Made a hollow mill for spring posts. Something is wrong. Help!!

[Rolf]

Working on the frizzen spring. The hole in the Lock plate is 2,5mm. I used the same drill for the hole in the Lock plate and the hole in the Hollow mill. I made the Hollow mill from silver steel.
It hardend nicely and cuts Sharp. I've cut six posts. Half  the post it cuts is 2,33mm in diameter and half the posts are 2,49mm.

Why the different sizes???
The posts are in the middel of the spring. I use cutting oil, 1000rpm and feed with very little down presure.
 2,49mm gives a nice tight fit. 2,33mm wiggels more than I like.
How do I get it to cut 2,49mm everytime?

Best regards
Rolf

[Acer Saccharum]

Rolf, it sounds like either your stock (the spring) is moving, or the cutter is orbiting around the pin. The farther the hollow mill is extended far out from the spindle nose, the more likely it can orbit around while cutting.

If the outside of the cutter is concentric with its bore, you might be able to steady the cutter with a bronze bushing in a plate that is bolted to your worktable. If you're doing this on a drill press, there is a lot of flex in such machines.

I hope some of that helps? Keep us posted.

Tom

[ron w]

i'd suspect that the hole in your mill wasn't drilled on the same center line as the center of the mill body.....(the hole drifts of center as it gets deeper into the mill)….. it starts out cutting on center and as it moves down the post it starts wobbling because the hole is also the mill's guide. is the smaller size of the post at the top or bottom (nearer the spring) ?. if the post gets smaller towards the spring the above might be the problem.

[kutter]

I've always relieved the hole in the hollow mill just back from the cutting teeth.
On a large bit, a boring bar works OK. On a small hollow mill I just drill back down from the top end and stop short of
coming out the cutting end.
Both to leave a small pilot shoulder to guide the hollow mill, but relieved back up inside so there's no interference betw the mill cutter and the post you are cutting.  It doesn't need a lot of clearance.

Otherwise the post you are cutting is the same dia as the pilot/guide hole in the mill,,,a force fit for the entire length.
The newly cut post, say 1/8d is being forced into an 1/8"d hole. The post strips off matl/galls as it is forced deeper in., ends up smaller in dia than supposed to be, or misshaped,or sometimes broken off up inside the hollow mill

Anywy, that's the way I've done it as I had problems with a simple hollow mill cutting a simple post.
A short post can be cut with the same dia pilot hole in the cutter I found, Say it's height and dia being about the same. But after that, I need to have some clearance for the post to turn in there.

See if the drill you used to drill the hole in the hollow mil will still enter than hole,,or perhaps the hole is diminished in dia now with material from galling and sticking the the sides.

[Rolf]

Thank for Your suggestions. I used a drill chuck in a vertical mill to cut the first posts. The posts have the same diameter at the top and the bottom. The problem seems to be caused by vibration in the chuck that cause the cutter to orbit the post blank. I made a New cutter following Kutters describtion that fitted in a milling collet. This solved the problem. Thanks.

Best regards
Rolf