Installing BWC rails on a PR

[waynec]

I'm just starting on my PR upgrade, a project that will include a new extruded aluminum gantry, a new Z axis and motor, an Ascension controller, and new BWC rails on the X axis.

My question is- How do I install the new BWC rails to make sure they are accurate? I'll be installing on the unistrut rails that are the stock PR rails. They seem to be very straight and coplaner, so I'm hoping I can just attach the BWC steel rails and shim them to be nice and straight. Is there more to it that this?

I'm sure other shopbotters that have upgraded to BWC rails can recommend a method to install them accurately.

Thanks for the help, as always.

WayneC

[bob_dodd]

Wayne do a search for "Ed Coleman" he has some pictures of his PR Bobd

[rhfurniture]

Wayne,
I have used bwc rails, but mounted them on new drawn bright steel sections. I am shortly to put them on my PRT Z axis and gantry rails, but will get a machine shop to mill a shoulder to mount them on. To me, "seem to be straight" and shimming are best avoided if possible. BWC rails are great though.
R.

[edcoleman]

Wayne:

Here's the post to which Bob was referring:

http://www.talkshopbot.com/forum/messages/28/7776.html?1116029913

If you do a search on the forum I think you should find other examples of upgrades (many more sophisticated). My upgrade is simplistic, but it works for my purposes.

Take a look and let me know if you have any questions.

-Ed

[waynec]

Thanks for the link. That's pretty much what I'm planning to end up with.

My question is how do I get these new BWC rails positioned accurately? That is perfectly level, parallel and straight?

I just picked up a laser level yesterday. I was thinking I'd set it at the correct height for the rail top, and mount the rail so that the top of the rail was just painted by the laser beam. Is this accurate enough?

I was planning to set one rail to a laser-set height, then use a sort of a temporary gantry to set the second rail by laser leveling the temporary gantry. Once this leveling is done, I'd go back and shim the rails to be parallel and straight, again with the laser beam as a reference.

Does this seem OK as a method? I'm certainly no machinist, so working for the kind of accuracy I think is needed is daunting.

Am I worrying more about this than I need to? I just think that if the rails aren't right, the gantry motion will be off- i.e. the gantry will be no more accurate than the rail installation. Is this right?

Thanks again for the help.

Wayne

[edcoleman]

Wayne:

For the rail positioning...I started with the stock PR which was set up level and square then offset the rails from the unistrut using nuts as a standoff. I adjusted level with a good 4 foot bubble level. How's that for low tech.

I'm sure that some might cringe at the accuracy of this method, and for machining tight tolerances this would probably not suffice. However, for my "craft" applications this was plenty accurate.

-Ed

[waynec]

OK. I'm probably overthinking this. My current rails seem to be OK too. I'll sure start here, and if that's good enough for the sign and woodworking I want to do, that's great.

Rignt now I have some problems with depth accuracy over the length of the table, so that if I spec a 3/8" deep mortise at two different places on the table I might get one at a half, and one at almost 3/8" deep. When I get to assembly it becomes a problem. I'd like to get within a 32nd over the table. But I don't think the problem is with the X rails, but with my bent gantry. Since I'm installing a new gantry, I expect this problem will improve greatly.

Does your method give you that sort of accuracy, say to a 32nd over the table? If so, I'll good with that.

Thanks for the reply.

Wayne

[edcoleman]

Wayne:

I have not noticed any problem with depth accuracy across my table.

Once you have everything set up reasonably square and level, then "flatten" your spoilboard everything should stay lined up provided you have no issues with your gantry.

-Ed

[waynec]

Good enough for me.

I started mounting the rails today. I made a jig out of a piece of angle iron that has several properly spaced holes in it. I clamped it tight against the X axis unistrut, and used a punch to mark the hole locations. Then I drilled and tapped them.

I tested the fit of the rail to the tapped holes, and they seem right on. There is no real way to adjust this other than to redrill or elongate the holes, but it does look nice and straight and level. So far, so good.

THanks for your input, Ed.

Wayne