View Full Version : Internal grooving in small bore diameters..

01-05-2007, 10:27 PM
I'm looking for some suggestions on how I can achieve a more repeatable and rigid clamping system for internal grooving in small bore diameters. I'm using a micro 100 internal grooving tool clamped in place w/ the set screws in one of the "aloris" type toolholders (boring ones w/ the v groove)for the quick release toolpost. At present the shank is 3/8" w/ the lockdown flat. I could always switch to a round shank. I need the tool to reach inside approx 1.1" in a bore diameter of .312". I need to hold the tolerences within .00025" over a 50 thousandths depth of cut. Are there any toolholders made that would achieve a better more rigid /repeatable clamping of the grooving tool...ie, collet system, bushing system. etc. Any help would be appreciated. I noticed that Sandvik has a coroturn XS system for small parts (grooving, turning, threading). Anyone own one of these??

Thanks in advance....Krems

Rich Carlstedt
01-05-2007, 11:38 PM
.00025 positioning Tolerance ?
First, are you doing this on a hardinge?
Do you have DRO to read this ? or are you using a stop?
Are you using a "real" Aloris or a improv?
Is it a wedge style holder?
What is Spindle play ?
How wide a cut ?

When you talk about that kind of tolerance, you need good equipment.
There are some great minature tooling makers, but they slip my mind as far as names.
We had some made by a firm near Chicago (starts with a "R") that was customed, but really worked.
I don't need the answers to the above, just that all sorts of things occur when requiring that tolerance.

You will probably want a custom Carbide boring bar..no inserts..you need rigidity !

01-06-2007, 12:20 AM
I've been making custom moulds for a while. I need the grooving tool to reach inside to cut the grooves. I have been using solid carbide grooving tools w/ a custom grind. Accuracy wise less than .0005" depth of cut is more realistic. Width of cut is not important. I'm cutting a .100" wide groove to a max depth of .065" TIR. But I need to be able to reach in 1.25" into the hole. I've been using the wedge type Aloris clone for a toolpost. There has got to be a better way to hold the carbide bit than the wedge toolholder w/ the v groove in it. Maybe one of the wedge toolholders w/ the round step down boring bar bushings. Any thoughts??


01-06-2007, 12:26 AM
As Rich says, special needs require special tools. .050 DOC? Is that the depth of the grove or the actual DOC you're trying to take per pass? :eek:

Another option is to grind the grove in.

01-06-2007, 12:53 AM
CC, The total groove depth is .065"" or ..0325"" depth of cut. Sorry If I confused anyone. I'm cutting 3 seperate grooves inside the bore. The max tool depth is 1.2" . I just want to be sure that I have a rigid clamping system so that the first and third grooves will be the same. I am trying to eliminate any tool flex. Thanks again


01-06-2007, 10:25 AM
You're reaching 1.25" into a 5/16" bore, right?

Then trying to cut a .100" wide groove with a diameter of .3125+.065, right?

And you want to maintain tolerances within .00025" (.000125"+/-).

What's the material?

How will you even check the parts for accuracy?

Is this a joke?????

01-06-2007, 10:51 AM
It sounds like maybe you want this tolerance in order to produce tight tolerance molded parts ??

I've designed some precision deep bore parts for optics but never this tight. Several thoughts though ... put a tiny geometry (small radius) in that tool to minimize deflection in the finishing pass, use a very fine feed, sharp tools, high speed. You did say width was not critical?

Or, use something similar in concept (don't know that they even exist) to an expanding reamer. It would have to expand, in place, after positioning at the required depth. The expansion would have to have a hard stop with the OD of the tool ground at that stop.

01-06-2007, 11:16 AM
krems, I think the only advantage you would have by going to a bushing type boring bar holder would be that the bar would be supported all the way to the front of the toolholder. With the type you are using now you are losing a little support because of the distance from the front of the tool holder to the set screw that holds it down. As far as the type of tool holder that you are using now allowing your tool to move any at all causing inaccuracies in your work, I highly doubt that. What little bit of rigidity that you will gain by swithching over may not be much but every little bit counts when working in a hole that size. My question is how are you measuring your groove to verify the tolerances stated?

01-06-2007, 11:39 AM
Are these lube grooves for lead bullet molds? Aluminum or steel? I assume you are clamping the two halves and boring simultaneously>

01-06-2007, 01:27 PM
Sorry about the terminology confusion. I'm new at talking the language. I am indeed making bullet moulds. Aluminum and iron. Mostly reboring existing ones. 4 jaw chuck, both halves clamped together.. I've probably made 100 moulds or more for myself/friends. I am a lead bullet long range target shooter. I grind my own tools for this...razor sharp.

I'm trying to get all the driving bands cut to the same depth so that when the cast bullet is made all bands will have the same finished diameter. I can get them to within .001" but I'd really like to see the finished diameter to within .0005". I think my cutting technigue is good but I'm getting a little flex on the tool as I reach into the cavity further. I realize it's hard to measure on a cast bullet because of all the other variables associated w/ casting ..alloy/temp/shrinkage. etc. I can't measure the mould cavity itelf I have to measure on a cast bullet. If you measure enough of them you can tell what type of job you did on boring the cavity.. A bullet can't be round enough. I'm looking for an alternate clamping system for the boring bar that is more rigid than using the wedge toolholder w/ the v groove in it. Maybe the bushing type holders or the 5c collet system that Dorian tool has for their quick release toolposts. I may be unrealistic in my expectations but am always looking for improvement.


01-06-2007, 01:49 PM
The depth of the bore will have no effect on the flex of the boring bar so long as the length of extension remains the same.

If the desire is more for uniformity than actual dimensional accuracy, use the same extension of the boring bar for all the grooves, and allow the part to revolve for some uniform period of time at the final depth of cut to compensate the spring.

01-06-2007, 03:13 PM
The wedge type holder shouldn't be flexing any at all if it is working properly. Especially with the kind of load you are putting on it making an internal grooving cut. If it was, there is no way you would ever be able to make any kind of heavy turning cut with it. If you are holding the ring size to withing
.001, I would leave well enough alone. I doubt you would ever be able to prove that holding them to .00025 was helping any more than holding them to .001 anyway, so why bother. I'm not trying to tell you not to do accurate work, but you have to have some tolerance when machining, and trying to hold a tolerance that you can't even measure seems a bit crazy.

01-06-2007, 03:35 PM
I never thought of it that way but you're right. I'm making the same cut w/ the boring bar clamped in the toolholder for every cut. My ignorance is showing again. Sometimes the obvious is hard to realize. I may have to change my machining technique to dial in the acceptable tolerence level. I may have differing shrinkage rates going on when the metal is cooling. The mould is hotter at the sprue plate than at the front driving band inside the bore. Quest for accuracy...isn't it fun.

japcas: I suppose I'm unrealistic in what I'm trying to do. I havn't bought a commercial bullet mould that was better than mine yet in terms of roundness and consistancy. I should be glad for that achievement and leave it alone. Too many variables come into play here. Yea it's crazy but always worth trying.

Thank again..........Krems

01-06-2007, 04:25 PM
You could always have a form tool made,(or make one yourself) with the requisite number of cutting edges to form your grooves. That way they'd always be in relation to each other. Personally , I'd use a toolholder that uses a bushing or a collet.

01-06-2007, 04:47 PM
Do you not use a sizer on your cast bullets? Usually the cast size is a lot less critical than the sizing die, the bullet can be in a diameter range several thousandths wide and the sizer takes care of the size.

Holding diameter to .00025" an inch away from support on a small diameter tool is pretty close to impossible, the only way you might be able to cut to your spec is if you made a tool that would fit the bore and advance a cutting bit out the side to a stop. But since most bullet molds weren't made that way maybe you could make a center closing fixture and use a 'cherry' as the larger makers used. Otherwise you could just hold .001 and be happy that you're doing better than most makers.

01-06-2007, 08:39 PM
You are barking up the wrong tree, but here's a simple, cheap way to stiffen your tool holder.

Take a piece of the largest diameter round stock (steel) that will fit in your tool holder, and drill it for the diameter of the boring bar shank. D&T for several setscrews (or it can be designed to use the tool holder setscrews). The resulting bushing will give the shank a little more support. It's not as rigid as a split sleeve bushing, but it's good enough for miniature bars.

Getting back to the real problem, and that is the flex and vibration on the slender stem of the boring bar. There is not a darned thing you can do about it except keep your tools sharp and make multiple light passes.

If you make all grooves the same diameter, the bullet WILL NOT cast the same on every band. Generally the bottom band will cast smaller, depending on alloy, etc.. If you want all bands to cast equal, you have to cut a tapered mold. And/or cut the darned thing oversize and size the bullets. The perfect as-cast diameter is a myth.

Regarding roundness, that is a function of mold alignment. Very few store-bought molds have perfect alignment. Your boring bar has little to do with it unless your lathe is wobbling or you are doing something terribly wrong.

01-06-2007, 09:07 PM
I thought I recognized the terminology. Looks like fairly long 8mm or .30 cal bullets.

As others have said, this problem is why sizers were invented. It's easier to make a round hardened sizing die than to get a perfectly round bullet cross section but getting the initial bullet diameter close is still important. I know target shooters want to use as-cast bullets but I never felt that light sizing was a detriment - maybe .0005. I used to cast bullets for the .45 ACP from linotype at .452 and size to .4515 - these rounds out of the right pistol would stay within an inch at 25 yds.

I also shoot 525 gr bullets from a 45-70 and they are hard to cast so I make them large and swage to size in a hydraulic press with a custom made die. This gives a very consistent size and density.

01-06-2007, 09:24 PM
Sounds like you are trying to do the impossiable . Just do it. Instead of casting bullets way not just machine some out of lead measure them and shoot them to see if any thing different happens. I thank you could mackine up some easery than make a mold and cast them and watch thm srink.

01-06-2007, 11:39 PM
One way to prove the mold might be to use one of the very low temperature Cerro alloys to cast a bullet. That would remove most of any effects caused by temperature differential, and give an indication of what the mold is actually capable of.

Sizing the cast bullet is just as likely to introduce variations as it is to eliminate them.

What type of gun/ammunition combination are you shooting?

"The Bullet's Flight From Powder to Target" was written by Dr. Franklin Mann in 1909, exploring the effects of variables on the path of the bullet. It is a fascinating book in the methods he used and the conclusions drawn. The quest for accuracy continues a hundred years later. Good luck.

01-07-2007, 01:56 AM
I think Mtngun hit it on the head. The perfectly round bullet is like chasing a unicorn. The bottom band definately casts smaller than the other bands. I just got done cutting two moulds and I've started cutting the bottom band slightly larger as well. One was for a 50-90 sharps rifle and the other for a 38-56 win 1886 rifle. I wish I could figure out the picture thing so I could post a few pictures. These things sure are pretty!! The diameters all came out w/ in .001" which is as good as I'm able to get right now. I'll still experiment a little to see if I can improve. I believe in sizing a bullet as well. I like to cast around .001" oversize myself then run through a sizing die as well. I just want to be able to hit the diameter the same on all the bands. I've been making a lot of hunting bullets moulds for winchester leverguns lately. Mostly the oddball stuff...40-70wcf, 40-82wcf, even a 45-75 model 1876 rifle bullet. But my favorites are the 50 caliber stuff 50-110, 50-90 sharps.

I think I'll try some sort of bushing/collet holder of some kind just for kicks. Can you use one of the split bushings w/ a shank that has the lock down flat on it?
No one has responded to using the 5c collet holder that Dorian sells for their wedge toolpost. Must not be worth buying. I still can't figure out how they close the collet. Must be a wrench system.

JC:..I've looked through that book you mentioned. Interesting read. I've been shooting a 45-110 sharps rifle lately. 540 grain bullet. 30-1 alloy. Black powder.


01-07-2007, 10:28 AM
krems, I just had a thought about how you could improve consistency on your ring grooves. If you are sizing the bullets to get the desired size after casting, why not size the ring grooves individually. You would need to make a split tool with handles like a normal split mold. Bore it to the size you want the groove to be after sizing. Then groove around it for relief to go around the bullet. This size isn't critical as it won't touch the bullet. Then, modify the mold so that the rings are .003 to .005 oversize. Then put them in your ring groove sizer and close it up, to size them all the same. You could make it so that it size one which would probably be the most accurate or so it sizes them all at the same time. It would take more pressure to size them all at once but an arbor press could be used if necessary. Might have to do some experimentiing to find out how much oversize the rings will have to be cast to make them consistent but that's part of the R&D. I know it sounds like lots of extra work but sounds like you are willing to go the extra mile. So, what do you think?

01-07-2007, 12:46 PM
I am going to be modifiying a couple of existing moulds soon and am interested in how this turns out for you. You seem to do quite a few molds, so I would suggest that you keep records what the resulting diameters of the bullet bands are. You may find a trend that would allow you to modify the cut band diameter on the next molds so you end up with uniform cast bands.

I am sure you are aware that alloy and casting technique also play a big role in the results. If you are serious about this high level of consistancy, I would suggest getting thermometer if you don't have one and work to try to use the same technique: casting temp, pour height, sprue fill, ect.. on each bullet.

If you are not aware of it, there is quite a bit of good info here, and a few of the fellows make/modify their own molds:


01-07-2007, 04:25 PM
Here's the tool for recessing. Now just scale this down...


Actually that's probably not a useable design for what you're doing, but the broad idea is good. That is, if you have a tool shank that fits in the hole and the cutter force is taken against the back of the shank and the side of the hole you eliminate a lot of other flex factors. This would need a redesign for size and perhaps a screw wedge to advance the cutter, but it would allow you to measure and calibrate it and use a mark on the screw for repeatability.


01-10-2007, 01:59 AM
Thanks guys for the advice. Just got back online after a long chat w/ the Dell computer tech. in India. Video card went dead...no monitor. I've lurked on the cast boolit site before. Lots of good info over there. If any substantial progress is made I'll report back.


01-10-2007, 05:04 AM
Here's the tool for recessing. Now just scale this down...


Actually that's probably not a useable design for what you're doing, but the broad idea is good. That is, if you have a tool shank that fits in the hole and the cutter force is taken against the back of the shank and the side of the hole you eliminate a lot of other flex factors. This would need a redesign for size and perhaps a screw wedge to advance the cutter, but it would allow you to measure and calibrate it and use a mark on the screw for repeatability.


Or he copuld just use a recessing tool like they use in screw machines. Sometimes called a swing tool."00" size ought to work in a SB.