Reply with QuoteI think you would be better off with the xl pulleys instead of the mxl. They are 3/8 wide instead of 1/4.
Ben
Pulleys
So, in the spirit of Pulleys. I have only a certain about of room to work with the height between slabs:
So, I found some gears that arrange as such:
The smaller one is PN: A 6D16-018DF2508
0.458 pitch
The larger one is PN: A 6A16-080NF2510
2.037 pitch
The belts:
https://sdp-si.com/eStore/Catalog/Group/213#
I picked a few random belts:
So, do you guys think the gears will work? I will pull the trigger and purchase them. I have to have the RM2505 screws turned down to a 0.3125 shaft.
I think you would be better off with the xl pulleys instead of the mxl. They are 3/8 wide instead of 1/4.
Ben
I agree with Ben, you need to design your system based on the loads you expect, not just what looks "pretty"!
Do a search on the thoughts of Mr. Gecko (Mariss Freimanis) as he has put a load of info on this site.
Art
AKA Country Bubba (Older Than Dirt)
I didn't go with what was pretty, I went with what would fit. Man... you guys think that much of me?? :P
My constraints were 1/4" bore for the servo shaft, and a main pulley that was under 2.125" tall. Really everything else I am open to suggestions.
Here are XL pulleys for a 3/8" belt.
smaller pulley outer diameter: 0.68
larger pulley outer diameter: 2.017
The height as pictured before is 2.125 MAX. The height from the center of the RM2505 screw is 1.1024. As you can see in the picture below, the top of the gear almost touches the underside of the slab above:
(pay no attention to there being 2 large gears in the picture - but the outline of the ballnut is accurate)
SO, since the gear will hang off the table, the underside can go below the surface of the bottom of the BK15 bearing block. I can shave (hopefully) 0.1" off the bottom of the bearing block and this should accommodate the table sliding over everything and not touch the belt.
Or, mill a tiny extrude where the block goes. Probably easier to toss the small block in the vise, know what I mean?
EDIT: I will have to mill out a shallow 0.26" deep, by 2" wide channel (or wider depending on the belt) for the length of the servo motor. Glad I looked into the dimensions of the servo. ugh. I hate milling steel, but oh well. It has to be done to sit the motor under the table
The large circle on the right gear represents the body of the servo:
Last edited by nateman_doo; 02-21-2014 at 09:25 PM.
Don't forget to take the thickness of the belt in for consideration. I do think your doing an excellent job with your setup and machining of parts.
Ben
Thanks! That is why I am going to shave the underside of the bearing & floating blocks about 0.1". That should make up for the belt. I have to mill a slot to clear for the servo, and it has to be wide enough for the servo to slide back and fourth, unless I make a plate with an adjustable take up pulley to take up the slack.Originally Posted by bhurts
Looks good so far. I know you had considerable trouble and trial and error on the way covers on your other build. Have you given any thought to how you will accomplish that on this build?
I think it might be too wide to do a solid way cover for the Y axis, but a nice sheet metal telescoping one on each side would be doable.
Just design that part now so you can make attachment points. Better to be done sooner than later I think. That is also a nice thing about learning to design full machines or axes in a 3D CAD program. Let you see what might work before you ever sink the first drill bit.
Look forward to seeing the progress. Good luck with it.
Lee
I am keeping the bellows in mind. I looked them up the other day in fact. They are 4" tall, and I have 2.125 to the bottom of the 1" plate, so I will be sitting at 3.125. The bellows for the X&Y will be 0.875" above the plate surface, which is no big deal for the Y, and the X will have an additional 1" thick fixture plate on top, so its good to go. That was another reason why I added those saddle plates. Also so i can remove any plate and re-seat them with dowel pins into that fixture plate.
I am ordering the ballscrews as soon as i get conformation that I can have an additional 0.5" extra long 0.25" diameter shaft, so I can mount the encoder on the far end of the shaft.
I rescind my endorsement for HTD belts, turns out they have more clearance than trapezoidal tooth form. Granted mounting the encoder to the ballscrew shaft would prevent this from causing positional errors but it may still increase servo dithering as one axis performs a heavy cut while another attempts to hold position. The GT series belts show the least clearance.
That's half the story, now consider belt tensile properties. This is a great table to compare the relative merits of belt tooth geometry when most other factors (belt material, pulley sizes, speeds) are held constant.
Looks like a lot of flavors for GT series belts but GT2 is the most readily available. Now lets assume those servos are tuned for some aggressive accelerations and routinely use 1/2 their max torque rating of 350oz-in, that is 10.9in-lb. To fit the space constraints looks like the small pulley will have a pitch diameter of ~0.6" so the belt tension would be 10.9/(0.6/2) = 36lbs. A 3/8" wide XL belt would only have a recommended working tension of 28lb*0.375 = 10.5lb! A similar size 5M GT2 belt 15mm wide would have a recommended working tension of 160lb*15/25.4 = 94.5lb.
Lastly, compare belt stiffness. The 5M GT2 belt has a modulus of elasticity 3 times that of XL assuming common fiberglass reinforcing members.
Belts may be a small component of the overall machine design but when maximizing rigidity, every bit counts. Not to say XL belt absolutely wont work but it may be a weak link of this system.
My preferred source for timing belts and pulleys: Timing Belt Pulleys | B&B Manufacturing
You sort of went over my head there for a minute. What I got out of it was you need a stiff belt to hold position. I purchased the belts that were Polyurethane and Kevlar tension member. What exactly is servo dithering? I think I understand what your getting at by your example. The belt has to keep the axis not moving very rigid.
Basically the XL belt will be loaded 3 times more than "recommended" during quick moves or heavy cuts whereas the 5M GT2 belt is within its rating. Exceeding the recommended tension intermittently is acceptable but will increase wear and possibly cause the belt to skip which are already increased risk factors when using the smallest pulley available. Also consider the 5M GT2 belt has less "play" and is 3 times stiffer. Since 5M GT2 and XL belts/pulleys take up about the same amount of space, your particular application would be best served with 5M GT2.
Dithering is when a servo is "hunting" to find or hold position - occurs often when the commanded position is between encoder pulses, there are externally applied loads that oscillate (ie big slow turning end mill hammering away on steel), or bad motor tuning. Mounting the encoder on the ballscrew will improve positional accuracy but the motor now turns more per encoder pulse so it will dither more. This is further exacerbated by any stretch or play in the belt.
I was looking at the sight you suggested, and it doesn't seem to have as desirable match to my constraints, for a 5mm GT2 pitch.
Here are my constraints:
servo diameter 0.25" bore
Ballscrew shaft 0.375"
Larger gear MAX with belt is 2.125 (thats with zero clearance, so id stay a few hairs under that)
None of them can match belt sizes, or it goes right to a larger diameter gear that will not fit as the table goes over the system.
I used xl pulleys for a long time with the equivalent size motors that you plan on using. I did not ever have any issues with the belt slipping a tooth. There will be some stretch with any belts you choose but it is minimal. Zach_G is correct that you are on the limits of what the belts will do but in my experience the only time I ever came anywhere near peak torque was during crashes. I have since gone to slightly smaller motors with high reduction gear boxes but I'm using r&p so its a whole different ball game. I also think you will have more issues with motor tuning if you put the encoders on the screws. Any movement of the motor that doesn't produce any counts on the encoder will cause the system to go unstable unless loosely tuned which may or may not be ok in your situation. The best way to get what your looking for is with a double closed loop with the encoder on the motor and glass scales on the axis as well. The only relatively low cost controller that supports double closed loop is k-flop as far as I can tell. Like everything we do here its worth a try doing it like you plan to now and if it doesn't work as well as you like you can move the encoder to the motor shaft. If you want to do the double loop later that can also be added without any major changes to what you're already doing.
Ben
Sound advise. What is r&p?
Putting the encoder on the motor was my backup plan, but at least I could try it on the axis. I was looking for positional accuracy, and am striving to put that on my top list, however you all bring up the good point about the servo hunting for position. I have to look into what a k-flop is, and what double closed loops are. Will it take an average of both encoder inputs? thats on my to-do list for tomorrow.
I already bought the gears I listed on friday night I think, and they are in the next state, so I will have them in hand to at least get my feet wet before I dive into the more sturdy gears Zach suggested. This servo and gears aspect is very new to me. its been steppers for the last few builds for me. Direct drive no less.
Experience usually trumps analysis and since you already bought the belts, may as well give 'em a try.
Nate, I don't see why you need the gears to fit under the table really. You can still get max travel from a screw without the belt ever going under the table. Placing the nut on the outside of the table will still allow a screw shorter than the rails to gain max rail travel. Traditionally speaking. If you are going for an oversize table, then I can see the need.
Lee
I like the way you think. I am very thankful to have all of you aboard on this build right from the start, and will try to look into every suggestion you guys make. I already ordered these, so if they dont cut the mustard, I can always return, or just buy new what u suggest, I just have to look elsewhere for pulleys that fit my shaft (or make some sort of adapter?). here is the parts list so far:
Lee, you are correct, the table does travel over the gears so I have to keep them smaller. The Z axis will be slightly different. The X/Y aren't fighting gravity, only fighting the sheer forces of my naturally light cuts in copper/aluminum. (0.04-0.06doc @ 20ipm) The Z axis is fighting gravity with a HEAVY head assembly. I will defiantly get oversized gears and the 'table' won't go over the rail in the Z axis application.
Scored a pair of 35mm rails! The Z axis will likely be limited to around 12"... BUT... I think I can make some use of fixture plates. Most of my work is done on thin sheets of copper around 1/8", to 3/4". I still want to be able to use my Vise, but that takes up a good portion of the Z, so if I just get a thick slab of material to bolt on top of the plate, then I can do my work on top of that plate, and remove it when I want to work on parts in the vise. win.
The rails are KWSE35's, that have 6 rows of bearings vs my X&Y which are NSK LH25 series and have 4.
Carriages - KWSE35
I was convinced when I got confirmation that the head assembly for the IH mill head is 235(!)lbs with the motor. The tormach is almost half of that.
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