End mill through pilot hole

[Toolguy]

The main reason is that the end mill has sharp edges all the way up and down the sides. Any variation from perfect will cut more on the sides of the hole. This includes spindle runout, collet or toolholder runout, chips packing in an opposite flute, chatter, etc. A drill bit only cuts on the end. It's all about tool geometry, and what is and isn't a cutting edge.

[754]

Use good holder with minimal runout.
tight spindle.. least amount of material remove, and you will be close or on size..

[eKretz]

Use a good holder with minimal runout, use a nice tight machine with everything that moves clamped down tight, make sure you are trammed in good, only leave minimal stock. Use a sharp endmill. You will cut a hole that is very near the size of the tool.

[ammcoman2]

I have had good results by locking the quill and feeding using the knee and my drill chuck (Jacobs JKP keyless which has close to zero runout ). I figure it is then as close to a jig borer as I can get.

Geoff

[Doozer]

I am glad I am not using your mill..

Consider it might not be exclusively the fault of the mill.
Most likely syndromal events lead to the undesirable outcome.

-D

[dero]

Thanks for all the replies, I am a complete novice , so all advice is well received .
The mill will be a new carbide 7mm , opening up a 5.5mm hole ,so it sounds like all should be ok .

[A.K. Boomer]

it will make a hole .003-.007 oversize.

Someone needs to drag their loose chattering quill or do a better job at cleaning chips out...

Old Marts allot closer with his "no more than .001" for a 10 mm hole...

[A.K. Boomer]

I have had good results by locking the quill and feeding using the knee and my drill chuck (Jacobs JKP keyless which has close to zero runout ). I figure it is then as close to a jig borer as I can get.

Geoff

Are you using a drill chuck in place of a collet? I certainly don't agree with that --- and I understand why your locking your quill and using the table but that's bad for chip accumulation unless you want to wear out your arms and Z - axis,,,

the quills there for a reason - and if you have to drag it you have to drag it - it does not take much to silence it and keep the machine running smooth, and you get to chip clear...

I get done with plunging parts and then I can take the part out of the vise, take the endmill out of the collet - stuff the endmill into the part shank first and then hang the part upside down from the endmill and it will usually take forever to fall out of the endmill with that all too familiar"popping" sound that's telling you it's about as tight as you could get it --- usually about .0005" over the size of the endmill ... this all of course depends on the weight of the part and the size of the endmill...

keep in mind im not a machinist im just a mechanic that understands machines....

[Doozer]

A machinist need to be a mechanic first.
If you don't understand mechanical processes
you will never be able to diagnose machining problems.
If you are a machinist without being a mechanic
you are just a button pusher.

-D

[A.K. Boomer]

A machinist need to be a mechanic first.
If you don't understand mechanical processes
you will never be able to diagnose machining problems.
If you are a machinist without being a mechanic
you are just a button pusher.

-D

I believe for the most part it's a huge head start, but also know there's guys (machinist's) that have been doing it so long they may not understand the finer details as to "why" but they have so many trials and errors under their belt that they know what works and what does not...

Being a mechanic for many years still did not prepare me for when the carbide hit's the metal,,, I had allot of "oh fuque" moments and still do once in awhile...

[A.K. Boomer]

In keeping with the OP, what I believe is important to remember about the two things I brought up that are very much responsible for boring holes oversize,

it's important to realize ALL quills are loose, they all have clearance that's how they function, even a brand spanking new high quality mill has clearance enough to make a difference in plunge culling with an endmill,

and with chip cleaning --- it's so very important --- alls it takes is one chip getting caught in a flute and going for a 360 degree ride real quick to "oscillate" the cutters flute on the opposing side and walla you got a hole that's blown... if it's a critical hole then peck the hell out of it and clean it out each time and go light...

Edit; I will also add - mostly due to chip load but also due to the endmill following and not "looking for a way out"

drill your pilot hole as close to the endmills size as possible with of course not going over... this will reduce chip size and load and also make the endmill follow without squirming...

[Doozer]

I believe for the most part it's a huge head start, but also know there's guys (machinist's) that have been doing it so long they may not understand the finer details as to "why" but they have so many trials and errors under their belt that they know what works and what does not...

Being a mechanic for many years still did not prepare me for when the carbide hit's the metal,,, I had allot of "oh fuque" moments and still do once in awhile...

I think you misunderstand me.
By mechanic I do not mean one who pulls wrenches.
By mechanic I mean one who understands mechanics of mechanism.
One who understands basic physics of motion and understands cause
and effect relationships of mechanical things.
Like a good machinist, a good carpenter has to be a mechanic first.
He has to understand what it takes to make a house stand up,
just like a machinist has to understand how to clamp work to the mill table
or fixture parts in the lathe chuck. You have to think to be a machinist.
You have to think to be a carpenter.

--Doozer

[A.K. Boomer]

Dooze a good mechanic does all the things you just mentioned and much more... that's part of the "understands machines" quote...


One can be an absolute guru in this area - but as I stated before - it does not prepare you for when the carbide hit's the material - or what kind of material - or speeds and feeds, or not even carbide - HSS - or on and on,,, this falls into the category of "machinist" and even the best of the best mechanics still have a huge learning curve in this area...

[Doozer]

Yes, of course. A good auto mechanic is absolutely one who understands the
mechanics of mechanism, and more.
There are a few outliers though. I have seen a mechanic beat the brake rotors
off a car, because he did not know that on this car, it had tapered bearings,
and you have to take the cotter key and nut off. Apparently he never ran into that.
Maybe most rotors come off the lug studs. But he beat the rotor off in chunks.
Poor car.

-D

[A.K. Boomer]

Another honorable mention --- plunge/peck with the quill in the least extension possible - things only get compounded the further out you go to where sometimes even dragging it will not help as the cutter will have leverage over the cinch up area...