Linear Bearings and slideways

I have just bought an NSK Linear Bearing and Slide for a test rig build. I am surprised that the mount holes for the bearing are 6mm diameter
(that is 6mm not clearance for 6mm bolts). This means that the units must be mounted with 5mm capscrews. Just seems wrong to me! Has anyone else used such kit and seen the same problem? The slideway is the same but counterbored.
Not had a chance to speak to NSK yet.
Richard
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On Sun, 06 Sep 2009 21:56:07 +0100, Richard Edwards

I would suggest that they need to be mounted with 6mm shouldered cap screws in accurately located holes. The whole ideal is accurate location rather than letting the slide flap about any way it want's to go :-)
Mark Rand RTFM
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On Sun, 06 Sep 2009 23:45:40 +0100, Mark Rand

I fully understand your comment however :
6mm Shoulder cap screws have a head diameter of 10mm. The counterbore in the Slide Rail is 9mm diameter. Also The "Index" edge of the rail is discussed to be adjacent to a shoulder on the plate that it is fixed to. The Torqueing sequence is designed to "force" the rail to the shoulder. We now have two registers, the shoulder edge and the shoulder screw shank "fighting" each other. Not good practise IMHO.
Standard 6mm capscrews do pass through the holes (threaded shank measured at 5.83mm) so it would be feasible to use them for the bearing, assuming that the threaded holes are machined cock on. 6mm standard screws could be also used for the slide if the heads are turned down. Nothing with regard to the screws is discussed in the mounting instructions.
All of the above goes against Unbrako recommendations for "Close Fit" tolerancing (quote below)
Close Fit: Normally limited to holes for those lengths of screws threaded to the head in assemblies in which: (1) only one screw is used; or (2) two or more screws are used and the mating holes are produced at assembly or by matched and coordinated tooling.
Here Unbrako suggest a clearance hole of 6.4mm
I hopefully will speak to someone "in the know" at NSK today.
Richard
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On Mon, 07 Sep 2009 06:43:50 +0100, Richard Edwards

Correction - The Counterbore is 9.5mm diameter. (still needs the capscrew head to be turned own to 9.3 or thereabouts.)

Richard
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On Mon, 07 Sep 2009 08:01:03 +0100, Richard Edwards

I seem to be contnualy replying to my own posts <G>
Spoke to NSK yeserday and they confirmed that the bolts should be 5mm. I intend making some thick washers!
Richard
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I have nothing to add on the bolt size, just didn't like seeing you talk to yourself :)
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On 8 Sep, 13:09, Cliff Ray <namnocATgotadslcouk> wrote:

Perhaps that the only way he gets a decent conversation.
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On Sun, 06 Sep 2009 21:56:07 +0100, Richard Edwards

I presume you are referring to 'Profile Rail Guides'. I've used them extensively and they do indeed have apparently large holes. The rails are dimensionally very accurate to one (reference) edge, but not particularly straight when free standing and the hole positioning is not particularly tightly specified - they cannot be used for aligment. The recommended mounting was in a groove, with the reference edge of the rail pressed firmly against one (the 'good') edge of the groove by a removable or expandable wedge between the opposide side of the groove and the non-reference edge of the rail. An open groove or butress will do fine if you can arrange a reasonable clamping method.
The loose screws were specified so that they could not influence the alignment and are solely for retaining the rail and keeping it flat against the accurately flat base of the groove. Screws fitting tightly in the holes would completely compromise the accuracy of the rail straightness which should be set by the reference edge and the mounting butress only - plus the flatness of the base of course.
The wider, heavier load capacity rails are obviously stiffer, but the same scheme was recommended. It may not be significant in your application - we were looking to maintain straightness of 20mm rails of under about 10 microns in two planes over about 6m, but at least give the rail a chance and use the correct screws so as not to inadvertantly distort it.
Richard (another one)
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Richard Shute wrote:

Thanks for that, I just learned a lot.
-- Peter Fairbrother
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On Wed, 09 Sep 2009 00:32:16 +0100, Richard Shute

Hi Richard
Thanks for the post, I have been away so delay in response. I fully understand that the rails should be registere to the index edge and that the screws are for clamping only, however I still cannot relate to the "excessive" amount of clearance!
I consider myself very lucky that I do not have to work to 10 microns over 6m <G>
Glad that I was not just talking to myself <G>
Richard
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