R8 Insert Deflection

[Grandpop]

I am at least the 2nd owner on an Enco 8 x 28 knee mill. I came with 3 R8 collets that all went in and out easily.

One of the first things I bought was a set of R8 end mill holders. Imagine my surprise when none of them fit up inside the spindle! After a lot of investigation it appeared that the holders did meet the Bridgeport specs for the end diameter, and the spindle bore was .0005 too small. The PO had ground the end of his collets down to fit!

After a lot of head scratching, my father suggested taking a length of wood dowel, slotting it to hold a piece of Emory paper, putting the emory in the slot and wrapping a turn or two around the diameter. I turned the end of the dowel down to fit a cordless drill to drive it. I didn't have a great expectation for success, but with nothing to loose, I wrapped a piece of 220 grit around it a gave it a try.

I was amazed that after a few minutes of up and down in the bore my smallest end mill holder was starting to fit. I ran the drill a little faster, swapped in anew new emory, and a few minutes later they all fit just right.

I have no way to get up in there to measure the bore accurately, but the largest end mill holder is a nice slip fit. To be clear, I would not be concerned if the bore ended up .001 over. Nothing I will ever make ever make on this hobby machine would be affected.

 

[mikey]

I have a RF-31 and use Crawford collets, a Tormach collet and ETM tooling in it and all fit without any issues. If something doesn't fit I would suspect the thing I'm trying to insert before modifying my spindle. But that's just me.

 

[wrmiller]

I have a RF-31 and use Crawford collets, a Tormach collet and ETM tooling in it and all fit without any issues. If something doesn't fit I would suspect the thing I'm trying to insert before modifying my spindle. But that's just me.

Yea, that is kinda what I was thinking.

 

[RJSakowski]

For reference, the quote in post #20 is dated 2006. I had run across the same thread. I would be interested in seeing that R8 spec in the 1952 edition of the National Machine Tool Builders manual.

I am curious as to who the Yank that was responsible for the Twonk spec was. A little later in the same thread was a suggestion that the Asian collet manufacturers might be grinding the R8 taper to 3-1/2"/ft. which would be a half angle of 8.297º rather than the accepted taper of 16º51'/2 or 8.425º.
One issue that I have with my Tormach mill is that the collets make contact at the large end of the taper which would mean the socket taper angle is too small. I had thought that this was a manufacturing error but now it seems that it may be a design error resulting from the Twonk spec. Hmmm.

 

[Doubleeboy]

Interstate stuff can be hit or miss . Lyndex is good and not too expensive.

 

[wrmiller]

Interstate stuff can be hit or miss . Lyndex is good and not too expensive.

I bought the ultra precision set from PM at the same time I bought my mill. Collets are Taiwan made and have near unmeasurable runout. Well at least none I could detect with my equipment anyway.

I did have to back out the keyway pin a bit though.

 

[Bill Kirkley]

Mikey, I am trying to determine if what I did had any affect on the precision of the spindle. The results do make me feel better. It indicates enlarging the upper end of the spindle has no affect on the stability of a tool in an R8 collet.

I used a scale to test the force on a tool. I determined this by the resistance I felt turning the handle, mimicking the resistance I feel running the machine. The range was about 115 to 196 pounds force. Cranking the handle much harder than I ever would the force was about 246 pounds.

I weighted the weight. It is actually 43.8 pounds. In my test, at a distance of 21.5 inches, there was a torque on the collet of 941.7 inch pounds. If you feed the tip of an end mill 3 inches long measuring from the end of a collet, this torque would place a force on the mill of 313.9 pounds (941.7/3). This is greater than the extreme force I measured (246 pounds).

So, I am relieved that boring out the spindle will have no adverse affect on its function.

Below are photos of my force measurement and my setup for boring the spindle.

 

[Bill Kirkley]

If they are reproducible then you are should be OK as too the vertical collet alignment in the static mode.

I believe it is the R8 bevel that determines the alignment of the insert. For all practical purposes it is a wedge. If the two components mate properly, there can be no tilting. That is why from the beginning I didn't think the upper end of the spindle had any impact on performance.

 

[mikey]

Mikey, I am trying to determine if what I did had any affect on the precision of the spindle. The results do make me feel better. It indicates enlarging the upper end of the spindle has no affect on the stability of a tool in an R8 collet.

So, I am relieved that boring out the spindle will have no adverse affect on its function.

Bill, I understand the intent of your tests and I'm glad your tests give you confidence. In the end, it is your machine and as long as it performs to your satisfaction then that's what counts.

 

[jknc]

I am at least the 2nd owner on an Enco 8 x 28 knee mill. I came with 3 R8 collets that all went in and out easily.

One of the first things I bought was a set of R8 end mill holders. Imagine my surprise when none of them fit up inside the spindle! After a lot of investigation it appeared that the holders did meet the Bridgeport specs for the end diameter, and the spindle bore was .0005 too small. The PO had ground the end of his collets down to fit!

After a lot of head scratching, my father suggested taking a length of wood dowel, slotting it to hold a piece of Emory paper, putting the emory in the slot and wrapping a turn or two around the diameter. I turned the end of the dowel down to fit a cordless drill to drive it. I didn't have a great expectation for success, but with nothing to loose, I wrapped a piece of 220 grit around it a gave it a try.

I was amazed that after a few minutes of up and down in the bore my smallest end mill holder was starting to fit. I ran the drill a little faster, swapped in anew new emory, and a few minutes later they all fit just right.

I have no way to get up in there to measure the bore accurately, but the largest end mill holder is a nice slip fit. To be clear, I would not be concerned if the bore ended up .001 over. Nothing I will ever make ever make on this hobby machine would be affected.

I know this is an old thread at this point - but this method worked amazingly well for me on my PM727. After bluing I felt like my spindle was concentric. I checked the collets for size and they were 0.949 - well within spec. Oddly over the past year of use the spindle was getting tighter. I used a dowel and some emory cloth and it cleaned it up perfectly. The collets slip right in now (it had gotten where I needed to use a soft blow hammer to get them in).

Instead of using a drill I just ran the mill about 200rpm and moved the dowel by hand up and down in the spindle bore.