[How-To] Translating Bolt Pattern

[Ben17484]

Very newbie question here, but I want to copy the bolt whole pattern on to my new bracket:

I don’t have transfer bolts, and I can’t get a good enough read on the size of the bolt holes. I have a Milling machine with DRO and rotary table, so I feel there should be an easy ish way to do this.


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[ErichKeane]

So making transfer bolts is pretty easy, so you could do that way. It becomes easier if you have access through the back (at that point, its just making a point on a bolt!, or just using a transfer punch).

Since you have the DRO, you can just measure and do it that way. If you can come up with center-to-center distance between two holes, you can figure out the diameter of the circle.

 

[Ben17484]

So making transfer bolts is pretty easy, so you could do that way. It becomes easier if you have access through the back (at that point, its just making a point on a bolt!, or just using a transfer punch).

Since you have the DRO, you can just measure and do it that way. If you can come up with center-to-center distance between two holes, you can figure out the diameter of the circle.

Thanks Erich. The centre to centre method seems to always be where I mess up. I do the outer edge of the holes distance minus the hole diameter and always seem to mess up. I’ll give it another go and see how i fair.

I didn’t know if there’s a simpler method that I’m just not aware of. Guess I’ll need to learn how to do holt hole patterns on my dro!


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[RJSakowski]

There are a number of ways to transfer a hole pattern to a new part. The simplest way is to use a transfer punch. I have made custom punches to fit a particular need. It does require that the existing part fit closely to the new surface to get an accurate transfer.

Another way is to measure the existing hole diameter and then measure either the inside to inside or outside to outside distance between adjacent holes. If measuring the inside to inside distance, add the hole diameter to get the center to center distance. If measuring the outside to outside distance, subtract the hole diameter to get the center to center distance. Do this for each of the sets of holes. If the distances all match, you can assume that you have an equilateral triangle and your bolt circle diameter can be determined with a little geometry and some math. (The bolt circle diameter will be 1.253 times the distance between holes.)

Another way is to put close fitting pins in the holes and measure their diameter and outside to outside distance between the pins. I prefer this as it is difficult to get a good measurement of the distance between small holes with a caliper as in the above paragraph. For the part that you show, the countersink is a good datum so I would use flat head screw with a nut on the reverse side. The head of the screw will register on the countersink so the diameter of the screw isn't critical to a good measurement.

Other methods include using your DRO as a comparator determine coordinates of the holes. I have a DIY microscope that mounts in the spindle and in conjunction with the DRO, I can determine features to +/- .0001". Based on the coordinates measured, the distances between holes can be determined. It is easiest to use a CAD program as the coordinates can be entered as points and the distances can be measured in the drawing so no math is required. Most CAD programs allow you to draw a circle based on three points on the circle and doing so will create your bolt circle of which, you can measure the diameter.

 

[Charles scozzari]

Hi, this might help. I set this up quick to help you out. Imagine the hub in the vice as your rotary table. Hopefully you have a chuck mounted on the rotary table. Lock the part you need to copy in the chuck. With the table on Zero move the mill table until your run the quill down with a rod or center punch in the mill chuck. Once you do that lock the X and Y of the mill table. Put the piece to get the new holes drilled in the R/T and lock it tight. Divide for whatever pattern is needed by rotating the table to the next hole 90 -120 - 72 - and continue until complete. As long as you don't move the X Y the pattern will remain. Turn that square stock you are using to the finished diameter so you can lock it in the R/T.

 

[Ben17484]

Thank you Charles and RJ for your detailed responses, it’s really helpful.

I decided to tried measuring the distance between the holes and using the bolt hole pattern on my DRO. I used the little machine shop calculator to work out the diameter of the bolt hole diameter and plugged the values in to my DRO BHP function. It was clear that something had gone wrong as the X,Y coords it gave me weren’t even close to the pattern needed. I then used the Bolt Circle Calculator on LMS and got the correct X/Y coords. After drilling the holes, they were a little off. Even though I’d measured everything several times and taken the average measurement, I’d obviously done something wrong. Measurements taken again, second attempt was much closer.

(Please ignore the bad state of the bracket at the moment, the bolt hole pattern was going to be the hardest part for me, so I wanted to get that done before cleaning everything else up)

Still not perfect, but close enough for this job. I need some more practice on my measuring, but I’m really happy to have a new skill.

Now for the DRO - I think I need an end angle set (in this case to 240) in order to get proper values. Otherwise, if I set it to 360 or 0, I get no Y coordinates. I’ll dig up the manual for the DRO and read up on it tomorrow. It’s nice to know the LMS site has all of the details to work out the coords if I need it though.


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[Charles scozzari]

Thank you Charles and RJ for your detailed responses, it’s really helpful.

I decided to tried measuring the distance between the holes and using the bolt hole pattern on my DRO. I used the little machine shop calculator to work out the diameter of the bolt hole diameter and plugged the values in to my DRO BHP function. It was clear that something had gone wrong as the X,Y coords it gave me weren’t even close to the pattern needed. I then used the Bolt Circle Calculator on LMS and got the correct X/Y coords. After drilling the holes, they were a little off. Even though I’d measured everything several times and taken the average measurement, I’d obviously done something wrong. Measurements taken again, second attempt was much closer.

(Please ignore the bad state of the bracket at the moment, the bolt hole pattern was going to be the hardest part for me, so I wanted to get that done before cleaning everything else up)

Still not perfect, but close enough for this job. I need some more practice on my measuring, but I’m really happy to have a new skill.

Now for the DRO - I think I need an end angle set (in this case to 240) in order to get proper values. Otherwise, if I set it to 360 or 0, I get no Y coordinates. I’ll dig up the manual for the DRO and read up on it tomorrow. It’s nice to know the LMS site has all of the details to work out the coords if I need it though.


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No problem, thanks for getting back to me. Charlie.

 

[ErichKeane]

Chinese DROs you either need to set the end angle correctly, oe do an extra bolt. So instead of 3, do 4, and 1 and 4 will be the same hole.

 

[Ben17484]

Chinese DROs you either need to set the end angle correctly, oe do an extra bolt. So instead of 3, do 4, and 1 and 4 will be the same hole.

I’ve just tried both things you mentioned and they both work for my DRO. Thanks for the info.

It’s a bit annoying that you have to do these work arounds, but I guess it’s something that’ll become second nature after I’ve done a few.


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[BGHansen]

Glad you got it to work! A cumbersome way I've done it is to clamp the original part to my mill table and the one to be worked/drilled in the vise. I found the center of the original part and made that my ABSolute coordinate (0, 0). Then moved over to the part to be worked and found its center, switch to INCremental coordinates and zero'd there. Then back to the original part in ABS, with a tap drill in a drill chuck and found the center of a hole. Switched back to INC and moved to that same coordinate on the part to be drilled. Pretty sure that's how I did it.

Bruce