Thread: Engineered Fits

For starters what size shaft or hole are you working with.

Your lathe will be able to take the .001mm cut,you may not be able to see it though.

The .001mm cut may not be constant over the length or bore due to wear in the machine or tool wear.

Have you searched at all for information on limits and fits.

Do you have Emery and are you aware that you can use it to polish shafts and bores.

Hi David,
Machinery's handbook will tell you everything you need to know about different class of fits. If you have a look on the internet you will find a free down load, it is something like 3000 pages. They are available to buy I think I paid $140 for my 28th edition but you can get the earlier ones on ebay for a lot cheaper.

Your doing it the right way by boring the hole first then the shaft, do you have a set of spring type bore gauges? Using these you can use the same micrometer to measure both the gauges and the shaft so your not fighting against readings of 2 different measuring tools, which may differ slightly.
When you get close and another cut might put you over, you can use emery cloth to bring down that last bit.
To get a good press fit I find it easier to cool the shaft and heat the bore, doing it this way gives you a wider tolerance range for the parts to fit than doing them both at room temperature.


Dave

Originally Posted by pipeclay

For starters what size shaft or hole are you working with.

Your lathe will be able to take the .001mm cut,you may not be able to see it though.

The .001mm cut may not be constant over the length or bore due to wear in the machine or tool wear.

Have you searched at all for information on limits and fits.

Do you have Emery and are you aware that you can use it to polish shafts and bores.

thanks Pipeclay, I am working with a variety of hole sizes, nothing specific. I have emery and did reduce a shaft this way - sort of worked ok. I have a fair bit of info on limits and fits. I just need affirmation that the way I am doing it is on the right track. Thanks also Dave J, it seems I am on the right track. I will keep at it. Doing a final emery/polish allows me to REALLY creep up on the fit even though is slow. Practice practice is the go I think.
David

Just remember about heating and cooling the other part. Within reason, something that wont even go together at room temperature will go in doing it this way.

I usually get what ever is necessary to press them together ready, go and put the shaft in a plastic bag in the freezer for about 20 minutes, 5 minutes before it's ready pre heat the part in the shed, go and get the shaft and re heat the part, take it out of the bag lightly oil it then press them together.

Dave

I'm like David. When I have to turn something that has to be a certain fit or an accurate size, I tend to creep up to it. Unfortunately, I frequently creep past it.

I find a cut of, say, 0.0005" often looks shabby because there's little load on the tool. That could also be a result of the carbide grade and insert geometry I'm using.

How do you blokes with lathes bigger and more rigid than our little Hercus 9 inch models turn a shaft to an exact size, no plus or minus, exact.?

Can you do it by dialing in one single fairly deep finishing cut and end up with the exact size?

Bob.

Now Bob there is no such thing as exact. You just get a better mic, or gauge block or....
I've been told when you are really getting down to it, the metal moves about. Like when you turn BMS it relieves the stress. So you have to sneak up on it. Of course it depends what level of "exact" you are talking about.

One tip I was given by a fitter, if you Turn a shaft under size, knurl it. Then turn it back down to size.

I think you'll find you can take much lighter cuts with HSS than carbide Bob.

Stuart

I would say your first problem is using carbide inserts as it's pushing the material away on small cuts, properly sharpened HSS will cut any small amount off.

The larger lathe I have is a lot better than my old Hercus but I still creep up on it.
I can never just dial it in and get exact measurement, there are to many variables involved. This is why I suggested for press fits heating and cooling the other part as it will allow for something slightly oversize instead of trying to fit it at room temperature and going undersize.
As for getting a nice sliding fit in another part you really need emery to finish the surface so it's smooth, otherwise it will fit first off, but then it knocks the highs off caused by the tool finish and is then a loose fit.
Another way is to leave the shaft slightly longer than needed, then when you get close you can do a small trial cut on the end to measure and try. Once you have it, you can continue the cut along the shaft.
When turning you have to watch out for heat, you may have the perfect size measured on the lathe after turning, then take it out of the chuck, go to assemble it 30 minutes latter only to find out it's now a loose fit.
If I need a precise fit I will rough turn it down leaving about 1mm on diameter, then go do something else and come back to it 1/2 hour or more and finish it. In a rush I have even taken it out and cooled it in a bucket of water.

Dave

From experience a lot of this close tollerance work comes down to a few Vital things.
1) The material being machined,characteristic of material will change as its being machined.
2) Knowing the characteristics of your machine,(does it cut parallel,does it cut tapered,does it need backlash removed).
3) Useing the correct tool for the job.
4) Useing the correct feed and speed.
5)Having the job set up the best way.
6) Having the correct measuring tools.
7) Knowing how to use the measuring tools in the correct way.

In regards to turning a job Exact with no +/- tollerance on a conventional lathe,I have had most results through bloody good luck.

A lot of the problem is being able to accurately measure the hole... I use Mitutoyo telescopic gauges for my hole measurement.

Others swear by Dial Bore Gauges.

Once you know your hole size it is then easy to make the other part to suit...

Also remember products like Loctite..They work...In fantasy land our lathes all produce a mirror finish and can cut to within 0.0001mm and we can measure to that degree as well.... In the real world we use loctite

Originally Posted by Stustoys

One tip I was given by a fitter, if you Turn a shaft under size, knurl it. Then turn it back down to size.

If that was the same fitter who rebuilt one of our windmill heads he cost us $1500 in crane hire when his "press fit" knurled shaft came loose after a couple of years of use (and his correctly fitted key fell out as well) Suffice to say I am disgusted with the quality of work done by some qualified machinists... You can get away with it if it is some cheap and easily replaced item, but for mission critical things it is a very bodgy job.

O.K We move exact to the side and settle for a tolerance of plus or minus 0.0001" or 0.002mm, something measurable in the back shed. It appears that creeping up to that final cut is the norm.

I had been discussing the issue of creeping with David "YarrD" a few days ago and I told him of a fitter and turner who had chastised me for pissing around with numerous cuts to achieve a set finished size. He had a big Colchester Mascot and his method was to determine the depth of cut required and take one big, lets say 3-4mm, cut to acheive the finished size. If you had a tolerance of a few thou, I guess you could take one deep cut on our little lathes. He was making trolleys for brick kilns, not watches, so I guess he could be confident with that one cut.

Dave J. I've been using carbide because it's easy and I'm lazy. I will grind up a few new HSS tips and see how I get on.

Loctite. I've been using the stuff for the last 40 years and couldn't do without it.

Bob.

Originally Posted by YarrD

I can measure the shaft fairly accurately and I have installed my DTI at the back of the cross slide so can dial up .001mm cuts (noting that my lathe - Hercus ARM, cannot really make .001mm cuts). But I can't measure holes as accurately (I have purchased an inside micrometer on Ebay but not received yet).

do you mean .01mm??? a thousandth reading DTI will move by the slightest change in temperature you are over thinking things remember an on size shaft will not go on the same size shaft working to 0.01 or 0.03 should be fine for most things

do you have formal training of measuring instruments? if not get to know someone who has and get them to teach you i disagree with Dave your measuring should be consistent will all your measuring tools and i mean ALL your measuring tools. to measure bores you can use verniers, telescopic gauges, internal mic's, pin gauges or the very accurate spring dividers im not joking they just as good if not better for most purposes than internal mic's oh and about internal mic's they are a bitch both to use and to calibrate ie an un calibrated mic will give you anything but the size you want the way you get around using un-calibrated internal mic's is to first use it to get your feel in the bore and then use a calibrated external mic to read the internal mic by the way if they are both calibrated right they will read the same measurement now all this is just a PTA if you ask me so my advice is use good old fashioned spring dividers you will learn how to "feel" the true bore size and transfer it to your mic but you can not expect 0.001mm resolution its just to small and a complicated measure to even begin talking about

just to sum up calibrate your external mic you will use to measure the shaft and bore right before you use it and always double check with with a good vernier or hell even a quality rule and remember 0.03, 0.01 is a tight size for a worn lathe

Originally Posted by Anorak Bob

How do you blokes with lathes bigger and more rigid than our little Hercus 9 inch models turn a shaft to an exact size, no plus or minus, exact.?

Can you do it by dialing in one single fairly deep finishing cut and end up with the exact size?

Bob.

only on quality industrial machinery we have a very worn Takasawa lathe at work but it will still dial in your size every time and you know what its dials are in 0.02 increments why because thats as close as you will need even on a toolroom lathe!!!!!!!!

we also have a russian "S-A-ST-A-C-E" (cant spell it) lathe again beautiful lathe you dial in 2mm and exactly 2mm will come off

Exactly is a big word, if you mean exactly to be "+/- 0.02mm" then I guess exactly is possible.
I believe Daves point is that you don't need to know what size the part is. As long as you are either making both parts or making one part to fit another part that you have access to, you can use calipers to measure and never know what size anything is. You only need calibrated measuring tools when you want to use the measurement from one to set another or you are boring the pulley and someone else is making the shaft.

RC I think the kurled shaft would have done fine until the key fell out. Although with the size of that hub you would think he could have bored it and fitted a sleeve.

Stuart

p.s. In industry 0.02mm might be as close as they ever try to get on a lathe. If they want closer than that it would be cheaper to rough it close and finish in a grinder. Most of use don't have cylindrical grinders...... yet

Wrong word. Had I thought about it, I should have said a size with a given tolerance.

Bob.