I decided to address another problem with my Hendey lathe. This time I turned my attention to my three jaw chuck. My chuck had two issues I wanted to address. The first is that it would not hold a part perfectly centered which is known as runout. Runout is measured by holding a ground bar in the chuck and placing a dial indicator to measure the surface. The spindle is turned through a complete revolution and the maximum and minimum values noted. The difference of these values is the runout. It’s easier to set the dial indicator to zero at the low point and then find the high spot. I measured 0.008″ runout with a 1.25″ diameter ground rod.
The second issue with the chuck was that the jaws were wider at the front of the chuck than the back. I’ve seen this referred to as being “bell mouthed”. This is easy to see because you can see light between the part and the jaws. To illustrate this I’ve slipped a 0.006″ feeler gauge between one of the jaws and the part. It goes in about 0.25″ and thinner gauges went in further.
To fix these problems the jaws are ground on the lathe. As there is slack in the jaws, they must be forced outward to take out this slack. I used a method I came across on Home Metal Shop Club‘s website. The author of the article suggested using shims to push the jaws apart as opposed to the more commonly seen ring. Using my friend Gill’s Bridgeport, I made a set of 1.25″ shims. In the picture below, you can see I’ve also marked the jaw faces with Dykem to monitor progress on the jaws.
Professional machine shops and well equipped hobbyists use tool post grinder for grinding the jaws. It is setup to be mounted in the tool post to easily attach it to the lathe. I lack such a tool and instead used the arrangement you see in the picture below. It’s a pneumatic die grinder that I’ve “attached” to the tool post. Though it looks rather shoddy it held the grinder tightly enough for what needed to be done.
I don’t have any pictures of the actual grinding because I wanted to focus on that. First, I covered my lathe as best I could to keep out grinding dust. Grinding is done with both the lathe and grinder spinning. I ran my lathe at ~100 rpm during grinding after trying several slower speeds first. To start the actual grinding, I advanced the grinding wheel to the back of the jaw until it contacted to locate my zero. I dialed out 0.002″ with the cross slide and slowly moved the grinder out with the feed wheel. From here I kept advancing the grinder out and making passes checking my progress frequently. At first I was only contacting one jaw but eventually all three were being ground. As expected, in the beginning the area ground was in the back of the jaws. As work progressed the ground area moved outward. Once I was happy with the results, I moved the grinder in, dialed out another thousandth, and made a pass out of the jaws. I figured that this would impart a slight taper into the jaws which would be wider at the back since the grinding stone wears as the pass is made.
Seen below is a picture of the results. Note that my shims now have a concave edge on them. Before I started grinding I noticed they would get hit first by the grinder. I pulled them off and ground them on my bench grinder. The jaws all showed an evenly ground area everywhere except at the very tips. The tips were banged up and I didn’t want to remove the material required to get the last little bit ground.
While I was set up for grinding, I figured I’d touch up the front face of my jaws. As you can see below, the front faces of my jaws near the middle are a little dinged up.
I made passes with the grinder taking a couple thousandths off each time until most of the front face was cleaned up.
Once I was done grinding, I wiped all the surfaces of the lathe down to remove any grinding dust. I also removed my chuck, disassembled it, and gave it a cleaning as well. I put it back on the lathe and proceeded to take another measurement of the runout to see if there was improvement. I was rewarded with a runout a small bit over 0.003″. I’ve seen this runout specified on some new chucks. So, I was pleased with my results.
The “bell mouth” was also gone. I couldn’t see any light between the chuck jaws and the part.
If you’re wondering why I don’t have zero runout, there are a few reasons. The first is that inside of the chuck is a large scroll which also wears some over time during operation. The jaws ride different portions of the scroll depending on how wide open the jaws are. Thus, the area the jaws were pressed against on the scroll when I ground the jaws is not the same as when the jaws are at some other position. Since the scroll has probably worn unevenly, even if my jaws were perfectly ground in one position you can’t say that they would be perfect in another position. This also means, that my ~0.003″ runout measurement is only at that one diameter. I measured runout with a ground 0.25″ rod and had less than 0.001″ runout. Another reason would be my grinding set up.
I’m happy with the improvements I’ve seen in my chuck. I’m not sure how old the chuck is but I imagine it is quite old. I’m glad to keep running it with the lathe. Even if my runout had not decreased the removal of the “bell mouthing” will result in a better grip on the work piece. I’m hoping this will translate into an easier time parting off on the lathe.
Unfortunately, while tightening down one of the locking nuts on the compound, I broke one of the T bolts. It looks like someone had fabricated it from a stud by welding a piece of steel on top of it. So, I will be fabricating a replacement T bolt in a future post. For that, I’ll be taking a field trip to Gill’s.