Advice Needed from a Machinist-gctid822488

[Norton_Rider]

I am in the process of drilling a 3/8" hole in 3/8" thick high strength steel. I do not know the alloy, but it is steel used for rifle and pistol targets. I set it up in my drill press and first drilled a 3/16" hole. I used an old bit that I had laying around, along with a bit of cutting oil. The bit went though the material easily. Next I tried to enlarge the hole with a number of my Hitachi brand, 1/4 and 3/8 bits, some new, some used. They barely penetrated the steel. I then went to the hardware store and bought two Irwin brand, Cobalt bits in 1/4 and 3/8. I got 1/3rd into the hole before the bits were toast. I also tried a titanium coated step drill and it did not work.

What should I do to drill the 3/8 hole the rest of the way? Are there better bits than Irwin that I should look for? I don't want to keep throwing $$ at this in bits just to find they don't work.

[Nickdixon]

Not a machinist, but slow and cool is the trick. Slow is obvious but keep using oil. You don't need special expensive drilling oil, motor oil (clean, moderate viscosity say 30W) and don't spare it. Sounds like you bits are getting hot and soft. With unknown alloy, you need something pretty hard for the bit, but it sounds like you know that. But you would not believe how hot high speeds can achieve. I would stop when I stopped seeing shavings.

[Pcpete]

Go a bit larger on your step. Like nike said, lots of oil, slow rpm and not much pressure. It may be that you are developing some case hardening where the bit heated the metal. That's common and a pain. You sorta have to tease your way through the hard spot. You also want to be using bits that are cut for steel rather than wood or aluminum.

[Nickdixon]

+1 on case hardening and its truly a pain.

[downriver4]

Ar500 steel plates? If making hanging targets, maybe weld chain links on edges.

[CptCrunchie]

"Norton Rider" post=822488 wrote:
I am in the process of drilling a 3/8" hole in 3/8" thick high strength steel. I do not know the alloy, but it is steel used for rifle and pistol targets. I set it up in my drill press and first drilled a 3/16" hole. I used an old bit that I had laying around, along with a bit of cutting oil. The bit went though the material easily. Next I tried to enlarge the hole with a number of my Hitachi brand, 1/4 and 3/8 bits, some new, some used. They barely penetrated the steel. I then went to the hardware store and bought two Irwin brand, Cobalt bits in 1/4 and 3/8. I got 1/3rd into the hole before the bits were toast. I also tried a titanium coated step drill and it did not work.

What should I do to drill the 3/8 hole the rest of the way? Are there better bits than Irwin that I should look for? I don't want to keep throwing $$ at this in bits just to find they don't work.

The bigger the bit, the slower you have to go. I run " cobalt at 400rpm max. and give it a veritable bath in cutting oil.

[SomeSailor]

Once you work harden it... you're in trouble. You could heat the area to a dull red and then let it cool slowly. That will anneal the metal and soften it a bit. Slow cutting and lots of oil is the trick. Easiest is just to weld a couple of links to it. If you're hanging targets, run down to Harbor Freight and buy a couple of cheap C Clamps to hang them.

Where are ya shooting plates at?

[Norton_Rider]

First of all, thanks for the advice. I have an old drill press that changes speeds by by moving the drive belt to different pulley diameters. It's a PITA so I don't change it often. In this case I'll follow the advice and slow the RPM down. I'll try drilling from the opposite direction; maybe I can handle the work hardening this way. If not, I'll try annealing. I have plenty of cutting oil, so that's not an issue.

The target is a faller that pivots at the bottom. I'm just modifying it by drilling a hole for an eyebolt. The eyebolt will be used to actuate other targets and devices. Eyebolts are preferred to a welded eye because they are easily replaced if damaged. It's one of the targets used at the Marysville Rifle Club for Cowboy Action Shooting.

[green650]

Is it squeaking or chattering when you still?

If the hole gets boogered any new bit you stick in catches on the sharp edges and breaks the points off your nice new cutting edge.

Start a new hole, you aren't making artwork are you?

[Mr._Darcy]

Like others said it may have gotten case hard from the first drill. Heat as mentioned to anneal or grind the case hardened surface away, or as was mentioned start a new hole. Also be aware that with some steel made from recycled metal there can literally be a bearing ball or roller in there that the first bit just missed. Used to run in to bearing parts even in T-1 when cutting with a torch. It would just blow out around the bearing piece. Try a new hole slow, lubed, and cool.

Greg

[Jeffw]

"Mr. Darcy" post=822533 wrote:
Try a new hole slow, lubed, and cool.

:lol: :lol: :lol:

[Knot_Happy]

I would suspect work hardening, once you have hardened it it is going to be difficult to get through. In a past life I was a journey level machinist, however many years have passed since.

A couple of things to consider high cutting speed produces heat, however slower speeds can break a drill due to chip loading. Favor slower speeds in steel, but use the least amount of pressure that will create a chip. Watch carefully at the breakthrough as it will grab and break the drill. Lots of cutting oil, this is more for cooling than luberication, however lubrication reduces friction and heat. A shop will use a water soluable oil and have it flooding over the area. This creates both lube and heat removal.

Not all high speed steel is the same, you get what you pay for, coated M42 grade might work if you don't work harden it, however it sounds like you have already hardened it. Solid carbide will cut through work hardened steel, but may be more expensive than you want to consider. I would not use a step drill for this situation, the step does not have a strong cutting edge and will fail quickly. The step is typically for finish cutting and sizing , not rough cutting.

If you are almost through and hole precision is not critical consider a dremel bit grinder to finish it out.

If the precise location is not critical for balance or some other reason. I would recommend moving the hole. You indicated it is a 3/8 dia, rule of thumb is move at least 2x the diameter.

Another option if it must be located specifically for balance, or other reason could be cut out the area with a torch and weld in a patch plate and drill it.

[Pcpete]

Norton, if you want to come to Mukilteo you can use my vertical mill. It might take some minor digging to get at it, and it's a manual, but it will do the job. Do you know if Mike and Carol Delong are involved with that group? I know Mike does some western shooting.

[Norton_Rider]

"Knot Happy" post=822684 wrote:
I would suspect work hardening, once you have hardened it it is going to be difficult to get through. In a past life I was a journey level machinist, however many years have passed since.

A couple of things to consider high cutting speed produces heat, however slower speeds can break a drill due to chip loading. Favor slower speeds in steel, but use the least amount of pressure that will create a chip. Watch carefully at the breakthrough as it will grab and break the drill. Lots of cutting oil, this is more for cooling than luberication, however lubrication reduces friction and heat. A shop will use a water soluable oil and have it flooding over the area. This creates both lube and heat removal.

Not all high speed steel is the same, you get what you pay for, coated M42 grade might work if you don't work harden it, however it sounds like you have already hardened it. Solid carbide will cut through work hardened steel, but may be more expensive than you want to consider. I would not use a step drill for this situation, the step does not have a strong cutting edge and will fail quickly. The step is typically for finish cutting and sizing , not rough cutting.

If you are almost through and hole precision is not critical consider a dremel bit grinder to finish it out.

If the precise location is not critical for balance or some other reason. I would recommend moving the hole. You indicated it is a 3/8 dia, rule of thumb is move at least 2x the diameter.

Another option if it must be located specifically for balance, or other reason could be cut out the area with a torch and weld in a patch plate and drill it.

Thanks for the advice. I was actually thinking of using a couple of aluminum oxide grinding stones that I have for my Dremel to grind through the hardened metal. Hole precision is not an issue in this case.

As you and others have pointed out, I may move the hole, since the location is not critical. I used to live by the mantra of 2D-ED (2 diameters edge distance) when I was an aerospace Liaison Engineer many years ago

[Norton_Rider]

"Pcpete" post=822692 wrote:
Norton, if you want to come to Mukilteo you can use my vertical mill. It might take some minor digging to get at it, and it's a manual, but it will do the job. Do you know if Mike and Carol Delong are involved with that group? I know Mike does some western shooting.

Thanks for the offer. Let me see how far I get with the other suggestions and I'll let you know.

I may know the Delongs, but I don't know for sure. We all use aliases in cowboy action shooting. Often we'll shoot with people for years without ever knowing their real names. My alias is "El Vasco." In English this means "The Basque One."