Difficulty Fly Cutting on a Benchtop Milling Machine

[erikmannie]

Yesterday I was using an econo-import 3” fly cutter with econo-import (1/2”?) tool shank & carbide insert to fly cut .007” off some 6061 aluminum. The mill is a PM-25MV on the factory stand bolted to the concrete floor with 2 bolts.

The quill was locked in the fully retracted position the whole time. After dialing in the cut, I would tighten the two z-axis locking bolts.

While it was cutting, both z-axis locking bolts would loosen enough to allow the head to drop. This would cause the DOC to increase up to .025” or so (too much for a fly cutter!).

Can you guys think of any way to fix this problem?

 

[erikmannie]

I tightened the bolts as much as I felt was safe to do so. I’m not going to go crazy and gorilla-tighten these bolts because I don’t want to abuse the machine.

I understand that this machine has a 25mm max cutting tool diameter, so maybe the 4” or so fly cutting diameter is well beyond this max.

Assuming that one is able to use a fly cutter on this machine (I have done so before with .002” or less DOC on mild steel), maybe a solution to this problem is to apply a bit of Teflon tape to both bolts every time. I think this would prevent the bolts from backing out.

 

[erikmannie]

This used to be my only milling machine. As I usually work on mild steel, I got a much bigger mill because I felt like this machine was vibrating, as I have it mounted.

Another way to solve this problem might be to completely re-examine how I have it mounted (e.g. check that the 4 bolts mounting the mill to the stand are tight).

I find it a little unusual that the stand bolts to the floor in only 2 points.

Maybe I could loosen the anchor bolts in the floor, and put a long, thin shim under the back of the stand, and then re-tighten the bolts. We want the base to make full contact with the floor.

What is to say that my garage floor is completely flat, or that the bottom of the stand is completely flat? Does anybody ever put a thin square piece of rubber under the stand to take up any slack that may result from the lack of flatness in either the base or the floor?

 

[erikmannie]

As it is mounted, any machining operation with an appreciable DOC causes the whole assembly to vibrate.

 

[jwmelvin]

Make sure backlash in the z-axis is taken up (approach the position from below). Are the gibs well adjusted? 0.025” seems like a lot of droop from the lock disengaging.

 

[benmychree]

Tar paper is good to put under things to make better contact. Regarding big cuts on a tiny machine, "you caint rollerskate in a buffalo herd".

 

[erikmannie]

Make sure backlash in the z-axis is taken up (approach the position from below). Are the gibs well adjusted? 0.025” seems like a lot of droop from the lock disengaging.

You may have found the problem (lash!). I was *lowering* the head to establish the DOC.

The gibs have the factory adjustment.

 

[erikmannie]

Tar paper is good to put under things to make better contact. Regarding big cuts on a tiny machine, "you caint rollerskate in a buffalo herd".

Tarpaper sounds like an appropriate solution to marry the concrete floor to the base of the stand.

I want to point out (confess?) that I have used the stand to walk (and drag) the [machine + stand] assembly around the shop a few times when I moved the machine. Maybe this crushed the corners or abused the bottom of the stand in some way.

I must have been quite concerned about this after it happened because I purchased a second stand. That is, I have an extra PM-25MV stand here that I use for lubricants as well as having a heavy collection of 5C collets on top of it. The bottom of this second stand has seen little to no abuse.

 

[erikmannie]

I think I will go out in the shop and take some pictures of the stand mounted to the floor, even though the point made by @jwmelvin about the lash was probably the problem.

It is weird that I have always been aware of the z-axis lash on knee mills, but missed it on the benchtop machine.

A question:
(1) Are we only able to take up the backlash by way of turning a handwheel clockwise? I understand that these leadscrews are almost always LH threads.

The reason that I ask is because one *always raises the table up* in order to take up the z-axis lash on a knee mill.

 

[erikmannie]

From Post #3 in

How do you determine and measure when you have backlash?

Is this correct or an oversimplification?: "You turn the handle in one direction causing the carriage or table to move, zero the dial on the shaft, turn it in the other direction until the table just starts to move, read the dial, thats the backlash." :tiphat: Nelson

www.hobby-machinist.com

Quote:

“Those of us lucky enough to have digital readouts on our mills don't need to worry about backlash when moving to a position but we still need to remember to feed against the force of the cutter and/or lock the table to keep bad things from happening…”