Wednesday, November 7, 2012

Squareness Comparator Part 3

I had some time to make a few additional bits for the squareness comparator gage this week. I have been wondering how the flexure membrane will feel with the fine thread adjustment screw. I used blue tempered shim stock for the membrane and the radius plate that sits against the part to be checked. The material is fairly thick for a flexure (.094) so I fretted some about how stiff it might be and how much torque it would require on the adjustment knob.
Normally I like to cut the blue tempered shim stock on the waterjet or wire edm if its a precision profile. In this case I just used the milling machine with a carbide end mill like a band saw. The blue tempered stuff is just hard enough to be a problem with high speed drills. You can do one or two holes per sharpening before the drill gives it up. Carbide works fine but I had a couple of hole diameters I didn't have carbide drills for.
Milling off the corners of the flexure. The block is just to clamp the part against the back jaw of the vise. With the vise stop and the thicker angle plate the vise wouldn't close and clamp the part.
Doing a little layout so I can mill close to the lines to remove most of the material quickly. I also wanted to get rid of the rounded edges on the shim stock so the corners mate up and look decent.
I needed a short extension plate to lay out the large curve of the radius plate. I debated whether or not I machined this plate, or just sanded the radius in it. Its not an important radius to the function of the gage. As the gage is used it is swept to find the closest point where the reading is then read off the indicator.
I left a little meat on the radius for final tuning once the indicator is mounted. The gage is fully adjustable but its always a good idea to leave the door open just in case.
Here is a shot of the flexure membrane and the radius plate installed. I like the blue coloring against the contrasting steel. Unfortunately its is not a very durable finish. The bluing scratches easily and will wear off if rubbed with gritty fingers for very long.

The adjustment knob turned out to be a little bit of an adventure. I did learn something which makes it all worthwhile. I decided early on that I would use Aluminum Bronze for the adjustment screw. It threads like a dream and is strong, hard and slippery. The rounded tip of the adjustment screw bears against the ground top surface of the gage base. I debated adding some type of anti-friction bearing at this point but opted to just use the hard bronze directly on the surface for simplicity. The challenge turned out to be from machinery and tooling limitations.
Looks simple enough right? My first decision set the stage for the next set of problems. I decided that I wanted to make the knob on our Hardinge lathe. If you have ever cut threads on a nice Hardinge then you understand why I made this decision. Well before I could use the excellent threading abilities of the Hardinge I had to knurl the od of the knob. I realized that I may have never had occasion to knurl aluminum bronze before which is a pretty tough material for certain operations. It took a while to find the knurling tool, and after I found it with some super coarse pitch knurls I was disappointed. After poking around for a while and not finding another matching set of knurls to use it was time to knurl or get off the pot. Since I wanted to test the flexure out it was important that some kind of adjustment screw got made.

The first step in a knurling job is to determine the correct diameter to knurl. I measured the pitch on the knurls and calculated a nominal diameter that should be pretty close. After imprinting the pattern several times and adjusting the diameter slighly I had what looked like a winner.
 After way too many passes I figured out something was wrong. What I learned was that the Hardinge lathe has something like a clutch between the cross feed screw and the slide. There is a movable connection that I had no idea was there. It only showed up with the increased pressure necessary for knurling. During knurling the tool pressure is basically perpendicular to the part axis similar to what you see during facing but much higher forces. I must have made eight or nine passes before I realized the slide was slipping on this connection. It has a bolt so it can be adjusted so I tightened that and tried again.
 I didn't get a great picture of the clutch but you can see the top of it in this shot where I labeled it. (Added additional picture 11-08) There is a bolt that connects the slide to the lead screw nut through this plate. The plate can move in the slot when the forces on the slide get too high. Bottom line is knurling was a no go on this Hardinge. Luckily the Monarch 10EE was open so I moved the setup over and tried again.
For a machine built in 1943 it is still rock solid and going strong. I didn't mess around this time and put a live center in it right out of the gate. The Monarch didn't have any trouble at all. I think the knurls may be dull also which would certainly add to my problem. After all this I went ahead and ordered some new crowned knurls in a couple of different pitches. The pitch of the knurls we had on hand was way too coarse which makes the knob look crappy. I will definitely be making a new one when the new knurls show up. On the plus side I was able to test the resoultion and action of the gage even with my crummy knob.

I did go back to the Hardinge to thread the knob. It really does have the cats meow of threading features. The little bushing sitting on the compound rest is my thread fit gage. For one offs I like to use a gage that has been tapped with the same tap that the hole that the screw will be used. If you use this trick its best to use the same material and the same length of engagement as the finished part has for best results.
The ugly knob. The knurl is much too coarse for the diameter and thickness of the head. I'm starting to think fluted might look better for this application anyway.

 A couple of shots of the offending knurling tool and its wood chipper pitch knurls.
At least I was able to test the flexure action and resolution. That's an Alina tenths indicator against the vertical rod. The 40 TPI works fine and allows setting to .0001 easily. If I do it again I might increase the pitch to 60 or 80 TPI because its a long way from the flexure pivot point. I suppose the other thing I could do is swap ends with the flexure and put the knob in the front. This would have the same effect as a finer pitch screw. And I wouldn't have to buy another special tap.
 Screen shot for whats coming up. The "feet" for the gage are carbide balls pressed into holes to retain them. After press fitting they are ground flat and parallel to the gage body. They provide a hard wear resistant surface to slide around the surface plate.

Stay tuned. Its not done yet.


  1. Hey Tom
    Have you ever tried to make a fine knurl pattern with the coarse knurls? You can do it if you choose the workpiece diameter carefully.


  2. Hi Pete,

    I've done it by accident. Any pointers on how to control the process which is basically a double pattern.



  3. I've done it accidentally myself. I think the key is to carefully pick the diameter of the workpiece so that the knurl pattern is offset by 1/2 'tooth' in one revolution. To control it you would need to know the pitch of the knurl pretty accurately. One trick I know of to do that is to lightly coat the knurl with dykem or similar, and roll it acrross a clean sheet of white paper. You can then measure the pitch by measuring the distance for, say, 10 marks. Then set the diameter (and therefore the circumference) of the workpiece to be an integer number of knurl marks plus 1/2. It's mostly been a curiosity for me, since I have a set of medium and fine knurls, but I kept the idea in the old storage vault in case I'm stuck with coarse knurl and need a medium pitch. knurl.

    Great blog by the way.


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