Will a Hi-Roc drill survive this situation?

[sfriedberg]

Sorry I don't have a photo to show.
I just ran across a hard-as-hell inclusion in a 1.75" square bar of CF 1018. Was using a HSS 1/8" wide plain milling cutter (slotting saw) to make some .203" wide, 0.156" deep grooves. Using quite conservative SFM, feeds and DOC, total of 6 passes. Did one side of the part 5" long, no problem, groove was clean, cutter looked good, stayed sharp. Got about 1/3 through the groove on the other side and the wheels fell off. The inclusion basically trashed the cutter, and I didn't help any by trying to bull my way through the issue. (In fact, the cutter is irreparable, unless I want to turn it into a convex cutter on my T&C grinder, because the teeth just don't have corners anymore.)
Here's the situation. I can see a shiny raised area at the bottom of the groove, about 0.050" wide and the width of the groove. I don't know how much of the iceberg is below the surface. I have a 1/4" HiRoc drill (straight flute carbide designed for RC60-class materials) and permission to leave a drill hole that extends slightly past the walls of the groove. I am considering using it to clean out the inclusion to the final depth of the groove. I have only used HiRoc drills on uniform flat surfaces, to get through case hardening and just once or twice to drill through-hardened material. If I plunge down to the inclusion with a 1/4" endmill so the single flute of the HiRoc is not bouncing off the walls of the current groove, can I safely use the HiRoc on a mix of soft 1018 and hard-as-hell whatever it is? Any suggested alterations to the HiRoc maker's recommendations on speed and feed in this situation?

 

[eKretz]

Maybe, maybe not. I don't foresee a problem, but who knows what is in there. If it's something like stray sand, it will eat carbide too. Any reason you don't want to just grind it out of the way with a thin cut-off wheel?

 

[sfriedberg]

Cut off wheel would be another option, yes. I'd have a lot less control over the available manual grinders, and would probably leave a lot more skid marks than one reasonably tidy drill spot. Also my control over when to stop (depth control) would be pretty sloppy. The slot is shallow enough that I could probably clutch up on a Dremel without too much kicking off the walls.
In addition to my lack of manual dexterity, the partially cut groove is only 0.125" wide at the inclusion as I didn't get a chance to make my overlap passes. So I'd have to grind away the "overburden" too. All that is definitely possible.

 

[eKretz]

I always liked a grinder best myself, but I've gotten pretty good with 'em over the years. We used to have a run of stainless sand castings that would come in that had all kinds of trapped sand in the area where we had to finish a bore. Standard practice on those was to rough out to -.030" then take a die grinder and notch any sand pits to below finish diameter. After a while I got it down to where the pits were almost always barely below and could hardly be seen. If the pits weren't ground back we couldn't keep a straight bore - the sand would eat the carbide on even a light finish cut. The cut-off wheels can also be cut or dressed down to a smaller diameter to make them more controllable. Maybe try the drill and see what happens, keep the cutoff wheel in the back pocket just in case.

 

[Ralph_P]

If the hard spot is metal, run the drill 3k to 5k rpm. If it doesn't cut at first try, keep pressure on it and the hard spot will start to glow red. It will then fall through it like butter.

 

[dgfoster]

I suppose you have access to a surface grinder. I'd probably profile a wheel maybe 10 thou narrower than the groove and a little deeper than the intended groove. To make the hardened portion of the groove, make a some some cuts to get to depth and finish the sides of the cut to dimension. Once the hard area is taken care of, do the rest of the groove(s).

That would make a very neat groove and allow precise depth and width control. It would also reduce the pucker factor compared to the drill. I do think the grinder wheel will make a more controllable groove than a cutoff wheel. But, if the groove from the cutoff is "good enough" for the requirements of the job, it would be quicker than setting up the surface grinder and dressing a wheel.

Denis

 

[Bill D]

Angle grinder cutoff wheel one the same arbor the slitting saw was using.
Bill D

 

[sfriedberg]

Angle grinder cutoff wheel one the same arbor the slitting saw was using.

That's an option I hadn't considered.

 

[Bill D]

That's an option I hadn't considered.

Grit will go everywhere. Cover ways with aluminum foil or heavy canvas and use a shop vac while cutting.
Bill D

 

[sfriedberg]

OK, I didn't need to try every option suggested. Thanks for the tips! I started with the simple stuff, and it worked.
Where the groove was already cut, pretty much to final depth, I used a Dremel-like tool (actually a Milwaukee M12 cordless) with a cheap 1" diamond plated mini-cutoff wheel to grind away the inclusion. While I never got a really clear look at the inclusion (shiny silver embedded in shiny silver, and no nitric etch handy) it seems to have been a fairly thin shard of whatever-it-was.
I plunged down to depth with a 1/4" endmill to check the uncut width of the groove, and poked around with a sharp carbide scriber/probe. Sort of inconclusive where the inclusion extended under there or not, or maybe got torn out when the endmill came down, given that one end was already cut away.
I put another 1/8" slotting cutter on to cut the remaining width of the groove and listened for sounds of more distress. Yep, there was another small spot that ate HSS about a couple inches further along the slot. This time, I didn't try to just bull through, quickly backed off the cutter, left the part in the vise, and did a bit more hand grinding. Corners on the cutter were slightly buggered, but I was able to complete the slot OK, and that cutter can be resharpened. The original slotting cutter is trash. If I remember on Tuesday (next time I'm in the shop) I will see about a photo of the no-longer-teeth.

 

[magneticanomaly]

I do not know what the part is, but if it is stressed, that inclusion, or what is left of it, will be a perfect crack-starter. If the material was bought to a spec, seller should replace it.