My Squares Ain't Square

[chodges]

We have a fairly new (purchased in January) 48" x 96" PRS Alpha with a 4-hp spindle, and we recently noticed that squares and rectangles we cut are really parallelograms (none of the corners are 90-degrees). The larger the object, the more this is apparent.

We have cut objects drawn in Corel, AutoCad, and FlexiSign, but they all produce the same results.

We are cutting these shapes from pieces of material that are larger than the finished object, so it doesn't seem that this is an issue of not getting the material perfectly square to the table.

And our hold-down strategy (double-sided engraver's tape) seems to be ok because we can repeat cuts with no apparent discrepancies.

Is there something we can do to remedy this?

Thanks!

[Gary Campbell]

Charlie...
Look in the manual, or on the SB site, for instruction on squaring the gantry. This is a common maintenence item. We check and adjust ours monthly.

There is a chance that deflection is the culprit. To eliminate deflection, using a test piece, v carve a shallow, (.020) large as possible rectangle on the vector, then cut the same as an outside profile using your normal cutting feed and speed parameters. Measure sides and cross corners of the v line before cutting the profile. then measure the cut out part sides and cross corners.

If the V carved rectangle is right on, then either machine or bit deflection is the problem. If the V carved rectangle is also off, then the gantry must be squared.
Gary

[jhicks]

Charlie, I agree its most likely your ganty out of square but one nice pattern to run for verification is
Design a simple 6" circle, a 6" square, and a 6" diamond.
Place them separately or one inside the other.
Take a small bit maybe 1/8" and cut each as an outside profile.
Now measure the square side to side and diagonally, same with circle and diamond.
If your dimensions are off you're likely not running a true square set up.
Of course another way to really dial in is to cut a 47 x 96" or as large as you can, and measure that and its diagonals. That will give you a much better perspective over longer spans and you'll find out what and where so you can adjust your gantry to correct it.
I further agree one should devise their favorite method of verifying square. we check daily on our PRT and are often off 1/16" to 1/8" at start up and definately check after any stall or unusual interuptions.

[knight_toolworks]

why is your machine off on square often? I c an see after a stall or such.

[ljdm]

On mine, the older frame doesn't always hold square when powered off. A lot of flex in the design. Even so, its a good habit to pull it against the stops to make sure it didn't lose square, just in case. (You never know what the gremlins are up to in the middle of the night)

[knight_toolworks]

I always pull it against the stops.

[harryball]

Mine is fortunately very solid and maintains square when powered off so long as no one futzes with it. Evenso, I pull it against the stops every few weeks to make sure. Getting the stops right was the hard part, from then on it's no big deal.

/RB