XY Square Tolerance?

[patricktoomey]

I have my ShopBot up and running and I thought it was squared well enough when I assmbled it but I got suspicious due to some cutting issues. So I cut a 46x94" rectangle on a piece of MDF and measured both ways diagonally. I came up with 1/8" difference between the two which I'm not happy with. My question is how square are you guys comfortable with, 1/64" off on a rectangle that size, 1/128" or do you shoot for even tighter tolerances?

[paco]

Hi Patrick!

I'd say that 1/64" off over an 46" X 94" rectangle is quite good with a Bot. 1/64" over this part is somewhat not that easy to mesure; I'm not aware of an 120" wide caliper yet! 8-D

One may be able to get 1/128" but has to consider the time involved...

From 1/32" and better (over the 46" X 94" rectangle), I'd say a Botter is in business!

Do you "sub" for the NASA?

[patricktoomey]

Hey Paco, thanks for the answer! Actually I'm only 45 minutes from Kennedy Space Center but I don't work for them ;-) You're right, measuring on that scale to 1/128th is tough. I cheated a little to get my 1/8" error measurement. I do some work with Corian and I have several alumninum straight edges between 4' and 12' long which are straight to within .001" so I threw one of the longer ones on there and lined up one corner point. Then I aligned the edge of the straight edge over the other corner of the rectangle I cut and made a mark on the straight edge at the corner. Then I rotated the straight edge to the other set of corners and measured from my mark to the new corner and found my error. I figure I could comfortably measure down to 1/64 that way but I wouldn't want to try to get much better than that. For the work I do I think you're right about 1/32 being good enough, anything after that will be a bonus. Now I just have to figure out what the easiest way is to make this adjustment. I just got done with a 2 hour wrestling match with my Z axis and a tramel arm. I have no visible lines after table surfacing now! :-) I figure a few days of misery now will pay off in the long run if I can get this thing cutting straight, level and square.

Thanks again for your advice!

[richards]

Patrick,

My 120x60 Alpha is a little out of square, even when I've adjusted the carriage as far as the bolt holes would allow. To set things up quickly, I always stop the machine at a certain point (x=-18.0, y=6.0) and then use two pre-set combination squares to check things before starting up again. The right side square is set 3/32-inch shorter than the left side square, which gives me square cuts.

Since the machine has significant flex, I re-check alignment everytime I hear a 'glitch'. (As hard as I've tried to eliminate electical noise since switching from the PC to a spindle, I still get random glitches that basically drive me nuts. The cold dry weather we're having doesn't help much.)

[drodda]

I am fighting this issue as I am trying to do much more inlay projects and the table being square and flat is very important. I stop my machine at the end farthest from 0,0 each night. I use my stops to square my machine up every day as it starts. The motors tend to hold the machine wherever I start the power and reset the controls. With that said I am still unhappy with the tolerances that I am getting. I know that I have a slight problem with my Z not being perpendicular to the table also. I have a fix in mind for these problems that I think will make it very easy for anyone to square and calibrate their machine but have yet ot find time to impliment the changes. It would require me to take my machine ofline for a few days and modify the machine.

Brady told me some time back that I would come to the time in my shopbotting where the shopbot is no longer new and I would start modifying it to make it work for me. I think I have come to that point in my shop. My only question is does anyone have a fix for making sure that your bot is exactly square and true. I am not talking within a 1/64th. I am talking within 5/1000th or is this not really a reality with these machines?

I have a 60 x 120 PRT alpha with a welded steel table.

[paco]

Dave,

I agree 100% with Brady's statement... but to get the Bot wihtin 0.005" square; you got yourself a nice challenge! Nothing impossible but that may fill alot of your spare time... keep us post.

I'd be really interested to learn about your idea for squaring the Z.

[richards]

Dave,
I agree with Paco. My Alpha would never square up to 0.005 inches.

Have you thought about mounting dial indicators on your machine? If they were setup properly, you'd be able to start your day within 0.001 (depending on your eyes and the size of the dial). Holding that kind of tolerance all day long is something entirely different. (Dial indicators cost about $15 each at the local Do-All machinists supply store.)

I use three pre-set combination squares on my machine. Before turning the machine off, I locate the spindle to a known spot. Before turning the machine on, I check each side of the X-axis and one side of the y-axis. In theory, the proximity switches should get you exactly where you want to be, but on my machine, in order to have square cuts, I have to nudge one side of the x-axis 3/32ths. It's not what I like to do, but it works.

[drodda]

I will post pictures after I have it all made and attached. Sometimes the simplest things are left out when people design the whole machine. My idea for squaring the Z axis is very basic but should work really good.

I have thought from day 1 that shopbot has really missed something when I read my manual and it said to place the z carriage into the y carriage and attach with 4 3/4" bolts in rather large slots. Then they tell you to use a level to calibrate it. Most on here including myself used a cheap magnetic torpedo level. These levels in a verical position are far from calibrated. When I true my table I have a series of lines that show up. These lines are not enough to catch your finger nail on but this shows me that the Z is not perfect or even close to perfect.

[drodda]

I was thinking about your comment Paco about the 120" caliper. Why can't you attach a long steel straight edge to the caliper. This would allow you to measure out of square as large as you need. You only need to be able to measure the difference. With 6 inches of movement you could easily do this. If your table is more than 6 inches out of square you have bigger problems than trying to measure the diagonal accurately?

While writing this I already made a modification to this idea. Put a sliding stop on the steel straight edge so that you could adjust it in or out to accomidate many different sizes.

when cutting your test rectangle you only need to cut the 4 corners. you can screw scraps to the areas that the corners should fall and then cut these scraps and measure the resulting diagonals. This saves you from wasting so much wood or marking up your table top with test the cuts.

[gerald_d]

Our test for squareness is the following:

Take two thin sheets of plywood/mdf, each as big as the table will take, laid one on top of the other and lightly fix them down to the table (screws/nails/vacuum/weight). Trim a tiny amount off all the edges of the boards making them feel as if they are exactly the same size as each other. Turn the top board over and lay it back down on the bottom board. Feel all around the edges to find out if they are still a perfect match. Turn the top board 180 degrees around a vertical axis and feel all around the edge again.

This test tells you about straightness of the rails as well as squareness of x,y to each other. We don't measure anything if it feels right.

[drodda]

Gerald,
I was building a part that riquired me to flip one piece and put it together with another piece like you are refering. this is what told me in the first place that my machine was off. Ever since that day and trying to correct that problem I have been trying to figure a way to claibrate it with an exact method. I have trouble with the loosen all the bolts on the Y carriage and smack it with a hammer method of calibration.

On a different note today while snooping around the Shopbot site I noticed a video where they say that they are cutting at 480 inches a second and jogging at 1800 inches a second. Is this correct or am I missing something? Isn't that something like 102.27 MPH. What are they trying to say with these numbers?

[gerald_d]

...that they have seconds and minutes confused? link (http://www.shopbottools.com/prtalpha_videos.htm)

My gantries are welded together - I square them to the x-rails by adjusting shim washers behind the v-roller wheels.

[billp]

Dave,
Are you sure that isn't per MINUTE? If not, it should be....

[dingwall]

Dave, This seems to work well for me. It's similar to using a compass.

I use the bot to drill four 1/4" holes in a square grid.

I put 1/4" dowel pins in two of the holes (lets call them hole 1 and 2).

The compass is simply a length of mdf with a 1/4" pin in one end with the other end trimmed so that when the arm is pinned to hole 3 or 4 there is roughly a .010" gap between the end of the arm and the pin diagonal to it (1 or 2).

Then you can measure the gap with feeler guages at each diagonal.

It helps if the end of the arm is slightly radiused.

Then I adjust and use the hard stops like everyone else to start the machine from square every morning.

[drodda]

Sheldon,

I must have missed your post. I am interested in your measuring technique. Do you have a photo of this? I am thinking I have a grasp for what you are talking but a picture would really clear this up. Thanks for the reply though.

I am trying to make my hard stops adjustable so that I can move one in the .001 range rather than loosening the allen thread locks that clamp it to the rack. They always seem to want to go back in the same marks that are there if you try to move them just a hair.

Dave