Tips on Sewing Biothane?

[TomE]

I am starting to sew halters made from 2-3 layers of Biothane Beta 520 super heavy (3.5-4 mm thick) on a Cobra Class 4.  I am having problems with skipped stitches, mainly when backstitching 2-3 layers near hardware.  I am using a holster plate that allows me to stitch closer to a center bar buckle.  I did make small adjustments to the hook timing, following @Uwe's video on Youtube.  Thanks, Uwe!  The machine sews leather fine.

My current setup for Biothane is the Hennigan Precision holster plate with feed dog, the Hennigan narrow presser foot set, #207 thread top/ #138 thread bottom, size 24 794 7x3 round point needle, and 5-6 stitches per inch.  This setup buries the knots in a single layer of Beta 520 super heavy.  I have also tried #277/#207 thread with needle sizes 27, 26, and 25.   I've done a limited test of cutting point needles but I like the thread tension with round point needles.  I have used the stock holster plate and harness feet that come with the Class 4 for the larger thread and needle sizes.  Still get occasional skipped stitches.

The information for needle choices that I've read in this forum and seen on Youtube is contradictory.  Most suggestions are to use round point needles that are 1-2 sizes larger than what would be used for sewing leather.

I haven't yet tried thread lubricant, and I haven't played with presser foot tension.  I don't notice flagging while sewing.  I mainly sew bridle leather on this machine and would prefer not coating the thread path with lubricant, but I am open to suggestions.

I appreciate your advice on machine setup and best practices for sewing Biothane.

 

[friquant]

2 hours ago, TomE said:

I am having problems with skipped stitches, mainly when backstitching 2-3 layers near hardware.

Is there any visibility into that machine, for instance removing a cover so you can see the skipped stitch with a flashlight as it's happening?

[TomE]

35 minutes ago, friquant said:

Is there any visibility into that machine, for instance removing a cover so you can see the skipped stitch with a flashlight as it's happening?

I don't have a clear plate to view the hook, and it seems to only happen with the holster plate.  I'm going to reconfirm that leather sews without skipping stitches using the holster plate. 

I am guessing that the skipping is caused by friction on the upward stroke through biothane, causing flagging even though I'm not feeling it.  With a size 25 needle and #207/#138 thread the knots were on top, even after considerable decrease in top tension, so I installed a size 24 needle and that buried the knots.  I guess I could switch back to the larger needle and try increasing the bobbin tension.  If I can't find the right needle and thread combination then I guess lubricating the thread is the next step. 

[friquant]

26 minutes ago, TomE said:

I installed a size 24 needle and that buried the knots

What size needle did you use for your initial timing? Does your machine require the shuttle to be shimmed when changing needle size in order to maintain the same hook-to-needle distance?

[Wizcrafts]

I used to sew Biothane and heavy webbing in a previous location. I found that I had to move up one needle size, and use a diamond or round point needle. Additionally, I had to double the foot pressure to keep it from lifting with the ascending needle. Running the top thread thru liquid silicon will help the knots move up with less friction. It might help if you adjust your check spring to give more slack top thread as it goes around the hook. Try it on a test strip.

Thread color also plays a part in all heavy sewing jobs. Black thread is often double-dipped to get total color penetration. This process stiffens the thread more  than lighter colors. You can usually feel the difference when you handle the thread to feed it through the guides on thread stand. Straw color is usually much softer than black. That translates into tighter knots being formed. Tighter knots are easier to pull up in dense materials, like Biothane and thick webbing.