Model Engineering Articles

Buying a New Milling Machine

I decided that the time had come to consider a new milling machine to replace my Mill/Drill which had in turn replaced my original home-made Dore Westbury some years before. The Mill/Drill was more than adequate for most of my model engineering jobs but lacked registration once the head was moved up or down! I tried to sort this by fixing a narrow guide to register with a fixed column rack, but at best it would only hold to within 0.005". Another failing of the Mill/Drill was the tendency for the tool to cut deeper than set when making a deep cut. This was largely due to the excessive play on the quill gear teeth and I attempted to cure this by applying a constant upwards thrust on the quill by means of a spring mechanism. As a matter of importance therefore, the new mill must be capable of holding registration, not cut deeper than set and be more accurate than the Mill/Drill. Another need was size, which to fit in my workshop had to be relatively compact. I had always considered it was better to buy a used quality machine and then attempt to renovate it, however, the market for small vertical milling machines had little to offer and those that do exist are both quite heavy and very well used, which would require a lot of work and access to large machines to resolve.

The only other remaining option was to buy a new mill, which based on cost, would be a Far East import. The model engineering fraternity have over the years slated these imports for poor castings (sand not cleaned off), rough finishing, cheap electrical parts and poor performance. Indeed this was a view I shared in the past, but it has to be recognized that the Far East manufacturing capability has progressed enormously. Based on the fortunes from selling the early, less than perfect machines has enabled them to buy very high quality production machinery which can produce repetition work output to a very high standard. A visit to Chester UK Ltd. revealed a collection of milling machine imports. I can recall a previous visit only a few years ago and thinking how poor the products were in terms of finish and general engineering design. This time, there were still some basic machines but I was pleasantly surprised by the standard of the larger machines. My eye quickly settled on a vertical knee mill, model 626. A cursory inspection found it was well finished with most bearing surfaces ground and a smooth 'operational feel' to the table movements. My visit also coincided with a special sales event and so I managed to root out a demo machine which included a DRO and variable speed inverter included. Since I was able to use all my existing tooling from the Mill/Drill, I decided to buy it and hope that on a more detailed examination it would meet most of my requirements.

Eventually the mill arrived and it was erected on it's cabinet base unit, it is quite heavy and fortunately having a chain hoist made the task easy. First job was to check out the general build quality and design and then check the accuracy against the manufacturers tolerances. In almost every test it was better than the stated tolerance and this assured me that as far as accuracy, it was very much better than the Mill/Drill. I was also pleasantly surprised to see that it had incorporated many good engineering practises such as adjustment screws at both ends of the gibs and taper dowel pins for location of the table end bearings. The quill arrangement and fine feed was much improved and displayed some small backlash but there was now a substantial internal coil spring which exerted an upwards thrust. The only downside to this spring was that as the quill moved down so the spring force increased. This would make the fine down feed relatively heavy going for repetitious boring jobs as the handle became progressively harder to turn. This must not be taken as a major failing since it was quite usable but certainly the hand wheel could not be moved effortlessly. The electrical fittings and cabling were top quality and it was pleasing that no corners had been cut to keep the cost low in this regard.

Overall, the machine is very well built and for my needs more than adequate. The variable speed inverter and three phase motor soon proved a winner and for the first time I could effortlessly change speeds whilst performing a cut. I was amazed at how much better cuts were when the optimum speed/feed was found. The inverter (Italian origin) was pre-set with regard to the many options available and motor speed range. The feature I particularly liked was the start/brake facility and this switch was promptly relocated so that it was more accessible. I made and fitted a tachometer from a kit provided by Dan Mauch of Camtronics Inc. which enabled me to accurately ascertain the speed ranges obtainable by belt combination and inverter speed control. The RPM display is incorporated into the main control panel which has the following controls: Run/Brake; RPM display on/off/select; Pump; Feed on/off/LED (warning that feed is engaged). A full range of speeds (110 - 2295 RPM) could be had from four of the belt combinations ensuring adequate torque at the spindle and sufficient RPM (for cooling) at the motor. For most normal use most of the speeds I needed were available from a single belt combination thus eliminating the chore of belt changes, however the best bit was the ability to change speed during a cut.

The DRO was from the Far East (like most electronics these days, which invariably includes top branded names) and included a good range of facilities including bolt hole circle mode. The linear glass scales claim to be accurate to 0.0002" plus the absolute positions are retained even after power off.

Most of my end mills have seen numerous resharps, thanks to my Stent cutter/grinder, and are inevitably less perfect than a new cutter. With this in mind I decided to use a 0.5" endmill cutter which had been well used and previously sharpened at least five times and set the depth of cut to 0.125" on a mild steel work piece. I use a low volume pumped coolant supply which is there mainly to cool the cutting edge rather than clear away all the chips, which I do using a small brush. The cut proceeded effortlessly and there were no signs of undue bearing complaints and the resultant channel was dimensionally accurate with a clean finish. My Mill/Drill had the tendency to cut heavy on one side during in feed and then on return it would cut on the other side! The new mill, on it's return feed showed no such failings and removed no further metal.

In conclusion. My initial concerns about buying a new Far East import have, so far, proved to be unfounded. I have no illusions that my new acquisition will last as long as, or undertake the wear and tear subjected to an established quality named machine but this does not matter since as a model engineer, I will not be that demanding in terms of use. The DRO and variable speed unit are definitely valuable bonuses and eliminate the normal concerns over feed backlash ,dial errors and the inconvenience of continually needing to change belts. It is clear that the Chinese have learned a lot from their early design experiences and manufacturing processes and have progressively improved. Looking at the range of 'professional' milling machines now produced in the Far East, the high quality specifications and the low price tags it is only a matter of time before they dominate the modern machine shop. What ever happened to Dean Smith and Grace lathes?

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