12-29-2017, 08:23 PM
It was a very pretty day here on the ocean so I opened the garage door to let the shop warm up and then put in a couple of hours on the project.
The ¾ inch thick baseplate has been sawed to the dimensions in the CAD layout. I made a rough layout of the hole pattern on the protective film covering the jig plate. Although I use the dials on the mill to establish hole locations, sometimes I lose track of the number of turns and the layout prevents making an unrecoverable error.
After drilling, each fastener hole was counterbored, exchanging the twist drill for the counterbore and then re-installing the drill at every location. I was reminded that changing tools in a standard Jacobs chuck was a huge PITA, years ago. Thank goodness for the advent of Albrecht/Rohm style chucks !
Just for appearance, I wanted the end of the base plate to be round and when I envisioned it, the process seemed simple enough. As I thought more about it, I realized that it was going to require a little more work. My plan was to slowly rotate by hand, the base plate against an end mill, the desired pivot being held in the milling vise. (This would be similar to rotating the compound on a lathe to turn concave and convex shapes.)
To create a pivoting point, the baseplate was first drilled and bored. Next, a steel bushing needed to be turned/bored for a tight fit with the pivot bolt (which was just a long 1/2-20 bolt with a nice smooth shoulder) and a press fit with the plate bore.
The reason for the bushing is to prevent galling and seizure that would likely occur if the aluminum base plate was pivoted directly against the steel pivot bolt. I reasoned that a VERY tight fit between the pivoting surfaces was required to prevent chatter. Incidentally, at completion the pivot hole will not be visible, the tool holder will cover it, no matter the position of the tool holder on the base plate.
The base plate could be removed now and set aside temporarily.
I used a few pieces of scrap to make a multi-layered assembly, a “sandwich”, for clamping the baseplate while allowing it to rotate around the pivot. The “sandwich consists, first, of a rectangular piece of steel for gripping in the milling vise, to which the pivot bolt is welded.
A scrap of 1/8 thick rubber was punched to produce a ½ inch hole:
The is the "sandwich" excluding the aluminum base plate. It consists of the pivot plate, a rubber friction pad, a clamping plate and of course a nut.
The bushing (the function of which was described previously) was turned for a heavy press fit in the aluminum base plate and bored for a very tight fit with the pivot (the pivot is the body of the bolt welded to the steel scrap, not the threads). Note that a shop-made adapter was used in the boring bar to accommodate small cutting tools, in this case a 1/4 inch HSS tool ground for boring.
OK, if you’ve suspected that I was winging it, you are exactly right ! All of this was just intuition since I’d never tried milling something by hand-feeding previously.
Pressing the bushing was too heavy for my small one ton arbor press so I simply pulled the bushing into the plate using the pivot bolt and a nut rotated with a long breaker bar. All done, note that the pivot bolt shoulder was such a tight fit with the bushing that it had to be tapped into place with a rubber mallet which was exactly the type of fit that I desired.
The baseplate, rubber pad and clamping plate were installed over the pivot plate in that sequence and clamped in the milling vise. The rubber pad between the clamping plate and the base plate protects the finish of the plate but more importantly provides friction as the plate is rotated against the end mill.
Friction is desirable so that rotation of the plate will be slowed, preventing too fast a feed. Friction can be adjusted during the cutting process by tightening/loosening the nut that applies clamping pressure to the “sandwiched” parts, functioning exactly like a brake. Here is the assembly minus the nut. The mill spindle is being centered over the pivot point.
After centering, the “Y” axis is locked, it is now on the pivot bolt center line and the table shouldn’t be moved in this axis again.
The “X” axis is craked over so that the end of the baseplate is near the cutter and the rounding process begins. I initially considered supporting the plate with machinists jacks but didn’t. My reasoning was that the helix of the end mill would tend to lift the plate, not push it downward.
I decided to make a pass or two and see if there was excessive chatter. If so, I would need to make something to support the plate from above. I hope that this makes sense. It turned out that no support was needed; when the plate started “shuddering” I simply tightened the pressure on the rubber friction pad until the chatter ceased.
As can be seen, I added a large wood clamp to the set-up. After trying a pass or two without it, the lack of control when rotating the plate allowed only light cuts to be taken and I was impatient. Addition of the clamp not only added leverage but spread my hands apart placing them in a position that allowed much more control. (Also seen in the photo is a socket wrench installed over the nut that holds the assembly together. At several points during the milling, I had to tighten the nut.)
From here on out, I took 1/8 deep passes at an estimated 10 ipm and the chips were flying out like a water hose ! NOTE: I would NEVER try this with ferrous material unless taking the very lightest of cuts. In fact I don’t recommend this process at all for inexperienced folks !
There wasn’t much of a problem making a conventional cut on the part but a climb cut of more than a few mils would NOT be a good idea, even with the “brake” that I devised. I note yet again that this is a risky operation and would better be performed in a rotary table. I read about it one time and just wanted to try it as a technique that might be useful in the future. If it isn’t obvious, one wants a lot of mechanical advantage to pull this off and very light cuts are advisable.
The distance from the pivot point to the cutter needs to be quite a bit less than the distance from the pivot to where the rotation is being applied, for leverage and control. As mentioned previously, the pivot interface was very tight, with as little clearance as I estimated could be achieved while still being able to rotate the part. If the pivot was sloppy, it would have shown up as poor surface finish on the milled edge.
Here is the plate at present, protective film still sort of in place. Needs a bit of sanding to clean up the milled surface after which I think that I’ll break all of the edges on the router table. Although the surface finish is not the best, I consider it pretty good for the technique used, LOL.
The ¾ inch thick baseplate has been sawed to the dimensions in the CAD layout. I made a rough layout of the hole pattern on the protective film covering the jig plate. Although I use the dials on the mill to establish hole locations, sometimes I lose track of the number of turns and the layout prevents making an unrecoverable error.
After drilling, each fastener hole was counterbored, exchanging the twist drill for the counterbore and then re-installing the drill at every location. I was reminded that changing tools in a standard Jacobs chuck was a huge PITA, years ago. Thank goodness for the advent of Albrecht/Rohm style chucks !
Just for appearance, I wanted the end of the base plate to be round and when I envisioned it, the process seemed simple enough. As I thought more about it, I realized that it was going to require a little more work. My plan was to slowly rotate by hand, the base plate against an end mill, the desired pivot being held in the milling vise. (This would be similar to rotating the compound on a lathe to turn concave and convex shapes.)
To create a pivoting point, the baseplate was first drilled and bored. Next, a steel bushing needed to be turned/bored for a tight fit with the pivot bolt (which was just a long 1/2-20 bolt with a nice smooth shoulder) and a press fit with the plate bore.
The reason for the bushing is to prevent galling and seizure that would likely occur if the aluminum base plate was pivoted directly against the steel pivot bolt. I reasoned that a VERY tight fit between the pivoting surfaces was required to prevent chatter. Incidentally, at completion the pivot hole will not be visible, the tool holder will cover it, no matter the position of the tool holder on the base plate.
The base plate could be removed now and set aside temporarily.
I used a few pieces of scrap to make a multi-layered assembly, a “sandwich”, for clamping the baseplate while allowing it to rotate around the pivot. The “sandwich consists, first, of a rectangular piece of steel for gripping in the milling vise, to which the pivot bolt is welded.
A scrap of 1/8 thick rubber was punched to produce a ½ inch hole:
The is the "sandwich" excluding the aluminum base plate. It consists of the pivot plate, a rubber friction pad, a clamping plate and of course a nut.
The bushing (the function of which was described previously) was turned for a heavy press fit in the aluminum base plate and bored for a very tight fit with the pivot (the pivot is the body of the bolt welded to the steel scrap, not the threads). Note that a shop-made adapter was used in the boring bar to accommodate small cutting tools, in this case a 1/4 inch HSS tool ground for boring.
OK, if you’ve suspected that I was winging it, you are exactly right ! All of this was just intuition since I’d never tried milling something by hand-feeding previously.
Pressing the bushing was too heavy for my small one ton arbor press so I simply pulled the bushing into the plate using the pivot bolt and a nut rotated with a long breaker bar. All done, note that the pivot bolt shoulder was such a tight fit with the bushing that it had to be tapped into place with a rubber mallet which was exactly the type of fit that I desired.
The baseplate, rubber pad and clamping plate were installed over the pivot plate in that sequence and clamped in the milling vise. The rubber pad between the clamping plate and the base plate protects the finish of the plate but more importantly provides friction as the plate is rotated against the end mill.
Friction is desirable so that rotation of the plate will be slowed, preventing too fast a feed. Friction can be adjusted during the cutting process by tightening/loosening the nut that applies clamping pressure to the “sandwiched” parts, functioning exactly like a brake. Here is the assembly minus the nut. The mill spindle is being centered over the pivot point.
After centering, the “Y” axis is locked, it is now on the pivot bolt center line and the table shouldn’t be moved in this axis again.
The “X” axis is craked over so that the end of the baseplate is near the cutter and the rounding process begins. I initially considered supporting the plate with machinists jacks but didn’t. My reasoning was that the helix of the end mill would tend to lift the plate, not push it downward.
I decided to make a pass or two and see if there was excessive chatter. If so, I would need to make something to support the plate from above. I hope that this makes sense. It turned out that no support was needed; when the plate started “shuddering” I simply tightened the pressure on the rubber friction pad until the chatter ceased.
As can be seen, I added a large wood clamp to the set-up. After trying a pass or two without it, the lack of control when rotating the plate allowed only light cuts to be taken and I was impatient. Addition of the clamp not only added leverage but spread my hands apart placing them in a position that allowed much more control. (Also seen in the photo is a socket wrench installed over the nut that holds the assembly together. At several points during the milling, I had to tighten the nut.)
From here on out, I took 1/8 deep passes at an estimated 10 ipm and the chips were flying out like a water hose ! NOTE: I would NEVER try this with ferrous material unless taking the very lightest of cuts. In fact I don’t recommend this process at all for inexperienced folks !
There wasn’t much of a problem making a conventional cut on the part but a climb cut of more than a few mils would NOT be a good idea, even with the “brake” that I devised. I note yet again that this is a risky operation and would better be performed in a rotary table. I read about it one time and just wanted to try it as a technique that might be useful in the future. If it isn’t obvious, one wants a lot of mechanical advantage to pull this off and very light cuts are advisable.
The distance from the pivot point to the cutter needs to be quite a bit less than the distance from the pivot to where the rotation is being applied, for leverage and control. As mentioned previously, the pivot interface was very tight, with as little clearance as I estimated could be achieved while still being able to rotate the part. If the pivot was sloppy, it would have shown up as poor surface finish on the milled edge.
Here is the plate at present, protective film still sort of in place. Needs a bit of sanding to clean up the milled surface after which I think that I’ll break all of the edges on the router table. Although the surface finish is not the best, I consider it pretty good for the technique used, LOL.