Close tolerance dowel pin fits?

Terry Keeley · Mar 8, 2020

Making some aluminum parts that have two 1/4" dowel pins and need a "no slop" (for lack of a better term) fit.

The pins are 0.0002" oversize but the closest oversize reamer I can find is 0.0005" over. Won't this make a sloppy fit? Especially that reamers tend to cut slightly over anyway?

I did a test piece and barrel lapped the holes until the dowel pin just went in and the mating parts fit was perfect, is there a way to get this with reaming? Or am I dreaming?

Mechanola · Mar 8, 2020

No, you thought right. Reaming is the process before honing or barrel lapping as you take it.

What you can do else is use grooved pins at the price of non-repeatability (pins will damage bores) or rolled sheets. Some call them roll pins.

Conrad Hoffman · Mar 8, 2020

Is that 2" x 1/4" or 1/4" x 2"? Might make a difference. The only good way to do this is probably honing. You can buy reamers in tenth increments (or custom) and they can cut on-size if everything is set up right and the planets happen to align correctly, but there's a bit of luck in involved. A zero slop fit that goes in with only light pressure requires crazy tight tolerances and good geometry. Now, if you can tolerate a couple tenths slop, things get much easier. Same if more force is OK.

DavidScott · Mar 8, 2020

My method for a slip fit is to use a new 1/4" end mill with radiused corners, at least for shallower holes. I adjust the size with feed rate and spindle speed. You can also adjust the size a reamer cuts by honing the cutting edges of the flutes. I do this by honing a 45 degree bevel on them like is done for carbide in steel. These are all methods for production but you did say some parts. You can also adjust the reamed hole size with the drill size, feed, and speed. If you are going for this level of fit and each part has more than one dowel pin you better set your hole location before reaming as drills are not that accurate.

Terry Keeley · Mar 8, 2020

Conrad Hoffman said:
Is that 2" x 1/4" or 1/4" x 2"? Might make a difference. The only good way to do this is probably honing. You can buy reamers in tenth increments (or custom) and they can cut on-size if everything is set up right and the planets happen to align correctly, but there's a bit of luck in involved. A zero slop fit that goes in with only light pressure requires crazy tight tolerances and good geometry. Now, if you can tolerate a couple tenths slop, things get much easier. Same if more force is OK.

Fixed it, two 1/4" pins.

I hear you, but honing is out, I'm just doing this on a mill.

Where do you find reamers in tenths? I don't see them anywhere...

Finegrain · Mar 8, 2020

Keep in mind that when there are multiple of such dowels,the spacing between the dowels will certainly be more difficult to hold to tenths than the hole size. Perfectly sized dowel pins will still interfere if the spacing is off by a few tenths.

Having said that then, why not just press fit the dowel pins? Don't need to hold things quite so closely that way. A few tenths under and you are good.

Regards.

Mike

DanASM · Mar 8, 2020

When I need to make a reamer smaller I chuck it up and spin the reamer in reverse with an oversized bushing over it with grinding wheel dust/sludge inside the bushing. It will reduce the size of the reamer. Check it until the reamer cuts where you want it too. Then mark the reamer so you know it has been undersized for future use.

I had a .2555 reamer and I needed it to cut .2510 -.2550

I got it to hold .2530 after reducing its size.

Terry Keeley · Mar 8, 2020

Finegrain said:
Keep in mind that when there are multiple of such dowels,the spacing between the dowels will certainly be more difficult to hold to tenths than the hole size. Perfectly sized dowel pins will still interfere if the spacing is off by a few tenths.

Having said that then, why not just press fit the dowel pins? Don't need to hold things quite so closely that way. A few tenths under and you are good.

Regards.

Mike

I hear that, it'd never work if it were 3 or 4 pins per part, it's only two.

I have to be able to take the pieces apart and re-assemble them occasionally so a press fit is out.

Larry S · Mar 8, 2020

Your Dowel Pins will not be 2 tenths oversize. They will most likely be in a
diameter tolerance range of -0.000 to +0.0002 or maybe +0.0003".
Your Reamer will have a Dia. tolerance as well so your individual fit will depend
on which pin you pick out to put in a hole.
You might Google Ball Sizing. You can get very accurately sized hard balls to
push through a hole to size it. Keep in mind that there is some "spring back"
in the hole so that the sized hole is a little less than the size of the ball.
How much ? You'll have to experiment. So you may get very accurate holes but
your Pin size varies in the tolerance range.

Larry S

edit: Ball sizing doesn't ensure a Straight hole though. The Ball follows the
reamed hole. Just as the Reamer follows the drilled hole.

Finegrain · Mar 8, 2020

Terry Keeley said:
Where do you find reamers in tenths? I don't see them anywhere...

MSC has them, not all tenths, but lots. Right now I see .2500", .2502", .2504", and .2505".

Regards.

Mike

Terry Keeley · Mar 8, 2020

DanASM said:
When I need to make a reamer smaller I chuck it up and spin the reamer in reverse with an oversized bushing over it with grinding wheel dust/sludge inside the bushing. It will reduce the size of the reamer. Check it until the reamer cuts where you want it too. Then mark the reamer so you know it has been undersized for future use.

I had a .2555 reamer and I needed it to cut .2510 -.2550

I got it to hold .2530 after reducing its size.

Thanks, that sounds like a better way to reduce the size than just stoning as I've done before.

Would it cut =/- a tenth or so consistently tho? If so what sort of technique did you use? I haven't been able to get consistent sized holes in the past.

Conrad Hoffman · Mar 8, 2020

M.A.Ford #272 series probably has you covered, but I'm pretty sure they'll grind any size you like, as will various other reamer companies.

tdmidget · Mar 8, 2020

Why not tapered pins? with parts in place cut the holes and tap them in.

DDoug · Mar 8, 2020

Finegrain said:
Keep in mind that when there are multiple of such dowels,the spacing between the dowels will certainly be more difficult to hold to tenths than the hole size. Perfectly sized dowel pins will still interfere if the spacing is off by a few tenths.

Having said that then, why not just press fit the dowel pins? Don't need to hold things quite so closely that way. A few tenths under and you are good.

Regards.

Mike

Yup, dowel & diamond pin is a very common combination.

DanASM · Mar 8, 2020

Terry Keeley said:
Thanks, that sounds like a better way to reduce the size than just stoning as I've done before.

Would it cut =/- a tenth or so consistently tho? If so what sort of technique did you use? I haven't been able to get consistent sized holes in the past.

It cut fine for me. It just needs to be lined up correctly. If its off slightly it will bell mouth the hole.

Or you can grind the reamer back further so the first part is the correct O.D. and the rest is reduced so any accidental taper would not bell mouth the hole.

I did this 2 months ago and had to go back a few times to get the taper out of the reamer. It is slow to grind the reamer in reverse like I described but i had no problem taking .0001 off. It doesnt come off fast and you just have to keep trying until its correct. I ran 500 holes 1.250" deep with no problems.

Terry Keeley · Mar 8, 2020

tdmidget said:
Why not tapered pins? with parts in place cut the holes and tap them in.

Holes are blind in one part.

digger doug said:
Yup, dowel & diamond pin is a very common combination.

Was able to get a couple test pins bang on with my new DRO and some careful technique. Will keep the diamond pins in mind tho if needed.

DanASM said:
It cut fine for me. It just needs to be lined up correctly. If its off slightly it will bell mouth the hole.

Or you can grind the reamer back further so the first part is the correct O.D. and the rest is reduced so any accidental taper would not bell mouth the hole.

I did this 2 months ago and had to go back a few times to get the taper out of the reamer. It is slow to grind the reamer in reverse like I described but i had no problem taking .0001 off. It doesnt come off fast and you just have to keep trying until its correct. I ran 500 holes 1.250" deep with no problems.

Thanks, I'm going to give this a try, I only have about 30 holes to do but lapping them would be a PITA.

Were the holes you did in 6061? Did you run it dry or with cutting fluid? Take the usual 0.015" or so (on the diameter) out? Thanks!

DanASM · Mar 8, 2020

Terry Keeley said:
Holes are blind in one part.

Was able to get a couple test pins bang on with my new DRO and some careful technique. Will keep the diamond pins in mind tho if needed.

Thanks, I'm going to give this a try, I only have about 30 holes to do but lapping them would be a PITA.

Were the holes you did in 6061? Did you run it dry or with cutting fluid? Take the usual 0.015" or so (on the diameter) out? Thanks!

12L14
1.250" Deep
500 Parts
Browne and Sharpe screw machine with Neat Cutting oil.

I only use water soluble coolant on plastic parts in a "Dry Base" machine.

I never have issues with tapping or reaming on my machines. I ream at full speed for drills with about twice the feed.

The old screw machine trick with cutting aluminum is to thin out your used oil with kerosene to allow it to get into the hard to get areas (Gotta watch out for sparks as this is more fire prone). Or just use the older thinner cutting oil.

If your running coolant I heard you should ream and tap at like 10-12% oil. It still doesnt seem like much of a difference from 7-9%.
I dont have experience with it.

deltap · Mar 8, 2020

Worked in a die shop as a grunt when I was a kid. The pin holes were reamed slightly smaller than the dowel pins so that they had to tapped in with a hammer. The parts were bolted on and then the holes were drilled and reamed through both parts. Jacking bolt holes were installed for removal of part at time part was made. I know there are threaded taper pins for easy removal, not sure about dowel pins. Spring pins only need a drilled hole. Use a new drill so holes don't become oversized.

ratbldr427 · Mar 8, 2020

You can buy ground spring pins for a more precise fit.A little experimentation on bore size and you should be able to get what you want without as far as ease of assembly without all the aggravation of exact bore size.

We have quite a lot of European machinery that uses them, haven't seen them on US equipment.
I haven't looked for imperial sizes but that shouldn't matter metric sizes are available.

David_M · Mar 8, 2020

Tapered dowel pins for blind holes:

https://www.amazon.com/Externally-T...54?s=industrial&ie=UTF8&qid=1583718171&sr=1-6

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