Sunday, August 24, 2008

Lessons Learned On The Sherline Vertical Mill

Lessons Learned On The Sherline Vertical Mill

And fortunately, not ones that required stitches or even bandages! Some of this would be applicable to other vertical mills as well, of course. I've been making a lot more use of the vertical mill of late. As an example, I needed to make a small bearing that provided some adjustment. Here you can see how I did it.

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I milled one piece of Delrin to have a 1.01" slot in it, and another piece of Delrin that was 1.00" wide. Each has a .50" diameter hole through it--through which a 1/2" steel shaft fits. Then I drilled and tapped 1/4"-20 holes in the bottom part. Some 1/4"-20 hex head bolts lock the bottom part to the shaft.

The top part has a 1/4"-20 tapped hole as well--into which a thumb screw goes. This allows me to adjust the friction against the steel shaft.

As is often observed, buying machine tools doesn't make you a machinist anymore than buying a piano makes you a pianist. I have moved from that category to rank amateur in the last year or two, and I thought I would share a few tricks that I have learned along the way.

The biggest problem with any milling machine is that you can guarantee that it won't be large enough for some project or another that you are going to try. Milling machines move in (at least) three axes: X, Y, and Z. What I have is a Sherline 5000 that has been upgraded to pretty much the specs of the Sherline 5400. It has quite a bit of motion in the Z axis (vertical) and the X axis (side to side) but relatively little motion in the Y axis (front to back). It doesn't matter what milling machine you buy--at some point, you are going to find that it doesn't have enough motion for your project. You could buy the milling machine used to make the Death Star for Darth Vader, and sure enough, there will be some asteroid too big to clamp in the jaws of the mill vise.

What, exactly, is a mill vise? It is like the vises with which you are doubtless familiar, that clamp a object in position so that drill, cut, grind, or file. But a mill vise is far more accurately made. The bottom of a mill vise (where it mates to the milling machine table) should be very, very parallel to bottom of the jaws, where the workpiece will be positioned.

The mill vise also has to hold the workpiece very rigidly in position. If it starts to move even a little, instead of cutting off ten-thousandths of an inch of material with your cutting tool, it will grab ten times that much--and either your cutting tool will stop turning, or the workpiece will get pulled out of the jaws. The net effect is ugly, either way. You may damage the workpiece. At a minimum, you will have to reposition it, and reclamp it.

The mill vise that comes with the Sherline has two irritations, one of which is easy to fix, and one of which is an opportunity to show how clever you are.

1. Take a look in this picture. See the socket head screw on the mill vise (the triangular black piece at the left of the picture)? That's what you tighten to clamp the jaws to prevent your workpiece from moving.

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You tighten that down with a little tool that Sherline provides. It works great..for a while. The problem is that putting enough force on the socket head to clamp workpieces in place tends to chew up the head. Over time, I found myself having more and more problems getting the vise to hold the workpiece, and I couldn't figure out why. The reason is that I just wasn't clamping the workpiece so well anymore.

The good news is that this 10-32 socket head screw is extremely common, available in just about any hardware store, and you can afford to buy a dozen of them for a couple of dollars. Keep them around; as soon as you start having problems getting a good clamp on a workpiece, replace the screw.

2. Another irritation is that the Sherline mill vise is pretty small. You can't really put anything wider than 2" into the vise. Is there a solution to this? Yup. If you have something wider, you need to build a fixture that is 2" wide that either epoxies onto, or screws into, the workpiece that you need to machine. Or you use the other clamping mechanisms that are offered to hold a workpiece to the table. There are larger mill vises offered by Little Machine Shop.

In addition to the irritations that are specific to this mill vise, there are some that are general--and which any mill vise is going to have.

1. I found out by asking around, and frustrating experimentation, that if the workpiece is more than about four times the height of the mill vise's jaws (which are about 1" high on the Sherline), that the forces exerted by the cutting tool will exceed the friction that the mill vise can exert--and the workpiece will start to slip. This is especially a problem if the cutting tool that you are using is moving parallel to the jaws. In this picture, you can see how I have turned the mill vise to the side, so that the cutting tool will be moving left to right along the table, and therefore perpendicular to the jaws.

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If you are doing what is called "plunge milling," where you move the cutting tool vertically to machine an edge, again, you are going perpendicular to the jaws, and there is less of a problem. Obviously, you can't get too enthusiastic like this. Try to plunge mill off a tenth of an inch of aluminum or steel, especially if you try to move down too quickly, and your jaws will lose grip. (Well, perhaps Darth Vader's Death Star vertical mill could handle it.)

2. Another aggravation is that if you position the workpiece exactly in the center of the mill vise, you will quickly discover that the back of the mill vise limits your motion in the Y axis--where most vertical mills are a bit deficient, anyway. The back of the mill vise will run into the vertical rail upon which the cutting tool goes up and down. So what's the solution?

Let's take a look at this picture again.

Click to enlarge

I've put the workpiece off center in the mill vise--and for some what I have been doing, it was way more off center than that. Notice how the back of the mill vise (just barely visible between the workpiece and the rail with the sockets in it) is actually sufficiently far to the left that it doesn't impact the vertical rail. Problem solved.

You will also find that turning the mill vise backward--so that the clamping socket head screw is pointing away from you--provides a lot more opportunity for positioning a workpiece. It's a bit clumsy to tighten and loosen the mill vise, but it does provide more ways to get a large workpiece in the right position relative to the cutting tools.

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