Introduction abstract advantages applications

fabrication of lathe coolant pump with fitting arrangement abstract

Coolant pump system.
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Normally, when using the lathe, I use a brush to apply coolant/lubricant to the workpiece. This works fine for most jobs involving straightforward turning operations. However, for some operations such as screw cutting, it seems that a third hand is required to apply the coolant because the other two are fully occupied. It is also easy to forget to apply lubricant when carrying out complex operations.

I did look into the possibility of buying a coolant system but all the commercial units seem really over the top for use on a minilathe. They were large requiring several litres of coolant to fill and the flowrates were measured in litres per minute whereas the coolant requirement on the minilathe was more of the order of 10 ml per minute.

I then started to investigate if I could make a simple and cost effective unit. The design criteria were as follows:

1. The unit should be small and compact - I only have a small workshop
2. It should be easily dismountable from the lathe when it is not required.
3. The amount of coolant in the system should be small, not more than 500 ml.
4. The flow rate of coolant should be variable within the range 0 - 100 ml/minute i.e. from a drip to a fine jet
5. It should be safe. This means a low voltage pump and fully enclosed motor.
6. Components should be cheap and readily available.

I could easily envisage the type of system required. A small reservoir with a large catchment tray would be placed under the lathe. Coolant from the reservoir would be pumped up to a nozzle located directly over the cutting tool. Excess coolant then collects in the tray and drains back into the reservoir. This all sounds very simple but the sticking point for many months was what to use as a pump. The answer was staring me in the face evey time I drove the car. The windscreen washer has a small compact low voltage pump capable of sending out a considerable jet of liquid. The only real questions I had were would a windscreen washer pump be compatible with mineral oil based coolants and how reliable would it be if run continuously.

In one of my junk boxes I had a couple of old windscreen washer pumps. Both had seen some previous service. I partially dismantled them and discovered that one was a rubber impellor type pump and the other was gear pump. I discounted the first pump on the grounds that the rubber impellor was probably not going to be good in contact with the oil based coolant. The second pump had plastic gears, probably acetal, and I decided to test this pump for application as a coolant pump. The first tests were made using water and at the rated voltage the flow was several litres per minute. After running a few minutes the motor was quite warm. This was hardly surprising since windscreen washers are designed for intermittant use. On reducing the voltage the pump flow decreased and at 2 volts is was still delivering over 100ml/min. At this voltage the pump would run for hours and not get hot. It was also virtually silent. I then changed the pumped fluid to a mixture of 50% white spirit and 50% motor oil and set the pump working a 2 volts for several hours. At the end of that period I dismantled the pump again and examined the gears and seals for any sign of damage. None was evident so I decided that the pump could form the basis for my coolant system.