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  #31  
Old Fri 06 April 2007, 00:11
Charlie T
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Dont they use the smaller bearings though? Size two not three?

That picture looks like it would have alot of flex with that leg so long.
  #32  
Old Fri 06 April 2007, 06:16
Gerald_D
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That tall leg was for a special application. But I would be more worried about the thin rail "cap" flexing to the right between its mounting screws.

Vadeem, you will have to recognise that guy's experience is very rare compared to the thousands of other people who have SB's. As Charlie points out, they use size 2 rollers. And they used bent 3/16" plate as the "angle iron".

Here is a comparison, to scale, of the two different systems and why an un-hardened rail does give excellent results with a MechMate:


The small wheel is the size 2 used by ShopBot, the big one is the size 3 used by MechMate. However, ShopBot did not grind a very accurate and sharp "point" on their 3/16" rails - when new, the rails still had quite a large flat area on top. After bedding in, the SB rails developed a much sharper "point"....and then tended to stabilise with no further "wear". (Actually, they were not wearing as much as smearing - like putty)

By using a thicker rail, and specifying that it must be "sharpened" with only a 1mm point on top, there is maybe twice as much steel in contact with the wheel when new, and there is no bedding in or smearing. Plus, the bigger diameter is also much kinder to the rail surface - compare different wheel diameters rolling on sand....
  #33  
Old Fri 06 April 2007, 09:47
Gerald_D
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Here is the tale of the tape:


The 1.5mm on the left is what we see on the tips of our original ShopBot rails where no roller has run before. Where the rollers have bedded down, that increases to about 2.5mm. The MechMate rails start with a lot more than that (+3.5mm) and just stay there.

Our MM rails, made on a milling machine, have some tiny tool marks on the surface. We originally thought they would disappear with time, but there is absolutely no change.
  #34  
Old Sat 07 April 2007, 20:08
J.R. Hatcher
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Does anyone know how shopbot grinds their rails??
  #35  
Old Sat 07 April 2007, 23:48
Gerald_D
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Apparently they built their own grinder. Controlled it with their own controller and stepper motors. The grinding head is said to consist of a pair of belt-sanders. Quite few shopbotters spoke of the need to dress the rails with an oil (or diamond) stone to improve the ride even when new.
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