Problem with off center hole

[SteveCarter]

Hi,

I am making peppermills and my method is as follows:

Turn to round between centers with tenons at both ends.
Mount in chuck and use live center.
Turn body, spigot and top.
Drill 7mm hole through top and spigot.
Part of top and spigot.

My plan for the tenon on the top is to then mount it in the chuck and make a recess in the spigot to hold the metal ring that turns the shaft (I don't like to see it sitting on the end of the spigot).

The problem is that when I look at the 7mm hole in the spigot, it is off center. Furthermore, if I mount the top in the chuck it is not centered either.

I have checked lathe to make sure that the live center and the drive center, they appear spot on.

Would appreciate any assistance.

Thanks

Steve

 

[Hu Lowery]

a few thoughts

Steve,

Twist drills are about the crookedest tool we use, does your drill point line up with the centerline of your lathe? I would center drill, use a short drill bit, then the longer one.

I felt like a dummy yesterday, got reminded there are two morse tapers on my lathe! With a pivoting headstock that likes to get out of alignment I have to check alignment pretty regularly. I did and trued things up. Kept fighting a mystery vibration though. Saw dust had built up in my tailstock morse taper and the live center in my tailstock was crooked.

Hu

 

[Dennis J Gooding]

Steve, not sure why the hole could go far wrong if you use a Forstner bit, but you can avoid the problem by drilling the hole before finish turning the top and then using a cone center in the live center to keep things aligned while finish the top.

 

[SteveCarter]

Steve, not sure why the hole could go far wrong if you use a Forstner bit, but you can avoid the problem by drilling the hole before finish turning the top and then using a cone center in the live center to keep things aligned while finish the top.

Hi Dennis,

It is not the forstner that is causing the issue - it is the 7mm hole that goes through the top for the shaft..but I will try Hu's and your advise by using a short drill bit and then a long one. And I will drill the hole before I start turning so that the cone center uses the hole.

Will report the results.

Thanks much!

 

[Michael Mendelsohn]

I found that the twist drill tends to wander off center as it tries to follow the grain. I have two alternatives that I use when turning the tops of pepper mills.

Method A
1. Mount the top piece by the tenon in a chuck and drill through with the 7mm twist drill.
2. Mount a scrap block in the chuck and turn a point.
3. Remount the top piece between the point and a cone center in the tailstock
4. True up the tenon (both end and sides)
5. Remount using the tenon using the cone center to insure concentric mounting.
6. Turn the piece, including the recess for the drive washer.

This method allows an pattern offset between top and bottom. If I am using a blank where top to bottom alignment is critical such as a laminated blank, I use -

Method B
1. Turn tenons at both ends of the top piece
2. Mount in chuck using the tenon at the 'bottom' of the top
3. Drill halfway through using the 7mm drill
4. Remount using the 'top' tenon
5. Drill 3/4 way through using a slightly oversize (8mm) drill. The drill will 'find and follow' the first hole and the oversize will provide some tolerance. The drive washer will cover the fact that the hole is oversize.
6. Turn the piece, including the recess for the drive washer.

In both cases use the skew long point to make a starter divot to center the drill at the start of the hole.

 

[Bill Boehme]

I am far from being an experienced peppermills maker (Im up to one so far, but have good intentions of making at least a couple more eventually). The one that I made turned out concentric, but I did a lot of planning ahead to minimize the things that cause misalignment. I have turned many tool handles and other things that have the same challenge.

Here are a few thoughts:
Do the drilling before turning the exterior and then turn the exterior between centers.
Headstock and tailstock boresights are never perfectly aligned so, as much as possible, plan the steps so that you minimize switching the drive end.
Woodturning chucks also have errors. The scroll gear has errors and the jaws have errors. If the jaws crush wood fibers that also adds to alignment error. When possible I turn between centers.
Drill bits deflect ... especially small twist drills. The difference in hardness between early and late wood will pull a drill bit. Drill slowly and clear the chips often to reduce side forces on the drill bit.

 

[hockenbery]

Like most have said,
I like to drill the holes first or early in the process and then do the centering on the drilled holes with pin drives or cones.

I usually use wooden pins but cone centers will work well on harder woods.

Al