What is the tolerance for needle bar height?

[williaty]

This is for a domestic Juki but you guys are the best sewing machine mechanic resource on the internet. The first test the service manual calls out is setting the needle bar height. The spec for setting the needle bar height is that at BDC the stop pin in the needle bar clamp is 19mm above the surface of the throat plate. The current setting is actually 18.61mm above the needle bar plate. In other words, the needle bar is 0.39mm (0.015") lower than the spec calls for. The service manual, unfortunately, doesn't list a tolerance for this or any other measurement. Since I'm trying to correct some problems I'm having with the machine, is this something I should try to correct or is it so close to right that my problem must lie elsewhere?

[DonInReno]

What kind of problems are you having?

For most sewing machines the rule of thumb (common to most service manuals) is to bring needle to bottom of its travel, then after the needle has risen 2mm (about 3/32” or thickness of a US nickel) the hook should be inline with the needle and 1/16” above the eye.

This will show correct needle bar height, hook timing and hook clearance with the needle.  As the hook passes the needle it should be as close as possible without actually touching.   It’s ALWAYS a good idea to only perform this check after replacing a new needle.   

There’s nothing wrong with checking the 19mm dimension in your manual, but I’d suggest looking at the above as a double check.

If your needlebar is only .015” low (so the hook is passing the needle closer to the eye) the only problem it might make worse would be increasing the chance of a skipped stitch.  If the machine isn’t skipping stitches then the current needlebar height isn’t a problem.  Even if it is skipping stitches, the culprit is most likely a function of hook gap and condition of the hook point.

I hope that made sense.

 

[sandmanred]

I like to see the hook point pass the center line of the needle right in the middle of the scarf (top to bottom).  There is normally a spec regarding how much the needle has lifted by the time the hook is supposed to meet the needle centerline.  Do they mention that value?  If it's machine that runs a typical universal domestic needle it should be pretty close to a Singer 31-15 where the rise of the needle should be 3/32 inch when the hook tip crosses the needle centerline.  Hope that helps.

[williaty]

On 3/1/2021 at 11:59 PM, DonInReno said:

What kind of problems are you having?

 

 

Several things that I can't figure out if they're related or not so I want to verify that the machine is properly set up to begin with. It's skipping stitches on thicker material such as 2 layers of quilting cotton with 2 layers of 1/16" cotton batting between. On two layers of cotton alone, it'll sew perfectly fine 99% of the time, then suddenly start leaving 1/4" tall loops on the bottom side of the fabric (top thread isn't getting pulled fully back up after going around the bobbin, I think), then magically fix itself and resume stitching perfectly like nothing has happened.

 

In the service manual, the procedure for verifying everything is:

1) Verify needle bar height at BDC

2) Verify needle centering left-to-right in straight stitch mode

3) Verify feed dog height

4) Verify feed timing

5) Verify hook tip is on centerline of needle when the needlebar has risen 1.6mm from BDC. Change hook rotational timing if necessary

6) Verify hook tip is 3.2mm above top of needle eye when needlebar has risen 1.6mm from BDC. Adjust hook assembly height if necessary.

 

 

I've done 1 through 4 already with the only discrepency being the needlebar being 0.015" low. I'm waiting on #5 for another tool to come in. I need a longer extension on my dial indicator to measure the 1.6mm rise on the needlebar to verify the timing position.

[CowboyBob]

10 hours ago, williaty said:

Several things that I can't figure out if they're related or not so I want to verify that the machine is properly set up to begin with. It's skipping stitches on thicker material such as 2 layers of quilting cotton with 2 layers of 1/16" cotton batting between. On two layers of cotton alone, it'll sew perfectly fine 99% of the time, then suddenly start leaving 1/4" tall loops on the bottom side of the fabric (top thread isn't getting pulled fully back up after going around the bobbin, I think), then magically fix itself and resume stitching perfectly like nothing has happened.

 

In the service manual, the procedure for verifying everything is:

1) Verify needle bar height at BDC

2) Verify needle centering left-to-right in straight stitch mode

3) Verify feed dog height

4) Verify feed timing

5) Verify hook tip is on centerline of needle when the needlebar has risen 1.6mm from BDC. Change hook rotational timing if necessary

6) Verify hook tip is 3.2mm above top of needle eye when needlebar has risen 1.6mm from BDC. Adjust hook assembly height if necessary.

 

 

I've done 1 through 4 already with the only discrepency being the needlebar being 0.015" low. I'm waiting on #5 for another tool to come in. I need a longer extension on my dial indicator to measure the 1.6mm rise on the needlebar to verify the timing position.

Try more foot pressure if that doesn't work then back the hook up alittle to 1.8 or even 2.0 it'll make a bigger loop for the needle to catch.

Edited March 3, 2021 by CowboyBob