New Project: Shop Made Flexbar Fly Cutter

[homebrewed]

Install the disk part in your vise so the long dimension is UP. Mill a slot across the narrow dimension where you need it. Now you have three flat sides for your cutter. Yep, the cutter isn't in a square hole. I don't think that matters, given the direction of cutting forces. Think of it as being similar to how cutting tools are installed in a lathe QCTP. The holders don't have square holes in them for the cutter, either, just a slot.

If that bothers you, you could make a deeper slot and back the cutter with a same-sized key. Both will be held in place with set screws, of course, so there'd be some additional machining involved. Not that getting yourself a square hole is the end of the machining you would have to do anyway....

 

[Nutfarmer]

So my budget is zero here, but I can do this.

I am going to make one of these:

View attachment 441388

My first concern is that I haven’t yet found a way to make the square holes. I don’t have a broach.

Drill a hole and file it square. Or drill a hole and braze an insert that is square.

 

[erikmannie]

They also make round HSS tool bits.





If I used these, I would have to bump up the diameter of the tool bit to 7/16” or 1/2” in order to have the same or better rigidity as a 3/8” square tool bit.

 

[jwmelvin]

So mill a 3/8” wide slot (say) 5/8” in, and then silver solder (braze?) in a [3/8” X 1/4” X plate thickness] steel block? I had not considered that.

I meant silver braze, really. Not the lead-free plumbing solder with ~2% silver, but something like Stay-Silv with 45% silver.

Same idea though, of a filler with good wetting so capillary action will fill the joint.

I’d have the screw transverse, not radial, so the brazed block just stops the slot from opening due to screw force. Bronze braze just from the outer diameter would be fine. Or a small weld, but like you say, that had a lot more potential for distortion.

 

[pontiac428]

Here's the best option. You can make these using your mill and lathe, that's what I would do.

DuMont 3/8" Square Broach 9⅜" OAL HSS 25/64″ PilotMachinist Minute Man | eBay

Find many great new & used options and get the best deals for DuMont 3/8" Square Broach 9⅜" OAL HSS 25/64″ PilotMachinist Minute Man at the best online prices at eBay! Free shipping for many products!

www.ebay.com

 

[homebrewed]

They also make round HSS tool bits.

View attachment 441404

View attachment 441405

If I used these, I would have to bump up the diameter of the tool bit to 7/16” or 1/2” in order to have the same or better rigidity as a 3/8” square tool bit.

Good idea! I'd be interested in seeing the cutter profile you come up with, along with any grinding fixtures you find you need to make for that.

I've experimented with a single facet grind on round HSS, much like the so-called tangential or diamond cutter, but the relatively shallow angle between the cutter and work complicates the tool design.

 

[pontiac428]

This is the grind I use for my Lopre cutter in aluminum (before honing and hand-radiusing). Normal RH tool with good clearance.

I would not use round cutters. Even with a shank flat the tool has too many degrees of freedom. Remember, the fly cutter tool's life is a brutal whup-whup-whup, that will jar anything loose. Square is stronger, stiffer, and locks up tight. If Lopre and Flexbar took the extra effort to broach square holes, I'd follow that queue in your design.

 

[Magnett]

Take a look at Joe Pi’s grinding a broach on a mill.

 

[pontiac428]

I'd be willing to say you can skip grinding if you don't have the equipment. I avoid abrasives on my mill, but that's just me protecting my investments.

Take a 8-12" piece of O-1 or W-1 in 3/8 square rod, rough the taper in the lathe and make your grooves. Cut the grooves with a little bit of rake on the cutting side, no more than 3-5 degrees using a grooving tool. Then plunk it on the mill and machine your steps using an angle fixture of sine bar. Flip the broach and set the angle to double your initial angle for the back sides. For steel, make your cutter clearance cuts at 6-8 degrees. Heat treat and hand hone, or grind sharp using whatever you have that will work.

 

[erikmannie]

Good idea! I'd be interested in seeing the cutter profile you come up with, along with any grinding fixtures you find you need to make for that.

I've experimented with a single facet grind on round HSS, much like the so-called tangential or diamond cutter, but the relatively shallow angle between the cutter and work complicates the tool design.

The one time that I used round HSS toolbits, it was in school with a very old timey instructor. He ground a square coming out of the cylindrical tool bit, & then ground the square part of the toolbit like anyone would grind a square toolbit for, in that case, RH turning.

Imagine this exact grind emanating from a cylindrical toolbit:



This is why, if I did go with a round toolbit, I would go larger than 3/8” diameter in order to have the same or better rigidity as a 3/8” square toolbit.

Here is a picture of the late instructor, Ken Gilliland. I was *very* lucky to have had him as an instructor.



As far as a cutter, I love brazed (or solid) carbide, but I have no green wheel. I need to throw away my dull carbide toolbits!

Also, my bench grinder is a hunk of junk with the lowest quality all-purpose wheel you can imagine. This is why I usually gravitate toward inserts.

Along these lines, maybe I should plan to use 3/8” indexable tool holders (“shanks”) with 1/64” nose radius inserts for fly cutting what is almost always mild steel. I really enjoy being able to work with a perfect cutting tip!