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Mind the Gap ! How much play in an axle ?

How do you accurately fit an axle to a flywheel ?

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Brian John04/08/2014 06:16:34
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I have a number of flywheels and stationary engines to be machined and constructed when my Sieg C0 lathe arrives. (The original had to be sent back as it was damaged in transit.) I am going to buy some 1/4'' silver steel and possible some stainless steel to use as axles.

1. If the hole in the flywheel has been drilled with a 1/4'' drill bit then what size should the axle be ? It must be less than 1/4'' otherwise it will not fit but how much less ?

2. Should I be buying 1/4'' bar stock for this or should I buy something larger and machine it down ?

3. What is the best way to put a flat section on the axle so that the flywheel grub screw will have something to sit on ? I do not have a milling machine.

Edited By Brian John on 04/08/2014 06:18:45

JasonB04/08/2014 07:43:31
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1. A drilled hole in the flywheel is not ideal, drills can cut oversize and wander. Better to drill in steps upto 6.2mm then ream1/4" which will give a good sized hole. or drill to 7/32 and finish with a "D" bit or 1/4" slot drill. Your silver steel should fit straight into a hole made this way. As you are down under 6mm may be better as you will be able to buy metal and tools locally.

2. To start with you will be better off buying in 1/4 SS or Precision ground mild steel as its not easy turning a long slender shaft with no turning experiance

3. You can just file a flat for the screw, either a pillar file which is narrow if you have one, square file or the cutting edge on a common hand file will do.

J

frank brown04/08/2014 07:53:00
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3. Do not put a flat on the shaft, it is better to measure carefully and drill a dimple in the shaft, this will then stop the flywheel coming of the end.

2. Use 1/4" silver steel rod its extremely accurately made.

1. Its best if the hole is 1/4" + .002, but it is unlikely that your drill will drill the hole so small, its more likely to be + .005" over size. For really accurate work it is best to bore the hole on the lathe, using the drill rod as a gauge. I would think as 1/4" equals 6.35 mm, a 6.3 mm drill, should be about right. 6.3 mm = .248" . if the hole turns out to be too small, then putting the shaft in the lathe and using a fine file will soon remove .002" from the diameter..

Frank

Bob Brown 104/08/2014 07:56:37
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Most drills will drill over size, by how much depends on the drill and the size of the hole, also drills tend to drill holes that are not round. Small holes for precision fits are usually reamed to size, ideally very slightly under size. An easy way to find out what size the holes are and what they are like, is to machine a piece of stock bar to fit the hole then measure it and see how it fits, you normally see three witness marks in the shaft.

I tend to put a small indent into the shaft to accept the grub screw rather than put a flat on the shaft.

Nigel McBurney 104/08/2014 09:48:37
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The flywheel should be a very good fit on the shaft,with no clearance,if made a running fit the grubscrew will pull the flywheel out of true when tightened, Start by facing the hub of the flywheel hasting ,ensuring the tool is dead on centre height so that no central pip is left, a pip will push the centre drill off centre, centre drill then drill 5.5 mill for about 4 mm deep,then get a very small boring tool and bore the hole to 6.2 this will produce a round true running hole to ensure that the reaming size drill 6.2 or 6.3mm will start dead on centre,then drill through and then ream to size. this should give a good fit on silver steel, note mild steel rod is always slightly undersize so silver steel is best,avoid stainless steel it is not a good material to run in bearings. if you have new drills or reamers ,drill and ream a couple of hole in some mild steel as very often new drills and reamer cut slightly oversize and a couple of practice holes will take minute grinding burrs off the reamer, Drill and ream cast iron dry, ream steel with a shot of oil from an oilcan on the reamer. Take time and care there is nothing worse than a wobbly flywheel

Eric Cox04/08/2014 09:50:36
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As it's just a fly wheel on a "small" stationary engine and not driving anything, does it need a flat on the shaft. If its finished and not going to be disassembled use bearing fit to "bond" the wheel to the shaft.

Neil Wyatt04/08/2014 11:05:54
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Standard reamers are made with tolerances of +0.0001" to +0.0005"* and leave the hole very slightly oversize, with a 1/4" shaft this is a close running fit, and a thin boss may still allow a little wobble. Jason's suggestion of a d-bit made from the same diameter silver steel as the shaft is best.

I find the most reliable solution is to make the flywheel the best fit possible on the shaft, and finish turn the rim of the flywheel with it on the shaft (or a dummy shaft) between centres.

Neil

*According to my Machinery's Handbook

JasonB04/08/2014 11:39:09
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Posted by Eric Cox on 04/08/2014 09:50:36:

As it's just a fly wheel on a "small" stationary engine and not driving anything, does it need a flat on the shaft. If its finished and not going to be disassembled use bearing fit to "bond" the wheel to the shaft.

The problem with not using a flat or dimple is that as you tighten the grub screw it well mark the shaft and form a burr. When you then try to remove the shaft at a later date this burr will scratch any holes it passes through which could well be the bearings. The same can happen if teh flywheel should slip, the screw will scrape a ring around the shaft

If the threaded hole is long enough you can sometimes get away with placing a small slug of soft aluminium or copper in the hole first so the grub screw just squashes this against the shaft. This is a useful method when setting eccentrics as it allows some adjustment then once you are happy with how the engine runs make a permenent flat/dimple.

J

Ian S C04/08/2014 13:40:42
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Brian, it depends what the flywheels are for, on my hot air engines I use 10 32 UNF brass slotted head grub screws(ex electrical fittings), these don't damage the shaft, I use these on flywheels and pulleys. On my motors these are removed, and replaced quite often. Most of the wheels are 3/8" bore, but some of the shafts are 1/4", so the gap is taken up with a split bush, I then use a steel grub screw.

When I rebuilt a Stuart Turner S9 I used a gib head key in the flywheel, can't remember the size, it may have been 1/8", the key way cut in the shaft with a home made slot drill, the FW had the key way cut, but I had to make a new crankshaft.

An IC engine needs a very secure fixing to the shaft.

Ian S C

colin hawes04/08/2014 14:02:05
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A slot drill held in the tailstock chuck is a useful way of ensuring concentric accuracy on small holes as it is effectively a boring tool and will correct any initial wander of the first drill. the idea is to centre drill, small drill, slot drill carefully with the tailstock barrel lightly clamped then drill the hole to reaming size followed by the reamer. Don't leave too much to ream as 8 thou (0.2mm) is a lot for a small (1/4 " reamed hole. Machine reamers can be used at quite a high speed. I use motor oil as a lubricant for reaming. Colin

Edited By colin hawes on 04/08/2014 14:03:07

colin hawes04/08/2014 14:05:08
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Don't know why I got a smiley. Editing didn't alter it. Colin

Neil Wyatt04/08/2014 14:47:54
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The system has a nervous tic every time someone posts a " followed by a ).

Neil

Roderick Jenkins04/08/2014 16:24:09
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I've found it's best to put a space between any pair of non-alphanumeric characters otherwise the system will insert a smiley at the slightest excuse : )       : (        ' )    

Rod

Edited By Roderick Jenkins on 04/08/2014 16:29:07

Neil Wyatt04/08/2014 16:38:30
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This is a subtler bug. The quotation mark doesn't get replaced, only the bracket. Your examples are all replacements for recognised smilies.

Neil

Brian John16/08/2014 05:44:52
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Should I be using a 1/4'' reamer or an adjustable reamer ?

**LINK**

**LINK**

http://www.ebay.com.au/itm/1-5-1-4-Cutting-Diameter-HSS-Adjustable-Hand-Reamer-Milling-Cutter-/390848792188?pt=LH_DefaultDomain_15&hash=item5b0067467c

Edited By Brian John on 16/08/2014 05:48:04

JasonB16/08/2014 07:27:33
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The first one will be what you want for reaming in the lathe lathe or mill, Adjustable ones are really only for truing up worn holes which then need an oversize shaft made to suit.

J

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