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Thread: Milling an Inside Radius
12-22-2005, 01:42 PM #1
As will become more obvious as you read this post, I'm new to the vertical mill - have had mine for a whole three weeks now, and just finished installing a Newall 3-axis DRO (Z-axis on the quill) a couple of days ago. Which brings up a question - one of the guys here posted a photo of a nice saddle block he machined to fit atop the ram of his B'port. It's sitting right over the hole the machine lift eye threaded into, and is the base he used to mount the arm that holds the DRO control head/console. He did a very nice job of finishing it, even machined a radius or bevel (couldn't tell which in the photo) around the top edge of the block. My question is how you would set up to machine the radius into the bottom of such a block to match the radius of the top of the ram? I'd like to make a copy of his saddle block out of 6061, and am also looking at machining a one piece scope base (also of 6061) to fit rifle actions that have different radii on the front & rear receiver rings.
I've seen a vidio clip of cutting an outside radius by turning the workpiece by hand online, but don't think that's going to cut it with either of these projects. I guess I could try to figure out how to keller the radius with multiple passes with a ball end mill, but would sure prefer a quicker, less complicated method, even if it required the construction of a jig or fixture - if it worked well enough, I'd like to make scope bases for sale. I'm starting to wonder if you'd need a CNC machining center to do what I have in mind.
12-22-2005, 02:44 PM #2
I've cut radius' (radii?) with a flycutter, feeding the spindle down.
But what is the radius you're looking for, quite a bit bigger than a flycutter I believe. Sounds like a lathe job.
12-22-2005, 02:51 PM #3
Yes, you can swing a convcave radius in the same manner. You will have to get used to making disposable fixtures, Make a bar with a pivot hole correctly located under spindle. Mount the bar of material(6061 in this case) onto the swing bar by tapping clamping, whatever. Swing the radius as in demonstration for your first operation. Then demount workpiece, now saw it off and finish it up. Wastefull of material? Perhaps, but things as this are necessary with out all the proper tools.
Another method is to use a toolmaker's vise laid on it's side bolted to aforementioned swing bar.
Yet another, which can be hard on mill, is to bore it with a boring head. Set the desired rad in head by trial cuts in wax or calculation. Walk the head into part with the X axis after sucessive boring passes til desired chord and rise of arc are achieved. In aluminum the finish won't suffer to greatly due to intermitent cuts.
12-22-2005, 02:53 PM #4
If the radius is a segment of a true circle then it can be cut with a boring head or a rotary table. Can you point us to the picture of what your describing? If a freeform, not a true radius then other methods will have to come into play.
no neat sig line
12-22-2005, 04:17 PM #5
You can always "trig" your x and y positions or easier yet if you have a CAD and step your way around the radius witj your fly cutter,boring head,end mill,or whatever your using.
I'm assuming you have a DRO but can still be done if you don't by "using the dials"
12-22-2005, 04:27 PM #6
For the scope bases, I would make a tube on a lathe, then cut 2-3 scope bases out of the tube with a slitting saw.
and FWIW 1 piece bases arent all they are cracked up to be
For swinging the radius for the DRO mount?? sounds like a rotary table job to me
12-22-2005, 05:43 PM #7
Depending on how wide the piece has to be in relation to the radius, you can get something close by selecting an appropriate diameter cutter and tipping the head. What that generates is an ellipse (or one of those fancy conic sections) but over a limited width it will be close enough to a true radius for what you're doing.
As an example, the base I made for my Barnard, same OD as a Remington, was done with a 1" endmill at 45°. I didn't have the larger radius at the back to deal with though. That could be done with a flycutter, either tipped at an angle or set to the diameter and plunged. Two ways to do that, either a vise standing on edge or a right angle head ... which you want to buy anyway, right?
12-23-2005, 02:08 AM #8
The Bport would be a lot happier using a regular end mill as opposed to a boring bar. The interrupted cut of the boring bar would put a lot of strain on the spindle bearings.
Use a rotary table or swing bar and rotate the workpiece 180° around the Z axis to create the radius, feeding the X axis to get the proper depth of cut. Multiple cuts will probably be needed, depending on the size of the piece.
12-23-2005, 01:09 PM #9
Guys, sorry I was too lazy to point you in the right direction of the post & photo I was referring to. Still don't know how to post a link directly to that post, but it's titled "Installing Newall DRO", and is in the B'port forum. Kevin Bucy was the member who posted the photo of his saddle mount for the Newall arm, and his post was the latest one on that thread when I checked a couple of minutes ago.
And thanks for all the suggestions - I was thinking I needed to invest in a rotary table, but wanted to wait until a project/job came up so I'd have a better idea what size table to buy. An 8" table would sure be easier to handle, but 10" looks like it'd have a lot more room for larger work pieces, plus the ones I'm looking at have six T-slots instead of four.
12-23-2005, 03:39 PM #10
I know what pc you're referring to...it's the little make up shim that saddles atop the Bridgeport arced ram thus providing a flat surface to mount a display arm. As I had suggested in the post above, you were interested in doing without the rotary table, so I stated the same swinging type milling can be done for this pc. You will have to make a fixture that has a pivot and to which you mount the work pc instead of swinging the workpiece on it's own pivot point. You would mount the "saddle" at the proper distance from the pivot to achieve the desired radius. Now mount the fixture on table so pivot is in line with say the Y axis of spindle and begin swinging and cutting the saddle by feeding the X axis till proper radius is achieved. The trick is how to mount the work piece to the swinging bar. A pc of stock longer than needed can be bolted/clamped to the fixture and final part then sawed off the "waste" mounting part.
Yes, a rotary table is infinetly easier as the others suggested, as you already know, but you had asked how to do it without.
12-24-2005, 12:16 AM #11
All of the above are good methods, but involve a
little extra work. If you can put the extra time in to making fixtures, great, but sometimes you
need "down and dirty". For this job, I'd take a piece of that clear plastic (I think it's acetate) off of say a stationary box, or even a thicker blister pack, and use a contour gage to lay out the curve on it. Rough it out with a razor knife, fit it in with a little sand paper,
(you can put it grit up over the top of the arm and work the plastic against it to get it right), and use this plastic template to scribe your aluminum block. If you've got skiptooth blade and a true running bandsaw, that's the next step, if not, skip that step and get out the largest ball end mill you have. Cut "to the line" and you'll be close enough with a little filing to finish. The larger the ball end mill is, the fewer passes it will take to produce a reasonably smooth curve, with less finishing work. If you have a die grinder and flexible flap wheels, finishing goes real quick. Put it up there and if it rocks, put the sandpaper on upside down again and sand it in. The top of that ram arm is not a precision surface, so I wouldn't waste a lot of sweat on precision methods unless you enjoy the challenge and want the experience. Just my 2 cents, your mileage may vary.....-JM
12-24-2005, 01:09 AM #12
Pardon me, but unless I missed something, your method requires
a piece of that clear plastic
a contour gage
a razor knife
a little sand paper
a skiptooth blade
a true running bandsaw
a ball end mill
a die grinder
flexible flap wheels
This is simpler than a rotary table?
And permit me to observe that many hobby machinists have no ball end mills at all.
12-24-2005, 10:46 AM #13This is simpler than a rotary table?
And permit me to observe that many hobby machinists have no ball end mills at all.
12-24-2005, 01:48 PM #14
12-24-2005, 03:08 PM #15He also said he didn't have a rotary table.
"I was thinking I needed to invest in a rotary table, but wanted to wait until a project/job came up so I'd have a better idea what size table to buy."
12-24-2005, 03:13 PM #16This should be the link to where the pictures are at.
12-24-2005, 04:11 PM #17
Marv Klotz has a web site where many simple formulas are available for solving shop math questions. His web site is http://www.myvirtualnetwork.com/mklotz/ . There is a program there called "Radius.zip" a Zip program for a DOS program to calculate the angle to tilt a cutter to to approzimate a odd radius, such as a rifle reciever. You can set how much deviation you want from a true radius (such as .001") and what size cutters you have, the program will figure out what angle to set the cutter to to generate the required radius. I have used it to cut rifle bases and it works.
One caution, if it shuts down after running and you can't see the results, you need to review the instructions at the top of Marvs site, he tells how to correct that problem. It involves computer settings.
12-24-2005, 04:58 PM #18
You say you just installed a Newall? Is that a C80? Because if so, one of it's features is what I call "MNC" - Manual Numeric Control.
The DRO will calculate contour positions for you. Check the manual. YOu will need to specify the start and end coordinates, radius, cutter diameter, and increment (.010" is a good compromise between mind-numbing tedium and a reasonably smooth finish).
The DRO will display coordinates, you move to those coordinates, then press the button to get the next set of coordinates.
If I need a very smooth finish then I'll make a jig or use the rotary table (soon, I'll have CNC..) but for milling a utilitarian inside or outside radius the Newall is a great help.
The saddle that is shown in the photo would be *easy* to do with the Newall. Just use a ball end mill and set up the radius in the Z-Y plane. After you move the cutter into position you will traverse the table on X, move the cutter, traverse back taking another cut (that has you alternately climb- and down-milling, you might prefer just down milling), and so forth until you are done. It shouldn't take long at all to make that piece.
- Glenn Minch
12-24-2005, 05:51 PM #19
Steve & Glenn - Many thanks for those last two tidbits of information! I'll check out the site you posted the link to - it sound like just what I need.
The Newall is indeed the C80 - I'll review the manual and look for that function.
In the meantime, while I can still get free shipping from Enco, I'm thinking of buying one of the Phase II 8" rotary table sets (with the footstock & index plates) in their current sale flier. At around 70lbs., it's probably as large a table as would be handy to set up & remove with any frequency.
12-24-2005, 08:11 PM #20
In your situation, I think I'd use a 3 or 4 inch length of 3 or 4 inch square tube, 1/4" wall, cut in half, with a hole drilled through it. Cut the "legs" to such a length that they contact the ram at the same time as the center of the flat part on top, so it doesn't bow, or just put a washer or two under it if they're too long. No contours to worry about, and very easy. You could grind the edges that contact the top of the ram if you want it to fit perfectly; even doing that, it shouldn't be more than ten minutes' work.