clearance hole

[misterroy]

Hi, I am a teacher and find myself doing metalwork this year. I have been supplied with the drawings by the exam board, but I think they have taken no account of clearance. In the vice in the middle of this page

there is a piece of 100 bar sliding in a 10mm hole.I think that the rear hole should be 10.5mm. The bar is the piece below the screwthread, the screwthread should also have clearance at the front end. Am I right? Metalwork is not my main teaching thing.

thanks

[Nightjar]

You should have been given tolerances for the hole and the bar. Both will be nominally 10mm, but the tolerances will allow the type of fit needed. For a sliding fit, the hole will need to be no smaller than 10mm but will be allowed to be slightly over. The bar will be given two tolerances, both of which will result in it being slightly under 10mm.

The front end of the screw thread will similarly have permitted tolerances but, as it can be a looser fit, the tolerances will be wider.

I don't have the tables of ISO fits for holes and shafts any more, but if they are not given on the drawings, they must be somewhere in the course work papers.

Colin Bignell

[The Natural Philosopher]

a cigarette paper is IIRC a couple of thou. A wipe with fine emery paper can make a 'wont fit' to a 'sliding fit'

[Nthkentman]

Hi, I am a teacher and find myself doing metalwork this year. I have been supplied with the drawings by the exam board, but I think they have taken no account of clearance. In the vice in the middle of this page

there is a piece of 100 bar sliding in a 10mm hole.I think that the rear hole should be 10.5mm. The bar is the piece below the screwthread, the screwthread should also have clearance at the front end. Am I right? Metalwork is not my main teaching thing.

Many of your concerns can be addressed by buying

It was my little bible when I was doing ONC production engineering and as a centre lathe turner and fitter by trade an indispensible reference.

[newshound]

That's a really serious omission if you are trying to prepare people for doing real engineering one day; less so if you are trying to teach "craft skills", where you learn about using emery paper. Real workshop training for mechanical engineers is probably a thing of the past in this country.

It's also a sad reflection on our education system that you have been "dropped in it" like this a) without the training or experience and b) with clearly inadequate documentation.

[newshound]

Yes indeed, although I rely more on my copies of Kempe's and Machinery's Handbook. Although it isn't actually going to help in this case.

[Bill Wright]

I prefer KY Jelly.

Bill

[misterroy]

Is the following correct for a Free running fit?

tolerance on a 10 mm bar for a sliding fit is h9 0, -0.043mm so the bar is to be between 10 and 9.967 mm

for the hole, 10 mm H9 tolerance permissible hole size 10.000 -10.043mm

ultra fine rizzla papers are 20 microns, so the cig paper clearance stands up to ISO standards. I could not find the thickness for the other papers.

[The Natural Philosopher]

dirty beast

[newshound]

They sound reasonable. Except that it is a pretty dodgy vice design; are we assuming that the clearance in the screw thread is large? In a proper engineering drawing this would be defined, as would the centres distance between the two holes, and the angular tolerance between the screw and the dowel (well actually the tolerances would be defined for the machined features on both plates).

[John Rumm]

Not to mention risking a rusty screw...

[Bob H]

Now now folks, be serious, lol

[harryagain]

Yes, in practice there would need to be a clearance. The rod would need to have a reasonably good finish.

In practice the hole would be drilled the same size and enlarged with an expanding reamer. This leaves the inside of the hole with a good finish.

Ideally the rod and hole would be different materials to help reduce friction (eg steel rod, cast iron vice body.)

[Nightjar]

That was written in a bit of a hurry, as I was on my way out. I have also found my Machinery's Handbook with the tolerances in it.

To expand, tolerances may be given either as dimensions e.g. 10mm -0.000

+0.015 or as a letter/number combination, which refers to an ISO standard. The tolerance I have just give would then be shown as H7. To complicate matters, fits may be based upon the hole being the nominal size (hole basis) or the shaft being the nominal size (shaft basis), giving two possible combinations for each type of fit. You will need to refer to the course work or the drawings to find out which is required.

By fit, I mean the relationship between shaft and hole. These fall into three main categories: clearance fits, where the two parts are meant to move freely; transition fits, which are intended for locating pins or similar; and interference fits, where parts are intended to be fitted together permanently and with some force applied.

These types of fit are further divided, according to need. Clearance fits can be loose running, which allows for very wide tolerances, free running, which is used for bearings where high speeds, large temperature variations or heavy loads are required, close running fit, which is use for precision bearings with moderate loads or speeds, sliding fit, which is used where accurate location is required with free movement and location clearance, which is used for stationary parts, which need to be assembled and disassembled freely.

I suggest that the bar is intended to be a sliding fit in the rear plate.

For a hole basis fit, that would make the hole H7 10mm -0.000 +0.015 and the shaft g6 10mm -0.005 -0.014. I.e, the hole should be between

10.000mm and 10.015mm in diameter, while the bar should be between 9.995mm and 9.986mm in diameter.

For shaft basis, a sliding fit would be G7/h6 i.e the hole would be G7

10mm +0.005 +0.020 (10.005 to 10.020mm) and the shaft h6 10mm -0.000

-0.009 (10.000mm to 9.991mm).

I would expect the bar to be an interference fit in the front plate, using a medium drive fit, which means the bar would need to be forced into the plate using medium pressure. This would normally be applied with a press. For hole basis, this would be H7/s6: hole 10.000 to

10.015mm, shaft 10.023 to 10.032mm. For shaft basis, it would be S7/h6: hole 9.983 to 9.968mm, shaft 10.000 to 9.991mm.

The students are probably expected to drill and ream the hole to size and to turn the bar to size on a lathe. As you will see, using shaft basis for the fits in this case means the shaft can be the same size along its whole length, while hole basis will require a step where it enters the front plate.

The screwed shaft should, I think, be a free running fit in the front plate. That would be H9/d9 for hole basis; hole 10.000 to 10.036mm shaft

9.924 to 9.960mm if this is also 10mm diameter. For shaft basis, D9/h9; hole 10.040 to 10.076mm, shaft 9.964 to 10.000mm, again assuming it is 10mm diameter. If the diameter is not 10mm, the fits remain either H6/d9 or D9/h9, but the tolerances will depend upon the nominal size and you will need to find a table showing them.

I hope this quick tutorial helps you sound as though you know what you are talking about :-)

Colin Bignell

[soup]

I remember doing LOTS of limits and fits during my degree[1]. Lecturers seeming favourite was "a rattling good fit" this was were things fitted where they touched. Jute handling machines had to have this degree of 'sloppiness' due to all the fluff build up.

[1] Was at Dundee college of technology (now Abertay university)so nearly everything was explained with relation to the textile field.

[misterroy]

Thank you all for your help, the internet is a marvel. I have plenty to be getting on with. There are reemers somewhere at work, looks like they are going to get used.