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Machinist guru help needed

Old 06-14-2005, 12:31 PM
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DesignMan
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Default Machinist guru help needed

Twice in a row now, I have broken carbide bits while drilling out the center of crankpins to improve engine balance. In both cases, the bits shattered just as the bit was about to break through the other side of the crank-web. Just enough was sticking out to allow me to enlarge the hole and drive out the broken bit, so the pieces were saved in both cases.

However, what am I doing wrong? Is my rotational speed too low, perhaps? I am going medium on my drillpress with 1/16" bits.

Some guidance would be very welcome.
Old 06-14-2005, 01:08 PM
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Default RE: Machinist guru help needed

A 1/16" carbide bit should be spinning as fast as that drill press will go. Some sort of coolant will help, and make sure your setup is as rigid as you can get it, as carbide is so inflexible and brittle. I find I have much better luck using a carbide center-cutting ball-nose endmill (after center drilling, of course), since the flutes are curved instead of sharply angled, which is where the drill is most vulnerable.
Old 06-14-2005, 01:28 PM
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Default RE: Machinist guru help needed

i also agree that your speed is probably too slow. 1/16" needs to spin FAST! and if it's not then you end up putting too much pressure on it get it to cut


dave
Old 06-14-2005, 02:20 PM
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Default RE: Machinist guru help needed

I was using lube, but obviously going to slow with too much pressure. My setup is a drill-press with a vice on a cross-slide, so it is reasonably rigid.

I suppose I could also use my mill, and then I could control the feed rate better.

Thanks for the advice!
Old 06-14-2005, 03:07 PM
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Default RE: Machinist guru help needed

As well as the speed I suspect the issue is the lack of control just at break through. Most drill presses I've used have a fair degree of play in the rack. This means that the bite of the drill lips at breakthrough will allow the drill to jump forward regardless of your efforts to control it. Try lightly tightening the quill lock to add some drag or increase the spring return pressure for critical jobs such as this. Or add a backing block of some sort to drill into so you avoid the breakthrough bite.
Old 06-14-2005, 04:22 PM
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Default RE: Machinist guru help needed

Using my mill is sounding better and better, if I have the speed needed. I think it's adjustability is limited compared to either my lathe or drillpress.
Old 06-14-2005, 06:14 PM
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Default RE: Machinist guru help needed

If you can predict where the drill is going to come out on the other side, and having a grinder mark there isnt a problem, try flattening the surface so its square with the bit. If im not mistaken, there is a bevel on the exit side of the drill on the crankface, and its walking and catching, due to an uneven breakout.
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Old 06-14-2005, 06:17 PM
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Default RE: Machinist guru help needed

Bruce has the right idea. Tighten the quill to eliminate backlash and keep the drill from grabbing when you break thru the other side.
Old 06-14-2005, 10:17 PM
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Default RE: Machinist guru help needed

To echo Vicman, tighten the quill lock to get some control at break through.

Tightnening up the return spring on the drill press will NOT help in this matter. The spring on any drill press I am familiar with is on the pinion of the feed. Because of backlash between the pinion and the rack of the quill, the quill will drop down when there is no longer any upward pressure on the tip of the drill. Another possible solution might be to put an external spring on the quill itself, just for these delicate operations. (This would be quite easily done if you have an external stop to hook the spring on to, as most drill presses have.) This would then have the benefit of backing up immediately when you let up on the feed, instead of having to pull the quill up from its "locked" position.

I was once working in a shop where a fellow employee had a very painful blood blister under a fingernail. He had the bright idea to relieve the pressure by carefully drilling a hole in the nail with the drill press - and proceeded to learn some of the above "first hand" so to speak...

Arlen

(In the interest of public service, here is a time tested remedy for the above condition. Unfold a paper clip that has about a .030 diameter wire with a SQUARE END on it. Heat up the tip of the wire and push it into the top of the nail. This will provide a large enough hole in the nail, but with any luck, will not go into and damage the tissue below. It'll probably STILL hurt though...)
Old 06-15-2005, 01:06 AM
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Default RE: Machinist guru help needed

HAH! I had a guy with the same experience in my shop!!! Only he didn't listen about using a drill bit less than .125 and ran the thing striaght thru his finger.[X(]
Old 06-15-2005, 04:54 AM
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Default RE: Machinist guru help needed

why use a white-hot needle when you've got heavy machinery on hand?



dave
Old 06-15-2005, 07:38 AM
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Default RE: Machinist guru help needed

I saw a Guy drill a fingernail too....I can still hear the scream !!! [X(]
Old 06-15-2005, 06:56 PM
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Default RE: Machinist guru help needed

Ah, sorry. I didn't allow for the various spring return designs. You're quite right all.

Not a blood blister story but I had a guy borrow my very small and very sharp jeweller's screwdriver to tighten a screw on his glasses or similar. The driver slipped and ran right through the cartilage pad on the end of his thumb and out through the nail...... He was a perfect match for a sheet of white paper for quite a while after that....
Old 06-16-2005, 07:41 PM
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Default RE: Machinist guru help needed

Using a mill to drill through the crank is probably the best tool to use. Drill most of the way through using the best cutting fluid you can find. "Molly Dee" is the best for hardened steel, but if it smokes, the fumes will "tarnish" the metal (vise and workpiece). When you get to the last .100" use the stop for the drill/quill, and come up with the table in small increments. This will prevent the drill from biting or wanting to jump in the break-through. Hope this helps. By the way, to fix a blood blister under the nail, use an ultra small drill in the # 80 (.0135) size and do it by spinning it with your other hand. NEVER in a machine!

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