How do I indicate in something SQUARE?

[ErichKeane]

So a while back, I picked up a pair of the Windy Hill Foundry 6" machinist square castings (https://windyhillfoundry.com/) to play around with. I finally got around to starting to machine them.

I started by milling, the 'sides' on a pallet plate, then am working on surface grinding the sides.

For the 'faces' I'm going to use the same plate, then use my X and Y axis to mill the sides, which should get me as accurate as my machine is for 'square'. However, I'm wondering how to grind the 2nd face? For the 1st, I can indicate across the face and just grind it that way.

I'm not sure if I should just do the same for the 2nd face. That would make it out-of-square up to 2x the error in indicating it in. I thought about indicating the 1st ground face using the spindle-up/down, however that would only get me as accurate as the surface grinder spindle squareness. AND considering that the 'head' up/down doesn't necessarily have to be accurate in that dimension, I'm scared that would be worse than the mill.

Every one of the youtube videos I've seen seem to use some giant 'machinist cube' and just bolt it to that, and rotate the cube. However, I can't seem to 1- find one of those available, 2- convince myself this job is worth whatever-many-hundreds/thousands one of those would cost.

Is there something I'm missing here? Does anyone have an idea?

Currently, I'm planning on mounting it to an angle plate that I'll indicate in at 90 degrees somehow or another

 

[jwmelvin]

Currently, I'm planning on mounting it to an angle plate that I'll indicate in at 90 degrees somehow or another

If you trust your angle plate (you can check it on a surface plate?), then you should be able to use that and you'd just have to indicate the surface to be ground so that it is true in the pitch direction. I guess if you don't trust it, you'd indicate the mounted square true in both longitudinal and cross-feed directions on the surface grinder; that approach would be relying on the milled angle as a reference.

If you are using the mill axes for the 90° reference, then is surface grinding just to improve the finish?

 

[Lo-Fi]

I don't think I've seen a good cheat for this. If you use an angle anything, your grind is only as good as the accuracy of your fixture. Indicating and adjusting or shimming may be your best bet if you've not for a precision doodad.

 

[Shootymacshootface]

I'm sure that you have already watched it, but This old Tony has an excellent vid on squaring a block on the milling machine. He demonstrates how difficult perfection really is to achieve. His measuring techniques are also excellent. It's definitely worth a refresher before working on a straight edge.

 

[ErichKeane]

I've squared up blocks before (and have vises and stuff to do that), but the issue is basically the shape of these. There is no 'opposite' face I can make it square to.

I wasn't particularly worried about the angle plate, since it could only introduce a 'tilt' in the short-dimension, so I figured indicating it square-ish would be close enough.

But the problem is with the square bolted to the angle plate, I have to set the 'twist' correctly.

If you trust your angle plate (you can check it on a surface plate?), then you should be able to use that and you'd just have to indicate the surface to be ground so that it is true in the pitch direction. I guess if you don't trust it, you'd indicate the mounted square true in both longitudinal and cross-feed directions on the surface grinder; that approach would be relying on the milled angle as a reference.

If you are using the mill axes for the 90° reference, then is surface grinding just to improve the finish?

I had a couple of ideas how to set the angle plate to as-close-to-90 as possible, but I am not AS concerned, as that is a minor-direction. I AM using the mill to set the 90 degrees, but I was hoping there was a way to make that more accurate. My mill is about as good as I could guess, but I was hoping to use this as a particularly-precision square for my surface plate.

 

[RJSakowski]

I made a "precision" square using my rotary table. I edge milled one side and rotated the table 90º. My RT is capable of resolving to 5 seconds of arc. If one can believe the readings, this should provide an angular error of .0005" over 10". Believing the readings is the rub though. Trust but verify.

It is possible to check a 1/2/3 or 2/4/6 block for squareness. The sum of the four included angles are equal to exactly 360º. A squareness indicator as used by TOT referenced in post #4 above normally has to be zeroed with a known good reference square. Not having a reference square at hand, one side of the block can be used as the defacto square. All four sides are measured and the deviations recorded. The block is flipped and the measurements repeated. If the sides are labeled 1, 2, 3, 4, you want the angles created by side 1 & 2, 2 & 3, 3 & 4, and 4 & 1. From the deviations from zero, and the length of the face, the included angle can be calculated. adding the four included angles, the original zero error can be calculated and corrected for in the squareness indicator setting.

 

[Shootymacshootface]

Joe Pie has another excellent vid where he takes advantage of the mills inherent accuracy between the X and Y axis movements.
I know that your question was intended more towards the orientation of the straight edge, but to me it is kind of a moot point. I believe that there is always a way to indicate a peice, part, or tool of any shape to get things started.

 

[petertha]

Edge Precision did a detailed 3 part series back in 2015 called Square Me Up. The method might be a bit over the top for most home shop applications, but he shows the underlying principles & logic in detail. One big takeaway point for me was to ascertain how perpendicular the fixed vise jaw is. We go through great lengths tram the mill head to table & dial in the fixed vise jaw laterally to X, but if the jaw itself is deviating from perpendicular for whatever reason, then then the successive part flip & rotate method likely results in a parallelogram section because it relies on this datum. Most jaws aren't very deep to begin with & using parallels reduces the contact area, so the collective tolerances can stack up. If you can count on your vise accuracy, it makes life so much easier.

 

[woodchucker]

so you started with the small edge, which makes it difficult to square up. Since you can only rely on a very short distance indicating to know if you are square.

Consider that that small edge is not a finished side. use it to roughly aquire the square, and mill the bottom. That now has a longer face in the short direction now. Then grind it. Now you can use that to re-tune your smallest face. When you are done, you can square to 45 or 60 your angled face.

Just my 2 cents, as that's all opinions are worth.

 

[Dabbler]

You can put your square on a magnetic SIN plate and indicate the vertical side. You then are free to grind the horizontal side and it will be square to tenths of a thounsandth.

Otherise you have to trust your angle plate.