Pen Mills Damaging Brass Inserts

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Hey All

For some reason my 7mm Pen Mill is damaging my brass inserts. (Gouging the full length of the insert badly.)

I’ve checked it in every possible way and it is spinning perfectly true. It is also at a perfect 90 degree angle.

I am running it at 3500 rpm but I can’t see why it would damage the insert.

Should I lubricate the insert?

Any suggestions?


“Pen Turning on the Rock”
 
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magpens

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When you say "brass insert" do you mean the brass tube ?

I really don't believe that a pen mill is meant to be turned at such a speed, but I am not saying that is the problem - I just don't see the need for such speed.

The pen mill is usually used for two things: (1) cleaning out the brass tube from possibly glue remnants, and (2) trimming the blank material down to and square with the end of the brass tube. . Usually, the shaft of the mill is just slightly under the inside diameter of the brass tube.

What are you using the pen mill for ?
 
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Ed McDonnell

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Have you checked the id of the brass tubes and the diameter of the mill? Using an 8mm shaft by mistake will do a pretty good job on chewing up the brass tube.

Given the rpm and your certitude of 90 degrees, I would imagine you are not using a hand drill.

Using it in a drill chuck on the lathe? Have you checked alignment of headstock and tailstock. If they are off, that could force the mill into the brass tube as you run it in.

Using it in a drill press? If so, then how are you holding the work? Possible alignment issues (not angular, rather hole offset from mill and held rigid so the mill bungs up the inside as you force it in)?

Pictures of your setup and the resultant gouging would help with diagnosis.

The best solution will ultimately prove to be abandoning the pen mill in favor of sanding the ends of your blanks. In the meantime, try using the mill in a hand drill and see if you still get the gouging when you insert it into the blank. You don't even need to test it on a good blank. Just see how the mill fits in a tube using a hand drill. You don't need 3500 rpm for a pen mill. Hand drill speeds will be fine.

Ed
 
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Joined
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Messages
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Have you checked the id of the brass tubes and the diameter of the mill? Using an 8mm shaft by mistake will do a pretty good job on chewing up the brass tube.

Given the rpm and your certitude of 90 degrees, I would imagine you are not using a hand drill.

Using it in a drill chuck on the lathe? Have you checked alignment of headstock and tailstock. If they are off, that could force the mill into the brass tube as you run it in.

Using it in a drill press? If so, then how are you holding the work? Possible alignment issues (not angular, rather hole offset from mill and held rigid so the mill bungs up the inside as you force it in)?

Pictures of your setup and the resultant gouging would help with diagnosis.

The best solution will ultimately prove to be abandoning the pen mill in favor of sanding the ends of your blanks. In the meantime, try using the mill in a hand drill and see if you still get the gouging when you insert it into the blank. You don't even need to test it on a good blank. Just see how the mill fits in a tube using a hand drill. You don't need 3500 rpm for a pen mill. Hand drill speeds will be fine.

Ed


I just ran a calliper on it and it looks like it is slightly larger than 7mm. (7.32mm to be exact.) I think this is the source of the problem so I will order a new one. May be a manufacturer defect.


“Pen Turning on the Rock”
 

monophoto

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What kind of 'damage' are you seeing?

I suggest looking very closely to determine exactly how the pen mill is affecting the tube. Is the 'damage' uniform around the entire circumference of the tube, or is it confined to one side? Is the 'damage' in the form of scoring marks inside the tube? Or is the 'damage' on the end of the tube?

A pen mill is intended to remove surplus material from the end of the blank, leaving an area around the end of the tube that is perpendicular to the tube. In doing that, the mill may also remove a small amount of brass from the end of the tube. You should stop using the mill as soon as you see bright metal at the end of the tube indicating that the mill has reached metal. Removing a small amount of brass is OK, but you don't want to remove so much that the actually shorten the tube. And note that this means that you will see some brass shavings in the material removed by the mill - that is also normal.

However, a mill has a second purpose - it is very easy to get some glue inside the tube, and the mill is also intended to scour out the inside of the tube. The diameter of the mill shank should match the inside diameter of the tube very closely, so if you have some dried glue inside the tube, the mill will remove it, but without significantly removing any brass from the inside of the tube. However, if you find that the 'damage' inside the tube is not uniform around the circumference of the tube, but rather is concentrated on one side, that suggests that the end-mill cutting edge of the mill isn't exactly perpendicular with the shank.

Without seeing the 'damage', there are four things that I can think of that could be happening:
1. What you think is 'damage' is actually the edge on the mill shaft peeling away dried glue from the inside of the tube - ie, a total false alarm. In this case, the scoring inside the tube will be around the entire circumference.
2. What you think is 'damage' is the mill machining a bit off the end of the tube. Again, this is normal (as long as you don't get carried away and make the tube too short).
3. I have the PSI mill, with both 7mm and 10mm shanks. The hole in the mill is slightly larger than the shank, so when it is locked in place with the grub screw, the axis of the end mill is very slightly displaced from the axis of the shaft - parallel but displaced. But I could imagine that a small amount of crud inside the hole in the mill could cause the end mill to be tilted very slightly, resulting in the mill and shaft not being exactly perpendicular. This could result in scoring inside the brass tube that appears on one side, but not around the entire circumference.
4. The third possibility is a stretch, but I will mention it anyway. At one point, I bought a length of brass tube at a hardware store that was nominally the same diameter as the tube furnished with pen kits, but I found that the wall thickness was actually very slightly greater than the thickness of the walls of the tubes in pen kits. As a result, while the OD was the same, the ID of the tube from the hardware store was very slightly less than the ID of the tube in the pen kit. Pen kits are generally made in Asia, while the tube I bought at the hardware store was made in the US. That showed up as a mismatch between the tube and my pen mill - the mill actually wanted to remove a bit more brass from inside the ends of the tube.
 
Joined
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Messages
142
Location
Newfoundland
What kind of 'damage' are you seeing?

I suggest looking very closely to determine exactly how the pen mill is affecting the tube. Is the 'damage' uniform around the entire circumference of the tube, or is it confined to one side? Is the 'damage' in the form of scoring marks inside the tube? Or is the 'damage' on the end of the tube?

A pen mill is intended to remove surplus material from the end of the blank, leaving an area around the end of the tube that is perpendicular to the tube. In doing that, the mill may also remove a small amount of brass from the end of the tube. You should stop using the mill as soon as you see bright metal at the end of the tube indicating that the mill has reached metal. Removing a small amount of brass is OK, but you don't want to remove so much that the actually shorten the tube. And note that this means that you will see some brass shavings in the material removed by the mill - that is also normal.

However, a mill has a second purpose - it is very easy to get some glue inside the tube, and the mill is also intended to scour out the inside of the tube. The diameter of the mill shank should match the inside diameter of the tube very closely, so if you have some dried glue inside the tube, the mill will remove it, but without significantly removing any brass from the inside of the tube. However, if you find that the 'damage' inside the tube is not uniform around the circumference of the tube, but rather is concentrated on one side, that suggests that the end-mill cutting edge of the mill isn't exactly perpendicular with the shank.

Without seeing the 'damage', there are four things that I can think of that could be happening:
1. What you think is 'damage' is actually the edge on the mill shaft peeling away dried glue from the inside of the tube - ie, a total false alarm. In this case, the scoring inside the tube will be around the entire circumference.
2. What you think is 'damage' is the mill machining a bit off the end of the tube. Again, this is normal (as long as you don't get carried away and make the tube too short).
3. I have the PSI mill, with both 7mm and 10mm shanks. The hole in the mill is slightly larger than the shank, so when it is locked in place with the grub screw, the axis of the end mill is very slightly displaced from the axis of the shaft - parallel but displaced. But I could imagine that a small amount of crud inside the hole in the mill could cause the end mill to be tilted very slightly, resulting in the mill and shaft not being exactly perpendicular. This could result in scoring inside the brass tube that appears on one side, but not around the entire circumference.
4. The third possibility is a stretch, but I will mention it anyway. At one point, I bought a length of brass tube at a hardware store that was nominally the same diameter as the tube furnished with pen kits, but I found that the wall thickness was actually very slightly greater than the thickness of the walls of the tubes in pen kits. As a result, while the OD was the same, the ID of the tube from the hardware store was very slightly less than the ID of the tube in the pen kit. Pen kits are generally made in Asia, while the tube I bought at the hardware store was made in the US. That showed up as a mismatch between the tube and my pen mill - the mill actually wanted to remove a bit more brass from inside the ends of the tube.


Now that I’ve examined it very closely, it seems like one of the cutting edges on the 7mm mill is ever so slightly skewed, resulting in an oversized cutting diameter. (7.3+mm due to a skewed cutting edge.) I think it is just a manufacturing defect.

Since they are cheap, I’m just going to replace it as it isn’t worth the hassle to try and fix.


“Pen Turning on the Rock”
 

bsshog40

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It sounds like you are using an 8mm pen mill. I just put calipers on mine yesterday because I plan on doing a couple pens that use 8mm tubes. I have a small assortment and none were marked. My 7mm that I'm using now, measured a little over 6.26? mm. My 8mm was a little over 7mm on calipers. Plus I mill on my drill press and have had no problems drilling blanks and pen milling at around 870 rpms. 3500 rpms to me just seems like an accident waiting to happen.
 
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Dehn0045

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Square on the lathe. Chuck and skew, perfect every time. Eliminates a lot of problems of other methods. Might take a little longer, but what's a couple minutes? Unless you're a high volume production type...
 

RSQWhite

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The outside diameter of a 7 mm pen tube is 7 mm the inside diameter is around 1/4 “(6.35mm) most pen mills have a 1/4” hole in the head, the shaft is also 1/4 inch. This set up usually allows part of the tube to get between the shaft and the head of the edge of the pen mill. The mill that I posted earlier won’t let this happen. The mill has a 3/16 hole in it but the big end of the shaft is 1/4”. This creates a step on the shaft leaving no gap for the tube to get into.


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Paul in OKC

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magpens

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I have never liked the idea of squaring blanks with an end mill, especially for wood blanks (danger of splitting end grain if cutter is not super sharp).

And so, I have never used them for that ... I square the blank ends by turning on the lathe. (I always round the blank before doing anything else, and round to a size I can grip securely in the headstock chuck.)

However, I do have a couple of pen mills ... I don't use the cutter for anything, but I sometimes (rarely) use the mill shaft to clean out brass tubes on the rare occasion when some glue gets far down inside. Most of the time, I use an Exacto knife to clean out little bits of glue near the end openings of the brass tubes.
 

mmayo

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Go to Rick Herrell for his sanding devices, if mine was lost or damaged I would buy both of his sanding devices again immediately. They are integral to quality pen making ( IMHO ).
 

jttheclockman

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Pen mills get a bad rap at times and many times it is operator error. First lets say never run a mill at those speeds. Asking for trouble. Second only use a mill with at least 4 cutting prongs. do not even buy the 2 head cutters. Third control the advancement and feel the material being cut away. Finally use the proper size. If possible use a carbide cutter and save the headaches. Yes sanding is an option and I use that too but I have used the mill many times and will continue and to this date have never had an incident.

Know your tools and how to use them. :)
 

Woodchipper

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I use the pen mill in the drill press. I made my own vise with a machinist's vise. I have had one blank split on the pen mill. I turned and drilled on the lathe....once. To me, that took too much time. I keep the mill sharp by hitting it with a CMT diamond file once in a while.
 

bsshog40

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I also use my pen mill in the drill press. My RPMs are about 700-800, in the area. The first thing I do is hold the blank in my hand, slide it up on the pen mill shaft, turn on the press, and move it up and down by hand to remove glue in the tubes. Then I mount in my pen vise, pen mill about 1/8" from my blank, and move the pen mill up and down until I know I have a nice entry. Turn on the press and clean/trim. Now I haven't done a 100 pens but the couple dozen I have done, I've had no issues with.
 
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