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Thread: What is good enough in making basic table adjustments?

  1. #11
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    Oct 2009
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    Thanks Cory for the level suggestion. The level I am using I got at Lowe's for about $80, and I bought it for the exclusive use on my Shopbot.... but certainly it can't be as good as a machinists level....... I'll have to look........

    Thank you Curtiss for your suggestion of the jig.......... I'll have to play with it....... Chuck
    Chuck Keysor (circa 1956)
    PRT Alpha 60" x 144" (circa 2004)
    Columbo 5HP spindle
    Aspire 9.0, Rhino 5

  2. #12
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    Oct 2009
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    Default What I used to tell if my rails were straight side to side

    Earlier when I had adjusted/set-up my Shopbot, I used an 8 foot long level,,, but my rails are 14 feet long..... so I had to do some approximating and guessing. A few days ago, as I prepared to re-align my machine, it occured to me that I could do something different, and better (I hope) which would allow me to more accurately measure side to side variations in my X rails, and not require the use of my level.

    I used two framing square stops, which are used in making repeated right angle cuts. These stops are cheap, from any big box hardware store, and are shown in the first picture.

    I used .003" diameter copper wire (all I had that was so fine, but it worked well enough and would draw tight and straight without breaking), which I attached under the knurled nut/set screw of the brass stop. I then clamped the brass stop to one end of my X rail.

    I then pulled the other end of the wire, down to the other end of the same X rail, attached that end of the fine copper wire under the head of the other brass stop, drew it tight, and clamped it to the end of the X rail. Then, using a scale, as shown in the last picture, was was able to accurately measure how far the X rail was from the wire at any point of the rail.

    And because I could put the scale right up close under the wire, I didn't even have to worry about paralax error when making my readings. I started off using the 1/32" - 1/64" side of the scale, but found it easier to use the 1/16th inch side.

    Also, while this was not a super taught steel wire, any sag in the wire (which was not readily visible) was in the downward direction, and would not affect the distance from the vertical surface of the X rail. IF I had such a fine steel wire, I could have drawn it taught, and probably have been able to use it to measure the top edge of the rail for straightness. But since as I understand it, getting the rails parallel is most important, this seems to be a useful method..... Though I probably re-invented wheel at best, it should still be useful.

    Thanks, Chuck
    Attached Images Attached Images
    Chuck Keysor (circa 1956)
    PRT Alpha 60" x 144" (circa 2004)
    Columbo 5HP spindle
    Aspire 9.0, Rhino 5

  3. #13
    Join Date
    Jan 2004
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    iBILD Solutions - Southern NJ
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    Quote Originally Posted by Chuck Keysor View Post
    Hello Brady. Thank you for all of your detailed directions and suggestions, which I greatly appreciate!
    1) As to the roll forged burs, I have them on every rail on my machine. On the X rails, they were much worse on the inside of the rails, and measured .005" to .010". On the Y rails, they are about .015" wide. I filed off all of the roll forged spurs on both X-rails. It took about an hour. (I'll do the other rails later). The top edges of all the rails were flat, so I didn't touch those, except for a few odd nicks here and there.

    2) I didn't mess with trying to improve the squareness of the X-rails.

    3/4) I got a friend to help me remove the Y-Z/spindle car, so I can square up the X car. That happened not long ago, so I haven't been able to work on that, and will first remove all of the roll forged burs on the Y rails.

    I took the attached picture of the junction of my X car strut connection to the X tubing. Based on your description, it seems as though I at least don't have the worst version.

    I don't know if my machine is 3G or 4G.... duh...……. but based on the "horrendous" comment, I am assuming that I have the 3G version.

    I was interested in your comments about replacing the strut with square steel tubing. I did a Google search on this, and found your Shopbot forum post dated 8-27-2005. But, for whatever reason, many of the numbers are replaced with $#%& symbols, so I don't know what the dimensions were of your stock. And I wanted to see pictures of what you had done, as I am not a welder/machinist, so seeing pictures would be worth a lot, as I'd have to have someone else do the welding.... But in that thread, you said there were pictures posted at www.bradywatson.com. However, when I searched for that, it appears as though you have shut down that site...……..

    You talked of adding extra V-rollers to the Z plate...….. and you wondered if my "T-rail" was still good after all these years.. I recalled seeing someone selling a newer version of SB Z plate assembly. If I have the "horrendous" version, with a worn "T-rail", would this be a desirable upgrade, or is that over-kill without having done other upgrades?

    As to gantry sag,,,,,,, when I bought my Shopbot, I purchased a dial indicator- trammel set. And I bought a nice digital height scale at the same time. Maybe a year ago, I did use the digital height gage to measure the deflection at the center of my X-car, with the spindle first parked on the side, and then rolled to the center. While I don't recall the readings, I did post them, and whomever asked me to make that reading was surprised at how good my reading was. (I didn't keep notes, and would have to go back and search the site to find the numbers.)

    As to the motor mounts, I'll have to look into that later, as I want to get my fundamental alignment in order, so the motor mounts, while good, don't seem to be in my critical path.

    I checked my motors, and they are all marked ASM 911AA. Would all 4 of them be swapped out, or just some of them? And, are these a direct mechanical swap out?

    Because of past comments I have gotten here on the forum, connected to my variable depth of cutting, depending upon where I am on the table, it has been suggested that I also remove my sacrificial spoil board down to the plenum layer, and put on a new spoil board... some have questioned that my spoil board may not be correctly glued to the plenum.... I'm just mentioning this, so you know I am also planning to do that after I get the other mechanicals in order.

    Thank you greatly...……….. (Thanks to the other posters as well, I will read through those next.) Chuck

    PS: In reading your post of 8-27-2005, based on only that one post, it seems as though the nature of posts have changed a lot in 13 years, they seem much less technical and intense today. There were lots of people who knew what you were talking about, and offering what seemed to be knowledgeable replies. Most of it was far over my head......
    Chuck,
    Your machine is an early Alpha, which used AS911 direct drive motors and the 3G style gantry. 4G and 2nd Gen Alpha PRTs used the 4G gantry, which consisted of CNC bent steel parts and aluminum extrusion & no utility strut whatsoever. Looking at your machine, it appears that someone has removed the long pieces of utility strut and replaced them with some kind of extrusion. I don't know if this was a mid-model change by SB or if someone did that themselves, but I have never seen it before were there is extrusion under the Y rails but utility strut on the Y car. It is definitely better than strut - but still not as good as a fully welded car.

    I'm pretty sure I used 3/16" wall 2.5x1.5" rect steel tubing. I marked it out, drilled and tapped it for 1/2-13 thread to bolt the rails on. You can use anything you think is reasonable for replacing what is there.

    It appears that you've at least got the 'better' rails on your X axis - with these appearing to be ground angle iron. If they have a square outside corner in where the 2.5 and 1.5" meet, they're angle iron. These wear WAY better than the soft, malleable CNC bent steel rails on the Y. There was a time when the bender couldn't do over 10' so in those cases, angle iron was used. It is very hard or impossible to find 2.5x1.5x.1875" steel angle with so many steel mills shutting down....so it isn't easy to just replace them. Inconsistencies in the steel that made up the PRT models got expensive - mostly because there was a lot of waste. Steel is rarely straight and true...which doesn't work out well for machine tool building. If you have any of the old table drawings back when you could buy or build your PRT table, you'll see tolerances for the steel called out on the drawing, like camber/twist etc.

    As for the Z, you'll have to decide if you want to add 4 more rollers or not. If you don't have any more adjustment with the 4, you can either fab up another t-rail, build up with weld and regrind the original or buy a RetroZ. If you're going to do anything other than the RetroZ, I'd add 4 more rollers. It makes the entire Z assembly glide better and resists wallering around during cutting.

    The AS911 motors are the most powerful Alpha motors made. The entire length is all motor. The newer tools have less motor and gear reduction. This gives more torque and better resolution. A straight drive Alpha with no reduction is almost unuseable - especially on the Z with a spindle. You you can tough it out and leave it alone, make your own belt reduction boxes like Mike Richards did (3:1) or buy gear reduction boxes at $400 each. If you do either of the latter mentioned, it will substantially increase both torque and cut quality & equal or better than a new Alpha because the motor itself is so big. Torque = less or no chatter and faster, more aggressive cutting. Resolution will give you finer carving ability.
    High Definition 3D Laser Scanning Services - Advanced ShopBot CNC Training and Consultation - Vectric Custom Video Training IBILD.com

  4. #14
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    Thank you Brady. You have given me lots of great advice and I have much to consider as I move forward.

    The attached image shows the complete profile of my angle iron x rails. I am glad to know I have something that is above average.

    I am the third owner of my Bot, and from my understanding from the second owner, it was made as a PRT, and then upgraded to an Alpha. So maybe that is when the aluminum extrusions found their ways onto my X car.

    Would it be problematic or foolish to upgrade the motors incrementally, doing the Z axis first? Thank you again, Chuck
    Attached Images Attached Images
    Chuck Keysor (circa 1956)
    PRT Alpha 60" x 144" (circa 2004)
    Columbo 5HP spindle
    Aspire 9.0, Rhino 5

  5. #15
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    Yes, those are angle iron and not the softer CNC bent ones. They last much longer. Again, keep in mind that those steel angles weren't all that straight to begin with, so it is possible to not be able to adjust out twist etc without getting really creative because there's no fastener where the bend/dip is.

    Alphas were twisting the 3G type gantries out of square, which is one reason for the change in the 4G models. The 4Gs had table gusset plates from the leg to the 3x5 crossmembers, CNC bent/formed motor mounts which were better than the 3G ones, but still not 'right' and AL extrusions on the X & Y cars. 8 rollers on the Z standard.

    FYI - SB sells bolt on hardened rail kits for PRTs for those who have worn out rails. Comes with CNC bent angle and hardened rail and gear rack pre-installed.
    High Definition 3D Laser Scanning Services - Advanced ShopBot CNC Training and Consultation - Vectric Custom Video Training IBILD.com

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