Reaming question

[julian downParticipant @juliandown56088]

Hi,

I'm looking to fit a 4mm mild steel bar into another piece of steel, to the depth of 5mm or so.The hole doesn't have to be blind. I need it to be a press fit, so it stays on. My question is how do I get the hole to the right size? I'm considering drilling it to 3.9mm and then using a 4mm reamer to enlarge but will this be large enough and is it best to use a hand reamer or machine reamer?

With thanks,

Julian

[julian downParticipant @juliandown56088]

[JasonBModerator @jasonb]

A hand reamer has a tapered end so provided you have a through hole you should be able to adjust the fit by how far you pass the reamer into the hole. Have a prcatice on some scrap first.

J

[FMESParticipant @fmes]

Depends on the actual size of the mild steel bar.

If it is 4mm then a drilling of 3.8/3.9 mm will give you a press fit without reaming. but check what size your drills are actually producing – most drill slightly oversize.

We normally drill 3.8mm for a 4mm reamer to H7 clearance on 4mm ground mild steel.

[Tony Pratt 1Participant @tonypratt1]

Yes, check diameter of the bar first, also Loctite will get you out of trouble if it all goes wrong.

Tony

[Clive HartlandParticipant @clivehartland94829]

I normally drill 3.9 mm and use a 3.98 reamer, not many people will have such a reamer but 0.02 interference fit is enough. If you are still unsure then Loctite is your friend.

Clive

[Michael CheckleyParticipant @michaelcheckley34085]

If your bar is supplied slightly under then a H7 fit would give you a clearance fit so as Jason says use the taper part of the reamer to get the press fit you want. Try it on a test piece first.

Ideally you would use a H7 fit hole and then use a bar slightly oversize i.e for a light press fit a p6 fit would be +0.012/+0.020.

An undersize hole of 3.8/3.9mm would be quite a heavy press fit for a bar of this size but it all depends on what size your bar is.

[julian downParticipant @juliandown56088]

Thanks for your responses everyone, when the bar arrives I shall measure it exactly. I had to look up the H7 reference, I take it the H's refer to the the fit, lower numbers being tighter. Where do the P's come in though?

Julian

[JasonBModerator @jasonb]

Capital H refers to hole tollerances

Lowercase h refers to shaft tollerances

Number 6, 7, 8 etc referes to the actual range of that tollerance

[julian downParticipant @juliandown56088]

Thank you.

[Mike PooleParticipant @mikepoole82104]

I found that a newish H7 reamer cut a bit oversize, a commercial 4mm dowel which measures 4.010mm was actually loose in the reamed hole (machine reamer). A friend of mine who is a toolmaker was not surprised and said they would put a reamer in a drilling machine and run in reverse and lightly stone it to get a firm push fit, sounds crude but they didnt pay for the reamers and the stores only keep H7.

Mike

[Neil WyattModerator @neilwyatt]

The H7 reamer is supposed to cut 4.000m to 4.012mm – a push fit, not an interference fit.

Either use Jason's cut on the tape tip, or use a number 37 drill.

I suggest experimenting on some scrap first.

Neil

[julian downParticipant @juliandown56088]

Yes, depending on the size of the bar the machine reamer might be too large then, a hand reamer might be a better bet. Number 37 drill?

[FMESParticipant @fmes]

Julian, may I recommend purchasing a set of Zeus tables **LINK**

You'll never regret it.

[julian downParticipant @juliandown56088]

Yes. I was looking at those, I just wasn't sure that metric clearance tables were in them – obviously they are! I think I might invest in one thanks, Lofty

[Michael CheckleyParticipant @michaelcheckley34085]

I’m not that familiar with number drills but is a number 37 a bit small? (2.642mm)

[FMESParticipant @fmes]

3.658 = No27

Edited By Lofty76 on 24/04/2015 21:43:36

[Roy MParticipant @roym]

Here is a tip that some of you out there may not know. Given the drilled hole sizes are the same, and all the machine settings are the same, a reamer will cut bigger if you use cutting oil (rocol etc.) as opposed to water based emulsion type coolants. This is a small amount but is sometimes useful. (I'm still toolmaking!) Roy M

[brickyParticipant @bricky]

If I can get the work in the fridge I get to a very close fit ,cool one and heat the other and this has worked for me.

Frank

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Roy M on 24/04/2015 23:09:11:

Here is a tip that some of you out there may not know. Given the drilled hole sizes are the same, and all the machine settings are the same, a reamer will cut bigger if you use cutting oil (rocol etc.) as opposed to water based emulsion type coolants. This is a small amount but is sometimes useful. (I'm still toolmaking!) Roy M

.

Roy,

That's very interesting !

I don't doubt you for one moment; but I would be interested to understand how/why it works like that.

MichaelG.

[Bob Unitt 1Participant @bobunitt1]

Any reason you can't turn-down the end of the shaft to fit the hole, instead ?

[Neil WyattModerator @neilwyatt]

Not sure what happed there, I must have got lost scanning across the spreadsheet. Try No. 23, 3.91mm, if it cuts to size an that's too tight pop a 5/32" drill through – with so little to remove it should finish dead to size which is 3.97mm, which should give you your press fit.

Neil

[Roy MParticipant @roym]

Ref; Reaming.  It may be that the coolant mix is not usually used at max strength, or closely monitored,but you have probably noticed that you get a different finish when using cutting oil and it is most likely the same phenomena. I know that the science of cutting fluids is complicated and my observations are only gained from experience and are not at all scientific! I would reiterate that the difference is small, but it is a trick worth knowing. Roy M

Edited By Roy M on 25/04/2015 15:08:52

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Roy M on 25/04/2015 15:07:40:

Ref; Reaming. It may be that the coolant mix is not usually used at max strength, or closely monitored,but you have probably noticed that you get a different finish when using cutting oil and it is most likely the same phenomena. I know that the science of cutting fluids is complicated and my observations are only gained from experience and are not at all scientific! I would reiterate that the difference is small, but it is a trick worth knowing. Roy M

.

Thanks for the thoughts, Roy

My own [wild] guess is that the Rocol lubricant enables the reamer to cut a near-perfectly circular hole; but the water based fluid allows some chatter [for want of a better term] and produces a very slightly lobate hole.

… any sense in that ?

MichaelG.

.

Trick noted for future reference, anyway !!

[julian downParticipant @juliandown56088]

Well, I only have a unimat lathe and it's a long shaft, otherwise that would be easier! That's interesting about the water-based coolant, thanks. The work should fit in the fridge, just about so I'll try that too.

One thing I've often wondered – if you heat up a piece with a hole in, does the hole get bigger or smaller? I can imagine the material as a whole expanding, but doesn't the hole get smaller as the material around it expands?

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