Drill bit to Reamer

[octaneman]

Hello everyone,



Is there a formula to determine the right reamer size in relation to the drill bit used ?

For example; I have a piece of steel 3/8 thick and 2" wide, I need to make a 1/2 inch hole to fit a dowel pin, what size reamer would I need to finish the hole and how would I calculate it. ? Is there a chart available ?


Thanks.

[PixMan]

There is no hard & fast formula. How much material to leave depends upon the type of material, hardness of it, the size and tolerance of the finished hole, and the reamer to be used.

When you say you have to make a hole for a 1/2" dowel pin, what type of fit do you require? Easy slip-through, snug-sliding or press fit? Most hardened dowels are toleranced as 0/+.0002".

What machine are you using to do this?

[octaneman]

The dowel has to be snug fit, the only machine I'm using is a drill press to make the holes and the reaming will be done by hand.

[Geof]

If you are reaming in mild steel, brass, or aluminum you can fairly easily take out several thou with the reamer so you can even go 1/64" undersize for a 1/2" reamer. Typically you would go something like 2% of the reamer diameter undersize for the drill between the limits of never having less than about 0.005" or more than about 0.025" to remove with the reamer.

For your application you may be better off doing the reaming in the drill press. It is very hard to run a reamer in by hand without creating a bell-mouthed hole; reamers are difficult to hold parallel with the hole by hand.

Clamp your work onto the drill press table for drilling and leave it there for reaming. You may need to position the table as low as possible while still getting the drill through the work so that you have clearance to get the drill out of the chuck and the reamer in. Do not use power to drive the reamer. Turn it by hand either by turning the top pulley on the spindle or putting a small rod in the holes in the chuck to act as a lever (unplug the drill for both of these). Feed very gently while turning the reamer and use a bit of libricant such as tapping fluid. NEVER turn the reamer backwards: If you find it jams pull it straight up out of the hole and start again feeding even more gently.

[PixMan]

If "by hand" you mean using the dress press and feeding it by hand, you'll be good with a Ø.5000" straight shank HSS chucking reamer. Run it at half the speed and twice the speed of the drill. The drill should be 31/64".

[packrat]

What was taught back in the 1960's during my apprenticeship - drill size for reaming -
3/8" dia and under use 1/64 smaller than reamer. 3/8" dia and over use 1/32" smaller than reamer. If you don't leave enough in the hole for a good chip load, the reamers won't cut to size.

[octaneman]

I really appreciate everyone's insight you've all been a tremendous help.

Thanks guys.