[How do I?] How to make a large-ish through-hole in thick-ish CRS stock?

[gr8legs]

Greetings all -

I need to make approximately 1.25" diameter holes through 1.50" thick x 3.5" wide CRS bar stock in the 3.5" face of the stock.

This is a job for my CNC, not drilling or boring. The hole size is not a standard drill or fractional size

I can do it with what tooling I have (a .750 diameter center cutting end mill with 3" length of cut) but that seems like a lot of time and horsepower wasted making big chips.

Ideally I could see doing this with a 1/4" or so diameter cutter and pecking down 0.1" or so per cycle but most smaller end mills have a limited depth of cut (3/8" to 3/4" or so) and it seems to me that once I am deeper than the length of cut of the end mill into the bar stock then the cutter shank will be rubbing on the ID of the hole I am making plus some of the chips created will be trapped between the cutter shank and the side wall of the borehole - guaranteeing a bad finish on the part.

Finding a center cutting end mill that will work brought me to this one: https://www.mscdirect.com/product/details/85646347 but 80 bucks for a cutter for a once-off personal project that I may crash and destroy on the first or second pass is a bit steep for my Scottish heritage. Is there a better way?

Thanks in advance

Stu

 

[Cooter Brown]

Chuck it up in the lathe and run a boring bar down in....... lol

 

[extropic]

I'm not (very) CNC literate but your task sounds very simple if your system can circular interpolate. I'd drill a .25 through hole on center (so no need a center cutting cutter). Using the .75 you mentioned is fine. Move to "on center and plunge to depth of cut. Circular interpolate to 1.25ish diameter. Repeat until through then cleanup ID to finish size.

I think it could be done in little more time than it took me to peck out this reply.

Edit: spelling

 

[mmcmdl]

Blow it thru with an insert drill .

 

[Cadillac]

Sounds like a job for a annular cutter. Like a hole saw but more rigid, hole is fairly precise, finish is similar to endmill.

 

[ChrisAttebery]

I’d drill as big a hole as the machine can handle. Then I’d do a circular ramp with that 3/4” end mill.

 

[RJSakowski]

There are a number of "it depends on's" here. What size machine, precision required, surface finish, how many holes, etc.

I would drill the largest undersized hole that I could and finish by boring. A drill is remarkably efficient at removing large amounts of material.

A hole saw will have a problem with chip clearance and in my experience. it leaves a poor surface finish. The annular cutter would do the job but that's a fairly expensive hole. A small diameter end mill will have a lot of stickout and chatter will be a problem. You would want an extra long or reduced shank end mll

 

[benmychree]

I see lots of broken end mills in any size below 1/2". Sounds more like a lathe job, although you do not say how long the part is.

 

[gr8legs]

I see lots of broken end mills in any size below 1/2". Sounds more like a lathe job, although you do not say how long the part is.

It's going to have to be a mill job as the part is 40" long and the hole goes at the end.

I don't have an 80" lathe nor the space to put one - and even if I did have the space SWMBO would certainly have something to say and it would not be 'that's fine honey'!

I will probably just treat this job as a circular pocket and use one of the milling cutters I have on hand. This part will be used as a new extended lifting arm for my rolling jack so it will reach the lift points on the GMC motorhome.

Stu

 

[JimDawson]

I would drill as large as practical, then as you say, pocket using a spiral down with a large end mill.