8020 Bearings

[bryanrabb]

I have had a problem with "slop" in my bearings, even after they are shimmed. My question is has anyone ever put a couple of allen set screws along each axis to snug the uhmw inserts against the extrusion instead of shimming? This way they could be adjusted for wear.

[vacpress]

having never seen the 80/20 bearings, i would give you a qualified 'try and see'

there will be more drag.. its like any linear system.. more friction on tigher linear componenets removes slop, but increases wear and tear... the trade off...

the other thing to mention is that there are at least 3 or 4 machines on here using 8020 bearings... check with those (brave) people...

8020 says their linear bearings are not for precision applications.

hopefully this response helps, not hurts...
i suppose another idea may be to get some delrin and try that instead of the UHMW...

Robert

[mxtras]

That's a good idea - the Delrin. I think UHMW is just too soft for this - Delrin is more rigid, isn't it - or am I thinking of something else?

Scott

[vacpress]

delrin is supposed to be amazingly ridgid, yet 'slippier' than the similar nyon. drylin is another synthetic bearing material... although i am not sure if it is available in a stock form... the IGUS drylin componenets ive encountered did not seem to be similar to the delrin components ive made..

delrin is supposed to have poisinois, topically irritating dust. so dont breathe it..

How big is the machine, what is it cutting, what is the cutting tool, feed rate?

It may be logical to consider spring loading one side, or both.. i suppose that also depends on the method of actuation.... If I had this problem, i would probably try the allen screws first, then get irritated my nice 80/20 bearings are binding, and try spring loading.. then i would find that my router is vibrating too much for springloading to be logical, and i would get really frustrated and spend $1,000 on ebay, purchasing THK stuff...


but that is terrible advice!

[Chris D]

Originally Posted by bryanrabb I have had a problem with "slop" in my bearings, even after they are shimmed. My question is has anyone ever put a couple of allen set screws along each axis to snug the uhmw inserts against the extrusion instead of shimming? This way they could be adjusted for wear.

It is obviously very hard to interpret exactly what you are saying is "Slop" based on what you have written. Keep in mind, you could add enough shims behind those pads so that they bind solid against the rails. So, by saying you shimmed it, and it's still sloppy, well, it doesn't make much sense.

On my big router, I have UHMW pads and strips for the interface on between the saddle and the ways. Some are shimmed some are backed with aluminum plates which are pushed against the pads via set screws.

In order to obtain good, helpful information, it might be a good idea to take some pictures of the machine and detailed shots of the area you are working on. Then, perhaps take a dial indicator and measure the slop - that takes some of the confusion out of things. Remember...

A machinist's idea of "Slop" is a world apart from what a carpenter calls "Slop".

Chris

[bryanrabb]

The delrin is posionus? That would have been good information to know before I milled it. None the less, it is far superior to UHMW, and I get all I want free. Win, Win! I tried the set screw thing, and it worked great, with the gantry mounted, their is no play that I can notice.

As for "Slop", how can i define something that is non-definitive? I tried using the ****ty 80/20 plastic shims, those things suck. I understand the machinist's point of view versus a carpenter. Hell, the little end thingy on a tape measure has more "slop" than you can actually measure with a rule. I just ment it was too loose to be worth a damn. I am planning on building another machine with THK rails, ball screws, etc. I'd just built this one, to help me build that one. Make sense? I will post some pictures tomorrow, after I take them tonight.

Thanks all

[bryanrabb]

Vac press, it will have a 30 x 30 useasble area, it will cut mostly wood and maybe do some engraving. Who knows. I am just building it for the sake of building it. I will figure out what to do with it when I get there. IT is Hack's plan with my mods. Using xylotex parts, so it won't cut more than 30ipm in all likelyhood.

[vacpress]

pictures?

i have been collecting parts for over a year.. slowly.. i go through periods of extreme brokeness.. and, then, also, i went to sweden for a year... which was a wonderfull distraction from the mildewy smell of congealed cutting lubricant.

im glad the delrin killed your backlash. this bit of info will be usefull for other 8020 bearing people...

the real question: why dosetn 80/20 offer delrin bushings?

[vacpress]

speaking of the fuzzy definition 'slop'... a simple chart describing different machine disorders would be neat and usefull for us arm-chair engineer types..

lateral skew, axial binding, etc... terms we can use to communicate... i suppose there is probably something like that in the machinery's handbook (what isnt in there?)

[Chris D]

Originally Posted by bryanrabb The delrin is posionus? That would have been good information to know before I milled it. None the less, it is far superior to UHMW, and I get all I want free. Win, Win! I tried the set screw thing, and it worked great, with the gantry mounted, their is no play that I can notice. As for "Slop", how can i define something that is non-definitive? I tried using the ****ty 80/20 plastic shims, those things suck. I understand the machinist's point of view versus a carpenter. Hell, the little end thingy on a tape measure has more "slop" than you can actually measure with a rule. I just ment it was too loose to be worth a damn. I am planning on building another machine with THK rails, ball screws, etc. I'd just built this one, to help me build that one. Make sense? I will post some pictures tomorrow, after I take them tonight. Thanks all

You will certainly get a good, tight motion with THK rails!

As for the slop in the "Hook" at the end of the tape measure - well, seeing as you mentioned that :-) The hook has slop to account for the thickness of the hook. That way, when measuring with the hook, hooked on something when pulling AND when the hook is pushed against something, the thickness of the hook is compensated for with the slop. That way you get accurate measurements both ways - sometimes slop is a good thing :-)

Chris

[bryanrabb]

Chris, I had to spend about an hour explaining that to our production people. We manufacture corrugated boxes, and our accruacy is +/- 1/16. I had to explain to them that they should not put their finger at the end of the hook and push the tape against it. But rather pull it. I may as well explained thermodynamics to them or asked them to create perpetual motion.

[bryanrabb]

Vac, the delrin didn't so much kill my backlash as using the set screws, and a Vix bit. Can't belive I tried to do it without one. THat was just plain stupid. I can't belive that 80/20 doesn't make them out of delrin, or impregnated uhmw. It can't cost that much more. THey charge so much anyway, they could just charge more for a better quality product.