The last thing I mentioned in that post was planning to make a set of thinner, taller jaws for the vise because the existing set was grabbing it below the centerline by a little over a quarter inch. That creates a force to push it vertically out of the vise - or cuts a notch into the fins. The SHCS that came in the vise were too tall to use on anything much thinner than the factory jaws, so I found some low profile button cap screws to order. I figured I could thin the jaws by about an eighth inch on both sides, giving me a quarter inch more opening. Oh, yeah, I ended up leaving the soft jaws 1-1/2" tall. I might lower that a little. While waiting for the screws to arrive, I made the jaws out of some quarter inch thick plate I had. They fit perfectly.
So now it was time for a test fit.
Seemed like it was all good. The gap between the bottom of the cylinder and the vise jaw figured to be 1" and I had a left over block of 1" bar stock. (Look on the left between the closest jaw and the cylinder). It felt very secure. It took me a while to figure out how deep to cut the fins and the 1-1/2" diameter part of the cylinder over on the right end because it's never really specified on the print. The print does show that the flat should be 1" wide, so a little geometry (and an online calculator) showed me I ought to cut it 0.190 - about 3/16" deep. (Seriously - how deep do you cut that? I think this is similar to what's called Geometric Dimensioning and Tolerancing, or GD&T. )
A friend convinced me to put an extra clamp across the top - I guess he was afraid of aluminum jaws replacing 3/8" steel - so I added that. I cut the flat over the base of the cylinder in a few cuts, .025 deep at at time (3/8 end mill) and it went very well. Which led me to decide to take off the 3/32 wide fins in a couple of passes. First pass at 0.100 depth followed by a second pass at 0.190. When I was done cutting off all the fins I thought I'd move the end mill toward the base as a spring cut (a second cut with the same settings; it removes some metal that gets compressed by the cutting forces in the original cut and "springs back" to a higher level), just to level out any cutting marks. Instead it dug in and ripped the cylinder open. Look at the shelf on the end where I stopped the cut. That's a deep cut at the far end.
You can see how bad it is. The Z axis on the mill never went below -0.190", so as I moved right to left along the cylinder cutting the fins off, the cylinder must have pivoted and lifted the left end. I sure couldn't see it happen. You can measure the diameter to the flat by putting the calipers on the fin and the flat at both ends. The end closest to the camera is 1/16" smaller than the end by the pedestal. The flat spot on the right end of cylinder after the fins was supposed to measure 1.000 and measures exactly that. Like I say, the entire piece was cut with the EM never going deeper than -.190".
In the aftermath of trying to figure out what went wrong, I focused on the idea that the cutter was getting lower as I made the cuts, which were right to left in the pictures here. To cut off the fins (3/32 or .094 wide) I raised the Z-axis of the mill from -.190 to -.100, moved to the next fin, made a cut, lowered the cutter back to -.190 and took another pass. As a diagnostic, I set an indicator on the spindle, raised and lowered the cutter back and forth between -0.100 and -0.200 about 40 times. The indicator never showed a change in position, which means the cutter wasn't getting lower every time I raised and lowered the mill's headstock.
My guess is that the cylinder pivoted because even with the taller jaws and the 1" bar stock spacer, the contact area holding it is still pretty small. Theoretically, the contact is only a line with one point on each fin; practically, the metal will spread out a little, but it's still a tiny contact area. Since the cut is deeper on the left, that means it was rising into the cutter, pivoting around the right end. There wasn't enough holding force to keep the cylinder from rotating. Maybe I should have made really light cuts? Maybe something between the jaws and the cylinder to keep it from sliding, like very light sandpaper or cardboard?
This was Friday and I've begun work on the replacement cylinder. I don't see any way I could fix this one. Maybe with welding equipment and an expert's touch, neither of which I have, but I do have more pieces of 2-3/8" diameter bar stock cutoffs that I can make into the new cylinder.
I'll be the first to admit that my main problem in doing machine shop work is that I Know Nothing. Well, not quite nothing, but I'm totally self-taught and when you're self-taught there are always holes in your knowledge. If anyone sees this and has ideas on how not to make this screwup again, let me know!
If I may offer some thoughts.
ReplyDeleteRotate the cylinder 90 degrees you can much better contact tightening
on the ends than trying to hold point contact on the diameter. Also use higher rpm and lower feed rate depending on your tooling. Don't try and take it all at once. You can mill off .03 depth steps or you can use smaller radial steps. Both will reduce tool pressure which will try and pull the part out of the vise. If you measure a taper from the left side to the right then the part has been pulled up as it was cutting.
Another thing of note. Is the spacer block and cylinder head the exact width as the adapter(?) on the other end? IF they are off slightly the vise jaw will not take up the balance side to side even with the c-c clamp.
SiG if you want to, you can email me and I can offer some tips.
I have known your pain :(
John
Thanks for stopping by. I had never thought of grabbing the cylinder in compression along the bore axis.
DeleteI'm building this by instructions in a book and the author has it clamped this way. The end of the cylinder, sticking out on the left, gets slitting cuts with a saw in the mill, so one setup allows him to cut the flat required and make the end cuts.
He has more spacers around the cylinder, including adjustable parallels, but they all seem to have the same problem of a contacting the cylinder in a line along both sides.
It's probably best to go to Gmail since I can send some pictures.
My email is theferalirishman. At. Gmail
DeleteMetal cutting forces are higher than intuitions developed from woodworking. After you clamp something, ask yourself, "Could I hang 500 pounds from that? "Could I tap that with a hammer without it creeping?" Because the cutter will be tapping it.
ReplyDeleteNot sure about the aluminum jaws. The jaws are thinner than the part being clamped, so the jaws will bend first.
The jaws are exposed (I mean sticking beyond the body of the vise) only on the ends and top, right? I can trim the ends back to just a little bigger than the vise with virtually no effort. Just use and end mill and trim them back a little at a time until I have .010 or .015 clearance from the vise just so that I don't hit the vise with the cutter. The diameter of the cylinder is 1" and the height of the vise body is 7/8 (IIRC - it might make more sense in metric). I'd really like the jaw to extend vertically to the centerline. That's a very small overhang compared to what I have now.
DeleteMajor suckage!
ReplyDeleteI've seen things like that happen and sometimes it makes you want to scrap the entire project.
On really frustrating days in other hobbies, I used to say, "I should have gone into stamp collecting. You buy the stamp. You paste the stamp in the book. Then you look at the book. How does that become frustrating?"
DeleteIn reality, my hobby is making things and getting better at making them. Building this part twice just means I don't get started on the next engine quite as soon.
Oh, I know how that goes. I've left the landscape behind me littered with dozens of failed/screwed-up/didn't work/blew to bits projects over the last 50+ years!
DeleteIt sure beats having to sweep up the remnants of an engine that went BOOM at 6500 RPM.....
Can you make concave jaws or inserts to go between vise jaws and the part?
ReplyDeletemachinist here. The cutter is a vertical screw, pulling up on the part. The usual fix is to use a v-block, or a toe clamp in addition to the vise, or, as previously pointed out, rotate the cylinder 90. In lathe work it is called too much lgth/dia. Clamping force is really friction, and the friction has to be greater than the cutting forces. Another situation is that round parts clamped vertically will rotate out every time, there, v-block is the fix. Really enjoy your blog, been reading for years.
ReplyDeleteThanks for your kind words about the blog and your technical input!
DeleteI had kind of come to that as a conclusion. The only contact surface is the line on each side of the cylinder that the vise jaw or spacer block squeezes. In theory it could almost be a point, but as the jaw tightens, the metal deforms a little and flattens out. Unfortunately my vise doesn't open enough to get my small Vee blocks in there. I need to come up with something like that, only small enough to fit. Which needs to be (mumble, mumble) inches thick.
Or a foot long piece of pipe on the vise handle and really crank down on that mother!
A V-block as suggested above is a good idea. Also, I might have cut the flat first, then done the rest of it.
ReplyDeleteouch.
ReplyDeleteThe way to fixture this is to turn a plug that sits at the very inside top of the cylinder, and projects out of the bottom twenty or thirty thou. Now clamp the head between that and the other vise jaw. It's the only way to clamp that you cannot cause elastic deformation of the part. You might be able to use a deepwell socket.
Couple thoughts.
ReplyDeleteIs there a piston going inside that bore? Boring that first, and them removing lots of metal from the outside can change the internal dimensions. Might not be critical, depending on needed tolerances.
Consider leaving mounting bosses or tabs on the outside, or inside, of the block/cylinder, to give you handles for secure holding purposes. Figure out how you will remove them, first. Generally, the later in the building process, the better.
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I'm guessing that the finished cylinder doesn't have to match all print dimensions and visuals perfectly. There is usually some leeway to play with.
As others have mentioned, machining is a forcible removal of material. The less secure the part, or the more delicate, the less material can be safely removed at any one time. Think of it as leverage. A lot of material at once requires lots of leverage. You are juggling lots of variables as far as strength, friction, clamping force, etc. Sharp tools also make a difference. Also, bit speed and feed.
When in doubt, nibble.
When in doubt, nibble. About to be my life motto - if it isn't already. I screwed that up on this attempt. I forgot an important fact while cutting the fins back and used too aggressive a feed.
DeleteThere will be a piston in that cylinder. The piston gets built a few parts farther along (following the book), and will be sized to fit the cylinder. I bored it a little smaller than my piston's rough bar stock (about 10 thou); the piston will be cut to match the cylinder.
The advantage is that not only do the finished cylinder and piston not to have to match the prints, they don't have to match any other model in the world. Lots of people have built these engines, but I'm not making interchangeable parts. They can be custom fit to each other.
If you aren't making mistakes, you aren't learning much, generally. If you aren't pushing the boundary's of your knowledge, you don't know what you don't know. Probably the most important area to pay attention to is figuring out what can hurt or kill you if you goof. Everything else is just time, money, frustration, or embarrassment. Learning to spot the mistake before you throw the switch is part of the reward you look for. That takes time and experience, and mistakes. Watching others do machining helps to acquire the knowledge base you need. Reading the machining blogs helps a lot. PM is really good.
ReplyDeleteSomehow I missed this comment - I'm also having the problem others have mentioned where blogger doesn't forward comments to Gmail or let me know comments have appeared.
DeleteThere's a real analogy to my former work life: we'd be building a prototype of something and it might take the better part of the week. You'd check it a thousand times, but you still have to flip the switch. The thing is, the absolute best case (for the program) is that it works and behaves exactly like you think it should. Sometimes, though, despite having checked it so many times, it gets wired wrong and something blows up. Sometimes something happens like a 100-pin edge connector gets wired as a mirror image and it can't work.
After a lot of years I came to rules to live by. Never assume it will turn out right. And never turn it on as the last thing you do Friday. At best, you "break even" and it does what you think. At worst, it goes up in smoke, you have to start over again and you ruin your weekend.
Making these parts is like that. No matter what we do, at some point we have to turn on the machine and start cutting metal. Sometimes it turns out right, sometimes something like this happens. I'd say that most of the time, even for me, it comes out right, or I know exactly what I did wrong.
It is constant learning. I always say, "every part is a puzzle". Some parts are several puzzles.