Older "PR" series rail upgrade

[edcoleman]

This may be of interest for anyone out there with an older "PR" series shopbot. For a long time I've struggled with keeping my 'bot square and true (I rack the machine every now and then, more than I care to admit
) and I've always had trouble with the wheel geometry (riding on the edge of the unistrut).

I've had a set of Bishop Wisecarver Dual Vee rails in the shop for several months and have finally gotten around to doing an upgrade. I know others have done this modification, but I don't recall seeing any pictures. So if you're interested, check out a few quick and dirty snapshots here:

http://www.colemanwoodworking.com/sb_rails.html

The mod was easier than I had expected and I'm kicking myself for not doing it sooner.

[Brady Watson]

Looks great, Ed!

If you need some square tube to replace the round pieces/channel on the X-car, swing by the shop next week. I've got plenty of shorts in the scrap bin that we can cut to fit.

-Brady

[rustnrot]

Originally there were wheels on the "bottom" of the unistrut, to keep, in my opinion, I do not know this as fact, the car from riding up when doing plunge cuts. Does yours have the wheels underneath and if so did you "reconfigure" them?

How much $ is the rail and where did you purchase it?

[edcoleman]

Tom:

Right now I don't have the bottom wheels installed. I have a couple of ideas of how best to attach them, I might just reuse the mouting plate from the original configuration with a larger bolt and standoff.

The rails are from a company named "Bishop Wisecarver" (www.bwc.com (http://www.bwc.com)). I purchased them second hand, so I'm not sure what they go for retail.

-Ed

[billp]

Ed/Tom,
I've been watching this upgrade process as I'd also like to upgrade MY PR machine as well. I'm thinking that it might not be that hard to just adapt .25" "flat stock" as bolt on rails. They'd probably work best if the tops were ground into the shape of the V rollers, but after looking at the actual grinding machine Shopbot used for years while at the Jamboree we noticed that the tops of those rails didn't seem to have a bevel on them, and the machine seemed to work fine when it was in use.
Ed's standoff technique should make it pretty easy to just adapt the existing hold down as he mentioned, and shim it into the proper position.
I thought I remembered someone mentioning that they had hand ground down the bevels on their rails. Has anyone tried this, or have any ideas for a technique?

[gerald_d]

Do-it-yourself rails......

I'd probably start with "bright" steel flat bar (that has been cold-rolled) - this is dimensionsally more consistent, and straighter than, "black" hot rolled steel. Then use something like a radial arm saw to hold a grinding disc while pushing the flat bar along a fence......... (Get flat bar that is about a foot too long and trim the ends off after grinding)

[billp]

Gerald,
Thanks for the info. I DO think this is a "doable"upgrade for most of us. I'd been thinking of looking for some sort of carbide(?) router bit that would be used more for grinding than cutting, and then do something similar in a router table. This way I could use featherboards to keep the rails in a "slot" and avoid kickback. Every time I ever tried to use a radial arm saw for anything except cross cutting it was always more "adventure"than I bargained for...

[gerald_d]

.....that's why I said like a radial arm saw - meaning a way of mounting a static motor. Have a look at "carbide burrs". I expect less kick-back than with fibrous wood and a coarse blade. Grinding steel is easier for me than working with wood.

[gerald_d]

Here are two possibilities; radial arm left, table saw right:


148

Remember to clean all the sawdust out of the table-saw before making sparks.

[gerald_d]

These (http://www.grindtech.com/download.asp?filename=BURRS.pdf) are the style of carbide burrs that I have in mind.

[billp]

Hmmmm,
I had a totally different idea when you first proposed this. I thought the blade would be set at an angle to the table and the piece would be passed along it, as though it were in the "ripping" mode. This would give it a larger bearing surface if a carborundum disc ( or something similar...) were used.
If I am reading your pictures correctly you are suggesting to use only the tip of the disc ? OR is that something like a grinding wheel mounted in the arbor? Either way I'm thinking that the flat "side" of said disc would probably create less chatter. Of course I only work with metal when I absolutely HAVE to so this is still hypothetical to me...

[edcoleman]

Bill:

Considering how the original PR design had the wheels ride along the unistrut, would riding along the edge of flat stock be really that much different? The edge of the unistrut did not perfectly match the wheel profile, why does the flat stock have to match?

I worked on squaring the machine today...wow, what a difference...with the perpendicular rails I was able to adjust for square in no time and I'm sure I've got it setup better than ever.

One other thing I think I'll add is a scale along each side of the x axis along the top flat edge of the unistrut. With the scale and a sighting lens on each side (like those on the fence of a table saw) I will be able to convince myself that the machine has not been knocked out of square.

-Ed

[billp]

Ed,
I think you're probably right. Even now my wheels on the Alpha are not a "true"match to the profile of the rail. I'm thinking that as long as there is SOME recess, and enough of a bearing surface to keep constant contact, the machine will be fine. It sure can't be any more tenuous than the 45 degree angle we started with...
I like your idea of the scale, but with the parallax factor thrown in you might still not be sure exactly where the axis is. I'd probably use your scale, and one of the little (3") "engineer squares" to check positioning, this way you KNOW exactly where the edge of your Y axis is sitting on the scale.

[gerald_d]

Bill, as you have indicated, we are not aiming to get a "perfect" profile with the grinding. Standard "flat bar" is relatively soft (no harder than unistrut) and the hard V-roller will soon "mould" the rail to shape. I am suggesting a normal grinding disk like you would use in an angle grinder, and I would guess about 3 - 5 passes each slightly deeper. The last pass of course being a very fine one. With the first few "heavy" passes the wear of the disk might be a bit much and the fence may have to be adjusted.

Experimenting in a table-saw is probably going to be quicker than discussing it here.

[gerald_d]

dud post

[gerald_d]

Ten minutes (including setup and teardown) at the table-saw:


149
150

[gerald_d]

151

[benchmark]

This is a picture of an old modified PR96, I found the V bearings were creating ridges on the Unistrut so I bolted some 2 x .25 cold drawn steel to the top of the unistrut. This ran for about 18 months without any problems.

Paul

[benchmark]

Here


152

[rustnrot]

Am I wrong in my opinion that the grooves that are cut in the unistrut are good, not bad. In other words the grooves make for "smoother" running??

[billp]

Tom,
As long as you don't rack your machine, or do anything to CHANGE the grooves things usually work well. Over the 6.5 years I've had my cable drive/PR machine there have been several grooves created, and after awhile you can have an effect similar to a train trying to switch tracks. Then things get weird...

[davidallen]

Ed and Bill,

mount a laser pointer with a line lens on your carriage. no parallax. I've seen some as 'novelty' items in both 'dollar' stores and Ebay.

da