Machining aluminum crankcases for model airplane engines questions

[jimbo7636]

Machining aluminum crankcases for model airplane engines question (see image of ML Midge crankcase with my notes):
There is a great article here about how to setup the work so the crankshaft bore is perpendicular to the cylinder, but I had concerns about the drill wandering too much during machining on the lathe. I think the concentricity of the 7/32" (0.219) plain journal in the crankcase is perhaps just as important as its surface finish. See article here: https://modelenginenews.org/midge/index.html

1. does it make sense to drill, bore, and then ream a hole, or if you drill then bore that is good enough?
2. does a drill wander more in steel than in aluminum?
3. for example, is it ok to drill a 1/4 diameter hole that is 1 inch long in aluminum and just ream it, or is boring always required?
4. how accurate should the quill runout on the lathe be? Like if its out by 0.002 inches over 4 inches is that too much?

 

[danallen]

If the drill wanders the reamer is going to follow the drilled hole. If you are going to bore then reaming is a waste of time. Boring is a sure way to get a round straight hole BUT that is a long way through a small hole. In your favor is the hole is a through hole.

 

[timo_gross]

Drill in Aluminium can wander, I drilled some holes (were it did not matter too much, but the part was scrap because the holes were off a lot)
It was an 8mm hole about 90 mm, or so, long. I think I pushed the drill too hard, and when (if) there was build up on the cutting edge it went downhill from there. Off very noticeable.

Here I come with my dangerous half knowledge. ( limited experience & internet recycled tips, so this is more a thought than confirmed method )
I got an acceptable wander about: Spot drill, then drill with chubby bit to acceptable depth, or even better bore a few mm deep with a boring bar to the diameter of the long drill. (taking time and keeping drill clean seemed to help a lot) Then follow with the longer drill.

Question to the experienced people: Does line boring make any sense for a 0.219" bore?

Greetings Timo

p.s. an ER8 slim collet nut might fit inside your lager bore, this is good when you want to spot drill inside, without too much stick out.

 

[jimbo7636]

Drill in Aluminium can wander, I drilled some holes (were it did not matter too much, but the part was scrap because the holes were off a lot)
It was an 8mm hole about 90 mm, or so, long. I think I pushed the drill too hard, and when (if) there was build up on the cutting edge it went downhill from there. Off very noticeable.

Here I come with my dangerous half knowledge. ( limited experience & internet recycled tips, so this is more a thought than confirmed method )
I got an acceptable wander about: Spot drill, then drill with chubby bit to acceptable depth, or even better bore a few mm deep with a boring bar to the diameter of the long drill. (taking time and keeping drill clean seemed to help a lot) Then follow with the longer drill.

Question to the experienced people: Does line boring make any sense for a 0.219" bore?

Greetings Timo


View attachment 153119
p.s. an ER8 slim collet nut might fit inside your lager bore, this is good when you want to spot drill inside, without too much stick out.

I did find the ER08 collet - it was 8.5 mm dia. Couldnt easily see what the dimension of the holder was. this could work. At this point if I have to bore the 0.219 hole I will probably make something with HSS lathe tool bit round bar like here:

 

[ajoeiam]

Drill in Aluminium can wander, I drilled some holes (were it did not matter too much, but the part was scrap because the holes were off a lot)
It was an 8mm hole about 90 mm, or so, long. I think I pushed the drill too hard, and when (if) there was build up on the cutting edge it went downhill from there. Off very noticeable.

Here I come with my dangerous half knowledge. ( limited experience & internet recycled tips, so this is more a thought than confirmed method )
I got an acceptable wander about: Spot drill, then drill with chubby bit to acceptable depth, or even better bore a few mm deep with a boring bar to the diameter of the long drill. (taking time and keeping drill clean seemed to help a lot) Then follow with the longer drill.

Question to the experienced people: Does line boring make any sense for a 0.219" bore?

Greetings Timo


View attachment 153119
p.s. an ER8 slim collet nut might fit inside your lager bore, this is good when you want to spot drill inside, without too much stick out.

You can get very low wander on your drill bits - - - the sharpening is the key (as me how I know!!!)
Have drilled to something like 3' with a 1/4" bit - - - a long very tedious job that was! (Wander was quite low!)

Re: boring your 0.219" bore - - - possible - - - but the way that you're doing it you're working with a 6 x dia stickout (actually higher because your par MUST be smaller in dia) when you pass about 4 or maybe its 5 x D in stickout - - - well you're playing in anal sphincter country (that's when your anal sphincter is clenched up tight the whole time your working!).

If this is a quantity one thing I would work on getting a nice very sharp drill and do maybe 0.10" pecks and a sharp reamer and you should be just fine.
Now if you're cranking out a bunch of these - - - time to buy some specialty tooling! (Also depends upon how picky you're getting on your tolerances.)

 

[indian]

I tend to make D bits for small main bearing bores

 

[stanstocker]

A straight flute carbide 13/64 drill will be very stiff and is unlikely to wander in any noticeable amount. Pricey perhaps but if you are really bothered by this issue it's a decent solution. It will leave enough material to ream or do a fine boring pass.

https://www.mscdirect.com/product/details/00971135
Actually a nice new name brand twist drill in this size will probably be more than rigid enough to avoid any notable wandering as well, and be far less expensive. Just do your careful center or spotting drill for location and poke the hole using WD40, kerosene, or isopropyl alcohol as lube. You should end up with a nice hole ready for final sizing.

Another initial opening up of the hole option would be to use a 3/16 end mill then drill and ream or bore to final dimension. Two flute end mills tend to be pretty good at poking shallow holes without any wandering. An end cutting four flute would be just as good in this application as all the cutting edges are in use so there are not offsetting forces coming to the party.

Avoid trying to step drill this hole, the smaller diameter bits will tend to wander. Your depth to diameter is just shy of 4:1 here, not exactly a challenging hole with heat and chip evacuation issues to ruin your day.

If you want to expand your skill set a bit look into making D bits, not my first choice in many cases but a very nice option to have in your pocket on a cold snowy day if you need to get a decent hole in something and don't want to wait for the big brown truck to get to your place.

 

[timo_gross]

A straight flute carbide 13/64 drill will be very stiff and is unlikely to wander in any noticeable amount. Pricey perhaps but if you are really bothered by this issue it's a decent solution. It will leave enough material to ream or do a fine boring pass.

https://www.mscdirect.com/product/details/00971135
Actually a nice new name brand twist drill in this size will probably be more than rigid enough to avoid any notable wandering as well, and be far less expensive. Just do your careful center or spotting drill for location and poke the hole using WD40, kerosene, or isopropyl alcohol as lube. You should end up with a nice hole ready for final sizing.

Another initial opening up of the hole option would be to use a 3/16 end mill then drill and ream or bore to final dimension. Two flute end mills tend to be pretty good at poking shallow holes without any wandering. An end cutting four flute would be just as good in this application as all the cutting edges are in use so there are not offsetting forces coming to the party.

Avoid trying to step drill this hole, the smaller diameter bits will tend to wander. Your depth to diameter is just shy of 4:1 here, not exactly a challenging hole with heat and chip evacuation issues to ruin your day.

If you want to expand your skill set a bit look into making D bits, not my first choice in many cases but a very nice option to have in your pocket on a cold snowy day if you need to get a decent hole in something and don't want to wait for the big brown truck to get to your place.

A new (to me) type of drill. It seems to be even doable with my "home shop" tooling. Do you think this can be DIYed with a Quorn style Grinder?

Carbide blank about 4-6 USD, Grinding wheel 60-70 USD, grinder between 1000 and 500 000 (depends on automatiion grade and model year )

 

[jimbo7636]

I tend to make D bits for small main bearing bores

would you be able to share a photo or sketch. I found this youtube video by clickspring but he was mainly talking about an engraving tool until that last part of the video where he did talk about a d-bit shape to bore a hole. thanks,

 

[stanstocker]

A new (to me) type of drill. It seems to be even doable with my "home shop" tooling. Do you think this can be DIYed with a Quorn style Grinder?

Carbide blank about 4-6 USD, Grinding wheel 60-70 USD, grinder between 1000 and 500 000 (depends on automatiion grade and model year )

If you are talking about the D bit style just knock them out with W1 or O1 drill rod, mill off just shy of half the diameter, harden, and then stone to a good finish.

If talking about the straight flute solid carbide bits I "guess" you could fabricate one on a Quorn, Stent, or similar T&C grinder, but it would be a lot of work.

While D bits will work as drills, they are really better used more like boring bars or single lip reamers that can cut aggressively.

 

[indian]

when I make my D bits I find it easier to get reliable dims if you harden then grind, making them from carbide is even better!

 

[jimbo7636]

If you are talking about the D bit style just knock them out with W1 or O1 drill rod, mill off just shy of half the diameter, harden, and then stone to a good finish.

If talking about the straight flute solid carbide bits I "guess" you could fabricate one on a Quorn, Stent, or similar T&C grinder, but it would be a lot of work.

While D bits will work as drills, they are really better used more like boring bars or single lip reamers that can cut aggressively.

Thanks for giving some details. I found some helpful info in the "George Thomas model engineering" book. So I also think clickspring followed George's advice on a flat-bottomed hole d-bit dill/reamer. I tried to model and dimension that item for the boll aero 18 main journal. let me know what you think. see images below. is the only cutting edge supposed to be the 0.1563 edge shown in my sketch?

 

[minh-thanh]

When I look at your attached picture, it seems that the engine is quite small..
My opinion is to lap the bearing to fit the shaft tightly, which ensures proper fit and surface finish of the bearing.

 

[stanstocker]

Thanks for giving some details. I found some helpful info in the "George Thomas model engineering" book. So I also think clickspring followed George's advice on a flat-bottomed hole d-bit dill/reamer. I tried to model and dimension that item for the boll aero 18 main journal. let me know what you think. see images below. is the only cutting edge supposed to be the 0.1563 edge shown in my sketch?

Your drawing appears fine and dandy. Use LOTS of light cutting oil / kerosene / alcohol with this as you are cutting aluminum with a zero degree top rake. Poking a 1/8 inch hole before using the D bit will avoid having the zero speed center zone fighting you the entire way through the hole and will probably give better finish. As the D bit is guided by it's OD and is quite rigid it won't be led astray by minor wandering in the clearance hole.

 

[Iampappabear]

The sharper the angle on a drill bit, they more they tend to wander, consider using a flat bottom drill, you might be pleasantly surprised at the results but you do need to start the hole with a traditionally pointed drill.

I can't compete with 1/4" through 3 feet but many times have I drilled 5/8" through 4-5 feet. When drilling holes this deep, its good to have hollow shanks and attach coolant lines also much easier on a horrizontal drill as the swarf does not fall back into the hole when retracting the drill to clear the swarf from the drill.

One other thing that just came to mind, when drilling castings, always potential for hard spots, not much you can do about that, but the sharper the edge on the drill, the less it will tend to push off.

 

[rklopp]

Use a new, name brand #4 screw machine length drill bit with 135 degree split point and follow with a 7/32 reamer, and it should come out just fine as long as your chuck has low runout (<.002?) and you can achieve a right angle with the cylinder centerline. That’s how I’ve done it many times over.

 

[jimbo7636]

Use a new, name brand #4 screw machine length drill bit with 135 degree split point and follow with a 7/32 reamer, and it should come out just fine as long as your chuck has low runout (<.002?) and you can achieve a right angle with the cylinder centerline. That’s how I’ve done it many times over.

thanks for the info. I have the 135 split point drill and will be acquiring the reamer shortly. I was going to re-check the quill runout earlier today but then work got in the way!

 

[indian]

to be super safe use a floating reamer holder!

 

[Steamchick]

Just an odd comment about boring... I.E. parallel bores, not my posts.
"If its out by 0.002 inches over 4 inches is that too much?" - YES!
Many thing a lathe is really accurate and useful if you bore into a workpiece held in the chuck. - Not so. The travel of the tool describing a straight line through the workpiece relies totally upon the accuracy of the alignment of tailstock travel or saddle (for saddle mounted tools) to headstock (quill)... 1 degree misalignment gives you a 2 degree included angle taper.... NOT good for maintaining compression in an infernal combustion engine, hydraulic cylinder, etc..
BUT a tool mounted in the Quill, that traverses a workpiece mounted on the saddle or tailstock will always produce a parallel bore. - you just have to set the alignment of the workpiece to the alignment of the quill/mainshaft. I.E. a rotating tool describes a true circle, which will produce a "true" cylinder when traversed through a workpiece in a straight line.
Whatever anyone else shows on U-tube, you cannot escape geometry. (And "Modern" production methods - that I was taught in the 1960s...).
Hope that helps you get parallel bores on your aero engines? Makes a big difference to compression and running-in. (friction/wear) and lifetime of bearings.
Cheers.
K2

 

[Steamchick]

Stanstocker. I have often used end mills instead of making D-bits - or grinding drills to a square end. My experience in the miller is very good - where I have run a small pilot through so there is no centre metal.
But I have made oversized and tapered holes with end mills used in the tailstock of my lathe (that has a tiny twist in the bed - measurable with a DTI). - As the mill is a very good sharp tool that cuts where it sees metal, and has no natural centreing, so can cut oversize... Conversely, I have used a bullet-centred drill that has some small amount of natural centreing to start the hole "at size" so the mill (if floating) can centre on the start made by the bullet drill. (maybe 1/8 in). Usually, this first part of the bore disappears as I allow an extra 1/8in for finish facing after boring to depth.
I once mounted an end mill in the tool post to use as a boring bar (an old chipped tool that only had 1 good cutting edge, and was just a bit smaller than the hole I wanted) - where I wanted the sharp corner to leave a clean corner and flat bottomed hole...
Any use?/Comment?
K2