Leadscrew?

[bearwen]

Ok well what do you guys recommend?
I was thinking of ACME 1/2-10 but was looking at maybe going with a 1/2-5 or perhaps 3/4-5 for better speed and less whip? I am going to build the JGRO to plans except possiblly make cutting area bigger to say 24" x (36"- 40") I need a cutting area this size for what I want to do. I am using delrin blocks for the leadscrew nuts as per the plans and I have 262 oz/in motors I may perhaps build this thing out of angle iron or use solid lumber ie 2"x6" etc instead of the MDF for flex reasons. I have a 1hp router that I will be using also. I am going to be carving wood mostlly please give your input. I am wanting to build a quality Jgro and fix some of the short commings people have stated about it in the forum while keeping it as cheap as possible (this is my first machine)

Thanks

[Firebird0095]

I would suggest buying precision leadscrews instead of acme screws. You can get them from Kerk Motion, and they are really not that expensive. If you don't have good screws you have no chance of having a solid machine. If you are carving and doing 3D contouring then you will want a good screw with the least backlash possible or you will not have a consistent surface. The Kerk screws are much cheaper than ballscrews, and they are just as accurate as a standard ballscrew. You may also be able to find the profiled lead screws in MSC but they are probably more expensive. Kerk has this teflon coating that makes the dust wipe right off too.

To decide on what lead to get you need to decide what you want your minimum move to be. Take the lead of the screw and divide by how many steps per rev that your motors have and see if that is good or not. If you are worried about whipping, then you need to calculate the critical speed of the screw and stay below that figure. I doubt you will be spinning them that fast anyway. For the 36-40" long axis I would recommend the 3/4 or larger screw. The extra expense is worth it in rigidity.

[Oldmanandhistoy]

I would not recommend solid lumber will move far too much but a good quality multiply would be a good choice IMHO.

Good luck.

John

[spalm]

Couple of points IMHO,

As the Old Man sez, use ply or MDF. Look for quality plywood with lots of ply’s per inch. There is nothing wrong with MDF (really). There is a reason it is so popular. You just need to brace it and finish it. The JGRO design will not be improved with using solid wood, as the wood structure is not its problem; it’s the unsupported pipes (rails). A few of us here have turned the pipe/skate bearing design inside out, where the gantry presses in on the pipes which are supported by a torsion box, as compared to the JGRO pressing out on the unsupported pipes. See ger21’s, mine, lionclaw’s or joe2000che’s.

As far as leadscrews, the easiest/cheapest is ACME. Fancy screws and JGRO is an oxymoron. ACME screws are very accurate, just a little less efficient than ball screws. Go with 1/2” 10tpi, they are cheap and you can use double nuts to attach to the bearings. The lower thread count screws will reduce whip but will probably require a different method of attachment and negate threading your own lead nuts ($). 3/4” is pretty large for your motors, the torque required goes up with diameter. That said, I went with 1/2” 8tpi 2start and love them. I am using Dumpter’s nuts and shaft collars for bearing attachment.

Good Luck,
Steve

[mike hide]

Seems to me solid wood Is much stiffer than MDF primarily because it is no grain direction . Cantelevered if will break under its own weight. Plywood on the other hand has directional grain characteristics , the problem with it is only half the thickness is working in any one direction at a time .

[Oldmanandhistoy]

Originally Posted by mike hide Seems to me solid wood Is much stiffer than MDF primarily because it is no grain direction . Cantelevered if will break under its own weight. Plywood on the other hand has directional grain characteristics , the problem with it is only half the thickness is working in any one direction at a time .

Please don’t be offended and I see your point but, having worked in the timber and construction industry for 24 year and being a qualified wood machinist I know a thing or two about timber.

I’m not going to go into detail about why solid lumber would be a bad choice but I would never use solid lumber for construction of a cnc machine and a little study would soon explain why.

I never give an opinion unless I am 99.9% sure it is correct. Having said that we all can make mistakes.

I am not saying you should take my information as fact and I am never wrong.

Before you decide on your chosen construction material please take a little time to check out the pros and cons for that choice.

Regards, John

[ger21]

Originally Posted by mike hide Seems to me solid wood Is much stiffer than MDF

Just try keeping it straight and flat. Go to Home Depot and by the straightest, flattest 2x6 you can find. Put it in your garage for 2 weeks, and then go see how straight and flat it is.

[rweatherly]

I built mine out of MDF, but it does flex. The modulus of elasticity for MDF is about 400,000 psi. The modulus for wood or plywood varies by type, but both are in the range of 1,500,000. Flex is directly proportional to that number, so the MDF will have almost 4x the flex of plywood.

That said, MDF is less expensive, can be cut and tapped easier, and if you build something like a torsion box, it will "solid" enough. The flex in my machine is in the gantry. I will probably re-make it in plywood or add a lot of stiffners.

[bearwen]

Well lots of wood stuff I am really considering welding it up out of angle iron or square tubing not sure. I have looked all around the forums and seen lots of great ideas I am thinking of using alot of different peoples designs and what not but build it out of steel. What do you guys think I figure if I do steel I could possibly machine light aluminum as well and fix most of the flex problems? Rails is a problem I was wondering if I use like a 1-1/2" or 2" or bigger pipe this should help reduce the flex in the bearing rails? Maybe I will just well the 1" tubing on top of the angle iron ? well please put in your 2 cents.

Thanks
Berry

[ger21]

Originally Posted by bearwen What do you guys think I figure if I do steel I could possibly machine light aluminum as well and fix most of the flex problems? Rails is a problem I was wondering if I use like a 1-1/2" or 2" or bigger pipe this should help reduce the flex in the bearing rails?

Building it from steel will not fix the flex problems. The flex is in the tubes the bearings ride on, and even increasing the tube size will not help that much. To get rid of the flex, the bearing tubes need to be supported, and supported well.

[bearwen]

Ger 21,
I thought about welding 2 pieces of angle iron together and then set the pipe on top and tack weld the pipe to the angle iron for support the problem I have is bearings how do I build the bearing races to ride on the pipe I need to have support of the bearings in the x,y,and z directions correct they cant just ride on top and both sides cause then the gantry could lift as you lower the z axis correct I thought about the jgro type bearings but with the supported tube as I mentioned but is the jgro bearing any good or is there a better DIY way?

[mike hide]

Originally Posted by rweatherly I built mine out of MDF, but it does flex. The modulus of elasticity for MDF is about 400,000 psi. The modulus for wood or plywood varies by type, but both are in the range of 1,500,000. Flex is directly proportional to that number, so the MDF will have almost 4x the flex of plywood. That said, MDF is less expensive, can be cut and tapped easier, and if you build something like a torsion box, it will "solid" enough. The flex in my machine is in the gantry. I will probably re-make it in plywood or add a lot of stiffners.

Actuall wood has several moduli according to which grain direction one is considering . MDF has little strength other than in compression because it is homogenious by it's very nature. Now comsidering plywood, and this varies widely as far as quality is concerned . Probably "baltic birch " with good quality wood used in multple plys is probably the best but one must remember even though it is stronger than MDF only half the thickness is working to the best advantage [longitudinal plies]. So in the long run as I uderstand it solid wood still is a better choice provided the design takes into account of it's movement . Basically across the grain with changes in humidity and to a much lesser degree temperature . Regarding finishes,whatever you do to one side it is essential to do the same to the other side otherwise you end up with a unbalance moisture absorbtion situation causing stability problems .

Someone mentioned getting 2 by 4's from home depot .Most construction materials are not dried to the correct level for precision construction as discussed on this group . If using solid wood it should be either kiln dried or air dried correctly to the correct moisture content for your area .I find even then it is good practice if possible to leave it for a time in the environment where it ia going to live before milling it . That being said bearing in mind every time it is cut ,planed etc The internal stresses have to readjust,the stresses at the new surface being zero. So as far as I am concerned I cut
less and less material off the closer I get to the required dimension and each time leaving it a day or two to stabilize between milling operations .

I am probably wrong here but I always though the oft mentioned "torsion box" was exactly that, a closed structure with everthing glued up including the end caps. What a lot of folks descibe to me are open box beams which deform under torsion loads and do not do as well as torsion boxes. as far as the ballscrew structure which transits the gantry along the bed I would imagine box beams are sufficient because the torsional loads would seem to be pretty low ....Please feel free to critique I have already made my major boo boo regarding butress versus acme threads, major brain fart ....mjh