Idea for Zero Backlash Acme nuts

[widgitmaster]

I have been tinkering with the idea of an easy to produce delrin nut that will offer zero backlash! It will have a 1.5 dia x 3/8 flange to fasten it to the router, and another flange which is sawed through to the thread and have two screws to clamp the nut tight.

I hope the image uploads ok, I did a printscreen in AutoCAD and saved it in bitmap format.

Eric

[High Seas]

Fidgiting WidgitMaster!
I suppose the threads (for the shaft) are to be cut before cutting the nut in half (along the shaft axis)? Would there be some minimal spacers or something to keep the nut from binding on the shaft as the screws are tightened? Looks like a pretty simple approach - guess the 3 concentric circles are for mounting and just dont show the hidden lines thru the flange? No flame intended - just curious.
Jim

[widgitmaster]

Yes, the nut will be threaded first, there should be no need for spacers, as the clamping screws can be tweeked as needed.
My CAD skills are not the best, but yes there are missing hidden lines ... oops!

I have several ideas for this concept, one uses a round delrin bar, the others use a rectangular one. I'm going to see which is easiest to make in my limited shop!

[Geof]

A problem with this approach is that you are adjusting the two halves of the nut radially so this means the crest of the thread on the screw is going to touch the root of the thread in the nut and cause binding.

[erase42]

here is my 1.5 cents. create a section that can be adjusted a small amout along the dirction of the screw shaft then clamped or held by a screw. the threads on each half of the nut would contact would contact a different side of the thread groove elimintating backlash without creating an unreasonable amount of friction and also providing a large contact area between the screw and nut to handle large forces without quickly wearing down. easily readjustable if needed too. tricky part is drilling and tapping the threads AFTER the nut is cut in half. possible i guess by clamping the halves together in a lathe chuck for best alignment? let me know if anyone tries it.

Ed

[widgitmaster]

Originally Posted by Geof A problem with this approach is that you are adjusting the two halves of the nut radially so this means the crest of the thread on the screw is going to touch the root of the thread in the nut and cause binding.

I don't confess to have all the answers, I'm just trying out different ideas!

My other thought was to turn an OD of 3/4 and cut an NPT thread, then put 4 slots on it and make a mating nut with NPT thread. That would close the pitch diameter a little more evenly. My ACME 1/2-10 plug tap is now cutting a little better, so there really is not much of an adjustment being made.

This does not take into account the force used to move the weight of the sliding unit! Each person has made them to suit their own needs, most of the people are not ME's and have not calcualted the force and stress into their designs. Yet everyone I sell to on eBay has asked me specifically what the endplay is on the delrin nuts I make to fit the rolled Acme threads! Their questions alone are the motivating force behine my experiments!


These $2.99 delrin nuts should be ok for the DIY wood & foam routers; however, I don't think I would use a delrin nut to cut the more resistant materials!

Eric

[Geof]

widgitmaster;

"My other thought was to turn an OD of 3/4 and cut an NPT thread, then put 4 slots on it and make a mating nut with NPT thread. That would close the pitch diameter a little more evenly."

Does this mean you would take two pieces of Delrin, one with an internal 3/4" thread, one with an external 3/4" thread, screw them together and then run your acme tap through. Now you adjust the backlash on the acme thread by slightly unscrewing the 3/4" thread and locking it some way. You would need 3/4" - 18 NF for this.

I have a knee mill, Bridgeport clone made in Taiwan, that uses this backlash adjustment method on the table feedscrew. It works very well because when you adjust it the two halves of the nut move together evenly and you maintain full thread contact around the whole diameter of the acme screw.

[widgitmaster]

Yes Geof,
My BP clone uses the same method, but the two nut sections are long, and are inside a tube (bore) by moving them together, the backlash is eliminated. That is a failsafe method, but I thought that it would be too long of a nut and too many pieces to make!

I realize I'm trying to re-invent the wheel, but something that will sell for as little as $2.99 and function in a simple environment for people on a limited budget, is my goal!

Not all those who build CNC routers have 30+ years in machine shops, they are building their dreams in their kitchens and closets, using the cheapest materials and using the least amount of tools!

My experience in all the shops over thirty+ years will be of most use if I can make a small, simple, easy to install, easy to adjust, widgit just for them!

Eric

[txcowdog]

Kitchens and closets huh? Well I've got that beat. I'm building mine in the attic.

[stirlingnut]

I used delrin to build the leadscrew nuts on my small engraver.I used the slot method,and have not had to adjust them in 3 years of hobby use. I made a tap from a piece of the acme screw material,and made simple blocks. I use a nema 23 motor for the X,and a nema 17 for theY,and Z axis,with plenty of power and speed for a machine this size.(X=12 inch travel,Y=8,Z=2.5)
Here is an old pic of one of the first things I did.I now have a better ball bearing motor on the spindle,and it works fine for me.