surfacing

[2006prt]

I have a 2006 prt alpha with a spindle. I use a 2.5" bit to surface the spoil board and I am starting to notice some problems. I can feel the pattern in some areas of the board but not the whole table. It's not bad enough to catch a finger nail but I would like to take care of it sooner than later. Am I correct in assuming the non-hardened rails are wearing in some areas more than others. That is the only reason I can figure it happens in some areas and not others. The table is a very strong welded frame that is straight, level, and co-planer.

Thanks
Todd

[bleeth]

Your assumption has a high likelihood of being correct, although it could also be a slight bowing over time of the channel support beneath needing fresh shimming to level the rails, particularly if you only have 2 legs on each Z. A really good long straightedge should be able to give you the answer. I have found that a wallpapers hangers extruded straightedge is a great device for this and other things needing a true straightedge and can be gotten through many paint stores.

[2006prt]

Dave thank you for confirming my suspicions. I will probably replace the rails with a hardened version to reduce the wear.

[ssflyer]

There's a much overlooked file in the SBParts directory called sample_exercis.sbp. It will run your machine back and forth from all the limits you set. They suggest you run it after setup and about every 2 months. It runs the machine over the entire length of travel, work hardening your rails and keeping them uniform...

[bleeth]

You're welcome. I'm going to throw a "while you are at it" idea or two.

First, if you are only going to put the hardened rails (for which I heartily recommend Superior Bearing as a great supplier) go to the next larger wheel size at the same time #3 instead of #2) and get the corresponding size rail. You will appreciate the difference. When you mount the rails, if you are just going to put them on top of your existing angle, you will need to modify the holes in the y gantry to increase the width a little. While you are doing this and after you are really sure that your gantry re-squared perfectly
Either add some kind of stiffening gussets or weld it so it is no longer just bolted together with a couple bolts. This will increase your operable cutting speed as well as jogging.

Do the Y at the same time. That was actually the first issue on my PRT and the Y rails being changed out was a huge improvement. If you go to the large wheels there you will have to relocate the wheel centers but it is also worth it. At first, I went to the larger rail but used the same wheels. Even that was a big improvement. It did mean that the width between the high and low side of the gantry changed and there is for sure a bit of "Now that I changed this I have to change that" but after I worked it out and got her up and running again I was impressed with the difference.

Ron's suggestion is a good one. I don't use that file but I did create a routine I called "jog test" which does pretty much the same thing but also throws in some big arcs and z movement as well. I'll admit I tend to use it as a checking utility after doing maintenance but then I don't worry about the hardened rails getting worn like the angle steel.

[2006prt]

Thanks for the advice dave. I have some 12' square linear rails that I am thinking about using for the x axis. I am just a little nervous about getting everything accurate.

[bleeth]

That's what I did with mine since I already had a heavy C channel frame.
Replaced angle with the square extrusion and bolted a 1/2" x 2 1/2" solid aluminum on top of it that was flush to the inside and stuck out on the outside for the rack to go under. I wanted a 2 3/4" wide piece for more clearance outside but standard sizes available are what they are.

Yes-it is "fun" getting it dialed in but once done was worth it.

[2006prt]

Dave

My current rails sit on 2 x 6 c channel 5/16" thick. I want to bolt my square thk rails to 1/2x3 aluminum and bolt the aluminum to the channel. My plan was to have the shopbot drill the mounting holes in the aluminum that I will then tap manually. In your experience is that something better off left to a machine shop or is the bot precise enough. The rails are 12' long so I will need to reposition it once. I figure if I drill the holes and true up one edge it should all be straight enough to provide a good reference from.

Thanks
todd

[MogulTx]

I drill precision holes in Aluminum ALL the time. The bot is happy to mill into aluminum. just remember to spiral ramp in. Go slow. Shallow cuts ( I use ..025 per pass and 1.0 / .3 ips. Usually around 14,500RPM...) I mill out a snug pocket in a 2 x 4 if I need to hold onto channel or bar stock, etc.. and press it down in there with a wooden hold down to keep it in place. Works great.

[bleeth]

Your aluminum bar should be true enough as is. I wouldn't mill the edge with the bot but drilling is not difficult. I use a drill bit in the right size collett for it. Much better than any router bit for doing that. I use drilling, not area clear and just plunge slow with peck strategy and slow the spindle down to drill press speed. With 1/2" stock I would be standing there with some aluminum rapid tap and apply a squirt as it starts each hole. Nice thing about doing it that way is you are never putting sideways force on your aluminum and so the tendency for the bar to move is eliminated. I did my block on the drill press though. Quite easy to set up a fence and guarantee all the holes were centered perfectly.
The object of using the square extrusion under the aluminum pillow block is leveling it on the c-channel and then just straight bolting the pillow block. Remember you also have to do tapped stop holes in the side of the block for the hardened v-rail.

I tap with a hand held drill motor. I love my milwaukee hole shooter.

[bleeth]

You need to keep track of the math involved. The last thing you want is to have a challenge in the distance from rail center to rail center varying from your gantry width by more than a minor adjustment or finding out that when you mount the motors the spur gear on the shaft doesn't engage the rack properly. Take some time here to verify and re-verify your numbers.

[2006prt]

Thanks for all your help. I will post some pics of my proposed system when work slows down a bit and I have time to break the bot down.

Todd

[bleeth]

Looking forward to it.