What's wrong with my thinking?

[Larryx]

I am preparing to do the machining on the carrier block. Owing to my lack of capabilities ( my ineptness, not my machinery) I am making a trial run in delrin. I have the  properly sized block blank , complete with the 10 holes drilled per print mounted in a 4 jaw which is mounted on a Hardinge collet indexer. The cylinder has been indicated to 0.002 concentrically.  I have a 20 position index ring on the indexer and every other hole is blocked which gives me 10 very accurately spaced stops. ( I know that because the Hardinge literature tells me that)  I align   2 holes with a magnetic square mounted (magnetically stuck) to the milling machine table bisecting the top and bottom holes plus the shaft  hole in the center.  As a side note, I positioned the 10 holes using the hole feature on my DRO.  My belief is that if I  use this setup, The milling cutter will be perpendicular to the desired flat, and a flat will be produced with each indexed position. I begin the indexing on the top hole, cycle through each position, but by the time I get to the 10th hole, I an nor longer bisecting the next hole.  Any suggestions? also, any else going to Cabin fever expo next week? 

[Dave]

Wrong indexing plate for the indexer? Arent some 40 to 1 and some 36 to 1?

[Dave]

Cutter  may be able to help you trouble shoot.

[Larryx]

I am using the 20 hole index plate and I am using every other hole.

[Dave]

Maybe the holes are off. measure some things with a caliper just to make sure they are close....

[Cutter]

Hole to hole edge with calipers should be consistent about .201

top & bottom

[Larryx]

Cutter hit the nail right on the bulls eye.  My hole to hole measurement is as follows: 1-2 0.213  2-3 0.203  3-4  0.210  4-5 0.206  5-6 0.208  6-7 0.210  7-8 0.220  8-9 0.201  9-10  0.206

  10-1 0.206  

 

The  technique I used was obviously ka-ka*. I drilled the holes originally with the 0.281  drill just to determine if the DRO  bolt circle routine would work, removed the  carrier from its mounting and returned at a later time, re-established ( or so I thought) the position and drilled out   the 0.420 dimension. Obviously what they say about removing the work prior to completion is correct. There goes $3.00 worth of delrin and a large chunk of my pride down the proverbial drain.

 

* insufficiently accurate

[Dave]

Cut the flats at least.... Fuck it, then you can still see if your set up is good

[Larryx]

Will do .


thanks to all who helped with my question

[Sparky_NY]

Cutter hit the nail right on the bulls eye.  My hole to hole measurement is as follows: 1-2 0.213  2-3 0.203  3-4  0.210  4-5 0.206  5-6 0.208  6-7 0.210  7-8 0.220  8-9 0.201  9-10  0.206

  10-1 0.206  

 

The  technique I used was obviously ka-ka*. I drilled the holes originally with the 0.281  drill just to determine if the DRO  bolt circle routine would work, removed the  carrier from its mounting and returned at a later time, re-established ( or so I thought) the position and drilled out   the 0.420 dimension. Obviously what they say about removing the work prior to completion is correct. There goes $3.00 worth of delrin and a large chunk of my pride down the proverbial drain.

 

* insufficiently accurate

The trick to consistency is to drill them THEN bore them.   The boring bar will not walk like the drill does.    Makes a huge difference.

 

A good job on that part is critical to a smooth functioning gun later.   That and the bolts are the most critical parts of the gun.

[Larryx]

I will follow your instructions.

 

It is not obvious to a newbie  that such experienced techniques are so critical. 

 

If I understand your comments, I drill the hole and then ream prior to  proceeding to the next hole (no movement of the work). It appears that a relatively short length reamer will be required to preclude  large movement  on the Z axis. My DRO is one of the Chinese version that displays position to 5 decimal places so the hole location should be accurately placed.

[Sparky_NY]

I will follow your instructions.

 

It is not obvious to a newbie  that such experienced techniques are so critical. 

 

If I understand your comments, I drill the hole and then ream prior to  proceeding to the next hole (no movement of the work). It appears that a relatively short length reamer will be required to preclude  large movement  on the Z axis. My DRO is one of the Chinese version that displays position to 5 decimal places so the hole location should be accurately placed.

Not reamed, bored with a boring head.   A reamer will still follow a drilled hole that is not true.    A boring head will true up any walking off center the drill bit did which is pretty much guaranteed to happen with deep hole drilling like this.

 

In general terms,  when holes need to be very accurately sized and exactly on location,  boring is the accepted method.   Drilling a starter hole to get most of the meat out is how to start of course,  just leave enough material for the boring bar to true things up.

 

I went through the same problems you are experiencing.    If I recall correctly, Cutter is the one that suggested boring the holes as the best method.    He was absolutely correct !   The hole positions were orders of magnitude improved.

 

Making the bolt carrier in 2 pieces also helps a lot,   the depth of the holes are much less than one piece.   Although its 2 parts instead of one, its actually easier that way with better results.

[bruski]

If I may interrupt this thread and say Ditto. 

 

Thanks,

 

bruski 

[Larryx]

 I will drill the holes suitable for boring ( per Sparky\'s suggestion)  I appreciate your knowledge and will follow your suggestion as to  bore the final holes. I was apprehensive regarding the 2 piece carrier but I believe I can do the work. Common wisdom seems to be  if one has multiple operations on multiple pieces or places, do all of the  similar tasks sequentially , rather than sequentially on each part or place . Transferring that technique to the task at hand, I will do  the following unless wiser heads suggest otherwise:  Make the sandwich comprised of the  front barrel plate, the rear barrel plate and 2 sections which will comprise the carrier block.  Once the proper clocking (locating?) holes are drilled and the appropriate  pin(s) are (is ) installed, I bore out each of the 0.420 holes.  I will machine the flats on that piece. I can then mount the  other piece of the carrier block and  go to work on the 0.280 holes and  bore them to proper size. Sorry to bother you on such elementary detail but  it will be too easy to \"fall off the sled\"  when working in what for me will be uncharted territory.

[Larryx]

per bruski\'s suggestion, I acquired an Erickson micro boring head at the Cabin Fever  Expo this weekend. IT is  adjustable to a tenth of a thousandth, so the small hole boring should be straight forward. strange how things work out. it started snowing Saturday morning with a couple of inches accumulation per hour so I decided to leave town before I got stranded in beautiful downtown Lebanan Pa. The last vendor I visited, Jim Beaver, who wears his beaver top hat at these shows. I asked if he had a small boring head (I had asked half a dozen other vendors the same question previously) and he produced this pristine Erickson model 50 version. I mailed it with the other items I acquired, so I should have it by the end of next week.  by way of explain action, the Expo is in Pa and I live in south Florida a distance of about 65 degrees F at this time of the year, Since I have to fly to get there, I have to ship (mail) the stuff I pick up.

[Cutter]

For what it’s worth

I would suggest a clearance hole at the bottom when boring the brass.

The chips need to vacate the hole, or holes can get ugly fast.

[Roller]

Amen to that!

[Larryx]

Started my Delrin practice carrier block and  very quickly realized I need  some additional experienced advice. As shown on the previously posted photos, I have the  un-machined block mounted in the 4 jaw vice. (I have a tail stock supporting the  \"smaller end\" _at least after machining  - this is not  shown in the earlier posted photos) Obviously this leave the  end being milled open to the  end mill.  . The print  requires that exactly 1/2 of the 0.281 holes  remain after the flats are milled. Is it sufficient to use a 0.281  round stock \"gauge\" which has had a flat milled 1/2 into it\'s diameter? ( D shaped)

[gbull]

That would work. You could also use a dial indicator depth gage with a pointed tip to measure the .2405 depth. Depending on your equipment there are many ways to do it. You could also make a dummy bolt milled to a D shaped end and then mill the carrier block until it slips thru.

[Larryx]

 I like gbull\'s suggestion. Not having any experience, I thought about doing that but did not know if it would work. Keeping in mind that the current effort is on Delrin, it is a bit more challenging to obtain a  flat machined surface. The end mill does a bit of smearing in the softer material rather than cutting as in metal. The lack of experience also me making the parts on faith and hoping all will come together and function when completed. Thanks again for the suggestions.