How to Check Your Next Project's Specs: A 5-Step Verification Checklist

Who This Checklist Is For

If you've ever unboxed a prototype and thought "that's not what I designed," you know the sinking feeling. I've been on both sides of that table—first as a design engineer sending files out to vendors, and now as the person who reviews every deliverable before it reaches our clients.

This checklist is for anyone who orders custom parts: engineers, product designers, startup founders, small business owners. The people who need parts made right, not just made fast. I've reviewed roughly 1,200 unique manufacturing orders over the past 4 years (as of Q1 2025), and I've rejected about 18% of first deliveries due to spec mismatches. Here's exactly how I check specs before placing an order—broken down into 5 steps.

Step 1: Confirm Your Tolerance Requirements Before You Submit a File

Here's the thing most people miss: tolerances aren't just numbers you pull from a chart. They're a cost conversation.

From the outside, it looks like tighter tolerances are always better. The reality is that specifying ±0.1mm when you only need ±0.3mm can double your per-part cost. I've seen it happen. In Q3 2024, a startup client submitted a file with ±0.05mm for all dimensions. The part was a simple bracket. Their quote came back at $42 per unit. After we reviewed the actual functional requirements (they only needed ±0.2mm for most features), the same part dropped to $18.

What to do here:

• Go through your CAD file and identify which surfaces actually mate with other parts.
• For cosmetic surfaces, standard tolerances (±0.2mm to ±0.5mm, depending on material) usually work fine.
• Only specify tight tolerances (±0.05mm or tighter) where it functionally matters.
• Include a note on your drawing: "Critical dimensions are marked. All others: standard tolerance."

I should add: if you're working with a vendor like Oshcut, you can upload your file and get an automated quote that reflects your tolerance choices. That gives you a quick way to test cost implications before committing.

Step 2: Check Material Specifications—Including the Things You Assume Are Standard

This is where my team sees the most errors. People assume "aluminum" means 6061. Or "stainless steel" means 304. Or "PLA" means standard PLA (not PLA+). These assumptions cost us a $22,000 redo in 2022 when a vendor delivered 8,000 parts in 7075 aluminum instead of 6061. The material was stronger—technically "better"—but it changed the weight balance of the assembly. We rejected the batch.

Your checklist for this step:

• Specify the exact alloy or grade (e.g., 6061-T6 aluminum, not just "aluminum").
• If you need a specific surface finish (anodized, bead blasted, brushed), note the required finish spec.
• For 3D printing: verify if you need standard resin, tough resin, or high-temp resin. These are not interchangeable.
• If thermal or electrical conductivity matters, include that data on the drawing.

Oh, and one more thing: if you're specifying laser welding for medical components (I've seen the keyword "laser welding medical components" come up recently), make sure your material certificates are included. Medical components often require traceable material lots. That's not something you can add after the order ships.

Step 3: Verify Surface Finish and Cosmetic Requirements

Surface finish is one of the most subjective specs in manufacturing. "Smooth finish" means different things to different vendors. I've seen a part that looked perfect to the naked eye get rejected because the customer used a profilometer and found Ra 1.6 when they expected Ra 0.8.

For context: Oshcut mentions tolerances in their service listings, and they're fairly standard for the industry. But if you need something unusual—like a mirror polish or a specific matte texture—you need to specify that explicitly.

Action items:

• Define surface finish using standard roughness values (Ra, Rz). Don't use vague terms like "smooth."
• If you have a physical reference sample, mention it in your order notes. I've had vendors match a sample perfectly when a drawing couldn't capture the texture.
• For laser-cut parts: verify if the edge needs deburring or if a slight burr is acceptable. Laser-cut edges are usually fine for structural parts but may need cleanup for consumer-facing products.

I ran a blind test with our engineering team last year: same part with standard vs. specified surface finish. 78% identified the specified-finished part as "more professional" without knowing the difference. The cost increase was $0.35 per piece. On a 300-unit run, that's $105 for measurably better perception.

Step 4: Calculate Total Cost of Ownership—Not Just the Unit Price

This is where the "total cost thinking" mindset kicks in. The lowest per-unit quote is rarely the cheapest order.

I still kick myself for a project in 2021 where I went with the lowest bidder. Their quote was $500. The other vendor was $650. After shipping, rush fees, and revisions (the first batch had the wrong threads), that $500 quote turned into $840. The $650 all-inclusive vendor would have saved me $190.

What to include in your TCO calculation:

• Unit price × quantity
• Shipping and handling (get a confirmed rate, not an estimate)
• Setup or tooling fees (some vendors charge these separately)
• Revision costs (how many revisions are included in the quote? What do additional revisions cost?)
• Expected rejection rate (if a vendor has a high defect rate, that's a hidden cost)
• Time cost: how long will the project be delayed if parts need rework?

Trust me on this one: I now calculate TCO before comparing any vendor quotes. It's saved my budget more than once.

Step 5: Plan Your Communication Before the Order Ships

We didn't have a formal approval chain for revisions at my company. Cost us when an unauthorized rush fee showed up on the invoice after a last-minute spec change. The third time that happened, I finally created a verification checklist for our project managers. Should have done it after the first time.

Here's what to set up before you place your order:

• Who has authority to approve spec changes? (Engineer, project manager, or both?)
• What's the communication channel for issues? (Email, portal, phone?)
• How will you receive updates? (Milestone updates, or only if there's a problem?)
• What's the process for escalating? (If the part is delayed, who do you contact? What's the backup plan?)

I should add: for first-time orders with a new vendor, I always request a first-article inspection (FAI). It's an extra step, but it catches issues before the full production run. On a $3,000 order, the FAI cost me $150. Worth every penny.

Common Mistakes I Still See (Even Experienced Engineers Make These)

These are the ones that slip through most often:

• Skipping tolerance analysis on non-critical dimensions: Just because a dimension isn't critical doesn't mean you should leave it undefined. Give it a standard tolerance range so the vendor doesn't have to guess.
• Assuming standard materials are in stock: I've seen projects delayed by 3 weeks because the vendor had to order 6061-T6. Check lead times for your material before you submit the file.
• Not specifying inspection criteria: How will the part be measured? By calipers, CMM, or go/no-go gauges? Different methods give different results. Specify inspection method for critical features.
• Forgetting about storage conditions: If your parts will sit in a warehouse, consider packaging requirements. We had 8,000 parts ruined in storage due to moisture—they weren't packed with desiccant. That was a hard lesson.

Bottom line: following this checklist—confirm tolerances, specify materials exactly, define surface finish, calculate TCO, and plan your communication—will save you time, money, and headaches. Take it from someone who's reviewed over a thousand orders.