Michael G
7th Nov 2015, 06:49 PM
In another post iolithblue asked if anyone could machine up a rondel. I think the conclusion of that post would be that it was easier, quicker and cheaper to whittle something out with an angle grinder and linisher but I'd done the maths and wanted to know whether I'd got it right or not (as well as check some ideas), so had a go with a bit of bright bar. I did several things that may be of interest to others, so I thought I'd post some pics.
So to start with, I set some bar up in the dividing head (so I could index) but worked out that to get the flats at the right angle I would need to tilt the head a little over 2 degrees. This is normally a pain but I've worked out a technique to allow slight adjustments that mean I could dial in precisely the angle required. You know those long nuts included in clamp kits? There is a use for them after all.
359385 359383
I tried a couple of ways to hold the bar stock while machining it. First was just with the dividing head, then with a separate clamp holding the bar. Eventually I settled for using the vice to hold a bit of angle as a support and using some locking pliers to hold the stock to the angle. Had to be removed every time I indexed but the locking pliers are good for that.
359382 359381
To get adequate support I machined half the job at a time (inspired by the lathe 3:1 rule). After getting the end to size I then moved back and did the rest. Because my angle was the same it met up very nicely at the final cut.
359380
Cutting was done in decreasing DOC as the section got thinner. The final cut was something like 0.4mm. Anything more and I was afraid that things would bend/ twist. This last pass was still cutting metal right to the end (call it an ultra spring cut). The material was moving as the sides were reduced (cold formed = locked in stress), so I could start with one side, do all the other sides and still take material off the first side on the same setting if I came back to it.
359379
The mark is at 150mm. The pointy end is around 1mm across, so I think just would have made it
359378
The groove down the sides was done by inclining the cutter. A slightly different angle (a bit over 1 degree this time) so the same technique was used to adjust the dividing head to suit. This time the angle was used as support and clamping was done on the tip where the groove ran out
359377
The finished item - everything was done with a 19mm cutter. By the time the groove was cut, it was a little worse for wear so the finish is pretty bad and it would take a lot of clean up to get it presentable (confession - it was not pristine to start with either). The yield surprised me though - the finished item weighs 64 grams. The bar stock that I started with would have been 650g for a piece the right size.
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Unfortunately while trying to do that clean up, the blade caught on the linisher and bounced in a bendy sort of way as it hit the floor :o
All up there was probably around the estimate of 6 hours machining involved, so it certainly is not a quick way of making one of these. I imagine it would be a lot quicker forging one.
Having looked on the web for information about these daggers, while they sometimes had edges they were basically a stabbing tool. Back in the fifteenth century steel was steel, so while there might have been good steel and bad steel I'm not convinced that anything is gained using a tool steel over a bit of medium/ high carbon steel for something like this.
Anyway -
The maths worked
Those long nuts do have a use
Machining an overhang longer than around 3:1 is trouble
Sharp cutters give better finish than dull ones
Michael
So to start with, I set some bar up in the dividing head (so I could index) but worked out that to get the flats at the right angle I would need to tilt the head a little over 2 degrees. This is normally a pain but I've worked out a technique to allow slight adjustments that mean I could dial in precisely the angle required. You know those long nuts included in clamp kits? There is a use for them after all.
359385 359383
I tried a couple of ways to hold the bar stock while machining it. First was just with the dividing head, then with a separate clamp holding the bar. Eventually I settled for using the vice to hold a bit of angle as a support and using some locking pliers to hold the stock to the angle. Had to be removed every time I indexed but the locking pliers are good for that.
359382 359381
To get adequate support I machined half the job at a time (inspired by the lathe 3:1 rule). After getting the end to size I then moved back and did the rest. Because my angle was the same it met up very nicely at the final cut.
359380
Cutting was done in decreasing DOC as the section got thinner. The final cut was something like 0.4mm. Anything more and I was afraid that things would bend/ twist. This last pass was still cutting metal right to the end (call it an ultra spring cut). The material was moving as the sides were reduced (cold formed = locked in stress), so I could start with one side, do all the other sides and still take material off the first side on the same setting if I came back to it.
359379
The mark is at 150mm. The pointy end is around 1mm across, so I think just would have made it
359378
The groove down the sides was done by inclining the cutter. A slightly different angle (a bit over 1 degree this time) so the same technique was used to adjust the dividing head to suit. This time the angle was used as support and clamping was done on the tip where the groove ran out
359377
The finished item - everything was done with a 19mm cutter. By the time the groove was cut, it was a little worse for wear so the finish is pretty bad and it would take a lot of clean up to get it presentable (confession - it was not pristine to start with either). The yield surprised me though - the finished item weighs 64 grams. The bar stock that I started with would have been 650g for a piece the right size.
359376 359375
Unfortunately while trying to do that clean up, the blade caught on the linisher and bounced in a bendy sort of way as it hit the floor :o
All up there was probably around the estimate of 6 hours machining involved, so it certainly is not a quick way of making one of these. I imagine it would be a lot quicker forging one.
Having looked on the web for information about these daggers, while they sometimes had edges they were basically a stabbing tool. Back in the fifteenth century steel was steel, so while there might have been good steel and bad steel I'm not convinced that anything is gained using a tool steel over a bit of medium/ high carbon steel for something like this.
Anyway -
The maths worked
Those long nuts do have a use
Machining an overhang longer than around 3:1 is trouble
Sharp cutters give better finish than dull ones
Michael