Reaming drill sizs

[Steve HuckinsParticipant @stevehuckins53362]

I have just drilled a part 15/64 (one size smaller) to ream it 1/4. Unfortunately the reamer took little or no material and also the final hole diameter was a bit over size. I have made a bush to go into the oversized hole once I’ve opened it up. I used a 7/32 (two sizes smaller) in the new bush and tested the diameter to find that a 15/64 drill shaft fitted fairly snuggly. I was able to ream with the 1/4 reamer and got a really nice fit.

Firstly Was the problem with the original drill that it cut over size or is not ground at the tip very well. Or am I in some way to blame. ? BTW It was a fairly new Dormer.

Secondly, would taking a larger amount than recommended with a reamer cause damage to it ?

I do go slowly when reaming and follow the speed charts when drilling. Also went in with a smaller drill size first.

Best wishes to all on the forum

regards

Steve

[Pete RimmerParticipant @peterimmer30576]

Well if the hole was already at the reamer size then something made it drill over-size. You should have had 16thou left for reaming.
I suspect it’s a bad drill. Do you have a letter A drill that you can try in a piece of scrap?

[JasonBModerator @jasonb]

For 6mm holes I would drill 5.8 so your 1/64th smaller than 1/4″ should have been OK, I suspect the drill is the problem.

[bernard towersParticipant @bernardtowers37738]

I would go for (the fairly new dormer), in my experience their sharpening is a bit random to say the least and depending how new where was is made they haven’t been made in this ciuntry for some time. And really it should be drill, bore, ream.

[noel shelleyParticipant @noelshelley55608]

A drill will never drill undersize but will often drill oversize and allowing only 1/64″ is not enough, as you have proved. Even a new, good drill bit will make an oversize hole, cutting lube for both drilling and reaming helps. Noel.

[Phil PParticipant @philp]

Quote
“And really it should be drill, bore, ream.”

Thats what my dad always taught me to do. Nobody seems to bother doing it anymore though.

Phil P

[DC31kParticipant @dc31k]

Page 2 here could be a good thing to keep in the useful file:

https://cdn.mscdirect.com/global/media/pdf/search/dormer/dormer-pramet-drill-tolerances.pdf

Be aware that the figures quoted are assuming ‘accurate’ machines and toolholding, so are best case. Apply your own reduction depending on what is in your shed.

[JasonBModerator @jasonb]

Never had a problem with the Dormer drills that I use all from Brazil, always have enough material to ream at 0.2mm smaller >6mm and 0.3mm smaller <6mm. These are the split point A002 and A022 range.

At 6mm and under I don’t usually bother to bore but do on certain larger sizes however I don’t see much point in then getting the reamer out. may as well bore to the fit I want as a bearing will want a bit more clearance on a shaft than a flywheel and I don’t have over and under reamers.

[BazyleParticipant @bazyle]

Quite some time ago the late John Stevenson explained that when he worked for a piano production factory he would test drill with dozens of the same size drill, presumably a reputable supplier, until he found one that gave the hole size they wanted for a particular fit. It would be good for some 300+ holes before repeating the process.

[Bazyle]

On Bazyle Said:

…until he found one that gave the hole size they wanted for a particular fit.

In the engineering literature, has anyone seen an account of honing (or perhaps grinding) the flutes of a drill bit to reduce its cutting diameter?

I guess because of the helix, you would need a hone with a long contact area.

In a similar manner, is there any guidance on how much to deliberately grind a drill off-centre to produce an oversize hole?

Are both of these a suck-it-and-see process?

[SillyOldDufferModerator @sillyoldduffer]

Possibly two problems interacting – softer materials tend to result in oversized holes, which also result from a blunt drill.

My guess is the ‘nearly new’ Dormer wasn’t in top condition.  What was the metal?

 

[JasonB]

On JasonB Said:

Never had a problem with the Dormer drills that I use all from Brazil, always have enough material to ream at 0.2mm smaller >6mm and 0.3mm smaller <6mm. These are the split point A002 and A022 range.

At 6mm and under I don’t usually bother to bore but do on certain larger sizes however I don’t see much point in then getting the reamer out. may as well bore to the fit I want as a bearing will want a bit more clearance on a shaft than a flywheel and I don’t have over and under reamers.

You can make a reamer cut a little oversize by striking hard down the front of the cutting edges at the cutting angle with a HSS blank or an old brazed carbide lathe tool. It raises a minute burr on the cutting edge and shaves more metal out of the previously reamed, undersized hole. Trial and error of course to get the right amount of pressure to get the right amount of oversize.

[JasonBModerator @jasonb]

But then I’d need a second reamer for the next unmodified hole so may as well just buy an oversize one rather than modify the original.

Anyway the bearings tend to be OK at a standard H7 reamed size , the flywheel I would want tighter so need a reamer that cuts smaller not larger and not sure how easy it would be modifying the 3mm and 4mm reamers I was using yesterday.

[HollowpointParticipant @hollowpoint]

I am surprised no one has asked what machine you used to drill the hole? An old clapped out drill press with 0.2mm runout at the chuck is obviously gonna produce a terrible hole! For example.

[Howard LewisParticipant @howardlewis46836]

The problem could be run out on the drilling machine, (Chuck or spindle) or the drill could have been ground with unequal length lips (In which case it will cut oversize).

Worth checking.

If the drill chuck, or spindle are faulty, the reamer might be off centre, and so act as a boring bar, rather than a proper reamer. (If this is the case, some flutes will contain more swarf than the others).

With unequal length lips, the point of the drill is off centre so the longer lip will cut wider ; hence an oversize hole.

Howard

[Graham MeekParticipant @grahammeek88282]

When reaming any hole I always start with a drill which is 0.5 mm smaller than the finished hole. The pre-Reaming drill which is usually 0.1 to 0.3 mm smaller (the amount depends on the finished hole size), than the finished hole. This means the pre-reaming drill has less work to do and is following a pre-existing hole. The use of a slot-drill just to start the hole will ensure the hole is in the right place, but I would never try this on a drilling machine. (Slot drills are usually undersize anyway).

In my 40 years of toolmaking I have never had a failure when following this procedure.

Regards

Gray,

[Steve HuckinsParticipant @stevehuckins53362]

Well I’ve certainly been given plenty to think about. Thanks to everyone for the valuable input. I am relatively new to model engineering with no background in engineering per se. So I’ve relied on YouTube and this forum for help. I have posted several times and had great feedback back.

To fill in some details, I am working on a PMResearch 3 small engine. To drill the pair of bearings for the shaft on the base I set it up accurately on my mill (VM25L). The column lowered to bring the keyless chuck near the work and then locked off. The engine base is cast iron and then with the fine feed I used a Center drill followed by a small drill then the 15/64 through both bearings.  Obviously I then used the 1/4 reamer but the result was as I have described earlier.

I now understand that drills are not always bang on size and need to go down a bit less to begin with and then test what I have before reaming. I will try to bore whenever possible as well.

I am making this engine in my workshop with a friend who has no experience of any form of engineering but was fascinated by the engines I’ve finished so far. He was keen to learn and have a go himself. It is really good to share some knowledge with him and see his excitement when he finishes a part. Just to have an enthusiastic colleague alongside and share the fun is great.

Regards

Steve

[bernard towersParticipant @bernardtowers37738]

Interesting that you reamed in the mill (under power??), what sort of reamer and what speed/feed?

[Steve HuckinsParticipant @stevehuckins53362]

Yes Bernard. A new machine reamer at slow speed with lubrication. Fine feed used.

[bernard towersParticipant @bernardtowers37738]

In that case if the reamer cut metal all the way round the hole the reamer is oversize or the holder/collet etc is not running true. You could try an experiment with a piece of scrap in the lathe, that will take the mill out of the equation. Best of luck.

[bernard towers]

On bernard towers Said:

In that case if the reamer cut metal all the way round the hole the reamer is oversize or the holder/collet etc is not running true. You could try an experiment with a piece of scrap in the lathe, that will take the mill out of the equation. Best of luck.

The first post says it cut little or no material when predrilled 15/64th.

The OP then reamed again using a smaller predrill and got a really good fit of his shaft

Suggests the reamer is OK as it was able to produce a good hole.

[JasonBModerator @jasonb]

Steve, you don’t say what if anything you used to centre the drill as it started the second bearing. If you just let the drill bit find it’s own start then it could have wandered slightly which could possibly have caused both holes to cut oversize as the edge of the drill would have been pushed against the side of the hole. For jobs like this I use a long series ctr drill to get a good start on the far side bearing.

You mention a pilot drill befor ethe 15/64th reaming size drill. If the 15/64th had been ground off ctr then that should not really make much difference as it should cut onsize if a pilot is used as per Graham’s method.

[Julie AnnParticipant @julieann]

An allowance of 0.2mm for a 1/4″ machine reamer is fine. If anything drilling 1/64″ under nominal leaves too much material. Either the drill is the wrong size, is mis-ground or isn’t running true.

A rule of thumb for machine reaming is half the speed and twice the feed for an equivalent size drill.

Julie

[Steve HuckinsParticipant @stevehuckins53362]

Ah …… that makes sense Jason. I only used a centre drill to start at the top bearing and followed all he way through to the bottom bearing with the smaller drill and then the 15/64. So perhaps I should have thought to use a long series center drill, which I do have, to spot the bottom one. Interestingly once complete the bottom hole was a better fit on the shaft than the top one.

I will follow yours and Grahams advice from now on thank you. Also thanks to Julie Ann for the speeds and feeds advice.

Cheers for now

Steve

 

[JasonBModerator @jasonb]

The picture I could not lay my hands on yesterday. Long spot drill in this case. Note I have also cleaned up the inside face of the bearing support so I am not drilling into a cast surface or have a step where the cap is a different width to the bed casting.

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