Question on drilling a deep hole.

[sanddan]

I need to drill a 4" deep .125" dia hole in a .375" dia rod. I am doing this on my 12x36 lathe. My first attempt I started with a regular length drill bit and after using a center drill to start the hole I proceeded to drill. Speed was 700 rpm and I must have pushed it too hard as the bit broke at about 3/8" deep. I slowed the speed to 300 rpm and started again, this time using very light pressure. I had to stop at about the 1/2" mark, honey-do's. Am I doing this correctly? If not, how should I attempt this hole? I have a 8" long bit for when the hole is deeper but I am afraid it will break even easier than the first try.

 

[JimDawson]

Howdy Neighbor! :wavinghi:

700 RPM seems about right for an 1/8 drill, faster in most materials would be OK. It does depend on the material. It sounds like you're pushing a bit hard. Cutting oil (or any oil) would help, use WD-40 for aluminium. Be sure to clear the chips (pull the bit out) about every drill diameter of depth.

 

[tkitta]

4 inch should not be a big deal. I drilled few times like this both no a lathe and on a drilling machine.

I used a bit of cutting oil but I also took the bit in and out a lot to clear the hole. I used bigger bit through both times - 3/8 and larger.

 

[G MORSCH]

Hello Sanddan:

Yes, deep holes are kind of a pain on a manual machine. But as Mr. Dawson suggests, they are certainly doable. Your Length to Diameter ratio (L/D) is 32:1. First of all, take a look at the manufacturers surface speed and feed rate recommendations. Speeds for plain carbon steels 180-280 BHN hardness (1008, 1010, 1018, 1020, 1026, 1040, 1045, etc.) range from 45 to 70 surface feet per minute (SFM) on Carbide Depot's website for High Speed Steel (HSS) with a 118 deg. point. We do a little math and find the lowest speed in the range in RPMs equals .125 x 3.14 divided by 12" = .0327 ft. per revolution. To achieve 45 SFM, divide 45 SFM by .0327 and choose the RPM that most closely matches the recommendation. 45/.0327 = 1,376 RPM. Since you will be hand feeding it with your tail stock, you will have to go with "feel". I always use a drop or two of Tap Magic on the drill point to reduce galling and adhesion, but most any manufacturers cutting fluid will help. Always use a center drill, as you did, on shallow and deep holes. Starting with a shorter drill is always a smart move but make sure that the drill has an accurate grind for those that regrind your own drills. And with a manual machine, peck often, and you will get the feel of how often to retract. If you feel any rumbling, retract, re-lube, do it again.

Good luck!

Best Regards, Gary

 

[12bolts]

Is this a through hole or a blind hole. I would not expect a 1/8 bit to drill concentrically for a depth of 4". If it is not overly important about finishing centralised then go ahead with your long series bit. But if you need a through hole to be concentric at both ends of your 3/8 rod then you will need to drill from both ends. Or drill from one end in an oversized shaft and then turn the OD between centres to get back to your .375.

.... I must have pushed it too hard as the bit broke at about 3/8" deep. ....

Most likely you just plugged the flutes with chips. Once you get to any sort of depth when drilling with standard twist drills you need to frequently withdraw to clear the chips. The flutes will not satisfactorily carry them out the back of the bit.

Cheers Phil

 

[sanddan]

Thank you all for the advise.

 

[mack318]

I would go slower than the recommended speed.

think of it like a bearing that is trying to seize up because it is full of metal chips

unless your using a flood coolant use only small amount of cutting fluid or close to none. because it will make it harder to clear the chips

flood coolant is used to carry the chips out. what i would use is compressed air turned down to about 30lbs [dont forget your eye protection]

i would also think about the sharpness of that cutting tip. nothing disappoints like a dull cutting tool. a dull drill point will seize and break for sure.

hope that helps

 

[Ed T]

I would like to reinforce the comment made above. The most likely reason that you broke the drill is not pushing it too hard, but failing to back the drill out to clear the chips. When you start to drill, you will see chips coming out the back of the hole as the drill goes in. The deeper you go, the harder it is for the chips to get out. Eventually, the chips bind on the drill and it fails. So, if you don't see chips coming out as you proceed deeper, you need to back the drill out and clear the chips. With very small drills it is customary to "peck" the hole through. Many small engagements to cut tiny chips and get them out of the hole before going deeper. It can be quite tedious, but not as tedious as trying to get a broken drill out of a deep hole. Also mentioned above, don't be disappointed if the drill does not come out in the middle of the exit side of the hole. It is very likely that the drill will wander off to one side or the other. In extreme cases, the bending imposed on the drill by drilling "around a corner" can also cause breakage.