Locating dowel hole sizes

I'm making a new aluminum adapter plate (for a unique engine/gearbox combination).
The dowels in the back of block are ground to .500" diameter.
I can drill the adapter holes to .505" (5 thou clearance) -
- in practical terms is that accurate enough?

To achieve that I first drill a 31/64" pilot hole then slowly open it up with a well lubed 1/2" drill.

If tighter is recommended what size reamer should I buy?
Do reamers produce a slightly oversize hole like drills do?
What reamer size should I buy - .5005" ?

Thx.

Drill under sized and ream the hole to the finished size. You can buy reams that are over sized. Using a drill is not a very good practice for accurate sizing.

The other option is to use an adjustable ream. The adjustable reams I have are hand reams and power reams. I would ream to .500 and use the adjustable ream for the exact finished measurement.

Thanks Steve - I just didn't want to assume that.
What finish size should I aim for - .502?

Go for .500 and see how it fits together first before opening it up any further.

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Originally Posted by dereklola

Thanks Steve - I just didn't want to assume that.
What finish size should I aim for - .502?

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I think that 5.002" should work. I would go for that or less and see how things work.

When I do engine adapter plates, I use a good quality 1/2 drill. But I am machining the part on a CNC mill and I am using a short drill. Thinking about the dowels on a Ford block, you might find that the distance between the 2 dowels on the block and your aluminum part will be hard to hold .002 inch accuracy.

You might be overthinking this a bit.

You can buy reamers in .0005 or smaller increments normally. Oversize on a .500 you can probably find .0002 increments on the plus side.

Steve is probably right on the tolerance. Depending on how far the holes are apart, you might get that much expansion in the aluminum.

Again - thanks - both. Will now get on with the job.

I do use short drill bits for jobs like this - and on my mill (but it's not CNC).
And I agree with your comment about the lateral distance problem which
is why I inquired if 5 thou dowel hole accuracy was OK. And for same
reason am going to use a diamond shaped dowel for one side.

It's not a Ford block - it's a Buick/Rover V8 - hence the need to make a
unique adapter.

Derek, what are you mating together?

Regular two land drills make a triangular shaped three lobe hole. Spiral land hand reamer will make a fairly round hole.More lands equal more smaller lobes and hence a rounder hole. Coefficient of expansion will be a more troublesome feature of trying to hold a close tolerance in aluminum over that distance.

You may think about using the block as a reamer bushing as you ream the holes in the adaptor. That would maximize your matching the center lines of the adaptor to the block. I assume you are trying to match the center lines of the two parts.

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Originally Posted by steve zemke

You may think about using the block as a reamer bushing as you ream the holes in the adaptor. That would maximize your matching the center lines of the adaptor to the block. I assume you are trying to match the center lines of the two parts.

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What a great idea. Have been trying to think how I would do this. The other thug that is important that others have pointed out is to sneak up on the correct oversize of the hole for the dowels. If you do get the holes too oversize, step dowels are available ( one size on one end and oversize on the other end. ) to save the day.

Well the adapter plate is finished - see pics in next post. Didn't take me all the 6 weeks since the previous post - I also rebuilt a bathroom and did some outside brick paving - retirement isn't all fun!

I still don't understand how a .500 ream, or .500 end mill, or .500 short drill can make a .500 hole. They all gave me oversize holes. Solved that problem by holding a fine stone against a rotating end mill to reduce it by about .002. Worked fine. The two dowel holes now fit the .500 dowels perfectly.

We got the dowel hole separation accurate by machining 'points' on short pieces of .500 steel rod (which was as accurate as the dowels) and inserting them in the dowel holes. Then got the centerline position by mounting an adjustable block on a loose plate (manufacturing aid) and centering it on the crank c/l. This loose plate was then used to center the adapter plate casting to the rotary table on the mill before machining the lip to locate the front of the FT200 housing. Much to my relief when finished it all just 'clicked' together.

Thanks to both Steves and others for your helpful advice - all of which encouraged me no end.

It is a couple of lbs too heavy but that doesn't worry us - better safe than sorry. Anyway we don't have FEA ability.

It would have been easier, but a lot more expensive, using a big billet of aluminum - and making the centerline hole first. But we already had the raw casting.

Pics coming on next post.

Adapter plate pics -

https://www.apexspeed.com/forums/att...id=85325&stc=1https://www.apexspeed.com/forums/att...id=85321&stc=1https://www.apexspeed.com/forums/att...id=85323&stc=1https://www.apexspeed.com/forums/att...id=85324&stc=1https://www.apexspeed.com/forums/att...id=85322&stc=1

You did that on a manual mill????

Art is not dead. What a masterpiece.

Yep! If this isn’t racing porn, I don’t what is.

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Originally Posted by Steve Demeter

You did that on a manual mill????

Art is not dead. What a masterpiece.

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Well I can't find any pedals on the milling machine so I guess it is a manual.

I have Derek’s Lola T91/50 now and had the great pleasure of visiting him at his home a few times. I can personally attest to the fact that he does amazing work that today’s engineers wouldn’t consider without a CNC machine! Very modest setup in his 2 car garage but he’s extremely capable and talented. Sometimes I just stare at some of the bits on the Lola and wish I had the time.. then I wonder if he might retire in Florida and buy the house next door. A guy can dream...

Drills, Reamers, end mills, etc. never make a round hole, and it is never the size stamped on the tool. Single point reamers can be adjusted to give the result your want, or you can do what Derek did. Adjust the cutter size to compensate for the machine error built into the spindle. Beautiful work Derek