Low-cost engraving head
by Alan Wragg

I HAD ALMOST finished my Quorn tool and cutter grinder - ‘almost’ because it required numbers to be engraved and a more powerful motor fitted. I used a very small (0.312in. dia x 0.008in thick) slotting saw to do the graduations, and the result was very neat. I did not want to stamp the numbers and spoil it so I starting to look into engraving using my CNC mill.

The first problem was that there was no software for engraving on the mill - a Wabeco using HSE software. The answer was to import graphic files and work on them.

A fellow SMEE member kindly gave me a HSS engraving tool to try out, and this revealed another problem. My mill’s top speed was 3,000rpm and it soon became apparent that was far too slow even for HSS engraving tools. For carbide tools the speed required was around 20,000rpm.

I had seen a step-up gearbox some time previously in a magazine, but that was complex and had expensive gears. And it probably would not reach the required speed.

Then, the idea for a solution to the problem came while looking at 2MT blank end arbors on the Arc Euro stand, specifically one 1.5in. dia. and 1.25in. long. The plan was to make a spindle using the same principle as the Quorn spindle i.e. pre-loaded bearings, but only about 3/4in. dia. and mount it in the arbor. This would be driven by a self-contained electric motor. The whole device would be mounted in the spindle of the mill, remembering to turn off the power to the mill’s own motor!


Construction

The bought-in components are:

2MT blank arbor                    £3.75

Magneto bearings                 £9.90

Timing belt 2.5mm                 £3.81

Timing pulleys                      £10.74

Model aircraft motor

10,000rpm, 7.5V                    £7.50                                  

6V motorcycle battery          £13.00

Switch and plugs                    £4.00


All other parts were fabricated.

The timing pulleys were chosen to give the maximum step-up ratio in the space available and it ended up at 1.875:1 giving a maximum theoretical speed of 18,750rpm. Initially Mike Kapp from the SMEE built a quick power supply, but we soon found that these motors draw a considerable current at start-up, and it was blowing 10W power transistors with ease. The interim solution was a 6V lead acid motorcycle battery which works well but does not reach full speed. It will eventually be replaced with a proper variable speed supply.

While there is nothing special about the construction it does require very accurate work for both the spindle and the collets. It has provided me with a fairly inexpensive high speed engraving spindle.

First test