I have already tried part # 1810 http://www.littlemachineshop.com/products/product_view.php?ProductID=1810

and it has way to much chatter and backlash.
LMS said the 1927 http://www.littlemachineshop.com/products/product_view.php?ProductID=1927

has been used in CNC conversions before, however this is now going to be my 3rd table purchase and im getting leary of the cheap tables (about time right).
What table is everyone using with good results in the 4 inch size?

Bummer - I was hoping to go with the #1810 when I build mine. I don't have any better options for you, but i'm subscribed and interested in what others have to say.

Mechanically, how does one increase accuracy and reduce backlash of a gear and worm screw device? there are no split nuts, and ball screws are out- That leaves tighter tolerances on the gears- is that it? What do you do about gear wear?

The only thing idea I came up with is to limit the rotational direction to one-way to eliminate the hysteresis, but that's an inconvenient solution at best.

you reduce the backlash by continuing the one direction of rotation. granted its not the bst approach but you need to find one with a better gear fit. get a name brand rotary table from KBC tools.

The larger sized rotary tables (6", 8" etc) have the worm spindle mounted in an eccentric sleeve such that when you rotate the sleeve in the rotary table main body the worm moves into or out of mesh with the gear wheel. Once adjusted you lock the sleeve in position. It doesn't solve all the backlash issues but it is a good start. I have my manual 8" Vertex rotary table, converted to 4th axis CNC, down to less than 0.001" backlash at 4 inches from the center axis (worst case). 180 degrees from the worst case position the backlash is effectively zero. It will need a lot of reworking to get it better than this.

So my advice is to avoid any rotary table that does not include backlash adjustment. Ask before you buy, or study the drawings/operation manual. Also ask for a copy of a typical test certificate/protocol. If they can't provide one don't buy it. It might be OK but its kinda to late once it arrives on your door step. What will you complain about "its not as accurate as you expected." The suppliers answer to this is "what do you expect for USD xxx." If you have a test certificate your reply to this is "I expected what the certificate says so I want my money back" Most suppliers will concede at this stage. Otherwise you may need to send the boys round (chair) .

Regards
Phil




Bummer - I was hoping to go with the #1810 when I build mine. I don't have any better options for you, but i'm subscribed and interested in what others have to say.

Mechanically, how does one increase accuracy and reduce backlash of a gear and worm screw device? there are no split nuts, and ball screws are out- That leaves tighter tolerances on the gears- is that it? What do you do about gear wear?

The only thing idea I came up with is to limit the rotational direction to one-way to eliminate the hysteresis, but that's an inconvenient solution at best.

I have a 4" VERTEX rotary table which I guess is just a smaller version of Phil's 8 incher. Very nice for the money and I plan to CNC it. It has the adjustment as described for the larger tables, worm in and out of mesh with the wheel. I think it is going to work when I CNC it.
I also made the mistake of buying a cheap one to start, but it was pretty poor even used manually. The most annoying thing is it was only about £20 cheaper than the Vertex.

It wouldnt be so much of a problem if the hand wheel was not so tight.
I can get the backlash and chatter down by adjusting the cam on the worm drive, but when it is tightened it takes 2 hands to turn the handwheel.

Maybe the solution is to add a gearhead to the handwheel before connecting the pully wheel. If the servo can't turn 3 or 4 to 1 ratio, I could just up the ratio to 10 or 20 to 1 and then adjust the software to compensate.
Does that sound like a good idea, or should I continue looking for a better rotary table?

Just a side note, when I took the rotary table apart, I noticed the manufacturer milled in some runs for bearings, but never put bearings in. The run is only milled on the table it self, not the body, so putting a set of loose bearings in changes the height of the table seat.

If you could make your gearhead zero backlash, it sounds like a plan.

reasonable pricing as well as quality
sounds like vertex to me too i'm sorry i didn't get one

you could score a nice big harmonic drive off of ebay and build something around that, they have next to no balklash, and can be found for under $40 some times.

I have the model #1927 from LMS. It is a very nice table with good B/L adjustment and very smooth movement.
I would say it should make a pretty good table to CNC and I do plan on converting mine.

Steve

How about the Sherline one? It's a 4" and available in CNC-ready flavor. I was thinking about buying this one after getting enough money for it.

I have made hundreds of parts with the Sherline 4". I machine aluminum aprox 1.5" diameter and 8", and use a tailstock for the opposite end (extra Taig spindle). I would say a quality built table for a 4".
It has adjustable backlash by loosing and raising or lowering the table itself-- not very good if your jigs rely on set height, but after it breaks in I have not had to adjust it in over a year.

I think the last time I measure for backlash it was around 0.008" at 6" out.

http://www.sherlinedirect.com/merchant.cfm?pid=127&step=4

This is what I have, it's a very nice table. Mine does have a couple thousands backlash but works great. I just snug it down so there is some drag.

I have the 1927 table, H/V rotary table converted to CNC(check my website) and it has been pretty decent, although I have not used it a whole lot, I did make a part on it this last week, 2" long 1" diameter with 1/4" end mill in 1018 steel and it did a very good job. I also had a 3" lathe chuck on there, so the end of the part was a good 5" out from the face of the rotary table.

This rotary table does have backlash adjustment and it works pretty well. It also has the clamps for the table(I removed mine) so you can clamp it if needed.

Jon