Test indicator dumb questions

Hello all,
With respect to dial test indicators, I am struggling with understanding
auto-reverse. It appears to be good, but what is it? I see lots of
mention of not needing to flip a lever; are such indicators obsolete
now? Is there any reason one would want one?
It might help to start w/ my import's behavior: the dial advances
clockwise from zero regardless of the direction of motion of the point.
It looked weird at first (the dial markings suggest it would do
something else), but I figured it was probably by design, having
something to do with an indicator version of backlash near zero.
When indicating a vise or a part, I use the cross travel to make contact
with the point and then continue moving to get a non-zero reading and
work relative to that. I don't bother to move the dial, though it might
be smart to "permanently" offset it so that I'm working at zero vs. 15.
While not in the market right now, I am thinking ahead to a rotary table
and wondering how I am going to align it. A centering indicator is one
option, but it looks a little long for a mill-drill. I might be able to
make it work by mounting it in an R8 collet, bumping in the table, and
then changing back to my ER chuck for normal work. With the mill's
dials locked, (if needed) I could move the table to free up space under
the spindle, change, and then start work with the RT's axis at zero on
the dials.
Is there an easier way? Is there another type of indicator I should
Reply to
Bill Schwab
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I can't think of a reason.
I'm not sure why they do this (I have one too) but I don't find it to be an issue. You can't use both direcitons at the same time anyway...
15 is excessive. A preload of a couple of thou should more than suffice.
Depending on what you're doing, there are a number of ways to setup a rotary table. I don't think you need a centering indicator though (especially given the price).
Typically you center the part on the rotary table first. Once that's done, center the spindle over the rotary table.
Many people make tooling plates for their rotary tables so they can cut into the table (only slightly) if required to mill a complicated profile. If you make a tooling plate, you can place a dowel hole in the center and then use a dowel pin to center the rotary table to the spindle. The part too, if you can accomodate a dowel pin hole in it.
I'd recommend getting one that will reverse without flipping a lever. Levers are annoying.
Reply to
Robin S.
15 is _convenient_ :)
Got it. Thanks for your reply!
Reply to
Bill Schwab
Perhaps I'm not reading you correctly, but the indicator's dial can be rotated to any convenient spot.
Reply to
Robin S.
The autoreversing DTI have a heart shaped cam that the tip rides against so that any motion from the center will push the cam back and thus cause a movement of the needle. These are very nice when you are just indicating as you don't have to do any thinking about what directions are happening with a particular setup.
-- Why do penguins walk so far to get to their nesting grounds?
Reply to
Bob May
I'm of the opinion that what they mean is that the indicator will function in either direction without moving a lever. A Starrett Last Word is an example of an indicator that must be manually reversed. A B&S BesTest is an example of one that will function in either direction without moving a lever. I own both and highly recommend the B&S over the LastWord. The mechanics of both designs have little in common with one another and the Last Word tends to give poor readings.
All too true----you can't use it in both directions at the same time, but it's a nice feature that you can use either one without making any changes (flipping a lever). A good example might be if you had a round object on a rotary table and wanted to dial both the bore of the table, then the OD of a part, in succession. All you need to do is move from one location to the other. The indicator is ready to read in either situation. That would not be true of an indicator with a lever, and there could be times when moving the lever might not be convenient. Working in a deep hole might be a good example of such a situation.
The amount certainly isn't critical, so long as one can read the amount of deflection from one end of the object being dialed to the other end.
When sweeping the surface of a table to square a spindle, it's obviously to advantage to have as little preload as possible so the tip can glide over the T slots, or other examples of a similar nature, but otherwise it makes little difference. Remember, you are not really measuring with a DTI under setup conditions, but looking for anomalies so they can be eliminated or minimized. The indicator, in use, behaves as a comparator. It's not really "measuring", although it does display in increments that translate into measurements. .
Any common DTI will suffice. It should be held in the spindle of the machine and inserted into the bore of the rotary table. Do not rotate the table, that won't tell you anything aside from the eccentricity of the bushing in the table (which should be 0). The spindle of the machine is rotated with the indicator in intimate contact with the bushing. The table and saddle are then adjusted until you get an equal reading on all four sides, or a constant reading, regardless of what it is. At that point, you should mark your table and saddle (wax pencil) so you have a reference point at which your dials will read 0-0, and each of them should be set such that you know which direction has eliminated backlash. My style is to always set the dials so they are reading in the right hand direction. That way you never have to remember which way you set the backlash. Only under very unusual circumstances do I set the backlash differently.
While that might work, once a part is set on a rotary table, the center hole is usually covered, so you can't use it to set up the table. It's typically best to set up the table as above, then dial in your part. It's generally more important to know the table is set properly, especially if you have several parts to machine. Assuming you make a minor mistake in the setup from a part, the same error is introduced to each and every succeeding part that is set up. It could spell the difference between a good part or scrap. Both methods will work, one has the edge.
Yep! Very effective way to make setups on a rotab, especially if you're stuck with a small table. My plate has random holes drilled and tapped for hold down clamps, and I don't hesitate to add them as required. The plate is aluminum, and considered perishable. Considering I have a 12" Bridgeport table, my plate has a 1" diameter hardened and ground pin for the plate, which locates from the center hole of the table. Setups can, therefore, be torn down and re-established without losing concentricity.
I couldn't agree more. I recommend the B&S highly. I've had two of them for countless years (well over 30) and the only thing that has gone wrong with either of them is the crystal shrinking.
Reply to
Harold and Susan Vordos
The analog in the world of electrical measurements is a "null detector"
The actual linear sensitivity of a last word type indicator varies a great deal across its range because of the cosine error between the angle of the point and the surface it's sweeping. But it will be most sensitive at one spot and if the needle nulls (never moves) during the measurement, one can be sure the feature and the machine axis are lined up. The non-linearity of the detector drops out.
Reply to
jim rozen
While that appears to be true of the B&S indicators I've used, as is often the case with a Last Word, the lack of movement of the hand is not an indication of perfect alignment. Once the error falls below a thou, they often won't display, so you are lulled into a false sense of proper alignment.
Frankly, the only thing I've found for which my Last Word is acceptable is for removing taper on grinding machines, or rough setups with a 4 jaw. I'm shocked that Starrett is still producing them, assuming they are.
Reply to
Harold and Susan Vordos
This reminds of a question that I have been trying to formulate. It first stood in my way with a plate that I "freed" from a rough cut piece of metal. The stock was too thin and big to be suitable for my usual approach to squaring in a vise, so I clamped it on the table (with plywood underneath of course). I arbitrarily set the dials to zero, and settled for allowing half the nominal size of the endmill to establish the dimensions. That was more than good enough, because only relative hole positions were important on the part.
For the right and front edges, it might have been (or seemed??) easier to switch to what you have termed left hand backlash. Is there a correct way to do that? As it was, for each cut that went against backlash, I backed up, overshot, and then stopped at the new position.
I suppose the same problem would arise if one wanted right hand backlash but somehow needed to edge find on the right side of something. So far, the best I know to do is backup, overshoot, and re-approach in the backlash-safe direction. For an RT, I assume this will be necessary on one side in each direction??
Please feel free to answer with something along the lines of "keep trying to formulate that question".
You made reference to getting by with small table. How does one know what is small relative to a given part size?
Reply to
Bill Schwab
That's why I said "last word type" there - I am very well aware of your feelings about starrett's offerings in those instruments.
And in fact I did purchase a BestTest indicator at work - and was suitably impressed by it's quality in comparison.
Still, I wonder if maybe your starrett instrument is sticky or dirty - mine will read reliably inside two of the thou lines on the dial.
Reply to
jim rozen
Unless there's a reason you need the dial to be direct reading from both sides, all you do is reverse feed when you get to the end. Backlash, in this instance, isn't an issue. Once you've measured the piece after taking a cut, you have an idea how much must come off to take the part to size, so you can then read the dial backwards. What that means is that you don't expect a dial to give you readings on each end of a part. If, by chance, you know exaclty how much backlash the machine has at a given point, you could calculate that into your reading, but it's not done routinely, if at all. If that isn't what you want to do, then how you detailed the cut is how it is done. You always go past the target mark, then feed back to it turning the handle in the direction you've set backlash. Using a DRO eliminates the worry of backlash as far as dimensions are concerned, but I wasn't afforded that luxury when I was trained. There was no such thing. I learned to machine without them, and still work that way.
If you're cutting a window, and you need to know where the cutter stops on four sides, what I do is measure and ignore backlash. Make a little sketch of a window, and write the dial locations on each side---and don't worry about backlash on the third and fourth sides. By measuring, you know where you are, and can turn directly to your finished size. That's not a big deal for one piece, but if you have multiple pieces to machine, once you've marked the dials, you can cut windows in short order. With a final pass, maybe a couple thou, climb milling, you can go into corners and out without leaving a trace of undercut.
Depends on how you apply the table. I think you'll find that there are times when you may set your backlash in one direction (as I detailed, fore example) and be able to run your parts without worrying about changing anything. One thing to remember is if you screw with the dials on the machine after you've located the centerline of the table as it relates to the spindle, you have lost orientation completely, so you have to start over. I highly recommend you learn to deal with backlash as you outlined it, just by overshooting it than turning back. If you establish that habit, it's much easier to work, and you eliminate, to a large degree, the potential for making scrap. Don't be too discouraged if at first you screw up a little. Most of us do.
Not necessary. In my opinion, that was a good question. The answer is simple. Those of us that work freely with backlash do so because of extensive experience. Some achieve the same thing instantly, while others will struggle for some time before it becomes routine. It's just one more of the things that separate machinists from the guy on the street. Like any trade, these are the types of things we master in the process of becoming journeymen.
One simple rule is if you're cutting the exterior of a large item and the table is smaller (envision holding the part by holes located within the part, such as holes in a sprocket, for example). The mechanical advantage can get out of balance in a hurry, and it becomes difficult to turn the table while conventional milling, and down right dangerous if you try to climb. Climbing on a rotab isn't a good idea anyway, but you can often do so to good advantage by setting drag with the table lock(s).
My rule of thumb for a rotab is real simple. Anything less than a 12" table isn't a good idea. I'm likely to get a lot of arguments with that statement, but if you've ever had to do any serious work on a rotab, you quickly come to terms with how damned hard it can be to make a setup. It's often almost impossible to find a place for clamps, for one. The larger the table, in most cases, the better. Keep in mind you have to handle them to get them on the machine----so anything larger than a 12" becomes really difficult. If the table is a compound, it's likely already impossible. If, by chance, you have to use the table on end, size then works against you, too. There's no perfect answer unless you happen to own more than one, each a different size. Bottom line here is each guy feels he can do everything he needs to do with the size at his disposal. I'd recommend nothing smaller than a 6" table, but I'd hate to be tied down to one that small. Maybe the best thing to do is visualize the projects you might be interested in running, then see how it works out with clamps. The typical rotab has slots @ 90 degrees, so you do have the added advantage of a second set of slots as opposed to the typical mill table.
Better check what I said before you thank me, Bill. I have my own way of working, and it isn't necessarily the same way as others. It works for me, but it might not for you. At any rate, let us know what you decide. Your opinion might be valuable to others in a similar circumstance.
Reply to
Harold and Susan Vordos
Let me try to paraphrase. You are saying that I can count off the desired size plus tool radius plus some wiggle room, take a cut, measure, and then back up based on relative dial readings. Fair?
I wouldn't expect that to work - that by no means implies that it would not work.
So far, I have found that I can see +/- 0.1 inch, and the dials do the rest. It is true that I sometimes have to switch from a 0.5 inch endmill to a 3/32 drill bit in order to be confident of the center of the cutter, but that's really not that hard to do (have I said anything good about ER collets today?) and beats adding to my scrap bin.
For those not convinced by the "I can get by w/o it" argument, you might better relate to my desire to get other tools, an RT, boring head and bars, etc., not to mention more woodworking tools.
Another thing that bothers me about a DRO: they can skip. It sounds like something that will start happening just about the time I grow to depend on it. Living in FL, lightning is a concern. A direct strike could knock out my motor, but a DRO would be much more likely to get fried.
There is also the problem of moving raw materials (mostly wood). Renting trucks is a pain, not to mention that I do not like to drive vehicles that do not belong to me. A pickup is probably in order. I like my car too much to work on it myself, but a bomb of a pickup will give me freedom to dabble in simple repairs (all together now - but NOT THE BRAKES!!!!).
Speaking of pickups, recommendations for make/model/year are welcome. Gas mileage is not really a concern, because I will not drive it much. Handling plywood will be a common task. Total cost to keep it reliable and safe is a factor, and being friendly to an inexperienced shade tree mechanic (with support from experienced friends) is a big plus. As shocking as the concept might be in FL, AC is negotiable, and would likely not be repaired. The one in my car gets what it need$ =:0
Sorry for the digression.
I _think_ I understand. On the sides backlash-safe sides, how do you account for the endmill radius?
Understood and agreed.
It hasn't discouraged me so far :)
Thanks for the warning.
I suspect that most things I would do would end up being mounted with holes inside the part - all the more likely if I buy a smallish table.
My shop crane would do it, but the point is well taken. OTOH, I doubt anything bigger would fit on my 31.
Maybe the answer is to get a small horiz/vert table and then see if I find need for anything bigger. There is certainly no hurry. For now, I can do what I have in mind with an extra setup or two and a rounding endmill.
I am grateful for your thoughtful reply independent of whether or not I agree with every part of it. As it happens, I find that I end up agreeing with most of what you say.
If I were to look for an area to disagree with you, it would probably be over the validity of mill-drill machines. You are correct to caution people about the limitations, but the machine is serving me well. If, as some here have predicted, I end up with a monster knee mill, the $900 for the mill drill itself will have been well spent.
Reply to
Bill Schwab
Yep! But your first cut on that side should come with the backlash in the opposite direction... What you're doing is establishing the relationship between the cutter and the part, but now in reverse, and any movement of the dial will translate into table or saddle movement, not taking out slop. By establishing the datum points on all four sides by this method, you can work directly from your dial, although you will be reading in reverse on two sides. Mental exercises, nothing more.
The idea assumes that backlash is constant, but I say it isn't. Screws wear, just as the nuts do. There will almost always be more backlash where the screw has been used more, say, at the center of the travel. Therefore, if you use a constant that represents backlash, you'd be introducing error (+ or -) everywhere except for specific places on the screw. A machine with worn screws and nuts is still capable of doing decent work, so long as you don't rely on them to locate over large areas. Again, this is a great place for a DRO---which doesn't care what the dial says.
One of the best habits you can acquire is to rely on a scale for movement. It won't tell you if you miss by a thou, but it can save you from mental errors, where you might transpose numbers (quite common for some of us) or if you miss by a turn of the dial. If you're drilling holes over an area, you might think you can see the difference, but a missed turn on a .200" dial isn't readily noticeable----especially if you believe you're on the right track. Keep a 6" scale at your disposal (larger, if necessary) and use it routinely. Tough habit to acquire, but the best one going. DO NOT trust your eye alone where dimensions are concerned.
You'll not get an argument from me in that regard. Frankly, I pity those that can't work without a DRO. The necessary skills required to be a machinist don't come about when you rely on such "cheats". I recognize how valuable they are, or can be, but I also have total respect for the knowledge, talent and skill that one acquires that can work without one. As I've stated previously, I wear my ability to work without a DRO as a badge of honor. Learning to machine that way doesn't come easily, not if you do good work.
I'll have to take your word for that. As I've stated, I've never used a DRO. I did experience a lightning strike, though, and I had to replace the board for the table feed on my Bridgeport. Freak deal, when I lived in Utah.
Brakes are no big deal, not if you're remotely familiar with how they work. I've done brake jobs all my life and had no formal training aside from one year of auto mechanics in my sophomore year of high school.
While they're not cheap, investing in an early Dodge with a full sized bed (hauling sheets of plywood) that is Cummins powered is a good combination. The later models don't appear to deliver the same economy, for reasons that I don't understand. I drive a '94 with about 98,500 miles on it-----and I get right at 22 MPG. I'm totally impressed with the engine, and the body appears to be built well. Only problem I can see is the cost of diesel is out of line with past history. I also have a '99, 1 ton, that delivers only about 15 MPG.
Depends. If I'm working only on a window and don't care how the spindle relates to the part otherwise, I've been known to reset my dial so the cut lands on 0 (taking into account the end mill radius) in two directions, then rely on measuring to establish the third and fourth setting. What I normally do is start with my spindle set at 0-0, on the part's edges, then deduct the radius and establish the two edges that are direct reading, often staying away by an even amount (maybe .010"/.020"), so I can then rely on the measurement from the established face, minus the amount of material left for a final pass, and measure back to establish the opposite boundary. I record my dial setting, measure, then change the numbers I recorded to reflect the desired setting. Once I get my numbers established, I like to take a cut all around, staying away by a prescribed amount, maybe the same .010" I mentioned, or even less, maybe only .005", then measure once again. That allows for minor corrections if you've missed something due to tool deflection or a mental error. Once I know where I am and I'm happy, I usually take my second to last pass by staying away from my mark by a thou, then climb mill each side to size, taking the last thou. Coming into corners is an art that you slowly develop. I generally back away by the amount of the backlash, so as the cutter comes into the corner, instead of undercutting, it pushes the cutter away ever so slightly, so you don't get that tiny undercut that is so common. It takes a little experience to do it successfully----but it's not hard to do. No, you don't get a step----the cut is seamless.
On occasion, you might find yourself having to work on edge-----maybe making a gear, what ever. The larger diameter table gets in the way in a hurry----so while I have a hard nosed attitude about table size, that applies to horizontal work only. The smaller the better when you're working vertically----so take your own advise and see how it goes. If you're serious about machining and stay with it, there's certainly nothing wrong with adding a second setup in the future, finances permitting.
It will have been especially well spent in that you can make your mistakes (and they're sure to come) on a less expensive machine, so if you do damage, you're losses are minimized.
Don't misunderstand my attitude about mill drills. I realize they're capable of good work, it's just that they're totally annoying to run once you've experienced a knee mill. It's like small lathes. I hate 'em. If I can't take a 1/4" deep cut in chrome moly with a negative rake tool, fast feed, I'm simply not interested in the machine. I had to make time when I ran machines, for it was my way of making a living, and I had to compete, or I was out of business. There's certainly nothing wrong with a fractional hp machine taking lots of light cuts for a guy that isn't in a hurry and doesn't mind the limitations of smaller or lesser machines. Look at it this way. If you have the necessary skills to turn the work out to specifications, can you show me even one individual that can look at the work and tell you on what machine it was produced?
It's far more important for an individual to gain the skills necessary to machine properly than to have the perfect machine. I've proven that all my machining career by getting by with my Graziano when I've had a life long love affair with an EE lathe. :-)
Reply to
Harold and Susan Vordos
It took a few minutes, but after several readings, I think we describing the same thing.
Speaking of slop, is it true that what you describe has the screw and nut surfaces in contact to hold the tool against the work?
My machine should be nowhere close to having such problems. However, it does perhaps suggest that I might want to be a little more agressive with the way oil in a few places. I can see the longitudinal screw, but the transverse is not so readily accessible - maybe from the bottom of the machine. Anybody with a 31 recall what it looks like from below? But to be of any real use, I would need a way to reach it through the stand. OTOH, that screw is shorter, so the oil port is likely to be more likely to be helpful.
Good point.
Another suggestion I found interesting is to use a plunge indicator, though that has obvious range limits.
I'm already doing pretty much that, and it has already saved me time and materials. The hard part has been getting the scale. I have been (ab)using my dial caliper for the same job, and look forward to placing my next order (which will include the scale) so it can get a break.
I am inclined to agree, though I don't yet know enough to cite examples.
A friend of mine had pretty much the same experience. Ok, maybe I'll loosen up eventually, but not right off.
Thanks. In reality, I'm going to keep an eye out for good deals and ultimately take any candidates to a mechanic I trust. If he talks me into buying a lemon, he'll never hear the end of it :)
Fuel economy should not be a factor on this vehicle. It is going to make weekend trips to building supply places and fall victim to my mechanical quackery; that's about it.
With the heavy trend toward SUVs and other large vehicles, the public didn't appear too worried about fuel consumption, and I suspect the auto makers got lazy as a result. Is there a difference in performance?
So you are leaving some metal and later correcting. Do you then simply assume that a 0.5 in endmill has a radius of 0.25 in and leave the final cut(s) to clean up the error?
Nice! I probably couldn't do it right now to save my life, but I see what you are doing.
I suspected as much - thanks for confirming it. In fact, my initial thoughts about an RT were for the vertical direction. This time at least, I'm fairly certain I can fake it other ways.
If I can't afford two rotary tables in my life, I don't want to know about it =:0
I thought of that one too, but didn't want to temp fate. The biggest benefit would be all of the questions I will know to ask about any machine I would buy. The growing pile of R8 tooling would not hurt either.
Don't say lathe right now - my evil twin (the one who buys all of the machines) might be reading over my shoulder :)
Reply to
Bill Schwab
Most likely so. You appear to have a firm grasp of what's being discussed.
Assuming the screw was accurately made, I'd agree.
You can't oil too much, not on such machines. It's messy, but it preserves their useful life. The best policy is for oil to never be black---if it is----it's showing signs of wear of the machine. I keep my ways wiped and oiled such that the Vactra used always looks like Vactra, never black. I also oil the screws on my machines when used, and will often stop during the course of the day and re-oil. It's the cheapest of all insurances. Think Oil-Dri (kitty litter).
That is the one cheat I permit myself, especially when running a lathe. It takes all the guess work out of linear dimensions, and is particularly useful if you follow up with second and third cuts. O ring or snap ring grooves, for example. Parting a piece after machining is another. My setups when making anything in quantity are much like one a person would make running a CNC---but it's all done manually. Care must be observed in setting up a long travel indicator that it is parallel to the carriage or table travel so you get an accurate reading.
Good idea. A scale will generally get in where other things won't, and it's much easier to keep near. I have mine in my shirt pocket always.
Lets see----we're discussing my 3/4 ton '94 Dodge with Cummins engine, as compared to my '99 Dodge, Cummins engine, and mileage.
The '99 has double valves and likely other "improvements" as compared to the '94. As far as I know, the engines are the same size----------but it's hard to compare them due to some variables. The '94 is a 2 x 4, standard bed, and automatic. The '99 I bought new as a cab & chassis, long wheelbase, on which I had a 10' enclosed box (a delivery van type box) installed. It's an all wheel drive vehicle, and weighs considerably more than the '94. It's a 5 speed manual. Gear ratios are the same, as are tire sizes. The difference in mileage showed up even before the box was added. I had driven it about 300 miles before delivering it for the box to be installed. The differences in mileage don't appear to be based on better performance, although, as I said, it's difficult to compare the two vehicles. I like both of them quite well, but would like better mileage from the '99. I'm tickled with the '94. Who else do you know that gets over 20 MPG with a 3/4 ton full sized truck? the '86 GMC I sold gave us just over 10, with a stiff tail wind.
No! I never assume the diameter of an end mill. It's too easy to measure them, and the same thing goes for twist drills. When I run my mill and the cutter size is of concern, I usually find one that is best suited to the job at hand, measure it, then allow for the diameter. If you get in the habit of trusting nominal sizes and you have your end mills sharpened, you'll eventually get in trouble unless you know the size in use. Keep in mind I'm speaking from the perspective of the guy that might be machining for gain, working to tight tolerances, secure in the knowledge that the work will be inspected. For the home shop guy that is making something that's not critical, it may not matter. Still, you're building work habits when you run the machines, so it's always a good idea to pursue perfection. Makes it easier when there's a demand. I've worked with more than my share of guys that had a "that's good enough" attitude, and couldn't handle tight tolerance work when it was necessary.
Try it each time you cut a pocket or window. I've never come up with a better way, and it's almost as easy as doing nothing but coming into the corner, which almost always leaves a tell-tale mark, or undercut.
Chuckle! Just keep thinking Monarch EE! :-)
Again, *Welcome*
Reply to
Harold and Susan Vordos
On 10 Dec 2005 08:43:02 -0800, jim rozen scribed:
Yes, Jim. You are right. There are very many cosine errors to be worried about using a dial lever gage (DTI). Some can be reckoned with by using a pear shaped contact point that may null out some of the angular problems. But only through a very, very small range .
Others can be dealt with by using the instrument in a parallel way... Ie; the more you put the point out of parallel, the more error you get on broader measurements. Tip here is only use them for very small, comparative measurements, not as a measuring tool on the bigger ranges. Keep it close to home.
As Jim says, the indicator is most linear in it's first few thou of travel, (past null) and if you need more than that, better to get a good dial gage that reads in tenths (0.0001). They are linear, accurate and unfortunately, quite expensive.
Best as always,
Reply to
Fred Fowler III

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