Needle hook distance

[Wizcrafts]

I want to point out an observation I've made over my decades of sewing. Unless the bobbin and top tensions are very light (think thin cotton thread), the needle often gets pulled slightly to the left at pick-off time. So, if the hook in that machine is set so it barely makes contact with an unthreaded needle, it will probably not make contact with the threaded needle as it sews material and gets pulled to the left. This fails if the tensions are so low that the needle isn't deflected by the thread. This puts the hook and needle in peril. Further, thicker thread will tend to pull the needle more than thinner thread..

I prefer to adjust the check spring for a big enough thread loop so that the hook never needs to be in contact with the needle to pick it off. I purposely bend the deflector shield if necessary to prevent the hook from contacting the needle.

[Nmgunguy]

So I ran a few “tests” on my 4500 to see what the impact of this hook-to-scarf delta had in my application and came to some really interesting conclusions that are somewhat different than what I had expected.   To start, I choose the largest needle I had(27/250) and adjusted the hook-to-scarf clearance to 0.5mm.  I have never really measured this setting before, only adjusted by “eye” but the 0.5mm seemed to be a much larger gap than normal for me.  With the stitch length at the maximum, I had no missed stitches in forward or reverse.  So then I changed to the smallest needle I have(22/140), swapped out thread both top and bottom and tested again.  This time, I had no missed stitches in forward, but I did have one missed stitch in reverse.  I’m not sure if this had any relation to the larger gap, or my not so great handling of the material

For test two, I put the scarf to hook back to “normal” for me and shortened the needle bar height by 0.5mm (raising the needle eye in relation to the hook point) and repeated the tests above.  This time it was very obvious that the adjustments were off as it missed nearly all of the stitches in both directions!

This tells me that for this particular scenario,  the relationship of the hook point to the needle eye was more critical for the hook to catch the loop than the distance from the hook point to scarf.  From a logical perspective, I can’t imagine that this would be the same for every machine as most of the machines that I am familiar with form a loop that is taller than it is wide.  This makes me think there is more tolerance in the height relationship than there is the width.  There are also so many other influences on the size of the loop that it is hard for me to know exactly which has the most effect.

In normal operation for me with both the 4500 and the 3200, I regularly switch between needle sizes without any adjustments other than tension and seldom have any issues.  Thank goodness for such a versatile platform! 

 

Anyway, very interesting topic for me and another great learning experience!  Thanks for all the input!

 

Now to put all the adjustments back so it can run……

[Gymnast]

I agree, that the normal timing adjustments of needle bar height and timing of hook is important. I just think these adjustments are well defined, because the same needle system got the same distance between needle but and upper edge of the needle eye when needle size is changed. I have had a harder time finding information about needles systems and the dept of the scarf and some of the geometry here, when you change needle size. So this is why I decided to discuss this in forum. 

I do not have a CB 3200 or 4500, but I have seen videos of them. They have got an oscillating hook and Uwe have made a nice video about it:

 

According to Uwe, the play of the shuttle hook regarding distance to needle needs to be low and I guess it means below 0.1 mm. The distance is adjusted by having more rings (race back) to be replaced behind the shuttle, and the rings got different thicknesses. Uwe comments about the orientation of the race back, that can influence reverse stitching.

Nmgunguy, did you adjust the distance by replacing the ring or did you do it in some other way?

I got a Bernina household sewing machine with a oscillating CB-shuttle hook. It have got a quite high play of about 0.3 mm regarding distance to the needle. For this machine I see the lowest distance when the needle go down and pass the hook while doing so. The lowest distance is determined by the part of the needle outside the scarf and near the needle eye. Perhaps you will se the same with this much larger machine, because it is also an oscillating hook machine.