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Jim Hubbell
05-01-2007, 08:21 PM
I am trying to build a collet for my Dremel-like tool. My problem is getting the 1/8" bore true. On larger pieces I drill, bore, then ream to size. This is way too small for any boring bar or tool I have. The first one I tried to build, I drilled then reamed to size. It was way off. I thought of a mandrel but again the small size presented itself.
If when drilling the hole one were to stop several times and rechuck the drill bit in different positions, would that maybe even out the bore?
How do you get a true bore?

Fasttrack
05-01-2007, 08:23 PM
"How do you get a true bore?"


Well, i'm in a US Government class thats a real bore... maybe you could check out your local community college for a similiar elective?










(srry i couldnt resist)

PHiers
05-01-2007, 08:25 PM
Jim, not sure if it would work better. Maybe try a two flute endmill? The only other thing I can think of is a d bit but damn that would be a small one!

BadDog
05-01-2007, 08:54 PM
I've got a tiny boring bar that might manage an 1/8" hole, but it would be close. Might look for one?

More likely, I would start with something like 3/8, then drill and ream 1/8" ID. Mount on 1/8" Cleveland mandrel and turn OD. Small mandrel would be cheap to buy, not that hard to make.

After ream, mount between centers and then turn OD. When done, mount in collet (or tiny 4 jaw?) to face, then slit on mill.

Main thought being, do the ID first and then use ID turn OD concentric. Even with a tiny boring bar, unless you are very close to on size with the hole, you won't be able to correct.

That's what comes to my mind, let's see how bad I get shot down... ;)

Peter Sanders
05-01-2007, 10:11 PM
Hi

More likely, I would start with something like 3/8, then drill and ream 1/8" ID. Mount on 1/8" Cleveland mandrel and turn OD. Small mandrel would be cheap to buy, not that hard to make.

After ream, mount between centers and then turn OD. When done, mount in collet (or tiny 4 jaw?) to face, then slit on mill.

Main thought being, do the ID first and then use ID turn OD concentric. Even with a tiny boring bar, unless you are very close to on size with the hole, you won't be able to correct.

That's what comes to my mind, let's see how bad I get shot down... ;)
I agree with that. :)

Mcgyver
05-01-2007, 11:38 PM
turning between centres is good idea. mandrel would be a bit of a pita as I've made these and it you want a bunch of extra stock to hold onto for the slitting operation. problem with the between centres is the you don't' really know what is going on with the bore, ie it may at an angle to the axis not just eccentric. then again maybe I'm just sore i didn't think of it :D

on getting the bore straight , I know 1/8 is small but there are boring bars for that. I confess i bought one of the miniature sets as i hate grinding hss blanks into boring bars, but a workable boring bar could be made from a bit of drill rod. its not going to take much load and will take several passes to work out spring, but it will work.

in my experience drilling straight holes is mostly a function of starting straight and the proper grind on the drill, not the chuck's concentricity is off a few thou. especially in the lathe where the revolving work tends to keep the drill centred. start the sharp centre (or better spotting) drill VERY slowly. also, drilling in stages helps, but might be pointless at 1/8". if you lack a T&C grinder or one of the expensive drill grinders maybe buy a high quality split point drill a size smaller, make a boring bar, then do as you usually do. ... as you noted, a single point tool is the way to get a straight bore

jimsehr
05-01-2007, 11:44 PM
First I would use a stubby small bar and bore in about 3/8 inch, too next size under 1/8 drill . Then drill with this size drill. This gets drill started on center. Then step up to 1/8 drill or ream to finish size. Also if you have an undersize end mill this will work if you let it bore by not going to fast that it follows old hole. End mill shold be sharp so it bores.
Jim Sehr

BadDog
05-02-2007, 12:25 AM
Earlier, "close to on size with the hole" should be "close to on CENTER with the hole".

One other thing that occurs to me about using a home made mandrel, you can leave it in for the slitting operation to help stabilize the thin collet wall.

Saw will need very fine teeth to reduce damage. Or maybe abrasive disk?

Also remember, the collet only needs to be on size for the first 3/8" or so, maybe less. (I think that's what Jim was getting at?) So clearance the back and, if you use a boring bar, you won't have it hanging out but about 4 diameters. Not great, but better than twice that.

Definitely a good point on that sharp drill. That's exactly why I bought my Christen Drill Grinder. Baring access to something like that though, I would probably buy a new one (and not a cheap import) just for the job (if I didn't have an virgin bit already).

Then again, maybe you have a Sinker (EDM) handy? ;) That would do it nicely, just burn and polish. I would love to build a mini-Sinker for home use someday, but man would that carbon be a mess. Really needs a separate room/shed to keep it under control.

darryl
05-02-2007, 02:41 AM
I would be starting with a 3/32 drill bit, go in about 1/8. Try to get it as close to centered as you can. Then chuck a short piece of 3/32 drill rod, music wire, etc in a cordless and grind one end to a point. Then grind a D on that end, just barely past halfway across the diameter. That's your boring bar. Chuck it up short in the tailstock chuck with the flat upwards and bring it up to the hole. It should look like it's on center.

Arrange to press against this short stub D bit with a piece of scrap in the toolpost somehow. All you want to do is flex it off center slightly. This is actually the adjustment to get the short bored section of the hole exactly the size you want. With the lathe running in reverse, dial it in carefully and slowly back away the 'push stick'. The D bit should bore the hole true, then center itself, leaving a dimple for a drill bit to follow. If you bore the hole to 1/8, then a 1/8 drill bit should be able to keep on track as you drill to depth with it. If you have a reamer of the right size, bore, then drill the hole slightly undersize for the reamer to follow.

Putting some side pressure on the D bit does two things. First it allows you to make an adjustment so it enlarges your initial hole to the size you want, plus taking out the eccentricity of that first hole. Second, it stiffens up the D bit so it can actually cut away the eccentricity without the cutting edge flexing away from the hole.

You can accomplish the same thing if you make up a holder for the D bit that mounts on the crosslide. The hole for the bit is made after the holder is clamped in to the toolpost, and the drill bit to make this hole is held in the chuck. Move the carriage toward the chuck to make the hole. You'll need a setscrew to keep the bit in place. I find though (for my lathe anyway) that the first method gives me tighter control of the D bit.

Once you get the last drill bit or the reamer through, slip a straight piece of music wire or equivalent into the hole, then run the lathe. You'll know if you got the hole straight.

BadDog
05-02-2007, 03:03 AM
Nice, I had forgotten about the incremental d-bit trick. Though I've never head the "push off center" part before. Very nice suggestion.

kap pullen
05-02-2007, 07:18 AM
I modify a 1/16, or 3/32 two flute endmill and use that for a boring bar.

Grind one flute away, and a bit of clearance on the top and side will do it.

A drill blank, or broken drill will do also.

I need a magnifier to do this.

Kap

Carld
05-02-2007, 10:30 AM
I would use a center drill to spot the end of the stock, then drill it with two steps to undersize about .010". Then I would ream it out and mount it on a mandrel chucked in a four jaw with a live center at the end. Machine the OD and ends as needed. Take it to the mill and cut the expansion slots. You'll have to work out the fine details yourself.

JCHannum
05-02-2007, 10:39 AM
The various suggestions for boring with a mini boring bar will ensure the hole being true and on center.

I would machine the collet in one setup from a larger parent bar rather than transfer it to a mandrel. Machine the outside and ID in one chucking to maintain concentricity. Do not use the tailstock. The bar can be transferred to the mill to slot, and the completed collet parted off. If any cosmetic trimming is needed a mandrel could then be employed.

Jim Hubbell
05-02-2007, 02:00 PM
Thanks much for the many ideas! I have no experience using a "D" bit. Or even what it would look like. I imagine a rod with a flat down to or slightly past center. The rod being smaller in dia. than the bore in which it will be used. A virgin drill bit for the initial hole might also help. I'll be "bizzy" with all these suggestions for a while.
Thanks again.

Jim