General Question for the Machinists Here

S

ScottRiqui

New member
This isn't specifically firearms-related, although I guess it could be. I'm curious about how to specify clearances on parts that are supposed to fit together when I'm talking to a machinist.

If I ask a machinist to make me a 1" diameter steel rod, and also to drill a 1" diameter hole in a similar piece of steel, should I expect the rod to a) fit into the hole, b) not fit in the hole, or c) maybe fit into the hole, depending on the tolerances of his equipment?

On a similar note, if I have something like a hole and a pin that I want to be an "interference fit" so that the pin can be tapped into the hole but won't come out on its own, how do I specify the sizes of the hole and the pin? Is there a certain "standard" amount by which the pin should be smaller than the hole, or do I specify the same size for both parts?
 
It's been a long time since I worked in a machine shop. And I wasn't a machinist. I was a student who had taken a course, passed a hands on test building something and was given the keys to the shop. I made one of a kind stuff.

If you had asked me to make a 1" diameter rod and a 1" hole, I would have asked if you expected the rod to fit in the hole. If you said yes, I would have asked which diameter was more important to you and how tight you wanted it to fit. I could do a better job at making the rod accurate to within a thousandth of an inch or so than drilling the hole - with the equipment at hand. If you wanted a weird size hole, I'd have had to grind a drill bit to the right diameter.

I wouldn't have known what to do with your pin problem without looking at a reference book. As I recall, unless you make the pin, the manufactured pin defines the specifications of the hole. If you also wanted the pin made, I'd have still had to use the book. If you didn't pick a standard diameter, I'd interpolate between two pin - hole combinations that were in the book.

Bottom line is you need to let the guy know what you want to do with this or how you expect it to fit together. And since there are two variables, you need to let him know which one is "fixed". If he's decent - and most are - he'll figure it out.

I'll never forget the instrument maker who ran the shop and his reaction to a student who brought him a uniquely shaped template to make a mask for an experiment. When asked why he needed this oddly curved mask, the student replied, "It will make the Bessel functions work out right and the data easier to analyze."

He's lucky he's still alive.
 
In the real manufacturing world, you would normally specify tolerances. The tightness of the tolerances depends on what you need. For gun work, tolerances are not very tight; the idea is to keep interchangeability of parts.

Jim
 
In the engineering world, the fit is usually set by a variety of parameters like thermal expansion, max allowable runout, assemblibility, supplier capability, etc.

I know a 1.000" bar in a 1.000" hole sounds like it fits, but not without heat, dry ice and a BFH!

Now, the easier way to determine this is by using a table of fits or handbook. Good book. It is kind of like a cheater's guide to engineering.
 
In the manufacturing world, the desired fit between the shaft and the hole has to specified.

Rather than type up a whole bunch of information for you, I'll point you to what you should be looking for:

ANSI Standard B4.1, Limits and Fits.

You can find all sorts of information by googling "ANSI B4.1," including an explanation of what type of fit.

Usually, if you go to a competent machinist and you tell him what kind of fit you want (press fit, interference fit, shrink fit, running fit, sliding fit, location fit, drive fit, etc) he can look in his standard fit tables and the amount to add/subtract from the basic dimensions for the type of fit you want will be found.

Edit: I see Nathan has pointed you to the _Machinery's Handbook_ - and I will too. All questions about machining, such as the one you asked, are answered therein - often in great detail.

In the edition referenced on the link to Amazon above, you'd want to start reading pp. 628 to 649.
 
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Depending on what you are trying to do,is this a one time deal or are you going to make a bunch?

What the other folks have told you about classes of fits is correct,but,for example,if what you really want is a 1 in round bar pressed into a bored hole in a plate,it may be good to find out what tooling your machine shop has.

Some machining processes give better dimensional control than others.

A press fit is a fairly narrow tolerance range,likely less than .001 on a 1 in press(I'm not looking it up)

So,if the best you have is boring the hole on a Bridgeport with a boring head,you can get pretty close with good tools,but working to less than .001 is like shooting sub MOA groups,it depends on a lot of things and you get a flier once in a while.

Now,if he has a hone machine,the hole can be held very close,in the tenths,so you would have good control over the hole,and if you bought 1 in drill rod with a ground OD,for example,you would hone the hole to fit the drill rod,or,a 1 in dowel pin.

Sometimes a machinist has better control over the diameter of the round bar,so it may be best to bore the hole first and fit the bar to the hole.

It might work best to let the man know what you need,like,a press fit,and let him figure out how to get there with what he has.
 
On a similar note, if I have something like a hole and a pin that I want to be an "interference fit" so that the pin can be tapped into the hole but won't come out on its own, how do I specify the sizes of the hole and the pin? Is there a certain "standard" amount by which the pin should be smaller than the hole, or do I specify the same size for both parts?
Click to expand...

What you describe above would be a spring pin. machined pins and the hole you are putting them in will eventually get to the point where the pin will not stay in the hole. Wear and tear of the hole and pin.

As stated by other posts the 1" hole and 1" pin ain't happening. First off what are you doing with the pin and part. You need to specify what amount of play you want between the 2 parts (tolerance ) . The average sheet of paper is .003 thick. (3 thousandths of an inch.) With reference to a hole that is a clearance on each side of the hole of .0015 (half the thickness of a piece of paper) Tight fit to say the least.

Again what are you doing with the 2 parts/pieces?
 
Shrink fit- the pin is a few thousandths (.005" or so) larger than the hole, heat the part with the hole, chill the pin with dry ice or liquid Nitrogen, then get them together fast. As the pin warms and the outer part cools, they grow together.

Press fit- Pin and hole are the same size, and you use a press to put them together.

Slip fit- Pin is several thousandth smaller than the hole, you can press it in and out with no problem, friction or trapped parts make them stay together.

Interference- the pin is slightly smaller than the hole, but has knurling, a bend, or a flattened spot on it that makes it fit tightly.

Of course, the first two require some good dimensional control abilities. Most guns use slip fit, interference, or roll pins (always fit tight because they can compress slightly). And occasionally you will see tapered pins, which only fit one way and get tighter as you drive them in.
 
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Just FYI, those units in Nathan's tolerance tables are micrometers* or microns. 25.4 microns equals .001".

*Pronounced microme'ters not microm'eters; the term "micron" is usually used to destinguish the unit of measurement from the measuring instrument.

Jim
 
if its a real machinist your dealing with, they should have the machinery handbook.

Exactly. If one looks at machinist's toolboxes, there is a special drawer in them (on both Gersterner and Kennedy boxes) to keep the book. Every machinist I know has at least one copy - and some guys collect the older books, which are now becoming rather valuable in their own right.

All someone who is taking such a problem to a real machinist need do is learn how to specify the fit and the size tolerances on the parts. The tables in the _MH_ will give the machinist everything else he needs to know.
 
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