The Framers Forum

I have an old Pistorious manual underpinner and it keeps crushing frames as it pushes frames in.

When I bought it the top piece that applied pressure from above was just a flat metal piece that came down. it was marking some frames so I fitted a felt pad to the bottom, but it’s still crushing frames.

If it's crushing frames the force from above is higher than the force from below.
Does it do it to flat moulding too?
Does it do it to different wood types?
Are you sure your wedges are in the correct way round so the blade is facing upwards?
I have a pneumatic one so I can control the pressure so it only ever crushes the moulding if it's very soft, but usually that doesn't cut well so is getting tossed before it reaches the underpinner.

I haven’t checked the wedges, I’ll have a look. I have noticed that they tend to go in with a “bang” which might be them in the wrong way I suppose.

It doesn’t really crush flat moulding since I added the pads, it’s usually these with a high spot on the profile.

sy_woodworking wrote: Wed 04 Jun, 2025 4:33 pm When I bought it the top piece that applied pressure from above was just a flat metal piece that came down. it was marking some frames so I fitted a felt pad to the bottom, but it’s still crushing frames.

The felt might not have enough "give" when pressure is applied. It depends on the thickness of the felt as well as its density. Most felt pads for a pneumatic V-nailer are in the 6-9mm thickness range.

I've sometimes had this problem with mouldings with very thin/sharp front edges - eg a bevel profile, or with some gilt mouldings (like the one you show) where the gilt finish has cracked and flaked off.

If I have a moulding that I think might cause problems then I use a triangular bit of ply about 3 inches on each side to spread the load from the top plunger with a similar triangular bit of mountboard to go between the ply and the frame as extra protection.

For this same reason there are also a few mouldings that I won't use any more, because I've found that they are too delicate to underpin reliably and so just not worth the bother.

I used to use an Framers Corner foot operated underpinner where I used a hard rubber pad between the round metal disc and the moulding and didn't have any problems.
If you wanted another underpinner you can have my old one free of charge, just have to collect from Hull