Newbie- Dowel Pins to the Rescue?

[webgeek]

I've been making a reasonable number of parts on my mill and I'm starting to get a process down. Sadly, it's also starting to show some real weaknesses and I need to address them. I've been using fixtures in 1/2" thick aluminum to hold parts. I've gotten pretty good at mounting and placing my fixtures as needed so no problems there. The issue is holding the work to the fixture in the first place.

Specifically, I've been using 4-40 screws to hold the work down to the fixture - this works BUT there is enough "wiggle" horizontally in a clearance hole for a 4-40 screw that sometimes the pieces move. The movement can be really nasty and basically destroys the piece - it's more than enough that the parts are visibly incorrect.

I've tried using more screws and this works but it makes the process of securing the work take a long time. To get more effeciency, I started cutting multiple parts (vertically) at the same time but this caused the slippage to come back as the parts would be under more stress for a longer period.

So that leads me to my questions - how the heck do machinists do this properly? I've been told dowel pins are the solution but I'm having trouble seeing how. I understand I mount the pins in the fixture and it will be a very tight fit. Let's say 1/8" as that's what I use for most things with the pins being a few ten thousands over 1/8 - this would make them permament for all intents and purposes which is exactly what I want. Then in the parts to be cut, I prep them with #30 holes for the pins. Is this correct? I'm kinda guessing here. #30 is .1285 - that's the next size larger than the 1/8" I can get. It's ~.0032" difference in the pin diamater as compared to the hole diameter vs. the 4-40 major diameter of .112" which is .013" off from the holes. Is that .01" really going to solve this problem?

To make matters worse, I'm looking for dowel pins and I run into "shoulder bolts" which look even better as they would hold the work and force alignment. Would these be the best choice?

So that's the conundrum - something trivial for you pros but really causing a newbie like me some silly quality-control issues with parts I'm otherwise very happy with. Thanks in advance for any advice!

Mike

[KVD]

Depends on the shape of the part. If possible make some aluminum jaws for a vise and machine the contour (or most of the contour) of your work into it.
Then clamp it and make chips. This works great if your blank/part shape will allow it.

If not you can grind or turn your own pins (out of green tool steel preferably) mount them in a fixture so the top of the pin is bellow your part with the pins tight enough to hold your accuracy. Tap the center of the pins for your preffered cap screw and use a washer and the screw to hold downward pressure. Two pins are normally plenty.

Good luck.

Edited: That .01 can make a big difference try to make that .001 or so, tight as you can without interefering with loading/unloading. Shoulder bolts should work as long as the hole in your part is tight enough.

Dowels are good for alignment but then you have to clamps or screws in other places to hold the part down.

[Shane123]

you could also make slide fixtures for some of your easier parts. i dont see too many people using these but they always worked well for the applications i needed.......

here is a really rough drawing in paint, but you get the point of fixturing made easy.....

[webgeek]

Thanks for the good suggestions and the drawing! I'm typically cutting .125" or .25" plate and either toe clamping or using the fixture with the plates screwed down to it. After more reading, it looks like the shoulder bolts are best - they will get me a very high tolerance fit while also holding down the part and still being removable.

I do have a question about the jaws stuff you mentioned - do you know of any good pictures of that in action? You hear about soft jaws all the time but I'm having trouble thinking of a part where those would work. Thanks again!

Mike

[KVD]

On the soft jaws, it works best on round parts. You just close your vise, leaving a gap(.250 or so can work bigger may be required) by clamping on scrap aluminum or something and then cut a circ pocket the size of your part and deep as neccesary. Sorry I don't have pics.

Variation of the trick works for longer rect. parts. Cut a groove down the middle of your jaws and you are left with a slot squared to the travel of your table and deep as you need.

[Shane123]

Originally Posted by webgeek Thanks for the good suggestions and the drawing! I'm typically cutting .125" or .25" plate and either toe clamping or using the fixture with the plates screwed down to it. After more reading, it looks like the shoulder bolts are best - they will get me a very high tolerance fit while also holding down the part and still being removable. I do have a question about the jaws stuff you mentioned - do you know of any good pictures of that in action? You hear about soft jaws all the time but I'm having trouble thinking of a part where those would work. Thanks again! Mike

most the time soft jaws are just aluminum or steel that hasnt been hardened to place in your vice instead of the grounded jaws.

as for my drawing, take a large plate and mill pockets into it for your parts. the pockets will be oversized on usually 2 sides so you have room to slide the part in, and the sliding clamp locks them against the wall of the pocket while applying downward pressure. you can do this for round parts as well, just make your pockets round and your sliding clamp round. you also will want to mill out tool clearance if your doing any side profiling. a better example would be to look at a tombstone for cnc mills. one half turn of your bolt and the part is loose and ready to be replaced with another, so on a big plate you could have a gang of parts, very productive for freeing up your operators to run multiple machines at once.

as for the sliding clamp, we usually make those at 30 degrees. you can make whatever slant you want, but i prefer steep for more downforce with less turning of the bolt. i have seen shallow pockets of 1/16 inch, but i usually make em about .125 to .1875 since some of my cuts can be pretty mean.

if you need to profile all sides of the part in one setup, then you will need a center locking mechanism thru the middle of the part, but same concept.

but since your typically cutting plate, my ideas wont be much help, lol. you will be better off with vises and large soft jaws in most occurances. we have a waterjet and a lazer so i dont usually have to mess with plate in the mill, unless its for tapping, countersincs, beveled edging or engraving.

[Shane123]

Originally Posted by KVD On the soft jaws, it works best on round parts. You just close your vise, leaving a gap(.250 or so can work bigger may be required) by clamping on scrap aluminum or something and then cut a circ pocket the size of your part and deep as neccesary. Sorry I don't have pics. Variation of the trick works for longer rect. parts. Cut a groove down the middle of your jaws and you are left with a slot squared to the travel of your table and deep as you need.

a simple v-style soft jaw helps tremendously also for one-off round parts also. another cool trick with soft jaws are dowel pin holes, and just place dowel pins in the holes as needed to hold the part

all our vises (mostly kurts) are 6" so the standard 3.875 spacing by .9375 from bottom pattern works for us on making jaws. we usually make about 40 new soft jaw sets a year, do all the counterbores and everything, and set them aside for when we get new part designs. Then usually within half hour, we are ready to start making chips!

[KVD]

Shane: I like that fixture now I have to try it.

[Shane123]

Originally Posted by KVD Shane: I like that fixture now I have to try it.

the dowel pin jaw or the v block jaw?

the dowel pin jaw isnt my idea, seen it about 2 years ago in modern machine shop monthly book we get, in an ad. except on their version, they have it filled with all the pins, and push in the ones that are in your way, and to reset it to all pins out, you blow it out with an air gun on the side because they have it air channeled in the jaw to the side. i just didnt want my pins that loose nor did i want to channel the jaw, lol. a simple pattern in mastercam for some holes on one jaw works or what i need. looks like a chineese checker board

[djr76]

Originally Posted by webgeek Thanks for the good suggestions and the drawing! I'm typically cutting .125" or .25" plate and either toe clamping or using the fixture with the plates screwed down to it. After more reading, it looks like the shoulder bolts are best - they will get me a very high tolerance fit while also holding down the part and still being removable. I do have a question about the jaws stuff you mentioned - do you know of any good pictures of that in action? You hear about soft jaws all the time but I'm having trouble thinking of a part where those would work. Thanks again! Mike

Another option instead of using shoulder bolts are Flat Socket Head Cap Screws if you are mounting with existing holes. The taper will prevent any movement.