Start Model Engineering

I am assuming at this point that you have made your move into model engineering and that you are now considering the expansion of your machining capability with the acquisition of a milling machine.

In my case I did try a vertical slide attachment to my mini-lathe and although many people achieve excellent results with such a set up it just didn’t feel right to me. I also tried to avoid the financial outlay of buying my mill by adapting an old bench drill. I bought a cross vice which gave limited movement in X (left to right) and Y (front to back) axis. The Z axis (up and down) was already present with the existing quill on the bench drill - although this didn’t provide the fine control required for successful milling.

After a few months of avoiding the inevitable I sought the advice of Bogs and he pointed me in the direction of Chester Tools and their ‘Conquest‘ milling machine. I took Bogs advice, bought a ‘Conquest’ and haven’t looked back. Not only can I now machine ‘flat’ bits it improved my level of precision to an amazing degree. Within days of taking delivery of my mill I was fitting digital readout displays (DROs) to all three axis. This enabled me to machine to an accuracy of 0.01mm - a hundredth of a millimetre. OK, no big deal to an experienced engineer but to me it was nothing short of a miracle.

On a cautionary note I must say I made a bit of a botch fitting my original DROs in that I didn’t provide sufficient protection to the scales and encountered problems of spurious readings caused by fine swarf and cutting fluid penetrating into the read heads. This shortcoming has now been rectified with custom made clear acrylic shields.

One other potential problem area lies in the nylon gears used to transmit power from the motor to the spindle. An attempt to take too deep a cut resulted in a jammed cutter and shattered gear wheels.  Although spares are readily available I decided to follow the route taken by many owners of this type of mill and convert to belt drive. A kit of parts is available with detailed fitting instructions which can be seen at http://www.mini-lathe.com/Mini_mill/Reviews/Belt_drv/belt_drv.htm. Not cheap, especially when you get stung for customs duty, but in my view a very desirable modification. The belt drive absorbs the impact of a jammed cutter by slipping and there is the benefit of quieter running.

My mill, like many others of its type, has a vertical column which can be angled for ‘off vertical’ machining. In my opinion it is much easier to angle the actual workpiece in a tilting vice. I bought a cheap one at Machine Mart and it does the job OK.  I also use a novel little programme application on my iPod Touch to set the required angle.

Most of my machining is with the workpiece held securely in the machine vice. To achieve a high level of precision it is of paramount importance to have your machine vice accurately set up on the milling table and for this a ‘finger’ dial test indicator (DTI) is required.

With the DTI held in the chuck I first of all check the vice level and use steel shims under the holding down bolts until I get a level reading. Once you have established the required shimming (is there such a word ?) this adjustment is unlikely to require changing but I do check from time to time to be sure.

Next, with the holding down bolts gently tweaked up, I check for squareness of the vice to the table by running the DTI along the face of rear(fixed) vice jaw. When satisfied with a constant reading along the length I finally tighten down the vice holding bolts.

As I first understood it there are basically two types of cutters. Slot drills and end mills. The essential difference as I understand it is that with a slot drill you can ‘plunge’ down into the workpiece but with an end mill the cutting edge must approach from the side as it doesn’t normally have a drilling capability. I can imagine experienced engineers cringing at this overly simple explanation.

In addition there are fly cutters and these basically achieve two results. One, used correctly they can produce a mirror like finish on wide flat surfaces. Two they can cover your entire workshop with fine glittery chippings. Apart from wearing eye protection I also put the acrylic shield in place to contain the spray of chippings. I feel the use of flycutters and grinding of the cutting edge will be the topic of a future addition to this page.

There is also a wide range of indexable cutters where you can replace the cutting tips or even rotate them to present a fresh edge to the workpiece. These are a sort of halfway house between slot drills/end mills and flycutters. Good for achieving a fine finish on wider workpieces without the mess created with a flycutter. Try them all, you will only find which suits you best through experimentation.

The Golden Rule of Milling

Now, I regard this next item as the golden rule of milling. Ignore it at your peril both in terms of damage to your workpiece but also your personal safety. I am referring to ‘Conventional’ Milling and ‘Climb’ Milling. This diagram illustrates the difference between the two. It took me a while to get a full understanding of the two types and, for me, I picture ‘Convential’ Milling as shovelling coal and ‘Climb’ milling as cutting peat (i.e. chopping down from the top.  However you choose to store this information in your mind the important thing is that you do observe the difference.  ‘Climb’ milling at one extreme, can whip your workpiece out of the vice and throw it with great velocity across your workshop - and it doesn’t care if you or anybody else is stood in the way. At a lesser level it can transform a carefully crafted workpiece into scrap in an instant - beware, you have been warned !