Does this make sense??

[Dman]

I am going to upgrade my leadscrews to 5/8 x 6 acme from 3/8 x 16 all thread. I need to make some sort of AB nut for the setup. I currently use the self tapped delrin plastic nut for my screw which is done on alot of the routers here on the zone. It does work well but I notice there is some tight spots that the router will sometimes lose steps if I push the feed up to 55 or 60 ipm. I have no problem running 45 (which I currently run at). I notice the screw whipping some at the higher speeds also. My idea is to make a housing for a standard acme nut to mount it to the gantry and a bracket to pull the nut towards the front of the gantry and take up the slop in the threads and also place another nut and housing at the back of the gantry pulling it towards the back of the gantry and take up the slop in the threads in the other direction. This should (as I have it figured in my twisted mind) take out the backlash from the system and at the same time stabilize the screw and eliminate whip. Please feel free to shoot holes in this theory.

[Madclicker]

I think this makes a lot of sense. I've been planning this approach for a while. I think it will also contribute an "anti-racking" effect to the gantry.

[Dman]

I think I am going to try it. Maybe this weekend. I'll let you know how it turns out.

[yukonho]

With the much more aggressive pitch of the lead screw, you will not need to spin it as fast to acheive the same IPM. That alone will eliminate the whip. Also you are going with a much thicker lead screw again making it far less prone to whipping.
The one thing you have not told us is how long this screw will be, which of course makes a big difference in it's likelihood of whipping.
Depending on the size of your machine, you may want to use HDPE (cutting board material) for the lead nut. It is low friction and eliminates backlash, I have been using mine for a long time now with no signs of wear.
The sticking on your allthread is likely caused by imperfections in the threads. Allthread is made quick and dirty and not to any tight tolerances. Going to a smooth ACME screw should eliminate sticking alltoghether, and the HDPE rides really smoothly on it.
Another note is that steppers are stronger the slower they run, so going to a more coarse leadscrew may improve your chances of running "missed step" free at the feedrates you want to attain.
Colin

[spalm]

I agreee with Yukonho. One more thing, did you mount the screw rigid to the frame (and not just use motor couplers)? Try spinning the screw without the leadnuts attached and find out what the max spin rate is. Try to make this as smooth running as you can before attaching the nut. I don't think you want to use the leadnut to solve a whip problem. IMHO.

Steve

[Dman]

My screw length will be 48" on the x and 36" on the y. I know that I can run the steppers slower now with the lower tpi screw and get the same speeds but I would also like to get the most out of the system I can and I thought this idea may help. I have my current screw setup with bearings a both ends with a little tension to help with whip. It does spins very well without being hooked up to the gantry. I really don't think that I will get the whip with the larger diameter screw and a slower speed on the motors but if I ever upgraded my steppers (currently 200oz hobbycnc motors) or went to a higher powersupply (24v now) I would like to not have to mess with my screw setup.

[yukonho]

Tensioning them a bit will certainly help. 4 feet is a long way for a leadscrew to go. Pictures would be great if you have any.
Colin

[Dman]

Just got a digital camera for xmas. Soon as I figure it out I will make some pics of my first attempt at cnc. I have had it running for about 6 months now and see sooooo many things I want to change. Another machine is in the plans. This site is just awesome for ideas. Can't even imagine making a cnc router without this site.

[ger21]

Just trying to spin a 4ft 5/8" screw is asking a lot for a 200 oz stepper. Those screws are heavy, and might need a much lower acceleration to get them spinning. You may not even be able to spin them fast enough to whip. I'd try it with a single nut, and see how it works. If you make a Delrin nut, try making a longer nut, about 2-3 inches long. That should help control any whip you may get.

[Dman]

I just got done in the shop and made a sample of a delrin nut and it seems to fit tight (no backlash) but spins pretty free. I will try this approach first I think. My goal in the end is: I would like to be able to rapid move 80/90ipm and cut at 50 to 60 ipm.

If I can get 50ipm (16tpi screw) now I could run my steppers half the current speed and still be running faster than I am now (6tpi screw). So by bumping the steppers to say 60% the current speed I should be withing my goals.

Do you guys think this is possible or is the added weight of the screw going to be that hard to overcome?