Reading PRT Track Burrs

[gerryv]

A light burr is beginning to develop over a one foot area of the non-hardened angle iron track on our PRT. It's on the outside and at the bottom edge of the grind. I'm not seeing anything obvious using a straight edge and light or when eye balling how the V-roller rides on the track but my eyes aren't young either. Does this wear pattern suggest (a) the track needs to be moved OUT or IN slightly at that location and which way (b) the track is slightly too "fat" at that point (c) the grind may be a bit convex there so metal is being extruded downward (d) something else?

No Jack, I don't want to buy hardened rails:D

[Brady Watson]

(d) Something else.

Take a bastard file and 220 grit sandpaper and file down those burrs - then polish with the sandpaper. The v-rollers forge the soft rail downward and this causes it to 'smear out' from underneath the v-rollers. This MUST be taken off to get smooth movement. File from bottom up on outside of rail - top to bottom on inside, due to L shape. Don't go crazy - just get a feel for what the file tells you and get into a rhythm. Then polish with the paper. It doesn't take that long to do properly. You want to concentrate your efforts on the region where the vertical part of the L just starts to meet the 45 deg bevel. If the top of the rail does not look like a truncated pyramid, and looks like this: /\ , then flatten the top so that it shows at least 1/32" flat down the centerline of the rail.

It ain't rocket science. Don't get scared. Just be deliberate and concentrate on what you are doing - you'll be fine.

Going forward...keep the rails clean & burr-free with a few wipes of sandpaper once a week or brown Scotch pad.

NEVER use pneumatic or power sanders or grinders - unless you work with steel EVERY day and know just how to dress the steel. POwer equipment can get away from you in a hurry & you can put dips in the rail, which isn't good.

-B

[gerryv]

Thanks much Brady for the good info. i've used a variation of this process wherein I apply a thin adhesive tape then a 120-180 grit cloth backed abrasive tape to a rigid steel 42" straightedge - basically a very long, very flat file :-)
I then use an 84" straightedge and a light to ensure that I'm flat from end to end. I get the tops of the rails flat first to within a thou or two and then use this as a reference surface for doing the bevelled edges the same way. my way of telling whether or not they are flat enough (besides using the longer straightedge) is to use light reflection to eyeball for width variations along the flat top. I learned this technique from an old machinist. In order to keep the bevels at 45 degrees, I was being lazy and just using a bevelled block of hardwood handheld against the track but hardwood blocks attached temporarily to each end of the "grinding" straightedge with slots that just slide over the vertical portion of the track would hold the 45 better. I'll do that next time :-)

What I'm up to here, being a retired old fart with lots of time to tinker, is to try to prevent the slightly uneven wear that would lead to an eventually low spot - so I'm trying to determine the root cause. Any thoughts on that aspect:confused:

[myxpykalix]

Gerald,
What makes you think i would recommend that?:rolleyes: lol
Thank goodness this is an issue i haven't had to deal with yet. :D
Someone like Brady would have more insight into the mechanical issues but the only thing i might add is if you are going to be doing any filing or sanding that you invest in a pair of those magnifying pull down goggles because if you are like me and the eyeballs are failing you need to pay very close attention to what you are doing. And a nice bright light wouldn't hurt either! Good luck!:p

[Brady Watson]

These things aren't milling machines and tolerances really are not that high. It's nice to see you are being careful...but ya really don't have to go to that extreme. If there is a low spot in the rail, just shim it underneath (part of original PRT setup - a step many skipped) to bring the rail up. This is a woodworking machine - it isn't as critical as you think it is...the v-rollers are designed to take up a certain amount of 'non-perfectness' and still deliver smooth movement.

There is absolutely no need to get crazy with the top of the rails, where the apex of the angle is truncated. Just make sure it isn't sharp because you don't want that apex riding in the crotch of the v-roller. You want it on the bevel.

The machine will run much smoother & quieter when you have them all cleaned up.

-B