter screws. You can figure that if you jump up to a .5" lead, and run 8th stepping motors/drives and 1:1 to the screws, that will give you .0003
resolution. In comparison, the .2" lead would be .0001" but, you would have to build an incredible machine to ever enjoy it. Coincidentally, I find your average "stiff sprung" Cherry switch to have
the same repeatability (.0003). So, based on the above, you can decide what you think you need for resolution. If you are not planing to use conventional Linear rails and bearings, the finer
resolution from the .200 screw will be lost in other parts of the machine. Oh, and you can expect the .5 lead screws to easily get you over the 500ipm mark but they will wobble over the 48"+
distance. Likely you would not run that high anyhow. I set my maximums at a comfortable 250ipm.

I just happened to remember,.....The MultiCam MT-22 uses .200 Lead in X-Y.... They use a 3/4" diameter screw and because it is a moving table design, they are probably close to 52" long. But, In
order for them to make enough useable torque (as the motors (stepper) lose power at high rpms) they run a higher volt power supply. I think it was 72 volt. This gets important because if you go
with the bigger diameter screws at .200 lead AND you want it to effectively reach 200ipm, your steppers need to have some pretty good torque at high rpm.

Personally, I would skip the regular ACME stuff and shoot for what Mcmaster is calling "Precision Modified-Acme Lead Screws and Nuts". They are hard to find in the Online Mcmaster catalog. Go to
PAGE 943. (type it right in the search window) http://www.mcmaster.com/

Pick your screw, ( I prefer a 4 start - .5" lead for X-Y, .200 or .25 for Z) The printed catalog is a little easier to follow. (These are reportedly from Kerk.) Then, select the correct "Wear
Compensating Nut" or "Anti-Backlash Nut". These are the lighter duty version (and less expensive) nuts than you can get from Kerk for antibacklash. However, they will run for many many years under
daily use, without fiddling with any backlash adjustments. Who needs that.

For example: #6350K22...The screw is $128 and the nut is $33. What kind of anti-backlash nut can you make for $33 ?? And, The screw part is only like $50 more than a regular acme ! Then you are
done with backlash ! Don't cut your costs in this area. It's not worth it.

If you are real frugal with your design, you just might be able to get by with one 6' screw for a 48X12 machine.

Enough rambling from me.............

Chris L

John Thompson wrote:

> I'm going to clarify my goals for a router table here...
>
> The purpose of this router is to cut model aircraft parts in balsa in
> plywood, with 3/32 and 1/8" thick being most common. Based on my limited
> experience doing the same with a Fadal VMC, about 20-25 ipm is the most
> pratical for the 1/8" bits I'm using (I could experiment some more
> though, but I don't think it'll go over 50ipm tops). Rapid travel is not
> a issue. Thicker material can be done in multi passes, but would be rare.
> The maximum size is 12"x 48".
> Using a trim router, with bushing to handle the smaller 1/8" bit.
>
> The X axis would be supported on 3/4" or 1" hardened shafts, from
> McMaster Carr. I would make some short rails to put under the shafts for
> extra suport. The leadscrew would .200 pitch (5 turns per inch).
> Y Axis would be 3/4" support shafts, and probably the same or smaller
> diameter leadscrew as the X axis. The Z could have smaller shafts and
> leadscrew, and would not need a backlash nut.
>
> Note, I'm trying to do this as cheaply as possible, that means finding
> the "unbuildable" components as inexpensively as possible. I can machine
> most of the parts, but the Acme stuff is out. I would probably buy the
> McMaster Carr 1018 leadscrews, but need to find the anti-backlash nuts
> to fit these.
>
> Other notes: accuracy of .005 is more than enough.
>
> Other questions...can I use bronze bushing to ride on the shafts instead
> of the platform blocks? I know they'd have to be open because of the
> shaft supports, but thats not a problem.
>
> Could a anti-backlash nut be made with Acme nuts and a spring, figuring
> out some way to attach the whole shebang together?
>
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