Need help Machine wont cut My elna 792d wont cut but chew fabric,I've not used it heaps,I did rolled hem & when i put it back to normal 4 thread overlock it wouldn't cut anyone suggest anything

3-thread rolled hem:

Remove left needle.
Settings as follows:
Upper looper 3-5
Lower looper 7-9
Right needle 3-5
Stitch length 1-1.5
Cutting width 3-5
Differential N

If you scroll down to "Serger," and download Workbook 1, there are instructions in there for setting up a rolled hem.

Bernina Mastery Workbooks

Your machine differs but the way it functions is very similar--at least the settings should be helpful.

I don't know your model in particular but generally this is how you set up a serger/overlocker to create a rolled hem. Remove the left hand needle and thread if 4 threads are threaded). Now there is usually a lever around the stitching area that you slide back towards you to remove the stitching finger back (disengaged). Then you adjust the top looper tension looser (about 2-3) and lower looper tension tighter (7 or 8). Leave needle tension at normal tension (5). You may need to move the cutting blade to the right too. The technique is to cut the fabric wider but with a narrow stitch so the cut edge rolls under within the stitching. You want the top looper thread really loose so it rolls right around underneath up against the needle thread, the lower looper thread needs to be tight and almost invisible, pulling the upper looper thread down. I usually do a few test runs and check the stitching. Once happy, I turn the stitch length right down to 1 so it is very dense. This uses a lot of thread up.

Ideally use a wooly overlock thread on the upper looper as this "fluffs" out once stitched and "fills" in covering the fabric edge completely.
some good images here
Sewing Basics Narrow Rolled Hems with Serger Sew Mama Sew Outstanding...

Use a rolled hem foot.

this sounds like an overlocker/serger machine you are using? if the fabric is catching on the overlocking tongue it could be a number of factors, have you lifted the top cutting blade out of the way and forgot to put it back into place? That would be my first thought, the top and bottom blade move against each other cutting the fabric. Rolled hemming usually requires the blade to be set on about 2, or middle of its setting range, the stitch finger is pulled back out of the stitching area, top looper thread is loosened off and bottom looper thread is tightened up, resulting in the top looper thread wrapping around the edge of the fabric and pulling the cut edge down under within the seam, forming the rolled hem.

Rolled hemming on any overlocker is usually achived the following way. Right hand needle only.
Blade over to the right usually to cut wider than normal but you can vary this to suit the fabric and stitch width you want.
Thread in this needle and the upper and lower loopers. If you can source any, use woolly overlock thread in the upper looper, this fluffs out when not tensioned and covers the fabric to give the look of solid stitching. Change upper looper tension to about 2 and lower looper tension to about 6 or 7.

The lever by the plate is usually to control the little stitch finger where the stitches are formed, move it backwards and it should move back towards you?? If this happens, then you want it in the retracted (towards you) position for rolled hemming so the stitching is much smaller and the fabric can roll. Normally this stitch finger holds the fabric firm for the loopers to form the stitch over for your normal 3 or 4 thread overlock.

Now test stitch and see how it looks. Tighten the lower looper thread so it lays right beside the needle on the underside. You may need to then tighten or loosen more the upper looper, you want the upper thread to wrap all the way around to the underneath against the lower looper thread and needle.

The cut edge of fabric should roll to underneath inside your seam. Once you've got this happening, turn the stitch length down to 0.5 or so to close it right up tight. This uses heaps of thread so I usually test everything else, then close it up at the end to minimise waste.

Hope this makes sense, good luck.

You can not do a rolled hem on heavier material. It is only for thin cloth. You have to do a 3 thread narrow edge on heavier cloth.

Just trim the fabric to the desired hem length plus one inch, then overlock around this raw edge with thread close colour match to the fabric.

You then have the choice of either stitching around with a stretch stitch on your sewing machine, or hand sewing.

Being lycra shouldn't change how you hem them unless the trouser leg is extremely close fitting and the hem seam is going to be under tension.

My first choice would be using a twin needle in straight stitch from top side but this finish can pop/break if the hem is under tension. But perfect for most knit hems.

If you hand sew then you can turn up hem allowance and put a length of fusible hemming web inside the hem allowance and press it to hold. Then do a back stitch hand sew around to hold hem in place.

If you sew with the regular sewing machine, then use either the stretch stitch or a very small zig zag, and a ball point or stretch needle.

On ready to wear you would see a coverstitch hem but your overlocker doens't have this stitch.

You need to disengage the cutting blade by openingfirstly opening the front and left hand side covers. pull on the larger of the two silver knobs and slide the red lever towards you. I'm not sure what the tension settings are, but I'm certain you need to reduce them a fair bit to aid in the rolling effect. I only use 3 threads when doing a rolled hem.
My description isn't too detailed but I hope it helps!