Making Chips‎ > ‎Knurling‎ > ‎

Techniques

My hope with this page is to give the novice like me a good starting point to minimize or eliminate the many variables in creating knurled parts.  Hopefully, if you read my dissertations above, you know it was quite a Journey...just to solve the Illusive Integer Values.  The other part of this journey was/is in learning, developing and practicing techniques that work consistently and keeping the Mini dialed in for the operation.

There are a ton of variables to the practical side of knurling.  I have mentioned quite a few along the way, but want to detail them a bit more in sections.  Please keep in mind these techniques really only apply to the Mini and my LMS knurling tools, but may have a broader scope.

The Tools

It is important to consider all of the tools, because like in "Tolerance Stacking"...if one, two or three things are off a bit, it's a whole lot worse at the other end, mainly at the knurl wheel and material.  Here are some basics I have found for the Tools.

The Knurling Tool
First and foremost is the knurling tool itself.  Its rigidity and roller tracking need to be tight and smooth.  These scissor types have a lot of joints, pins, clips, and threaded parts which require lubrication and periodic inspection. They also allow for a lot of tolerance buildup. 
  1. The lateral play at the scissor joints should be minimal and the set screw for the tool post shank should be tight and square to the body. 
  2. For lubrication I prefer light machine oil particularly on the wheels & pins but like a dash of White Lithium for the threaded knob and screw, and the 3/4 round anchors on the scissor arms (for pressure).  Make sure the wheels spin freely with no rough spots or excessive play.
  3. Keep the wheels clean and free of debris.  Knurling aluminum and brass will create flakes of material that get caught down in the teeth and along the edges.  It will also "Colorize" the wheel, leaving a silver or gold patina.  I use a file card to clean the teeth and remove flakes, then mild solvent to remove the rest.  Also inspect the teeth for wear, chips and deformations.
  4. You can check the roller tracking by lightly squeezing the scissors together (or tighten the knob) until the wheels touch and teeth are engaged, then spin them with your finger.  They should be directly (square) over one another and not move too much side to side while turning.  When changing wheels always inspect the pins and wheels for scoring and always put them back in the same position and direction they came out. 
  5. I also used Chris' Tip to change the "one time" snap rings to US snap rings, then I changed again to 1/4" E-Clips and have had no problems.  They are much easier to R&R than fiddling with baby snap-ring pliers.
  6. Another mod I did was to add thin washers above and below the spring to keep it from putting angular pressure to the scissor arms and cocking into the slots.

The Mini and Tool Post 

With the Mini it's about: runout on the chuck, compound rest squareness, and the gib's.

  1. The chuck runout can be managed by using a ground rod and test indicator, adjusting if necessary. (See the 4 Inch Chuck page for more detail)  The less the better.  Remember we are Forming so excessive runout will create highs and lows around the circumference, even though the tool self centers...mostly.
  2. The compound rest squareness is important.  I use a Wixey digital protractor, or a small machinist square will work too.  The Stamped Protractor on the Mini is pretty worthless so with a square or protractor you can set your compound rest 90° to the the cross slide. 
  3. As for the gibs, making sure there is minimal play in them will get you a long way toward a good knurl.  If it rocks or shifts slightly during knurling, the teeth will change the attack angle and create high and low points. I use the LMS technique for adjusting gibs found in the Mini Lathe User's Guide.

The Tool Post, "Quick Change" or "4 Way", is the last line of defense to getting good knurls, in my opinion. 

  1. It must be tight and square to the Compound rest and the work.  You can do this by loosening the cinch bolt and placing a square to it and the material or the compound and assuring the 90° between them.  It's important to get everything square to the work.
  2. Put the tool in the holder and snug up the set screws.  Then square it to the Post using the flat of the scissor arm and then tighten the set screws.  Also make sure it is perpendicular (vertically) to the post.  Sometimes baby chips get caught back inside the holder and will  tilt the tool slightly.
  3. Make sure that the holder and tool are on horizontal centerline of the work as well.  This is a little tricky with the scissor type because it will technically float to centerline.  However, I found that mine sticks a bit on the high side (not sure why, spring maybe).  What I do is tighten down the threaded knob until the wheels and teeth engage, and then open it slightly until it's possible to get a small piece of shim stock (.010-.015) between them and snug it down again.  This leaves a little blade sticking out so I can then use a feeler gauge (.015-.020) vertically between the stock and the blade.   If the feeler gauge is vertical when you run the cross slide in until it touches, it's darn close.  If not, adjust or shim your holder to get the feeler gauge vertical.  Old school but works pretty well.  The tool has slots for the screw and knob pass-thru and will rock slightly even tightened down.  Basically you are looking for the centerline of the shim stock to be in line with the top of the shank in the holder from a side view.  If the two wheels aren't aligned horizontally to centerline one of the wheels will have more pressure applied during the process making the knurl uneven or "Heavy Handed", even with the self centering feature.

Procedures (Making Chips)

I use the following procedures to consistently minimize the variables further.  So far it works well for me on aluminum, brass and mild steel for all three pitches.  It is only for single width knurls (3/8")...basically thumb screws and such.  I will cover longer runs and Delrin later. 

  1. Load the material into the chuck, minimizing how much is forward of the jaws for the knurl area.  If necessary, use the tail stock and a live center to support it. 
  2. Use the Calculator and turn the material to the recommended diameter you want your knurl and pitch to be.  For best results try to get within .001-.002 of the calculated diameter.  I do get reasonable results as long as it's not over about .oo3-.004 in either direction.
  3. Load the scissor holder with the appropriate knurl wheels and apply light machine oil to the bearing surfaces. Then go through the squaring processes above.
  4. Once all the tools are checked, aligned, and set up; open the jaws of the tool until the wheels will clear the stock, then move the cross slide, compound rest, and tool over the stock to the area to be knurled.  Try to keep the compound rest back over the edge of the cross slide to give it as much rigidity as possible and shorten the "lever arm".
  5. Move the cross slide to get the centerline of the wheels as close to the 6 & 12 O'clock position as possible.  Its better to be slightly toward the front than behind the stock.  Tighten down the pinch wheel until the wheels just touch the surface of the material.  Make sure both wheels just touch the surface.  Then turn the knob (1/4-1/2 turn) to take a good bite into the material and rotate the chuck by hand about two full revolutions.  You will see a decent first impression of the knurl.  Carefully inspect for double tracking and even distribution across the face of the tool.  If it's double tracking you likely have, over or under shot your target diameter.  If it has uneven distribution, recheck/adjust your squareness...tighten it a bit more and rotate another rev or two until it looks good.
  6. Once you have a good first impression:  Tighten the knob another 1/4-1/2 turn, make sure the Mini speed is its lowest (150-200 rpm), then turn on the Mini for about for about 10-15 seconds (about 50 revolutions).  Stop the Mini and inspect the knurl, checking the peaks and valley's and tracking.  You can turn the chuck by hand to inspect. They will likely be about half formed.  Tighten the knob another 1/4-1/2 turn and turn the mini back on for another 10-15 seconds, stop and check again, repeating until your knurl looks like a "Good Knurl".  If you over tighten, it will look mushroomed on the ends and see evidence the knurl is shifting side to side from the pressure with partial lines at the edges.  The difficulty here is not knowing our tooth depth and the relationship of the threaded knob to tooth depth...so its all about going slow until you get there...patience will pay off.
  7. Once satisfied with the knurl open the jaws of the tool and move it away from the work.  I use a small brass wire brush to clean up any flakes and any debris.  You can turn on the Mini and brush away from the material.

General Tips

  1. As for lubrication/cutting fluid, I generally do not use it on brass or aluminum.  I did initially use WD40 but found the flakes actually stick worse to the tool and material.  The Mini is moving very slowly and not very many revolutions so most flakes and debris are easily removed with a short blast of air, during the operation.  (Always were safety glasses as those flakes will fly everywhere!) On steel, depending on the hardness I will use WD40 for softer steel, and cutting fluid on more dense materials.  I have successfully used no fluid on small, mild cold rolled pieces (1018 or 12L14)...it just feels better to use fluid and probably easier on the wheels.
  2. A nice feature built into the spreadsheet is a "Material to be removed" row.  It basically tells how much to remove from the specified diameter.  Most stock materials need a clean up pass to true it up.  After the clean up, mic the material but do not move the cross slide dial until entering that number into the spread sheet, then go back and turn the dial accordingly using the "Material to be removed" value to take the final size cut.
  3. Sequence of Operations is pretty important with knurling because of needing to be close to the chuck.  I generally do my knurling last but not always.  Thumb screws for instance: I take an initial clean up pass and then turn down to thread depth, thread it, add a relief, any fancy turns, then move to knurling the larger diameter, then part it off.  It is difficult to chuck up knurl without deforming your nice work so consider your part carefully and when to do your knurling operation.
  4. When using the tail stock and live center, try to plan your part layout so the knurl end is close to the chuck.  For instance, the Carb Tool I made could have been done with a live center after I had counter bored the end, leaving the Knurled part closest to the chuck.  It also helps with chatter when parting off at the end.