Making a coupler between motor and lead screw

[nikolatesla20]

Hi, I'm trying to think of a good way to make a solid coupler between my stepper motor and lead screw. I know several designs just use a piece of rubber hose, but I need a solid coupler since the opposite end of my lead screw is not going to be supported (no thrust bearing because my cnc is for lightweight hotwire cutting only).

Anyway..my lead screw is 1/4 inch thread rod and the motor shaft is 3/16". So currently I took a 1/4 coupler (like a really long nut) and just drilled a set screw hole in it for the motor shaft. Of course even the I.D. of the coupler is still a small bit bigger than 3/16, so if I tighten the set screw down too much the coupler becomes "off center" or tilted just a bit. But over the length of 2 feet of rod, it's a lot!

I'm trying to think of a way to make a coupler for 3/16 to 1/4 - the 1/4 inch part I would tap out so it just screws unto the lead screw.

Really, I'm thinking a lathe is the only real way to be able to do this..

At first the thought of using a drill press, but it's hard to keep the bit centered properly - for example, if you drilled the hole for the 3/16, and then switched bits to drill out the 1/4 (Drill all the way thru with the 3/16 so you don't have to move the part when you switch bits). I know in the past I've tried this before and it still didn't really have concentric holes.

I also have found one post in the forum about this sort of thing but I don't remember what they suggested to the fellow.

Is there any other technique I can use?

-niko

[Runner4404spd]

i cna make you a bushing like that if you want. email me at jczub@hotmail.com

[Mcgyver]

Well, you nailed it, the easiest way is with a lathe....and you've already got a generous offer!

You should achieve concentricity of the two IDs' in the drill press by drilling the 3/16 all the way through then opening part of it up to the tap drill size. Better yet, open it up in a couple of steps and then ream. Unless your drill is very poorly ground it should almost perfectly follow the 3/16 hole- I don’t know why this didn't work for you in the past - are you following the steps of centre punch/centre drilling/small drill/final drill? Good drills accurately ground and not run too fast? As it’s tough to perfectly centre something under the spindle, for a part like this I’d let the vice float so the tool picks up the centre pop/centre dill/pilot hole. Still the lathe is the easiest way to go.

I would use 620 loctite to attach to the motor. you can remove it easily with a bit of heat if needed. The 620 needs minimal clearance, so the reamed 3/16 hole should fit nicely on the adapter and maintain concentricity. For the tapped end on the threaded rod, use a lock nut tightened against the adapter rather than trying to figure out how to hold on to the leadscew to tighten it. Make the adapter and locknut from hex stock and you are good to go.

[nikolatesla20]

Thanks for the tips - actually yes I was using a nut against the adapter on the lead screw end, and for now trying a set screw on the motor end, but I'm thinking the loctite is probably better.

As for help maybe once I see how much slop my machine has when done. I'm almost done with it..the x axis runs very well but the Z axis is a bit cheaply made

-niko

[erase42]

actually niko, ive seen your design, and being a foam cutter for a living, im pretty sure with your machine you could get away with the rubber tube design. i wouldnt normaly suggest it, but i really think you will be fine. for a stiffer connection, you might use a larger hose with some resistance to stretch and compression. use some smaller hose to "shim" the shaft sizes out to fit the larger hose. the only reason I even suggest the hose is because of the binding you will no doubt get with a hard coupling. I know there were concerns about your smallish motors, with a solid coupling you will no doubt have some problems when your carriage gets close to the motor end. also how hard would it be to support the other end of the allthread? a metal tab, some washers and a couple locknuts would do you a ton of good with very little work.

[nikolatesla20]

Yes, I think eventually I'll go with the metal tab and some nuts. I actually have the machine running now using some basic solid couplers, I got the Z axis done today, it was very quickly designed. It's not the most accurate machine in the universe, but it's my first machine and so far it's working ok! At some speeds the motors I think resonate because they just buzz back and forth for a bit before moving. But I'm not sure if it's the software acting up, since I'm using KCam and others have said it's not the best. Eventually I'll try TurboCNC I suppose.

One advantage to the tubing and supporting the end of the lead screw with a "thrust" bearing (or tab and nut for light loads) is it should remove any sway in the lead screw movement unless of course the screw is really bent.

I can run the motors around 5in/min or 6in/min before all heck breaks loose, and even then for some reason at certain speeds the motors act funny (the buzz back and forth thing)

-niko

[nikolatesla20]

Whoa,

I just downloaded Mach2 to give it a run, and it works great! No motor weirdness at all, and I could easily run at 6in/min after running the motor tuning. The accelleration feature really helps to not miss steps.

Definitely have to stick with using this software for now. I'm very impressed! Drew up a sprocket in CAD DXF format and had it drawn out using a marker at 6in/min , measured it all around, and it looks great.

This stuff is awesome!

-niko

[erase42]

yes mach2 and mach3 are both very very nice programs. i think mach2 is only $150 US
Art, the creator is constantly upgrading and adding functions that people ask for. They have a great support forum on yahoo with questions usually answered within the hour. Mach3 is even better so maybe look into that before you pay for a licence.
What kind of motion do you get in you leadscrews when you try to run that fast? do they whip around a lot or are they too short to do that.

[nikolatesla20]

The lead screws are fine, my machine is only about a 8" by 6" total travel. I'll take some pictures later tonight to show you guys what it looks like. The X axis lead screw is only about 14" long or so, so whipping is not a problem on the machine.

-niko