Great difficulty cutting 18mm MDF with T101B jigsaw blade

MDF clogs blades so you need one with coarse teeth. If you have a pendulum action on the saw, then turn it on. Don't force it. You may also find you get a better straight line following a drawn line rather than using a straight edge to run the machine along.

True for *cheap* jigsaws - a proper professional one however is a completely different class of tool and quite capable of cutting a straight line.

The OP's ancient AEG is no slouch. I have one too. £100+ 40 years ago IIRC

You use a router for that.

almost normal for jigsaws. if hole is critical I cut a template in

If hole is square then if possible use circular saw.

I do find reverse tooth blades (down cutting blades) that cut the opposite way are much better on MDF .... I buy them for laminates, but find them good on MDF

But what do you cut the ply template out with....?

Jim K

Using a jigsaw with a fence is a classic mistake. Follow the line by steering it.

NT

I use a router and a fence..

I have a hole cutting attachment which allows you to cut the hole direct. There will be a minimum size, though, which may be too large with many larger routers.

Lidl recently did what they called a combination cutter which will route holes from 50-160mm radius. Only snag with it is the 4.8mm collet size - most places don't stock bits.