mechanical questions...

[charleyy]

okay, my background is architecture/woodworking, so i am not conversant with mechanical practices, theory, and common solutions that many of you pros and high-end amateurs are experienced in, so don't laugh too hard at my stupid newbie questions... (you can laugh some...)

1. on many designs -including the 7th sojourn of JOHN CONRAD KLEINBAUER (inventor of the homebrew cnc (sic))- there is both an end support where motor(s) and/or end bearings for the drive shaft are attached, *and* a motor mount plate of some sort... (here's the stupid part) what function does the motor mount plate serve ? why can't you just mount the motor directly to the MDF/alum/plywd/whatever end support itself and call it a day ? (i can see that you need a separate mounting block if you have the rod/rail suspended by the 4-screw adjustable mount, but otherwise, it just seems to be an extra piece of stuff for no apparent purpose)

2. for the delrin/whatever 'nut' to attach to the backplates being driven by the threaded rod, is there any benefit to making that nut somewhat (or much?) thicker ? would it help absorb potential 'whipping' actions of the rod (especially as it got longer) ? would it be more betterer if you had TWO such 'drive nuts' at each end of the moving table (or the other axes) ? would that distribute forces better ?

3. in contemplating the problem of attaching motor shaft to threaded rod, is there a possibility that making a coupling out of delrin or other high density plastic stuff would stand up to the forces involved, yet provide the -apparently necessary- flexibility to allow for misalignments ?

i am thinking that taking a block, drilling for the diameter of the motor shaft on one end, and for the rod diameter on the other end, then just glopping it up with some epoxy *should* (?) work okay... besides the drawback of being made a 'permanent' connection with the epoxy, would such an arrangement work okay ?

4. for an approximately 16"x24" moving bed machine with a dremel that is intended to do wood (and maybe some plastic/acrylic), what would be a good range of 'power' (ie oz's of holding force) for the motors to be ?

5. don't laugh too hard, but i am thinking about using a set of drawer slides (actually, they were used for a trash rollout) as the 'linear bearings' for at least the sliding table. they are of a high quality and have virtually zero slop in both axes (side-to-side and up-and-down), and is already in a pre-made frame that i just have to stiffen up with a couple pieces of plywd, slap on a table, drill for the rod to come through one end, and i've got a sliding table for little effort and expense... it *does* have a slightly variable resistance when one portion of the full-extension slides pull out far enough for the other portion to engage, but i'm not sure if that is an insurmountable problem or not...

any advice, speculations, and warnings are appreciated...
thanks

charley
charleyy@alltel.net

eof

[marvinstov]

Charley,
For Question 1 & 5. I built a machine using some high quality drawer slides. You can see some pics in the members gallery. It works great, just not as durable as some other methods nor would it be suitable for doing metal or high precision work.

http://cnczone.com/gallery/showphoto...cat/500/page/1

As for the motor block. I did use them on mine to get as much thread in the usable area as I could but you can see that it wasn't necessary because the nut itself never comes that close to the limit. If yours happens to be the same I would think that you could omit it unless it serves some other function.

Marv

[High Seas]

Originally Posted by charleyy ...i am thinking that taking a block, drilling for the diameter of the motor shaft on one end, and for the rod diameter on the other end, then just glopping it up with some epoxy *should* (?) work okay... besides the drawback of being made a 'permanent' connection with the epoxy, would such an arrangement work okay ? eof

Charley - Welcome aboard! The difficulity of using a plastic and and adhesive like epoxy - is getting it to stick! I use delrin for molds and resin casting and very little release agent is required - so theres a bit of a problem.
You might consider spider couplings - a three piece affair, 2 metal and a rubber joint in the center. Fairly inexpensive - check www.mcmaster.com check out page 1039
The rub with them is they don't hold well in tension - so you'd have to work out some sort of mount for the z axis motor.

I mounted motor directly - but my set up is a bit different - see System2 in the gallerys.

cheers - Jim

[ger21]

1) You don't want the motor to support the leadscrew, so you usually mount a bearing and then you need to offset the motor.

2) Depending on how strong you're motors are, you could add a lot of friction in the system. Other than that, should be no problem.

3) Like Jim said, glue usually won't stick to the Delrin. I'd stick to the JC method of the tubing and clamps, if you're building one of his designs.

4) 100 minimum, but more never hurts. It also depends on what you're driver can handle. You might want to look at http://www.xylotex.com. Best performance for the money.

5)I've seen a lot of machines made with drawer slides, so it will work.

[marvinstov]

Charley,
Gerry is right about not supporting the lead screw with the motor. I don't know if it's clear in my photo but my lead screws are supported and aligned with the motor by the opposite end of the lead screw and the lead screw nut. In other words, when I remove the motor coupling, the lead screw is in line with the motor. The thrust portion is taken care of by a thrust bearing arrangement on the lead screw opposite the motor. If the lead screw is not supported separate from the motor, your motor bearings may not last very long.

Marv

[ger21]

I forgot to add... JK has spent a lot of time on his designs to make sure they work. Unless you KNOW you have a better way to do it, I'd build it the way he designed it.

[High Seas]

AH HA! Now I know why all the discussion on the other thread regarding; bearing support at the MOTOR end! Gotta put that high on my list of "things to fix up" right away. Everyone learns something here!
Cheers - Jim

[chuckknigh]

Originally Posted by charleyy okay, my background is architecture/woodworking, so i am not conversant with mechanical practices, theory, and common solutions that many of you pros and high-end amateurs are experienced in, so don't laugh too hard at my stupid newbie questions... (you can laugh some...)

Well, then you're already an engineer. That will help you tremendously! You understand things like deflection, and real world tradeoffs.

1. on many designs -including the 7th sojourn of JOHN CONRAD KLEINBAUER (inventor of the homebrew cnc (sic))- there is both an end support where motor(s) and/or end bearings for the drive shaft are attached, *and* a motor mount plate of some sort... (here's the stupid part) what function does the motor mount plate serve ? why can't you just mount the

OK, the job of the motor is to *turn* the lead screw...nothing more. Having to support the screw as well, means that undue stresses are put on the motor bearings...stresses a simple "mounting plate" would have solved.

Think about it this way. First you build a machine which could be worked with handcranks. That means that your screw would have to be supported on both ends. THEN you attach a motor to it, and make it move under computer control.

2. for the delrin/whatever 'nut' to attach to the backplates being driven by the threaded rod, is there any benefit to making that nut somewhat (or much?) thicker ? would it help absorb potential 'whipping' actions of the rod (especially as it got longer) ? would it be more betterer if you had TWO such 'drive nuts' at each end of the moving table (or the other axes) ? would that distribute forces better ?

The thickness of the nut, after a point, is of no relevance. From the context of your question, I'm assuming you want to know if making it longer (more threads) would be of benefit.

The honest answer is yes, and no. Remember those real world tradeoffs I mentioned? Well, the cost will increase...the friction will increase (bigger motors to accomplish the same thing = more cost, again)...the complexity will increase (more parts interacting, means more things can go wrong). It's an unnecessary expense, and an unnecessary complication.

So long as your axis is reasonably well supported in the first place, the problems you mentioned (except for whipping) are more or less theoretical.

BTW, having a bearing at each end of the screw allows you to tension the screw...reducing whipping. Sizing the lead screw correctly will practically eliminate whipping, anyway!

3. in contemplating the problem of attaching motor shaft to threaded rod, is there a possibility that making a coupling out of delrin or other high density plastic stuff would stand up to the forces involved, yet provide the -apparently necessary- flexibility to allow for misalignments ? I am thinking that taking a block, drilling for the diameter of the motor shaft on one end, and for the rod diameter on the other end, then just glopping it up with some epoxy *should* (?) work okay... besides the drawback of being made a 'permanent' connection with the epoxy, would such an arrangement work okay ?

I don't remember anyone doing it this way, but it seems as if it could work. The tradeoff is that your lead screw and motor shaft would be permanently attached to each other. If you want to change motors, or upgrade your lead screw, you are SOL.

If you want a simple and effective way to couple them, make sure they're about the same diameter, and use a piece of fiber reinforced air hose with some hose clamps to join them. It's so cheap that it's practically free, and its flexibility will allow for some minor misalignment -- you can make a more elegant mount, later, if you so choose.

5. don't laugh too hard, but i am thinking about using a set of drawer slides (actually, they were used for a trash rollout) as the 'linear bearings' for at least the sliding table. they are of a high quality and have virtually zero slop in both axes (side-to-side and up-and-down), and is already in a pre-made frame that i just have to stiffen up with a couple pieces of plywd, slap on a table, drill for the rod to come through one end, and i've got a sliding table for little effort and expense... it *does* have a slightly variable resistance when one portion of the full-extension slides pull out far enough for the other portion to engage, but i'm not sure if that is an insurmountable problem or not...

Why would we laugh? Lots of people have built machines using drawer slides. They work pretty well, too!

http://www.pathcom.com/~vhchan/cnc2/cnc2.html

Just remember about those tradeoffs, and plan ahead for upgrades.

-- Chuck Knight

[charleyy]

thanks, gearheads, i didn't consider the support of the threaded rod when asking about the motor mounts; obviously a critical consideration...

even though i bought JOHN CONRAD KLEINBAUER's plans, i am going to deviate from them because i am making as much of a junkyard machine as possible... (all of it scrounged but the motors, controller, s/w and roller blade bearings...) i understand how he has (more or less) engineered his machines to work fairly well as designed; but certainly the general design is amenable to variations that should work as well (or better ?)...

i guess it will be easier to spring for a couple bucks for the tubing/hose clamp couplings that are proven to work satisfactorily; besides, i *do* already have a couple dozen hose clamps buried in the shed somewhere... somewhere...

since i got she-who-must-be-obeyed to pick up some lag bolts last night, i can go out and attach the new drill press to the shed floor and finish setting that up... just came up with an idea (okay, i copied the method of a cheap mitre box i have) of using a shopmade cam clamp as holddowns for the stock on the moving table, and the drill press will make the job of making them pretty easy...

even though i have yet to do a lick of work on *this* machine (other than the exhausting tasks of daydreaming, sketching, and imagineering), i'm already thinking about the next one... let's see, if i use I-95 as the X-axis, and I-10 as the Y-axis, with the Space Shuttle gantry for the Z-axis, that will give me an effective area of 300 miles by 200 miles by 150 feet ! ! !
i can cut out my own little state ! ! ! hhh

...okay, step away from the cnc forum before somebody gets hurt.

charleyy
charleyy@alltel.net

eof

[Chagrin]

Just throwing in my 2 cents...

1) Push on the shaft of your stepper - many (all?) of mine have about a 2mm give to them. Aside from bearing wear this little bit of play is undesirable.

2) Delrin would be too stiff. The hose/clamp method is the most economical, and as to "why" it's necessary it's due to the near impossibility of getting the motor and shaft perfectly aligned -- even the slightest bit of angular difference results in a seemingly huge amount of runout somewhere in the system.

[foamcutter]

Charleyy,
I was going to add something but everybody else has covered it all well. Don't worry about not knowing much and don't be ashamed about it if you are an engineer, we will speak slow enough and repeat ourselves so you can understand! Only Kidding guys, chill. LoL? Seriously - No Question Is A Dumb Question! Someone here will have an answer that will help. Go for it and good luck! Ron

[chuckknigh]

Originally Posted by charleyy i guess it will be easier to spring for a couple bucks for the tubing/hose clamp couplings that are proven to work satisfactorily; besides, i *do* already have a couple dozen hose clamps buried in the shed somewhere... somewhere...

If you ask nicely at Home Depot/Lowes, they'll usually GIVE you a few inches of that hose for free. Seriously. Especially if you get a frustrated wannabe engineering student to wait on you. Make sure they understand you want only three 1 inch pieces, and let them know what it's for.

Eh, even if they don't give it to you, it's still around 50 cents for a foot of it.

even though i have yet to do a lick of work on *this* machine (other than the exhausting tasks of daydreaming, sketching, and imagineering), i'm already thinking about the next one...

Sounds about right...and it only gets worse while you're building! If you've got the engineer gene, you'll redesign your table DURING construction, about 2-5 times.

Welcome to the club.

-- Chuck Knight