Friday, June 24, 2016

Stopping and Measuring

I've just completed a fun project - my first machinist project in steel instead of the softer, easier-to-work metals like aluminum or brass:


You're looking at a fixture I made for my metalworking lathe. However, if you don't know much about lathes, it's hard to tell where the lathe ends and the new part begins! It's a removable, adjustable fixture (1 in the photo is the body and clamp knob) with a carriage stop (2) and a dial indicator (3), which measures the movement of the carriage at (4), and the indicator is removable using the thumbscrew at (5). That nice Ames dial indicator was a gift from Kelly Williams. (Thanks, Kelly!)


It started with a couple of chunks of steel that Phil Oles gave me. (Thanks, Phil!) In the photo below, I've already cleaned up the big chunk, and the smaller piece still needs to be squared up and the black coating removed:


Once I got the stock prepped, I marked out the big notch that fits over the "ways" in the lathe, and made a sad discovery: my little mill isn't stiff enough, nor is the vise big enough, to cut such a notch in that hard steel, even taking tiny bites. I took it back to Phil, and he cut it on his big mill. (Thanks, Phil!) Here's the raw material once it returned to the Grant St. Garage:


I was working from a plan that used two socket head screws to attach the small clamp piece to the main body, but that meant every time I needed to adjust or remove the stop fixture, I'd need to loosen  those  two screws with an Allen wrench. I saw a YouTube video with a similar fixture with a knob to clamp it, and I decided to go the same route. I made an aluminum knob, and tapped it for a steel screw made from a  1/4-20 bolt:


I used red Loctite to permanently bond the screw into the knob. The more common blue Loctite is semi-permanent, but red makes a bond that takes high heat and strong clamp to break. It ain't coming apart! Can anybody explain why red comes in a blue tube, and blue comes in a red tube?


I drilled a clearance hole (letter G in size for 1/4-20) in the body, and tapped a through hole in the clamp. I decided to use a pair of brass pins, bonded into the body and floating in the clamp, to keep the clamp from twisting. Here it is with the holes drilled, and to the right, assembled.


There were two more holes to drill and tap in the main body, and you can see both in the photo below. The one barely visible on the right side is for the stop, and I'll talk about it in a minute. The one about to be tapped in the photo is for the dial indicator, tapped 1/4-20:


Once the holes were done, I needed to mill a step for the dial indicator, 1" wide and 0.275" deep. My mill can handle that, since the piece is clamped very securely, as long as I didn't take more that 0.015" at a pass. 


Since I can't mill an inch wide, each 0.015" pass had to be made twice, moving the table between each pass. Too late, I realized I could have saved a lot of cranking by milling the outside almost to the bottom, and then doing the same thing on the inside. A final pass would make the bottom flat. As it was, it took about half an hour to do it my way, and I had to make notes so I wouldn't get lost:


Finally that task was done, and I got to work on the stop screw. I had tapped that hole #10-32 for a very specific reason. The 32 in the size means it take 32 full turns of the screw to move one inch. Thus, each turn moves 1/32", which in decimal is 0.00313". That means that one-third turn is almost exactly one thousandth of an inch. It's actually 0.00104", which is plenty near enough. I just needed an easy way to turn it one-third of a turn.

I decided to make a thumbscrew with indicator marks in thirds. First, I made the 1/2" thumbscrew in brass, knurling it and tapping it 10-32. But before cutting it off, I put it in a collet in a hex collet block. I mounted  that in the mill, and used a pointed end mill to scribe a line on the face of the thumbscrew, rotating two flats between each cut:


When finished, I cut off the knurled thumbscrew, darkened the scribed lines with a Sharpie, and used more red Loctite to permanently affix the screw into the thumbscrew. The round screw head is exactly what we want as the bearing point of the stop:


I rummaged around and found a spring that worked perfectly to keep the stop from turning from vibration:


So, the fixture can be used  two ways: as a dail indicator holder, or with the indicator removed, as a carriage stop. In the lead photo, you can see how I can precisely measure the travel of the carriage with the indicator. That's very useful if, for example, I need to face-mill exactly 0.010" off a workpiece.

The photo below shows how it will normally sit on the lathe, with the indicator removed. You can see how the carriage stop will not allow the cutting tool to advance into the chuck. When the cutter does that, it's called a "crash" and it's never good news, so I'll leave the stop there as a safety measure. When I need to make multiple passes on a workpiece and end at the same place each time, I can move and adjust the stop to ensure that as well.


One more photo, of another dial indicator that Kelly also gave me. (Thanks, Kelly!) It is astonishingly precise, with a resolution of a "tenth" (0.0001") and only 0.050" of travel. I probably won't use it much on the carriage, but I'll use it a lot in precisely indicating alignment!


If you look closely (click the photo to enlarge it), you can see that the indicator is a Hamilton. In earlier days, that was indeed part of the Hamilton Watch Company based here in Lancaster. But in the 1970's the instrumentation division was sold off and moved to New York, so it's unlikely this particular indicator was made here. Still, a neat bit of local history resonating in the workshop!















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