How do I adjust the crown on my flat boxes?

We often have people ask us about how to make adjustments to their flat boxes. In this article we'll give some basic pointers on how to make adjustments to most any manufacturer's flat box. 

Get to know the box.

With a new box, especially if it's from a new manufacturer, take some time to get to know the box.  Get to know how it runs on different joints, on different settings so that you have some frame of reference to go by. Check the joint you are coating with the edge of a straight knife to see if you have a deep joint, or a high joint. Then check it again after you run it, and see how your box responds with different settings, on different joints, in your job context and with your mud consistency. Your job context and your finishing process will determine what settings you need to get quality results. For example, if you do commercial work with a lot of stand up flat joints, each flat joint since it sits on an individual stud can be significantly different. You might need to run one flat wide open, and another flat on 3.

Check your box for basic maintenance.

Also, before making adjustments to the factory settings from the manufacturer, make sure your box is properly maintained. Check to make sure the brass bar is held securely by the skid. If the brass bar is loose and moves where it meets the skid, you won't get consistent edge pressure, and you will have a box that leaves edges. If the skids are wore down or bent out so that they can't hold the brass bar firmly in place, replace them. Also, put a fresh blade in the box. You'd be amazed what new blades and fresh skids will do for old flat boxes.

You can adjust the adjustments.

Most every manufacturer uses some form of "crown dial" system that allows you to adjust mud flow right out of the box. TapeTech®, NorthStar™, Columbia and Drywall Master all use a 1-5 setting crown dial, and leaving it disengaged completely giving you a "0" setting. Not only do flat boxes have a crown dial to make adjustments to the amount of joint compound they apply, but on those flat boxes I mentioned you can also adjust the brass bolts that set the spring tension for the crown dial. This means if your box puts on too much mud on setting 1, but not enough mud on setting 2 you can adjust the tension somewhere in-between setting 1 and setting 2 to get the exact amount that you want. By loosening the nuts on the 2 brass bolts you can increase the amount of tension that the crown dial spring puts on the brass blade holder. By tightening the nuts on the 2 brass bolts you can reduce the amount of tension that the crown dial spring puts on the brass blade holder. You want to be sure when making adjustments to these brass bolts that you adjust them in small increments and that you adjust them equally. You can also adjust the nut on the crown dial itself to adjust the tension. 

The spring can bend over time.

The crown dial spring can bow over time, especially on the 7", 8",  and 10" boxes as they have a thinner spring. You can remove the spring by removing the crown dial assembly, and then you can flip the spring over, replace it, or even add a second spring. When storing the box between jobs, it's best to set the crown dial to where there is no tension on the spring so that you don't wear your spring out prematurely.

The bar itself is also adjustable.

You can also make significant adjustment to how much joint compound your flat box applies by manipulating the brass blade holder itself. Some manufacturers send their boxes with a significant crown in the brass blade holder, and some manufacturers send their flat boxes with little or no crown in the brass blade holder. We would only recommend doing this as a last resort if you can’t accomplish the settings you need by adjusting the crown dial, and adjusting the brass bolts. You also want to be very careful doing this as the brass blade holder assembly is an expensive part. You can kink, or even break the brass bar pretty easily, as it has slots cut into it to make it more flexible.

Adding crown to the bar:

If your flat box is on 0 with the crown dial not even engaged and it is not applying enough mud, you can add crown to the brass blade holder. Using a towel or gloves to prevent cutting your hand, with the blade facing your palm, starting at the end where the blade meets the skid, slide your palm slowly towards the center of the box applying more pressure as you go. Do this from both ends of the box towards the center, and from the center back out both ways to create an even crown.  Look down the brass blade holder to make sure that you haven't created bends and kinks in the bar. You may have to loosen the tension of the brass bolts to accommodate your changes. 

Reducing crown in the bar:

If your flat box is on 4 or 5 and you find that it is still leaving too much compound you can reduce the crown in the brass blade holder. Again protect your hands as the brass bar can be sharp. Carefully hold the flat box by both ends of the brass blade holder with the crown dial facing up, and the blade pointed away from your body. Reduce the crown by pressing the brass blade holder away from the brass bolts with your thumbs. On larger flat boxes, you can do this adjustment a couple inches in from the ends and also again out in the center of the brass blade holder. Look down the brass blade holder to make sure that you haven't created bends and kinks in the bar. You may have to tighten the tension of the brass bolts to accommodate your changes.