What is the RhAT?
The RhAT Tool Sets each have five adjusting tools to adjust your machine's Rotary Hook.
They allow you to precisely adjust the Timing and the Gap between the Rotary Hook and the scarf of the needle. The gap can be adjusted in a range from the minimum to the maximum recommended allowance. One of the the RhAT tools replaces the needle in the needle bar when the adjustment is made. It provides a positive stop for the point of the Rotary Hook.
If you choose to, you can now make small changes to the radial position (the timing) of the Rotary Hook as well to fine tune the results for every embroidery head on
your machine. This may further improve the quality of your embrodiery.
Cause and Effect
If the gap is too small, the machine will have a tendency to have broken, shredded or
frayed threads. If the gap is too great, the machine may fail to make a stitch. Additionally, if the point
of the Rotary Hook is off radially the quality of the stitches is affected. A few degrees late and the
stitches may be loose. If the point arrives early (a few degrees before), the machine may miss or fail
to make stitches.
A great deal of trial and error was often required to adjust the
gap and the radial position of the Rotary Hook in relation to the needle. On single head machines (that
only sew one garment at a time) it might have taken four or more attempts to adjust the gap. The only viable way
to determine if the gap was correct was to sew something and observe the results. The problem is compounded
on a multiple head machine that may have 2 to 18 separate heads, each with a
separate Rotary Hook that
requires adjustment. It may be very difficult to get all heads to sew uniformly.
The GAP Explained
The gap between the needle of a commercial embroidery machine and the
rotary hook is usually adjusted by sight, trying to line up the sharp point of the rotary hook radially while
maintaining a gap of approximately .1mm to .3mm (about .004” to .011”) between the point of the
rotary hook and the scarf of the embroidery needle. This number may vary slightly depending upon the machine
When creating a stitch, the needle goes to it’s lowest point, then rises about 2 to 3 mm (.078”
to .118”) from bottom dead center to form a loop behind the needle. It is at this raised position that
the point on the Rotary Hook needs to be aligned radially (in addition to maintaining the gap described
above) to be directly behind the needle. As the point on the Rotary Hook passes behind the
scarf of the needle
it captures the formed loop. The Rotary Hook on a commercial embroidery machine is usually attached to a smooth
ground shaft and held in place by three flat tipped screws. The Rotary Hook can be rotated and moved in and out
when these screws are loosened.
Damaged Rotary Hooks
The gap may need to be reset when anything in the gear train on the machine
is disturbed or when a needle breaks and is jammed into the rotary hook. A damaged Rotary Hook may have to be removed
to have nicks or dings polished away to avoid thread breaks. Tajima (a leading manufacturer of commercial embroidery
machines) states in their troubleshooting guides that “Thread breaks are often caused by a scratch or burr on
the rotary hook”. The Rotary Hook may have to be changed and a new one installed. Gears may be damaged and need
to be replaced as well.
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