measureements for dowel holes...

[Brad-Man]

I have the need to create a jig for locating dowel holes so that individual aluminum cylinder heads for my 2 stroke motorcycles are centered on the cast-iron lined aluminum cylinder assemblies.

I intend to use aluminum dowel pins .25" in diameter.

I have searched and I have not found an answer for my question:

The dowels I will be buying are slightly oversize for a tight fit in the hole I drill in the cylinder assembly, but how much clearance should I allow for the dowel in the cylinder head?

Is .001" enough?

[SteveM]

I don't know the answer to your question, but the question I have is why would you use aluminum dowel pins?

Aluminum tends to gall on aluminum. Also, they will wear out faster.

Good, accurately and consistently sized steel dowel pins with smooth finishes are already available.

Steve

[TomB]

I have the need to create a jig for locating dowel holes so that individual aluminum cylinder heads for my 2 stroke motorcycles are centered on the cast-iron lined aluminum cylinder assemblies.

I intend to use aluminum dowel pins .25" in diameter.

I have searched and I have not found an answer for my question:

The dowels I will be buying are slightly oversize for a tight fit in the hole I drill in the cylinder assembly, but how much clearance should I allow for the dowel in the cylinder head?

Is .001" enough?

I use to design locating fixtures for industry and it was well known that the problem with using two dowels was that you could not control the distance between them as closely as one could control diameters. With two dowels you had to use diameters that allowed for the worst case oversize dowel + the worse case undersized hole (say .0005) + the worstcase over or under spacing of the dowels and holes relative to each other (say +/- .010). Figure all that out and you probably must have 10 or 20 thousands of clearance mostly from the spacing factor and then the part will be rotationally loose. The solution was to use one dowel and one diamond pin. The diamond pin was rotated into the position that its two fitted sides prevented the part from rotation around the dowel. With that approach you could use a 1/2 thou or less clearance on the holes and +/-.005 or +/-.010 on the spacing between the holes.

Tom

[Richard_W]

I have the need to create a jig for locating dowel holes so that individual aluminum cylinder heads for my 2 stroke motorcycles are centered on the cast-iron lined aluminum cylinder assemblies.

I intend to use aluminum dowel pins .25" in diameter.

I have searched and I have not found an answer for my question:

The dowels I will be buying are slightly oversize for a tight fit in the hole I drill in the cylinder assembly, but how much clearance should I allow for the dowel in the cylinder head?

Is .001" enough?

Should work fine. They make reamers in sets that are .001 under, on size and .001 over. I tend to shoot for .0005 to .001 for a slip fit.

Although I would use the standard hardened, chromed and ground dowel pins.

Richard W.

[Brad-Man]

Thanks guys.

[tornitore45]

Re read Tom comment!

It does no good to have 0.0005" on the dowel/hole diameter if you are aunable to hold 0.00001 between the two dowel spacing.

[10 Wheeler Rob]

I have seen industrial locating jigs that used one round pin and the second pin was a flattened diamond shape to just pick up two sides of the hole perpendicular the line between the two hole centers. This eliminated the second pin form binding due to error in the hole spacing on the parts.

But in this case the question is how accurate dose the head to cylinder alignment need to be, if its a few thousands, then clearance on the oversize hole could be more than 0.001.

Rob

[Harold_V]

I have seen industrial locating jigs that used one round pin and the second pin was a flattened diamond shape to just pick up two sides of the hole perpendicular the line between the two hole centers. This eliminated the second pin form binding due to error in the hole spacing on the parts.

Correct. That's what TomB was describing in his post, and what is commonly used by those of us who do or have built tooling. The diamond, if narrow enough, allows for a substantial amount of error in hole location, yet still provides for proper orientation.

But in this case the question is how accurate dose the head to cylinder alignment need to be, if its a few thousands, then clearance on the oversize hole could be more than 0.001

If a drill fixture with liner bushings, along with slip renewable bushings for drilling and reaming is used to establish the hole locations, a thou clearance may be adequate, even without the use of a diamond pin. Key to success is in achieving identical results for each set of holes. A thou clearance for the dowels in that instance is acceptable, although a reamer will generally provide slightly less, assuming it's cutting size. That's because dowel pins tend to run about .0002" above nominal.

For pins less than ½", I prefer a reamer that is no smaller than a half thou under nominal, otherwise you can raise issues when pressing the pins. A half thou is a very acceptable press fit for a ½" pin. Less is highly recommended for smaller pins.

The OP suggested the use of aluminum dowel pins. I wouldn't make that choice, not for any reason. They are too easy to upset and will most likely lead to problems that may not be easily resolved. Use stainless if steel isn't acceptable.

Harold

[shootnride]

TomB or Harold,

Either of you guys have a photo of these diamond shaped pins in use ?

Thanks

Ted

[GlennW]

http://www.carrlane.com/catalog/index.c ... 285356405F

[Harold_V]

It's quite easy to make a diamond pin if you have a cutter grinder at your disposal. You can do it with a surface grinder, too. That was common practice in the toolroom where I was trained. Start with a standard dowel pin.

While I don't normally recommend a pin be soft, you can even make them on a mill, using an indexing head if you don't mind using machinable stock. It's best if they are hardened, however.

Harold

[Richard_W]

If you have a mill with a DRO then there isn't any reason not to use standard pins. Diamond pins are for tooling where the part is removed and another is installed often. In this application drilling and reaming will give acceptable results and be with in a few tenths of location just using DRO. I have made many drill, mill and lathe fixtures with out a problem this way.

Richard W.