Reaming for a sliding fit

I'm attempting to dowel a tool I am making so that it can be unbolted and then re-assembled in the correct orientation without any clocking in again. I'm using 2 off 6mm dowels and would like to know if there is a way of reaming a hole on one of the parts to get a sliding fit - without having to purchase an expensive reamer! I have calculated that the sliding fit needs to be about 6.025mm (I think!)

Any thoughts or suggestions welcome....

Matt

Assuming that you have one reamer, for the 'tight' holes, Matt ... I would be tempted to either:

• lap the 'free' holes to a suitable size, or
• make all the holes the same size, and polish one end of the pins down a little in the lathe, before assembly.

MichaelG.

If it is a hand reamer that you have you can make use of the tapered lead. Ream right through the bit you want a sliding fit, most off the shelf 6.0mm reamers will be H7 tollerance so you should get a sliding fit. On the part you want the pin to be a press fit don't ream in past the taper which will mean the hole is a bit smaller and the pin will be a press fit.

Edited By JasonB on 05/12/2016 20:42:44

Good point, Jason

Definitely worth checking the fit of the reamer before making any decision about process.

MichaelG.

I was surprised that a new h7 machine reamer made holes that were on the loose side for dowels, I took this up with a jig fitter who spends his life fitting dowels and he said they would stone a reamer to get the fit they wanted, as they don't buy them it was not a problem to adjust them, he even said the would run it in reverse in a drill and stone it. You may get a better fit with an old reamer than a new one.

Mike

You could ream both holes for a sliding fit and loctite the dowel into one one of them.

Paul

The hole size can be influenced by a few factors. Obviously if the reamer is worn undersize, it will be difficult to make it cut big. The hole size can alter depending upon the type of material being cut, and also the amount of material you leave on for reaming and the speed and feed rate. I have mentioned in a previous post that the type of coolant used will also affect the hole size, neat cutting fluid(rocol etc.) will tend to cut a looser fit and I have used this to get a nice push fit. Also remember that hardened dowels are normally ground over nominal size. These are just practical tips learned through donkey's years of tool-making. Roy M.

Hi
If the old reamer cuts under size use an hss tool bit you can bump up the reamer by running the tool bit on the front of the cutting edge bump it up
Nobby

Thanks guys, there are some very interesting and useful tips here. The test reaming I did was with a hand reamer (new Dormer 6mm) and putting the whole reamer fully through gave a press fit for the dowel. So an increase in size for the sliding fit would require either the lap, a larger reamer. I like the idea of 'bumping up' the cutting edge of the reamer. I have a spare 6.00mm one I can try this on. As the pins are hardened, it will be easier for me to sort the holes!! I may have a machine reamer and I may try a few test holes with that as well to see what differences I get.

Matt

Taper reamer.

You could simply loctite one half in. Part way through Dore Westbury development they decided that it tends to self centre things. Seems to be true.

I believe it's possible to buy dowels that are sized for a drive fit in reamed holes but it needs pretty good equipment to make sure that what ever is driving the reamer does so dead true. From what I have seen roll pins often doubled are usually driven into drilled holes now. Seems that they are sized so that another can be driven down the centre to negate the effect of the split in the pin.

John

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Industrial standard Hardened & ground dowels are meant to be a light drive fit. Dowels are made marginally oversize to achieve this ( a few " tenths" ) . Sliding fit dowels will not give a precise location as there will be some clearance involved. The use of H7 class reamers will also give additional unwanted clearance. H6 reamers should be used. How hard is it to drive the dowels out ? 0.025mm clearance will give quite a loose fit. Knock out holes should be provided. It is better to drive the dowels out rather than have plates becoming canted over and jamming and damaging the holes. Normally H7 holes will give loose dowels. All the old time toolmakers had worn H6 reamers to give a nice drive fit and they guarded these with their lives ! It all depends on your application - you choose ! An old dodge to give a tighter dowel is to tap a ball ( from ball bearing) over the hole - This would be frowned on in professional quarters as repeated dismantling would give a loose dowel and poor location. If the toolroom foreman saw this someone would be in big trouble !

Edited By Dinosaur Engineer on 06/12/2016 00:12:57

Edited By Dinosaur Engineer on 06/12/2016 00:13:23

Get a hardened dowel pin and rub it up the front face, ie in the flutes of a reamer to increase the diameter.

To decrease, rub it on the top lands, only 3 or so swipes per flute.

Toolmakers have been doing this for decades.

Better to stick with a tight fit on both parts for the dowel hole to give positive location.

Then you can grind the end off the dowel pins and drill and tap them so you can screw a high tensile cap screw in there for extraction, either with a simple puller made from a bolt and piece of tubing or a slide hammer than attaches via the threaded hole.

Or you can drill and tap threaded holes in one piece being dowelled together to use with jacking bolts to get parts apart.

Or drill the dowel hole right through the two parts but ream only part way through, so dowel sits in both parts but not all way through. Then you can knock the dowel out from the other side using a punch.

An old trick to get a reamer to ream oversize was to put a piece of paper lengthways along one flute of the reamer so it covers one cutting edge of the flute. Seen it done with machine reamers, not sure about hand reamers with the lead-in taper though.

Or you could just by extractable pins.... ready drilled & tapped!

Mark

> The test reaming I did was with a hand reamer (new Dormer 6mm) and putting the whole reamer fully through gave a press fit for the dowel.

I would look at possibility of doing a combination of John and Marks suggestions.

Source some self extracting ( tapped ) taper dowels and matching reamer.

Make your self a little slide hammer for extraction to take tapped studs for the dowel removal.

Result is likely to be a more positive and consistant repetitive fit.

Dinosaur - I know nothing about H6 & H& but I shall do some reading up to understand this. For a slide fit, what oversize should I be looking for is 0.025mm is quite loose. The material is cast iron, BTW (both parts)

John S - I'm going to do a trial by rubbing up the flutes to see what effects it has.

Hopper - Does this paper then push the cutting edge outwards and then create the larger hole?

Neil - Why the frown?

MalcB - I would like to keep one side of the dowel hole blind and as such I don't think tapered holes will work. I'm also trying not to keep buying reamers.

Thanks again.

Matt

What you want is some precision silver steel. I did have a source of it but they were taken over by some German company. It was 6mm dia as close as I could measure it. That is very likely to be a slide fit in a reamed hole and as I said the other half could be loctited. The clearance on a typical reamer will be about 1/2 the tolerance band.

Wish you luck rubbing but it is possible to reduce the size of a reamer by careful use of a stone. Not much though. All this pain is why sane companies switched to roll pins at least 20 years ago but it does depend on what is being located. The difficulties also explain the reason for taper pins which is what was at least recently being used to locate the parts of drop forge dies.

I was tempted to say that JS has been talking to the fairies but thought I had better not.

You could  of course turn up and polish what ever fit you want.

John

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Edited By Ajohnw on 06/12/2016 09:27:35

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Matt,

[Now that I know the holes are in iron, not hard steel]

My father [a 'scientific instrument maker' by trade] showed me how to make a simple reaner for 'fitting' purposes ... Take a piece of Silver Steel that's a good sliding fit in the reamed hole, and then score a longitudinal line down it ... the raised burr acts rather like a cabinet-maker's scraper, and will remove 'just a lick' from the bore.

MichaelG.