Plastic Part - Using conical location bosses as targets for secondary & tertiary datums ISO draw
Plastic Part - Using conical location bosses as targets for secondary & tertiary datums ISO draw
(OP)
Hi Everyone
The attached pdf shows a simplified version of a plastic part I am working on. This part mates with a pcb with optical components which needs to be located precisely relative to features on the plastic part. As you can see in pdf I am planing to use the top of the screw bosses as targets for datum A (primary), none controversial I hope. In creating datum B (secondary) & datum C (tertiary) I would ideally like to use the conical (because of draft) location bosses next to the screw bosses. This is because these features are the nearest things I have to a direct link to the mating pcb, if I use other features I feel that I will be de-emphasizing the function of the component for ease of drawing & inspection. Options I have so far thought of are as follows:
Option 1 (easy, but suspect this is not GT&D legal)
Use the axes of 2 location bosses to create datum B (secondary) then use the axis of the back location boss to create datum C (tertiary)
Option 2 (fiddly, but maybe is GT&D legal)
The top of the location bosses are small (dia 2mm) flat circles I could place datum target in the centre of the circles to create the datums (2 targets for datum B, 1 target for datum C etc...). If this approach is 'legal' would it be best to position the targets using GT&D or 'normal' dimensions?
If anyone could give me a pointer as to which of the above 2 options (or maybe a different option) is the best way to proceed it would be much appreciated.
Thanks in advance.
The attached pdf shows a simplified version of a plastic part I am working on. This part mates with a pcb with optical components which needs to be located precisely relative to features on the plastic part. As you can see in pdf I am planing to use the top of the screw bosses as targets for datum A (primary), none controversial I hope. In creating datum B (secondary) & datum C (tertiary) I would ideally like to use the conical (because of draft) location bosses next to the screw bosses. This is because these features are the nearest things I have to a direct link to the mating pcb, if I use other features I feel that I will be de-emphasizing the function of the component for ease of drawing & inspection. Options I have so far thought of are as follows:
Option 1 (easy, but suspect this is not GT&D legal)
Use the axes of 2 location bosses to create datum B (secondary) then use the axis of the back location boss to create datum C (tertiary)
Option 2 (fiddly, but maybe is GT&D legal)
The top of the location bosses are small (dia 2mm) flat circles I could place datum target in the centre of the circles to create the datums (2 targets for datum B, 1 target for datum C etc...). If this approach is 'legal' would it be best to position the targets using GT&D or 'normal' dimensions?
If anyone could give me a pointer as to which of the above 2 options (or maybe a different option) is the best way to proceed it would be much appreciated.
Thanks in advance.





RE: Plastic Part - Using conical location bosses as targets for secondary & tertiary datums ISO draw
Second, use all four tapered posts, at a set diameter as datum target lines B1 thru B4. In the DRF, reference A and B(M). All d.o.f. are eliminated, and it reflects the actual functionality of your workpieces. This works with either '94 or '09.
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Plastic Part - Using conical location bosses as targets for secondary & tertiary datums ISO draw
1/ Using all 4 boss tops as Datum A targets - Agreed
2/ Using 'target datum lines' sounds interesting & had not occurred to me. However, I could do with a bit more detail as to how the datum lines are positioned on the part. I am guessing from your description that this solution work something like this:
- 'use all four tapered posts' = This puzzles me (so I suspect that I am misunderstanding), why do I need all 4 posts? Surly 3 would be enough, 2 for the secondary datum & 1 for the tertiary datum.
- 'at a set diameter' = I am guessing this is equivalent to a set hight up the conical location posts?
- 'datum target lines B1 thru B4' = Laying a datum target line across two location posts at a dimensioned height?
- 'In the DRF, reference A and B(M)' - Understood no problems with this, I assume that '(M)' is MMC
Am I reading your response correctly or have I got it totally wrong?Any further clues to the ignorant gratefully received.
RE: Plastic Part - Using conical location bosses as targets for secondary & tertiary datums ISO draw
Use all 4 posts because you don't know which posts will actually make contact with the holes of the PCB. If you arbitrarily select only two or three, then you have a significant chance of not being right.
Using all 4 gives you a pattern of datum features establishing the datum, without priority of one over any others. This is reflective of how the part actually mates. The primary datum (A) takes away 3 dof. The pattern of datum features eliminates all 3 remaining dof at once.
I would suggest putting the datum target lines at the same height as the primary datum target areas. The simulator for the 4 posts would represent the 4 holes in the PCB; the 4 lines of contact should be at the MMC size of the holes in the PCB (worst-case fit).
(M) is indeed MMC.
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Plastic Part - Using conical location bosses as targets for secondary & tertiary datums ISO draw
RE: Plastic Part - Using conical location bosses as targets for secondary & tertiary datums ISO draw
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Plastic Part - Using conical location bosses as targets for secondary & tertiary datums ISO draw