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mytqik
Joined: 26 Jun 2004
Posts: 127
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Nick,
Sorry I haven't got back to you earlier, I have been in 3 different countries in the last 5 days. So I dont know if I am coming or going.
I will try to answer your question. For any drill, it is only the outside 3rd that is effective. That is why drilling pilot holes is trade practice. By drilling a smaller hole, then a larger one, it is much more effecient use of machine time. Also it means that your smaller (& therefore cheaper) drills wear out, before the bigger ones.
As for feed rates, it is hard for the backyard workshop to guage. While we can setup rotational velocity, by changing the belts/gears in the drill press, very few have a vertical power feed. so therefore I would recomend a feed rate that does not produce blue swarf. This is with cooling. The drill bit should be able to hardle this.
As for your mill Nick, alot of what I just said goes out the window, as no doubt you will using tugstin carbide replaceable tips in your cutters. These tips are designed to be run at a much higher speed, so aim for producing blue swarf (obviously this only applies for steel). Those indexable tips also demand the use of flood cooling, so your workshop could get meesy, unless you add some splash guards.
When I get back to Australia, I will post some hard data about feeds & speeds for your mill. I would strongly recomend that you go to the local TAFE library & sign up as a social member (I know this is possible with a few up here). These librarys are full of very usefully information as they need to teach 16yr old apprentices. If you cant be a member, just browse & photocopy as required. Also try & get the prescribe textbook for fitting & turner apprentices. They give an excellent grounding in all form of machining. I still refer to mine, even though I haven't been on a machine in years.
Hope this helps.
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Fri Oct 22, 2004 12:48 am |
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mytqik
Joined: 26 Jun 2004
Posts: 127
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I agree there certainly is an art to sharpening drills. Here is is (assuming you have a bench grinder):
1. Always sharpen drill bits in the direction of rotation, not along the axis.
2. Always sharpen from the trailing face to cutting face. ie, hold the drill at an angle so that the stone will contact the trailing face first. Then quickly spin it around towards the cutting face. Practice with a new bit without the stone moving, & try to keep the drill bit flush with the stone.
(As a safety issue, always the front face, not the side face of the stone. The use of the side face will dramatically weaken the stone, generating stress cracks, which leads to one shattering. If you have ever seen one let go, you will no what I mean. Also, to check for cracks in your stones, dismount the wheel & suspend it via a loop of sting through the bore. Then lightly tap the stone with a piece of metal. It should ring, if not, there is a crack in the stone which stops the ring being generated within the stone.)
3. Most important, do not let the drill bit get hot. Heat is the biggest killer of cutting tools. When doing drill bits, it is best to do multiple quick, light grinds, as opposed to one haevy slow attempt. Always dip the drill bit in water between grinds.
4. The ideal result is a 118 degree included angle at the tip, so cut a template out of heavy card, steel etc & chain it to the grinder.
5. To quickly tell it is sharp, the cutting edge should be a constant line from the centre out the the edge ie no waves or chips out of the cutting edge, when looking end on. When looking side on, spin the drill in your fingers along the long axis. The trailing edge should be higher than the cutting edge. This difference between heights will dictate how long the drill stays sharp & its maximum cutting speed.
For wood the difference can be greater than for steel. If you can imagine the drill cutting a hole, if the difference between the trailing & cutting is large, it will allow the drill to cut a larger chip, however as the leading edge is thin, it will not last long. This is ok for plastics & wood when there is no wear.
For steel the difference should be smaller. ie 1-1.5 (depending on diamtre) from trailing to cutting. This will allow a thinner chip to be cut, but it will last longer as the thickness of the cutting edge is thicker.
The reason a drill bit becomes "blunt" is that the difference in height from trailing to cutting becomes too small for the drill to cut a chip & "dig" into the material being cut.
(I think I am getting "long post spokie-itis")
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Fri Oct 22, 2004 11:22 am |
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mytqik
Joined: 26 Jun 2004
Posts: 127
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As for the point off centre, that is where practice makes perfect. You are already helping reduce the chances of it occurring. Regular light sharpening is much better than letting it go blunt & then trying to sharpen it.
Also dont sharpen one side then the other. That is how the point becomes off centre. If you do a light grind on one, then rotate it around & do the other it *should* keep the point on centre. Maintaining a sharp drill is much easier than rescuing a damaged one.
As apprentices we were constantly being asked to do it be hand. While a sharpener will do a much better job, doing it manually gives you a much better "feel" for the machines you are using & also a much better understanding on how things work & what happens when you deviate from the norm. Also makes you work well under pressure.
As a side line, I met an english toolmaker about 3months ago. He is now an engineer here in China. Pride of place on his desk was a 5" cube of stainless steel. It had the most amazing finish on it, highly polished. The story behind it was it was the final test of his apprenticeship. He had 8hrs to turn a piece of hexagonal bar stock into a perfect cube, +/- 0.5mm in any direction, all faces parallel & perpendicular. This is not too hard with modern machines, but he did it by using only hand tools!! He even polished & then lapped it to a mirror like surface. It looked like an ordinary piece of metal, but was truely a work of art once the story was known. The point is that every one needs to crawl before they walk.
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Fri Oct 22, 2004 12:42 pm |
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