X Extrusion sagging

[EricSchimel]

I have a 2008 era PRS Standard 4x8. This was the one that came with only 4 legs, not the 6 that you see on them now. I was checking my bot for square and level and I put a 6' Stabila level on my X axis extrusion. It's sagging in them middle on both sides.

It definitely needs some support in the middle. Has anyone ever bought, or had made a set of addon legs to go in the middle? They don't seem that hard to fabricate by a metal shop...

[Gary Campbell]

Eric...
A quick search using "table leg" returned this and a number of other threads. From the 2007 archives: http://www.talkshopbot.com/forum/showthread.php?392-PRSa-Center-Leg-Mod&highlight=table

[EricSchimel]

HA! I tried a bunch of other searches, none that simple though. Thanks again Gary!

[jerry_stanek]

I have used a cable system to jack up the center of trade show trusses. It would also work to adjust your table.

[bill.young]

Don't forget that your ShopBot doesn't have to be pool-table flat. Since you use the tool to surface the spoil board, the table surface will always be parallel to the motion of the tool.

That said it certainly wouldn't hurt to add a center set of legs if you have any flex and sag as the gantry moves down along the beam. My vote would be to call ShopBot and see if they would sell you a set of legs that are already drilled for the extrusion..it could be the easiest and quickest solution.

Bill

[dlcw]

I used 2x4's to make a third "leg" for my 2009 PRS Alpha. The part that contacts the underside of the X rail is cut to a V point. I tap the bottom of the leg toward vertical until the X rail is straight. I have an 8' level that is extremely straight. You can also just site down the rail.

But Like Bill says, before you start a big job that requires a lot of precision, surface your spoil board so it's "flat" according to characteristics of your machine. Before I start any cabinet cutting job I flatten the spoil board and use 1/8" MDF (surfaced both sides) to create a sacrificial board that the material to be cut sits on. This keeps me from actually ever cutting into the spoil board itself. Works great.

[EricSchimel]

So my spoil board is surfacing just fine. The problem I've run into is that I'm trying to mill some small HDPE parts with an onion skin on them. It works but it's definitely highlighting where my table it sagging... Sometimes I get a good skin and sometimes I go all the way through and lose parts.


This is all despite resurfacing fresh before each job. My 6ft level shows that my rail is sagging a healthy 1/8 so I think I need to address it... I suspect over 8 feet it's even more. Shopbot does sell a leg retrofit kit that is predilled and has all the hardware I need....

I'm hoping this makes my reliability go up as I'm cutting these parts weekly now... If it was just a one time thing I'd probably live with it...

[dlcw]

If it's sagging that much you can go with the inexpensive solution I did or get the retrofit third leg. Then surface your table flat.

[jerry_stanek]

I had some unistrut that I had left over and put leg levelers on them for my middle legs

[EricSchimel]

Yeah, I think the sag I'm seeing is pretty extreme. I'm getting a quote from ShopBot on legs, and I'm going to look into fabricating something of my own...

[EricSchimel]

Ok, I think I found a good solution. An entire leg set from ShopBot was a bit pricey (and frankly more support than I need to solve this problem)

I ended up getting a local steel fabricator to make me two legs and a cross member for a couple hundred bucks. I grabbed some feet from McMaster and the associated hardware. The feet will allow me to micro adjust the leg height to exactly where I need it. It'll all bolt up the same way my existing legs do, and can be installed without pulling have the machine apart:

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[EricSchimel]

Installed! I was able to get my table perfectly level with the adjustable center legs!

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