[vc_row][vc_column][vc_column_text]By Spencer Erling, Education Director, SAISC[/vc_column_text][vc_empty_space][vc_btn title=”Printer friendly version” align=”left” i_icon_fontawesome=”fa fa-print” add_icon=”true” link=”url:https%3A%2F%2Fwww.saisc.co.za%2Fwp-content%2Fuploads%2F2017%2F12%2FDrilling-holes-in-hard-wearing-plates.pdf|||”][vc_empty_space][/vc_column][/vc_row][vc_row][vc_column width=”1/2″][vc_column_text]If you have ever tried to drill a hole in a hard wearing plate and found that after the hole starts no further progress is made, in fact all you seem to be doing is ‘polishing’ the inside of what has been drilled, then read on and find out what to do to prevent this from happening in future.
What does the name of hard wearing plate mean?
Hard wearing plate is sold under various trade names followed by a measure of the hardness. Common levels of hardness are 350, 400, 450 and 500 (Brinell hardness).
I guess there are very few fabrication shops that do not have a good old fashioned ‘boiler shop drill’ (otherwise called radial or column drilling machines) which can be set for various revs per minute as well as adjustable feed speeds. But without the advice that follows the scenario of the introductory paragraph is a common cause of failure.[/vc_column_text][vc_empty_space][/vc_column][vc_column width=”1/2″][vc_single_image image=”4921″ img_size=”large”][/vc_column][/vc_row][vc_row][vc_column][vc_column_text]The name of the game is to reduce vibration to ensure efficient drilling through the plates, and the way to do this is:
- Keep the distance from the drill head to the column of the machine as close as possible.
- It is important to securely clamp the plate to be drilled on steel blocks located as close as possible to the holes to be drilled.
- Minimise the distance between the drill tip and the support arm – most easily achieved by using very short spindle stick out and short drill bits.
- To prevent the drill bit from breaking, it is good practice to disengage the feed briefly just before the drill breaks through.
- Restart the feed when any vibration or play has ceased.
- It is important to use liberal amounts of coolant at the drill head. There are specially designed coolants that do improve productivity even though they appear to be relatively expensive. A suggested rule of thumb for coolant flow is the number of litres per minute = drill diameter in mm.
[/vc_column_text][vc_empty_space][/vc_column][/vc_row][vc_row][vc_column][vc_column_text]What about the drill bits?
For the occasional once off holes, standard HSS (High Speed Steel) drill bits are adequate.
For production runs HSS-E (micro alloyed) or HSS–Co (cobalt alloyed) drill bits are recommended. The 8% cobalt drill with a small helix angle and a robust core that can resist high torques is ideal.[/vc_column_text][vc_empty_space][/vc_column][/vc_row][vc_row][vc_column][vc_column_text]Table of recommended drill feed speeds in mm/rev and the revs can be selected from the table:[/vc_column_text][vc_single_image image=”4919″ img_size=”large”][vc_empty_space][/vc_column][/vc_row][vc_row][vc_column][vc_column_text]What is the next step up from entry level machines?
The more stable the machine, the easier to drill the hole! Machine tools such as boring mills and bed type milling machines will definitely improve productivity, especially when using cemented carbide drills.
In our industry holes will always be larger than 12mm diameter and so ‘indexable insert drills’ are recommended for high productivity at the expense of a wider tolerance than the slower brazed or solid carbide drills. Since most structural applications will be liner plates this lower precision will not be an issue.
- Once again coolant plays an important role in the economical drilling of holes.
- Cutting speeds and feed rates are available on request from the writer.
- Counterboring, countersinking and tapping bits are available for cemented carbide bits. Tapping can be done in blind-hole or through-hole applications using the correct bits.
Still not coming right?
Contact the writer for useful troubleshooting suggestions.
The writer wishes to thank SSAB for this information which has been extracted from their brochure Eng10-2009[/vc_column_text][/vc_column][/vc_row]
