QCTP boring bar holder modification

[John Conroy]

I got this boring bar holder with my quick change tool post set and have never used it because it only fits 3/4" diameter boring bars and I have none that size. I have a cheap set of boring bars that fit my boring head that has 1/2" holes. I made an adapter sleeve to fit the QCTP holder and hold 1/2" boring bars. I used a chunk of the original Y axis lead screw from my Ferro mill. It is 35mm diameter and large enough to produce the needed 1"OD and 1/2" ID needed.

You can see how worn the old lead screw is in the center.

My goal was .998" OD to match the original sleeve and I hit that dead on.

I used extra long drill bits to make the ID bore .500"

Then I used a slitting saw to make a .040" slit length wise end to end.

A great way to spend a Sunday afternoon.

 

[PeterT]

Nice. I'm glad you posted. I've got the same boring bar holder & this is exactly what I'm contemplating but for those round stock, longer boring bars with the HSS tool bits. Now the dumb question: there is no slit in the block to clamp down on the stock sleeve. How exactly do the outboard bolts exert their clamping pressure to the sleeve? ...which judging by your pic, has a longitudinal slit.

Also, seems to me (it was a while ago) I removed the bolts & the sleeve was in there but good. I wasn't sure if it was a strong interference fit or rust or some detail I was missing? Did yours come out easy or require persuasion?

 

[John Conroy]

The bolts thread into 2 bushings which have a tapered top that wedges against the side of the split tube. To get the tube out I had to loosen the bolts a couple of threads and tap the head of the bolt downward to break the tapered fit of the bushing to the split tube.

 

[PeterT]

Aha! Glad I asked. I'll be following your lead to re-employ the door stopper.

ps - are those carbide tipped stubby boring bars? Mine are no-name brand offshore & cut like sh*t. I played with the rotation angle & height setting a bit but the classic on-center setup is ugly. They seem reasonably sharp to the touch & geometry about right by eyeball. I re-dressed one of them with my green wheel just to try & seems to improved it. But I don't really know what I achieved, its not like I altered the angles very much. I have a HSS boring bar & its night & day better on all materials. How do yours perform?

 

[John Conroy]

I have only used them in my boring head on the mill and they seem to work fine there. I have only used an insert type boring bar on the lathe, until now I never had a way to use these elcheapo ones. I'll let you know after I use them. I'm really happy with the boring head I got from KMS at the same time as these boring bars.

 

[schor]

Nice job on the adapter. I have the same qctp and the provided sleeve on mine will not grab a 5/8" bar tight enough.

I use mine for 3/4 bars and when I have 1/2" bars I just use the holder with the v-groove.

 

[BadDog]

Good job. I've made quite a few of those bushings, and they work very well. But a few suggestions. First, turn it so that the slot is aligned with the clamping bolt wedges. You want the wedges to bear on opposite sides of the slot to close the slot down on the boring bar. And, on thicker walled bushings, you also want to cut another slot part way through on the opposite side to make collapsing the through slot much easier. I've got bushings that go from 1" down to 0.250, and those are pretty much impossible to clamp tightly without thinning the hinge. I usually cut the hinge slot within about 0.90 or so of going through.

 

[BadDog]

Regarding the use of the little import boring bars. They can be made to work, but in my experience never ideal. The better quality ones are, of course, better. But even the cheap one can give decent service with light cuts. But the green wheels are not going to get you there. At best, they can be use for rough shaping, not sharpening. Diamond wheels are really very cheap now, and will truly make a positive difference that you can't believe until you see it and use it. There are plans here for building your own A6 (0r other) diamond wheel grinder. But you can also do it by hand with a progressive set of diamond hand laps. I usually use the diamond wheels for almost all of it, but not infrequently touch up a fine edge or put on a corner radius using a hand lap. It's surprising how fast they will cut. But regardless of how you get there, you want a very smooth very polished sharp edge, and don't expect heavy cuts at all. And you must make sure you have sufficient support, but no rubbing beneath the cutting edge, and the top plane should be oriented on center to give zero rake. Also don't forget to watch for trailing clearance, and remember that the cutting edge is on the end of the bar, not the side.

 

[PeterT]

You mentioned the cutting plane orientated to center. One of the things I eventually noticed about my offshore Sh*t brand boring bars, the braze-on carbide is not set properly, meaning the resultant cutting edge was NOT centered relative to the shank center.... and it wasn't a small amount. In a lathe setup you can make some height + tool rotational adjustments to mitigate the issue, but its still not ideal & there are better tools. In a boring head, there are fewer options. Its either going to rub, dig & basically yield poor cutting performance. This video explains it better. I'm not saying all imports are bad, just check yours for this problem first. Cutting issues might be more than edge quality, speed & feed etc. if the basic geometry is out of whack.

 

[BadDog]

That's a very good point. With adjustable height lathe tool posts, you control the important relationships so you can address these limitations.. But on a boring head you have no "height" adjustment, only axial rotation which ties together other references that are pretty much set at manufacture. Frankly, for boring heads I've found the Bokum and other premium bars are the ticket. Using a boring head is already managing a lot of critical relationships that can be difficult anyway, so you ideally want to give yourself the best chance of positive outcome possible, but it depends on budget and what you have on hand when you need it.

 

[Bofobo]

I have used these bars as well, I found them to be useable once I removed excess material from the bar, but that method cost me 2 cutters, just brazzed horribly most likely. I don't have a quick change system I am using the original 4 space-rotating tool post that the cx701 came with. Included is a little block that works with that system I went to busy bee and picked up their little hss boring bar kit (pricy by comparison but better cuts and longer shafts, (including an internal threading bar and 2 for hogging) a con is the bar holder is dinky and I have to shim it up. But in a comparison test I would never spend my money on the KMS set of stubby bars again.

 

[PeterT]

I had one of those square blocks to hold a round shank boring tools. They have a few downsides. They don't grip shanks particularly well (at least mine didn't). Part of that might have been the bad ID finish, but also the set screws are exerting force squeezing the clamshell in the middle which isn't ideal. The bigger PITA is as soon as you loosen the setscrews, the boring tool is free to rotate & you loose your cutting tip vertical height position relative to bore center. The block also takes up room in the tool post notch that could be better filled with a fatter, more rigid boring bar. Sketch shows an improvement where screws exert clamping pressure off on the side. The assembly can be removed & returned to same height position. This design allows you to modify the extended clamping chunk to your specific tool post if that's a constraint.

The insert boring bar knock-offs are quite inexpensive these days. This 2 bar set on AliExpress is 30$C shipped to your door including 2 CCMT inserts. They are typically round shank but flat sided so you can clamp directly in the tool post holder, no in-between stuff involved. Many folks will say insert carbides are not for small machines, but I don't necessarily agree. I've screwed around grinding HSS & trying to re-dress those brazed carbide boring tools. Its just really hard to compete with an insert with all the correct geometry built in to begin with (rake, relief, nose radius...). Will the average home shop guy be pushing inserts to fully utilize their capabilities? Probably not, but when you can buy a handful of inserts on ebay for similar cost as HSS blanks these days, it doesn't really matter as much. If you blow the edge off or wear it out, screw on another & move on with life. Don't get me wrong. HSS is super useful for custom tool point shapes & can provide great finish if you know how to grind them. But for everyday boring, I'm happy with my insert style bars. I bought some uncoated inserts for aluminum & they cut awesome.