View Full Version : Play in linear bearings
27-04-2010, 09:07 PM
I am unable to find any information on how much slack there is in linear bearings. Also of concern is that there seems to be no way of adjusting the slack, no gibs such as you find in lathes & milling machines.
For a very small machine, say 200mm in X, Y and Z, is there a better way than precision ground rail and linear bearings?
28-04-2010, 12:25 AM
How much slack are you talking about? and I presume you arnt talking about the the round rail? Linear bearings are adjusted by changing the ball size, but thats normally just to set the pre load. I've played with the thk ones and there not to bad. although even the worn S/H ones I have don't have any slack as such, just roll easier
To my mind there is no better way than precision ground just cheaper options. for such a small footprint precision rails don't work out to cheap as the carriages are the expensive bit.
bit more info needed as you might be able to get away with using only one carriage per axis if the design and use allow.........
28-04-2010, 01:05 PM
I was thinking about the precision ground round rails, 20mm for the longest axis - probably for the others as well, though I may go down to 16mm for the Z. My guess (if I am wrong please tell me) is that unsupported is the best way to go if short rails are used (my longest rails will be 400mm) while the open bearings for supported rails would give more slack but reduced spring in the rails themselves.
29-04-2010, 12:13 AM
Dam thats just complicated things. I thought you were talking about the profile rails and carrages. Ignore the comment on adding or changing balls for preload, thats only for the profile rails I think.
Unsupported rails are ok 'ish' for short runs and thats why some use them for the z axis. Not sure for 400mm tho.....is that travel or total length. you mentioned 200x200 at the start.
I dont think that open bearings and supported rail are a problem as long as they are not subject to large side loadings (note there are varying quaility of these bearings and strength of the mounting blocks.)
To make a decision as to the best rails, you need to know what style of machine, what you plan to cut and what tolerance you require.
If its a fixed gantry cutting mdf to 0.5mm tolerance then you would probably get away with unsupported rails. mention Ali and forget it, there have been quite a few builds that have started with unsupported and then changed later at extra cost.
I seem to remember doing a cost comparison vs performance and the profile rails didnt come out much more expensive because you can get away with one one block and a much smaller size rail. and looking at the whole picture the lower mass also helps with smaller ballscrews, motors and controllers:smile:
29-04-2010, 11:26 AM
The 200x200 is travel, most of the work will be a lot smaller. Materials primarily aluminium, some PCBS, jewelery and the odd difficult part such as a titanium air turbine 22mm in dia (4 axis work). Cutters uniformly tiny and cutter speeds high - though titanium will present problems if the computer occasionaly stops to pray to Redmond. I will spend this evening looking at profile rails.
29-04-2010, 02:51 PM
if you are using very small (2mm?) cutters the cutting forces are tiny - a 2mm, 2flute cutter at 20000rpm and 800mm/min feed with a 0.1mm DOC endmilling on aluminium ineeds about 10w of cutting power and will exert about 1.5N cutting force (if the cutter doesnt break first!). At those loads unsupported rail is possible - the deflection at the centre of a 200mm length of 20mm rail is <0.001mm at 25N working load
29-04-2010, 08:17 PM
Should have guessed from the 200x200mm foot print that it was an engraver....:whistling:
Oh well that will teach me to start rambling without asking all the facts:smile:
There are ways to reduce deflection in unsupported rails like adding centre supports and if you can make the end fixings for both rails out of 1 piece and give it a good 50 to 100mm grip on the rail then the fixed end moment is able to help resist the deflection as well.
the only problem is the more you add the more it costs. Have you seen this site http://www.students.tut.fi/~kontkant/jv2.php.html
fixed gantry and only one rail and one block for the x and y axis. got to be comparable in cost to 2 round rails and 2 blocks and definitely much stronger.
29-04-2010, 09:18 PM
Engraver? Yes, I guess it is, but I tend to think of it more as a thing with a spinning cutter shapes that stuff by moving it three ways against the cutter. I must admit that I rebel against the idea of having only a single rail, too much like a narrow guage railway. On the other hand, Zapp have got some interesting profiles in their used/surplus section which I am trying to work into my Turbocad model. The topology of the machine in the site you pointed me to is similar to mine, except for the single rail and that my construction for the non-moving parts will be of 15mm steel plate.
29-04-2010, 09:37 PM
Most of the jobs I want to do will need cutters smaller than 3mm, and resultant high cutter speed so there shouldn't be too much load on the rails. However, I am beginning to be impressed by the advantages of precision profile rails - more for the reduction of free play than minimising deflection. It would be nice though if the manufacturers of the various rails gave us real numbers as well as drawings.
29-04-2010, 11:27 PM
Additional to my last question. I see that there are some THK 25mm sets ol linear guides and truck units on ebay, each set consisting of two rails and four trucks. Everyting seems to be right about them, but my only real question is, can they be cut to length? I know that it would almost certainly be possible to actually cut them with an abrasive wheel, but can this be done without damaging them?
29-04-2010, 11:30 PM
Cutting them is not a problem, I use a 1mm wide cutting disc in my angle grinder, goes through them like butter.
29-04-2010, 11:38 PM
Who's angle grinder ?????????
30-04-2010, 07:47 PM
I take it that you know about all the differnt types of thk? If not you want the 4 way equal loading like HSR or SHS, the SSR and SR can only be used horizontally, but if cheap enougth they would be ok for the x axis.
I under stand what you mean about the single rail but once you see a 25mm HSR you will realise just how strong they are,especially given the small cutters you are using. That said, I would probably use a pair of 15mm rails for the x axis and then single 25mm one's for the y and z
If you mount the x rails and blocks upside down then the block ('s) is (are) always directly below the cutter giving max strengh at all times
30-04-2010, 11:03 PM
I have ordered a set of two 510mm rails type HSR25 with four TA carriages. I haven't really settled on what geometry yet, but £90 for this set was too tempting - I can likely re-sell if they are not what I want.
I spent several hours pouring over the datasheets from various manufacturers and found THK particularly helpful - even though the TA carriages don't seem to be on their site (possibly discontinued or special order?) The most worrying thing I have seen so far is that linear guides seem to be extremely intollerant of mounting errors - less of a worry if only a single carriage is used.
I get the point about mounting the blocks and tracks upside down. Presently I am modeling various parts for my various options on Turbocad in the hope of settling on an optimum geometry soon.
01-05-2010, 11:37 AM
Good old ebay.
any system involving two separate bars, rails or what ever needs to be perfectly paralel. Even dovetails with adjustable gibbs only average out the highs and lows when worn. I think the THK's are good at absorbing small amounts of misalignment but you do need to get them fairly level and paralel in the first place.
the 5 bears site has a good 'how to' on setting up rails, using one as a master and dti gauges to set the other.
Are you doing a build log as well?
01-05-2010, 01:06 PM
I guess if I demand high accuracy the bits will have to be set up accurately. The latest info I have seen implies that the rails and carriages should be mounted on the same material - and very preferably steel.
I will start a build log before I even start cutting - sort of thinking out loud in Turbocad.
01-05-2010, 02:56 PM
If you are looking for super high accuracy then you will be facing the same problem as I have for setting up my lathe. and that is that even the bigest and best piece of precision ground steel will twist and defect if not bolted down evenly or to a flat surface. so it would seem it is more important to have a good solid base and if you torque everthing the same then it shoudnt matter if its Ali or steel (just my 2 penneth, I'm sure others will correct me if im wrong:smile:)
Have you thought about using a slab of granite worktop as the base and then building it in situ?
01-05-2010, 05:01 PM
I think the significance of not using different materials is that aluminium has a much higher coefficient of expansion than steel - this could be important as my workshop temperature has varied from 4 degrees to 30+ degrees. as far as the order of accuracy that I want, the magical 1 thou will do. I have got an old optical workbench (just a 1000mm by 600mm by 200 mm 'T' slotted extrusion, heavy enough to need two people to lift it) that may as well be the sub base.
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