Dressing the rails on a PRT

[sbd1]

Hi guys, Happy New Year!

I've been spending some TLC time lately going over & truing up my 2005 PRTalpha 48x96 after getting a skookum new digital Stabila 4' level (0.05 of a degree/4' or 0.1 mm/m accuracy).

I read a few times in here (mostly from Brady) that it's relatively easy to hand file the rails while performing regular maintenance. What I'd really like is some hardened steel ones, but there's just no room in my budget for that now (and being Canadian means that anything from the US is super-duper expensive).

What I'm after here is detailed instructions/pictures/drawings, etc... of how to dress the rails so that they are flat from end-to-end (just talking about the X rails here). I'm not talking about un-straight (bent or wavy from side-to-side) rails, or even shimming the rails, I'm referring to cleaning up the top/bevels of my X rails so that my YZ carriage runs perfectly straight from X,0 to X,96. My confusion comes from the fact that the v-rollers don't actually sit on top of the rails (the vertical height of the YZ carriage is affected by bevels on which the v-wheels rolls, not the top of the rails), so placing my fancy new level on top of my worn & undulating rails doesn't actually give me a true indication as to where I should be removing material, much less, how to remove material with a hand file accurately from a beveled rail surface.

I have checked the table surface (after resurfacing it) & it appears to be very close to flat, so I guess that's really all that's important, but I would like to know how you guys dress your rails when they get worn.

Cheers,
Daren

[Brady Watson]

Daren,
There are no pics or videos, as that takes time & resources to do...You've probably already read enough to get the general idea. Without seeing the actual rails, it is hard to gauge the situation. If the machine was setup and maintained properly, there should not be any dips in the rail due to things being far enough out of adjustment/kilter to cause non-linear wear on the rails.

All instruction or suggestions I have given over the years have been along the lines of cleaning up roll-forged metal that sits at the bottom of the rail pyramid only - as that is where the material winds up and causes issues. On very worn rails the top of the pyramid will be sharp; having roll forged the flat down to a point. The sharp edge can be arbitrarily ground down with a bastard file, then in either case, buffed with a Scotch pad.

Straight-edging the rails can help identify low spots, as long as another adjacent part of the rail (angle leg) is used to verify that the angle itself isn't dipping. In many, many cases, the tool itself (X-car) is not riding on the rails properly because it is slightly out of square or something is out of adjustment. This should be checked first and verified using the 3-4-5 square method.

In any case, there is very little material to remove when dressing the rails. This is why the word 'dressing' is used, because it is in no way 'grinding' or removing a lot of material. Many times the rails look and more importantly, feel worse than they really are. With careful and deliberate work with a file and sandpaper/Scotch pad you can get things moving smoothly again. Avoid the urge to use any power tools...

-B

[sbd1]

Thanks for taking the time to clarify this for me Brady, no pictures are needed as your description was excellent. I had actually typed up a more detailed reply, but after clicking the wrong button it's all gone, so I'm try to summarize it again here.

Hand file off rolled steel bottom edges & sharp top edge - got it. Strangely, the drop in height of my X axis rails at the ends (where my machine does most of the work) doesn't transfer onto my table surface, but I assume that's because the Z axis is centered between the 2 widely spaced Y carriage v-wheels which reduces the effect.

Did you ever build & offer for sale those harder-steel rails you were working on?

Hey Dave (bleeth), would you be able to post a photo of the hardened rails on aluminum pillow blocks on 80/20 extrusions that you use on your machine?

Cheers

[Brady Watson]

I never sold any hardened PRT rails. I've made a few sets from scratch with ground edges, but never offered them publicly. SB has a hardened rail conversion available that is a direct bolt on. 2hrs and it would be a brand new machine...but it isn't a cheap undertaking last I checked.

A number of places sell the small rail strips which can be bolted to the original angle rails or something else. BWC, Pacific Bearing, Superior Bearing etc all come to mind. You won't find lengths longer than 6' through the usual channels...You have to go through a supplier or direct.

-B

[bleeth]

Here's the thread they're in:

http://www.talkshopbot.com/forum/sho...ight=upgrading

As a side note I ended up with Alpha motors, drivers, and SB built control box as the last upgrade.

[sbd1]

Thanks Dave, nice looking setup you've got there. I spoke to SB the other day to see how much their hardened rail kits costs, and yeah it's spendy, so I'll have to do something else more affordable & local.

Brady: In this post http://www.talkshopbot.com/forum/sho...=hardened+rail you said "I was thinking of fabricating replacement T-rails for PRTs since I could use a couple on my own machines. If I am making some already, what's a few more? If interested let me know. FYI - these will be out of A36 steel, precision ground - NOT that soft malleable CNC bent steel that wears out very quickly - which you probably have on your machine now.

One thing that really makes a huge difference in longevity on the PRT T-rails is running 8 V-bearings opposed to 4. 4 bearings was the original setup, but 4G PRT & later PRT Alpha machines had 8 of them. It makes a BIG difference not only for wear, but for stiffness. Combine 8 bearings with good steel with higher Rockwell hardness & it will outlast the soft steel by a factor of 3:1 or more. At least this has been my experience."

Did you ever make these rails out of A36 steel? If so, how'd they work out for you? And what axis were you talking about that has 8 bearings on them?

Cheers

[Brady Watson]

Did you ever make these rails out of A36 steel? If so, how'd they work out for you? And what axis were you talking about that has 8 bearings on them?

Yes I did. They work very well. My original PRT has A36 on it and I've only dressed them once or twice in the past 15 years. They are still good & smooth. Since my machine is a 5x16, the rails are 18' long, which was too long to get in the CNC bender at the time...and I am glad for that now.

I didn't get ANY requests for rails, so I dismantled the jig and setup I had to make room for money making equipment. I made a few sets for a friend and myself and that was it. There's no profit in it for me (especially doing a set here or there) so I have zero interest in making any. My method is different than any you'll find on the web, but there are examples of how to grind your own if you look hard enough. Mechmate comes to mind as one of them.

You want 8 rollers on the Z axis tower. It makes the t-rail wear more evenly and last probably 7-10x longer if it is properly adjusted and not roll forging the t-rail.

-B