Trepanning small diameters -- make custom tool or buy nnular cutter?

[durableoreo]

I need to make a 1.375" hole x 5" deep in 3" 4140 round bar. I've watched a lot of David Wilks videos* lately, so I thought about making a tool. But annular cutters are available that cut 3" deep, which is also a good option.

I'd rather make something but time is relatively short and success is not guaranteed.

An annular cutter would probably be fine but I'm worried about all those teeth and swarf removal. I'd be running it on a 9x20 Harbor Freight lathe, which has limited torque. Maybe another option is but an annular cutter and remove all but 2 of the teeth.

While researching this, I found a video of some guy testing annular cutters**. He uses the 15-in Craftsman with a "1 HP" motor at 250 RPM. Under power feed, some of the annular cutters (15/16") jammed. My lathe is similar power, I think, but HP numbers can be deceiving. I've read that mag drills typically run at 375-400 RPM.

How would you approach a job like this with hobby-grade machines?



* https://www.youtube.com/@userwl2850
**

 

[mmcmdl]

Why not just drill it ?

 

[Aukai]

If it's only a 1 off project, maybe your biggest drill bit, and bore to size?

 

[Karl_T]

I'd drill what you can then bore it out on your lathe

 

[RJSakowski]

A 5" deep hole in 3" diameter steel is asking a lot from a 9 x 20 HF lathe. Consider work holding. Since you are making a central hole, the tailstock is out. This leads to the steady rest for support. You will undoubtedly need to use the outside jaws so your grip range is limited by the height of the step. My 5" 3 jaw has less than a 3/8" step height and my 6" 4 jaw has less than 1/2".

Cutting is about torque, not horsepower. Even if the motor rating is honest, you need torque multiplication to be able to take advantage of it. If the motor is controlled by a variable speed drive, you can forget it. Even if you obtain different speeds via pulley selection, the drive belt s used on small lathes are not up to serious torque delivery. In short, you will most likely be disappointed with the performance of the annular cutter.

If this is a one off job, farming out the work to a local machine shop is probably less the cost of the annular cutter. Boring with a boring bar would take longer but require much less torque. A five inch deep hole is asking a lot from a your lathe though. There will be a lot of flex in a boring bar of a size that will fit your lathe.

 

[mattthemuppet2]

do 9x20 lathes have a backgear? If they do you should be able to push a 1" drill through that if you step up say 3/16>1/2>1". It'll be a bear to drill that deep, but you can mount the drill (somehow) in your toolpost so you can retract and clear chips easily. You may have to do the last step with the 1" drill with your tailstock though, as you'll be limited by your carriage as to how much pressure you can put on the drill.

if you don't have a backgear then you're best off drilling it halfway to 1/2" then boring it out to say 1.25", then flip it, do the same on the other end and swap to a 1" boring bar to cut to final ID.

It'll take you a while, but it's perfectly doable. I managed to turn a 2-1/2 to 3" (can't remember) chunk of stainless into a mortar (and pestle) on my Atlas 618. Took a long loooong time, but I did it and it's still being used.

 

[extropic]

I need to make a 1.375" hole x 5" deep in 3" 4140 round bar. I've watched a lot of David Wilks videos* lately, so I thought about making a tool. But annular cutters are available that cut 3" deep, which is also a good option.

I'd rather make something but time is relatively short and success is not guaranteed.

An annular cutter would probably be fine but I'm worried about all those teeth and swarf removal. I'd be running it on a 9x20 Harbor Freight lathe, which has limited torque. Maybe another option is but an annular cutter and remove all but 2 of the teeth.

While researching this, I found a video of some guy testing annular cutters**. He uses the 15-in Craftsman with a "1 HP" motor at 250 RPM. Under power feed, some of the annular cutters (15/16") jammed. My lathe is similar power, I think, but HP numbers can be deceiving. I've read that mag drills typically run at 375-400 RPM.

How would you approach a job like this with hobby-grade machines?



* https://www.youtube.com/@userwl2850
**

You didn't mention tolerances for hole diameter, form or finish, but those (bold) parameters seem mutually exclusive to me.
4140 will work harden with light cuts so you need more lathe.

 

[MyLilMule]

You're going to have a hard time doing anything like that on such a small lathe. I would think a boring bar would be preferred if your tolerances are remotely tight.

 

[mattthemuppet2]

You didn't mention hole diameter, form or finish tolerances but those (bold) parameters seem mutually exclusive to me.
4140 will work harden with light cuts so you need more lathe.

Does 1.375" hole x 5" deep in 3" 4140 round bar not mean a 1.375" ID hole, 5" deep in a 3" OD round bar?

Another bit of information that would be helpful is is this is a through hole or blind hole. I'm guessing through hole or your wouldn't have suggested an annular cutter.

 

[extropic]

Does 1.375" hole x 5" deep in 3" 4140 round bar not mean a 1.375" ID hole, 5" deep in a 3" OD round bar?

Another bit of information that would be helpful is is this is a through hole or blind hole. I'm guessing through hole or your wouldn't have suggested an annular cutter.

You are correct. My comment would have been more clearly understood if I had written: You didn't mention tolerances for hole diameter, form or finish tolerances, but those (bold) parameters seem mutually exclusive to me.

Thank you for identifying your misunderstanding.

Fixed.