Upper Thread Breaking On Thicker Veg Tan

[charter]

Hello all. I have learned a lot in the past year or so - much of it from the experience and generosity of LW members - but not quite enough to address a problem I'm having.

I have a Juki 1541S which as far as I can tell is in perfect working order. It sews most thicknesses and types of leather beautifully. But when I have tried to sew a recent project with about 1/4" veg tan pre-dyed stock (2 layers of 7/8 oz) the upper thread breaks consistently after an inch or so under power. Using the handwheel works but is a chore for this thickness.

On the machine currently is a left zipper foot, #138 bonded nylon thread and a 135x16 #22 diamond needle. I have tried all tensions, from way too tight (breaks immediately using the handwheel) to loose enough to start showing the upper thread on the bottom (doesn't break but is a poor stitch). I have tried a #24 tri needle also - same behavior. The needle appears to be properly set in the needle bar and the eye is perpendicular to the stitch. There does not appears to be any dirt, lint, fouling, or burring on any parts. The needles are brand new.

The one possible applicable issue I saw somewhere as a cause of breaking upper thread is "inconsistent sewing speed". Because of the thickness and hardness of this stock the servo motor is laboring and it's hard to get a slow stitch.

I have not tried: Similar thickness natural stock, bigger/smaller thread, other scenarios.

Does anyone have any further ideas or advice? I am running out of scrap stock trying different scenarios ;-)

Thank you for information on this problem and for all the great information you've already provided.

[Wizcrafts]

Try using a Schmetz #23 leather point needle. Also, run the thread through a lube pot, or buy some pre-lubed thread. This makes it easier to fight the friction and heat.

Crank down the pressure spring or springs on the feet. If the leather lifts with the needle it causes skipped stitches and frayed thread.

Try using a double toed foot.

Reduce the bobbin tension, then balance the top tension.

[charter]

Thanks for the quick reply. Will order a lube pot - probably overdue anyway. Is silicone the right lube?

I will try all of the above. Adjusting the bobbin is a fantastic and promising idea. Yet another thing learned from a Wizcrafts post and very much appreciated.

[Wizcrafts]

I use liquid silicon lube, which is sold in quarts or gallons, from industrial sewing machine dealerships.