Flatback needles with no scarf requires very close needle to hook clearance adjustment to avoid skipping.
Specifications
Originally, there was just the 705 needle, which was basically the same as the 15X1. Pfaff came up with the idea that you could "scoop out" a section of the needle above the eye, (we call this the scarf) so that you could set the hook in closer to the needle. The stitch is formed when the friction between the fabric and thread on the "back side" of the needle during the first part of its upward stroke, causes a loop to open, thru which the hook should pass. Naturally, the larger the loop, the less likely the hook is to miss it. The size of the loop is from the needle surface to the extended thread, so if there is "less needle" there (the scarf) the loop is bigger, and the hook has a better chance to catch it and not drop a stitch. Pfaff called this needle the 130R, and it was basically the same as the 705 or 15X1, but with a scarf. It wasn't long before most of the manufacturers recognized this as an advantage, and Schmetz soon came out with the original 705H, which was virtually the same as the 130R, would work fine in the Pfaff, and also in machines like the Elna and Husqvarna, as long as their hooks were set in close to take advantage of it.

However, the original 705H needle did not work well in the Berninas, or any other machines with transverse oscillating hooks. The problem was that the oscillating hook makes one half rotation each way, per stitch, equaling one rotation per stitch in distance traveled. The rotary hooks make two revolutions per stitch, so they travel at twice the speed. As a result, the "timing error" on a transverse oscillator is twice as great as that in a rotary. You can see this by watching where the hook intersects the needle at LCR positions in a rotary and an oscillator. At the extreme L & R position, the point of ntersection on the oscillator is twice as far from the optimum spot on the needle as it is on the rotary. The scarf in the 130R & the original 705H needle was just long enough to ccommodate the rotary hooks at their extreme ZZs, so if you tried to set the machine to use one in an oscillator, the tip of the hook would graze the needle at the top and bottom of the scarf. This is where the old story came from that the H needle would damage the Bernina hook, and some people still believe this, even though this shape scarf went out, probably about 1970 or before. Schmetz was anxious for Bernina to be able to take advantage of this technology, so they elongated the scarf to allow clearance for the slower moving oscillating hook.

Now came something really "off the wall". Somebody high up at Bernina got the idea that the H on the 705H needle stood for Husqvarna, so they did not want these H needles in their machines. In fact, they insisted that Schmetz continue to make the old non scarf needles and "give them a letter too", so they became the 705B. As a result of these changes, if the Bernina's hook is set in close to take advantage
of the scarf, the 705H is definitely the better needle, but you
definitely should not use the 705B in these machines. If the hook is not set in close, the "far side of the loop" is at the same position on the H or the B, so it really should not make any difference which one you use.

Don't feel bad if you have to read this a few times before you
understand it. Some people have been in the business for a lifetime and still do not.

I will be happy to answer additional questions.

Bill Holman
Madison, Wisconsin
Allan P, Kalkaska MI
will recommend your website to others. very satisfied with purchase. thanks.