Hole saw mandrel

[michigangeorge]

I seem to remember seeing a special mandrel which allowed a smaller hole saw to fit inside a larger one to act as a guide when enlarging an existing hole. Anyone seen one of these?

[Yeadon]

I have a couple mandrels, and all are basically just big enough to squeeze a second, smaller hole saw inside on the same threads. They're just standard old mandrels, nothing too fancy. I saw a tip about this in Fine Woodworking, I think.

[Rational Root]

Cut a hole (size you want) in a piece of 1/2" ply.

Clamp the ply in place over the old hole.

The ply will now act as a guide for the hole saw.

It's a little fiddley, but I've done it a few times, It works OK.

[Paul Scheuer]

Or clamp the plywood behind the existing hole and let the hole saw guide on its pilot drill.

[michigangeorge]

Cut a hole (size you want) in a piece of 1/2" ply.

Clamp the ply in place over the old hole.

The ply will now act as a guide for the hole saw.

It's a little fiddley, but I've done it a few times, It works OK.

DUH? good thinking RR - your common sense solution will work great for my use as I only need a 1/4" hole depth. THANKs

[michigangeorge]

Or clamp the plywood behind the existing hole and let the hole saw guide on its pilot drill.

Another good idea, but not for my current problem - Thanks

[dstreck]

http://www.leevalley.com/wood/page.aspx?c=1&p=52518&cat=1,180,42316&ap=1

[redbopeep]

Use two mandrels back to back: The front one has the blades towards you, the back one does the cutting. If your mandrels have a quarter inch pilot then use a 1/4" solid rod (e.g. the smooth shaft part of an extra long 1/4" drill bit (that was broken off on one of your many other projects....:D) works fine) between the two. Be careful when starting because you can torque off this 1/4" shaft pretty easily else it works fine.

We used this system to do the 6-1/4" spigot counterbores for our 6" portholes. In that case put (jam!) a 6" hole saw in the porthole and drill with the 6-1/4" one. It spun on the 1/4" coupling shaft. We also used a variation of this for our 3" dia x 30" long shaft log :eek: and the counterbores for stuffing box and stern bearing on both ends. Worked great.

[michigangeorge]

Dave - thats the one I was looking for! Thanks
Redbopeep - great idea as I am also using it for the stern bearing counterbore.
Thanks

[Bill Perkins]

The forum comes through again ! I've decided to up size my bilge pumps after operating my new boat for a couple of years .This means bigger through hull fittings . The Lee Vally tool certainly seems like the simplest solution . I have in the past just glued temporary plugs in a hole (sawed out with a hole saw from scrap ) ,then used the required bigger hole saw normally . That does work.

[willmarsh3]

I had a similar problem a while back when installing a new speed transducer in a steel boat. The existing hole was too small.

I did something similar to Bob Perkins but did not even bother with glue.

I just made a plug of a piece of wood about 1 1/2 " square by 6" long with the corners rounded a bit. I tapped it in as far as it would go and cut it off about 1/4" above the surface. I used a hole saw with a drill bit in the middle. The drill bit went in the wood and held it in center as the saw cut the material around the existing hole to make a new, larger hole. The whole job took maybe 15 minutes.