Embroidery machines are a combination of an industrial sewing machine and a computer-controlled XY frame or pantograph. There are a large number of machine manufacturers, each with their own internal machine formats. Most machine manufacturers handle the .dst or digital stitch Tajima format, with an import function to convert these to internal machine code. Due to the original input source for these machines being 8-bit paper tape, there are a limited set of commands for encoding embroidery design data for machine embroidery. There are a number of special functions that can be encoded in a .dst file:
Originally, the “colour” was not encoded in the .dst, so a supplementary printout would be supplied with a digitized file to assist an operator in encoding the colour changes required to correctly sew out a design.
The “trim” is not always available on a machine, so once a design has been sewn out, then manual trimming would be completed.
Newer machines have automatic trims, colour changes, and additional special functions such as punch, sequin, and appliqué may also be available.
Each stitch is encoded as a relative position from the last stitch, and typically is encoded with a maximum stitch length of 12.1mm for a single stitch, with a resolution of 0.1mm in X and Y for pantograph placement.
The embroidery machine toolchain typically consists of:
Digitising is likely to be outside of the realm of the Smoothie project as far as implementing embroidery machine operations, however, it is included to assist with an understanding of some of the concepts that will later result in machine move operations. This stage is where the customer’s artwork is digitized into stitches, usually using a digitizing application, such as Punto, Wilcom, or a wide variety of other toolsets.
Punto specifically can take in vector-based artwork from Illustrator/Corel etc as the basis for a design, apply additional digitizing hints for stitch directions, and apply stitch types to the artwork.
Some example digitized embroidery designs
The simplest form of stitching, the running stitch is a row of single stitches, much like the output of a normal sewing machine. This may follow a path to provide an outline or detail highlight for a design.
The satin stitch is a progression of tightly placed sawtooth stitches along a path, so that the frequency (or stitch density) in the sawtooth is approximately the same as the thread width (typically 0.3mm or so) and the amplitude of the sawtooth is the “satin stitch width”. Complexity arises when a satin stitch follows a curve, as the density is different across the amplitude, so a compensation is required to be implemented with the stitch placement so that “bulking up” does not occur. A compensation technique is to place shorter stitches every second or third stitch, so that the thread does not attempt to fill the same location in the fabric when the stitches are sewn.
The fill stitch is a progression of running stitches to fill an area with color. With a single direction fill, each subsequent row of stitches is offset (as per running stitch) by the fill density, and a return run of stitching follows offset by the density calculation in the next row. Complications with fill stitching include navigating to start and end points, so that fills may be split into sections, and the machine can exit where required for the next stitch section. Curved fills, as per satin, the decision on how to approach maintaining density in stitch placement whilst producing the desired visual effect.
The underlay stitch is a special group of stitching, usually of a lower density and inset from a fill or satin. The goal is to both secure the fabric to the vilene underlay support, and also to suppress the pile of the fabric, to reduce show through of the underlying fabric color through the completed embroidery.
Exporting a design file is likely to be outside of the scope for any Smoothie embroidery control project. Once the digitizing process is completed, a machine format file would be produced, along with a design sheet, for transfer to an embroidery machine for sampling or production. Typical transfer methods may include:
Each manufacturer of machines has their own internal formats, but most will receive some standard file formats from digitizing software. It should be noted that embroidery files for machine production are typically not scalable, as the stitch placement is based on embroidery thread width, so any change in size will either reduce coverage or overload the material with too many stitches to produce a desirable result.
A very simple form for communications, which makes no assumptions of specific machine capability, and encodes special functions as “STOP” commands for later interpretation when reading the file in. The function encoding is why the supplementary design sheet is usually provided with the .dst file to communicate colors to be used, output file size, and other design information like thread used and start point information.
Some documentation of the file format is located here. I have not yet verified this to .dst codes in hand.
Depending on the implementation with Smoothie, the import function is likely to be required in order to take a generic embroidery design and make it ready for the specific machine implementation. The .dst file, for example, is similar to a .gcode file, however, is not complete run information, as the color change and special functions are not yet available as digital information.
The import stage of a design commences with the selection of an exported embroidery file. This is then scanned and a set of questions will usually follow:
This then would result in an input file ready to run in the machine. More feature-complete embroidery machines would also allow:
Specific machine implementations may require the selection of pantograph drivers:
In a production environment:
Once the sample has been signed off and the machine has returned to the start position, the next items to be embroidered are loaded onto the machine, and the start button pressed. Embroidery continues until a thread break occurs or until the cycle completes. Thread breaks are repaired, rewound, and restarted as per the pre-production description above. In the last production cycle, there may not be a complete machine full of embroidery to be run, so some heads will be turned off, and should not sew during the final embroidery cycle.
The embroidery machine hardware consists of a number of elements:
The XY Pantograph is used to position the work relative to the embroidery machine needle. The embroidery machine must record the position of the pantograph and stitch count following any machine moves, so that work can be resumed in case of an emergency stop. Some machines will implement encoders to feed back the current position to the machine, others will work relative to the start position and rely on the operator to know what they are doing. Different pantograph implementations may be required on a single embroidery machine to account for physical and positional differences for:
Each of these hardware attachment options may want to implement a positional boundary (max bounds) beyond which the machine will not travel. (This would require encoded position in hardware for implementation in software)
The core of the embroidery machine is an industrial sewing machine. This typically has a positional encoder along with a direct drive motor. Larger machines will have a brake attached to this drive motor to stop the machine as required. The rest position of a ZSK machine is approx 64 degrees past TDC. In operation, these typically place 500 - 1200 stitches per minute.
The thread is delivered from the needle frame above the work, with a bobbin thread carried below the work. In each stitch cycle, the needle is moved through a Z-axis of motion, piercing the work, and bringing the embroidery thread down to the bobbin case. The bobbin case is mechanically tied to the needle so that they are in correct alignment at all times. When the needle begins its return journey upwards, the friction of the fabric restrains the embroidery thread and forms a loop for the bobbin hook. The hook passes through the loop, and carries it under and around the bobbin thread, then releases the loop. The thread take-up levers which are also timed mechanically to the needle and bobbin are timed to pull upwards once the embroidery thread passes around the loop, and with correct thread tension settings will pull the stitch closed, before pulling additional thread from the supply.
Any time the needle is at or below the work, the pantograph must not be moving. Even the slightest move (acceleration or deceleration in positioning the pantograph or inertia in the supported work) while the needle is in the work will cause the needle to deflect and possibly break, or scar the bobbin case. This means that there is only a very short window for each stitch to position the work ready for the next stitch, so the pantograph module essentially needs to start to move in XY, arrive at the destination and have the work finish moving as well before the needle reaches the work again on the next stitch.
On a multi-head embroidery machine, the connection between the sewing motor and the needle drive bar is enabled by a solenoid. If the solenoid does not release the needle, then the stitch does not occur on that sewing cycle. On a single-head embroidery machine, the needle bar may be in constant mesh with the sewing motor. In this case, the entire sewing motor would not actuate in a jump stitch operation.
When a stitch is required that is longer than the maximum stitch length (12.1mm), a Jump Stitch can be encoded. This results in the Drive solenoid not actuating for an intermediate stitch and so the skipped stitch can be up to the length of two single stitches (24.2 mm). Jump stitches can look bad as the thread is not held to the work and can catch on fingernails etc, but is sometimes required.
Trim operation is performed in order to finalize the color being sewn. A stitch will be performed, but prior to the thread returning from the bobbin, a knife cuts the remaining end, and a solenoid-controlled wiper will pull the end of the thread out of the work and back into the thread keeper. This requires a timed operation between the mechanical sewing motor, the trimmer, and the catcher.
The Trimming operation must not occur when the needle is in the path of the trimmer, or needle breaks and damage to the knife will occur.
Production embroidery machines typically have 6, 11, 15 needles per head. There will be a needle position sensor to determine which color is presently selected. When a color change is required, a number of operations will occur:
A needle change must not occur when the needle/sewing motor is not in the home position.
Typically the first stitches on a new design or following a trim or color change will occur at slow speed. This gives a chance for the lock stitch to form and the thread to be tied into the work. After these start stitches (3-5 stitches), full-speed embroidery will commence at the set embroidery speed. Some machines will have a potentiometer control